3.1.4 TRANSPORT INTO AND OUT OF CELLS

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What are the factors affecting rate of active transport?

pH/temp (tertiary structure of carrier protein), speed of carrier proteins, rate of respiration (ATP production)

potato more negative water potential/hypertonic; water enters by osmosis; cells extend/are turgid; little/no water remaining in potato/fully plasmolysed/all water has moved out; cell wall prevents further shrinkage or, water potentials are equal/equilibrium/isotonic; no net movement of water/no further osmosis;

Both active transport and facilitated diffusion....

use transmembrane proteins OR transport hydrophilic / charged molecules.

Q: Plantago maritima is a plant that grows in salt marshes around the coast of Britain. It is able to survive in its natural habitat without wilting by maintaining a higher concentration of solutes in its cells than in the surrounding water. Explain why many species of plant are unable to survive in this habitat.

water potential in cells is higher than the surrounding water; water moves out of the cells; by osmosis/diffusion; water cannot be taken up by the plant/ plant loses water; linked to metabolic reaction/photosynthesis;

Describe how blood capillaries are adapted by diffusion.

—> Ventilation + circulation of air and blood —> maintain a high concentration gradient —> alveoli / capillary have thin walls (1 cell thick) —> reduces the diffusion distance —> Alveoli have huge surface area (folded)

Name the equation for making serial dilutions.

(Total vol / known conc ) x desired conc = vol of known solute concentration (60 / 1.2) x 0.8 = 40cm3 Make 30cm3 of 0.75 mol dm-3 NaCl from a stock solution of 1.8 mol dm-3 (30 / 1.8) x 0.75 = 12.5cm3 solute so (30 - 12.5) = 17.5 cm3 water needed

Q: The diagram shows part of a plasma membrane. Give one function of the molecule labelled Y. (1 MARK)

(surface / extrinsic protein) for cell recognition / binding to hormones / identification

Q: Figure 2 below shows the absorption of certain nutrients from the lumen of the small intestine into the blood. Name the membrane transport processes taking place at A and B in Figure 2. (2 MARKS)

- A = Co-transport - B = facilitated transport (Transport protein A is transporting both sodium and glucose across the membrane of the epithelial cell at the same time, so we can identify this as cotransport. The movement of glucose into the blood at B involves a transport protein but no ATP so it must be facilitated diffusion)

Describe the role of channel proteins (some are gated) and carrier proteins in the cell membrane in terms of the entry and exit of molecules in a cell.

- Allows movement of water soluble / polar molecules / ions down a concentration gradient (facilitated diffusion) - Provide structural support - Allow active transport across the membrane - Cell recognition / receptors

Explain the structure of cholesterol within the cell surface membrane.

- Binds to phospholipid hydrophobic fatty acid tails

Q: Blood plasma contains glucose dissolved in water. Glucose is a polar molecule that is taken up by muscle cells and used in the synthesis of glycogen. Describe how glucose enters musxcl cells through the membrane. (2 MARKS)

- Carrier proteins in membrane - Glucose enters by facilitated diffusion

Describe how ATP is used in active transport.

- Molecule binds to carrier protein and ATP attaches to the membrane protein on the inside of the cell - Binding of phosphate molecule to protein molecule causes the protein to change shape so that access for the molecules is open to the inside of the membrane but closed outside.

Describe active transport.

- Net movement of molecules / ions against a concentration gradient - From low to high concentration - Via carrier proteins - Using energy from the hydrolysis of ATP to change the shape of the tertiary structure and push the substances through

Q: Give two differences between facilitated diffusion and osmosis. (2 MARKS)

- Osmosis only involves movement of water; - Facilitated diffusion involves proteins;

Q: Figure 1 below shows a cell surface membrane and some molecules of a small nonpolar substance known as substance X. Give a possible name for substance X.

- Oxygen - CO2

EQ: Figure 1 I evidence for monoglycerides being lipid-soluble molecules. Suggest how. (2 MARKS)

- Rate of uptake is proportional / doesn't level off (so diffusion occurring) - (Lipid soluble molecules) diffuse through / are soluble in phospholipid bilayer

Active transport uses ATP in two ways, explain them.

- Used directly by carrier proteins to move molecules as an immediate source of energy - By using a concentration gradient that has already been set up by direct active transport (co-transport)

Q: Describe how proteins are arranged in a plasma membrane and the part they play in transporting substances into and out of cells. (6 MARKS)

1 Some proteins pass right through membrane; 2 Some proteins associated with one layer; 3 Involved in facilitated diffusion; 4 Involved in active transport; 5 Proteins act as carriers; 6 Carrier changes shape / position; 7 Proteins form channels / pores; 8 Protein allows passage of water soluble molecules

EQ: Explain why the structure of a membrane is described as 'fluid-mosaic'.

1. Idea of phospholipid molecules moving = fluid; 2. Idea of proteins (embedded) in membrane in separate locations = mosaic;

EQ: Figure 1 shows part of the structure of a plasma membrane. Add the missing information to complete Table 1 (3 MARKS)

A ‒ Phospholipid; Hydrocarbon chain - E; C ‒ (channel/carrier) protein;

Q: Identify the structures labelled in Figure 1. In each case, state one function of the structure that has bene identified. (5 MARKS)

A1: phospholipid head; attracts water and polar solutes A2: phospholipid tail; attracts lipids soluble molecules and barrier to polar molecules B: glycolipid; receptor / antigen / recognition C: carbohydrate molecule: receptor / antigen / recognition D: glycoprotein: receptor / antigen / recognition E: cholesterol; regulates membrane fluidity F: extrinsic protein; bind to carbohydrates / act as receptors G: intrinsic protein (channel / carrier); osmosis / active transport / diffusion

What is a solute?

Any substances that is dissolved in a solvent.

Difference between active transport and facilitated diffusion:

Both use carrier proteins but facilitated diffusion occurs down a concentration gradient, whereas active transport occurs against a concentration gradient. active transport required ATP.

What are aquaporins?

Channel proteins that allow for osmosis across membranes.

Why is the fluid mosaic model referred to as 'fluid' and 'mosaic'?

Fluid: Molecules within membrane (phospholipids) can move laterally / slide over each other / sideways (fluid) Mosaic: membrane contains mixture of both Proteins embedded and phospholipids bilayer, vary in shape, size and pattern

What are intrinsic protiens?

Go through phospholipid bilayer from one side to the other. Function: carriers to transport water soluble materials across membrane, enzymes.

Q: Compete the table by placing a tick in the appropriate boxes. (2 MARKS)

Look at image

Draw a fluid mosaic model of a cell membrane.

Look at image.

Define the cell surface membrane.

Phospholipid bilayer with embedded proteins etc. Selectively permeable; enables control of passage of substances in and out of cell. Barrier between internal and external environment of cell

Q: Which component of a cell membrane denatures as temperature increases? (1 MARK)

Proteins

Q: A group of students investigated the effect of sucrose concentration on the change in length of cylinders of tissue cut from a young carrot. They measured the initial lengths of the carrot cylinders, then placed one in each of a number of sucrose solutions. After 18 hours, they removed the carrot cylinders and measured their final lengths. Some of the results are shown in the table. The carrot cylinders were left for 18 hours in the sucrose solutions. Explain why they were left for a long time. (1 MARK)

Reaches equilibrium

Q: Cholesterol increases the stability of plasma membranes. Cholesterol does this by making membranes less flexible. Suggest one advantage of the different percentage of cholesterol in red blood cells compared with cells lining the ileum. (1 MARK)

Red blood cells free in blood (not supported by other cells) so cholesterol helps to maintain shape

EQ: Suggest the function of cholesterol in the plasma membrane. (1 MARK)

Restricts the movement of other molecules making up the membrane;

What are the factors affecting rate of facilitated diffusion?

Surface area, concentration gradients (until the number of proteins is the limiting factor as all are in use / saturated), number of channel / carrier proteins

What are extrinsic proteins?

Surface of bilayer / partly embedded. Function: mechanical support to membrane, conjunction with glycolipids, as cell receptors for molecules eg hormones

Define osmosis.

The passive Net movement / diffusion of water molecules across a selectively / partially permeable membrane down a water potential gradient from high water potential to low water potential. - passive, down concentration gradient

What happens if a cell is placed in a hypotonic solution?

There will be a net flow of water into the cell by osmosis, and the cell will gain volume, may swell & eventually burst. If the solute concentration outside the cell is lower than inside the cell, and the solutes cannot cross the membrane then that solution is hypotonic to the cell.

EQ: Students investigate the effect of different concentrations of sucrose solution on epidermal cells from a red onion bulb. The students dilute a 1.0 mol dm- sucrose solution with water to give a range of sucrose concentrations. The students put pieces of epidermis in the different concentrations of sucrose solution on microscope slides. The students put a graticule in the eyepiece of the microscope before observing the cells. A graticule is a glass disc with a numerical scale on it. Figure 1 is a photograph of some epidermal cells in water, seen through the eyepiece graticule. Figure 3 shows the same epidermal cells when the students replace the water with 1.0 mol dm-s sucrose solution. The students observe the onion epidermal cells in each of the six concentrations of sucrose given in Table 1. Cells that look similar to cell A in Figure 3 are described as 'plasmolysed'. The students look at 20 cells in each concentration of sucrose and count how many are plasmolysed. The students calculate the percentage of plasmolysed cells for each concentration of sucrose. Figure 4 shows the students results. Table 2 gives the osmotic potential of each concentration of sucrose solution. (a) At equilibrium, 50% of the cells are plasmolysed. At equilibrium, the osmotic potential of the onion cells equals the osmotic potential of the sucrose solution surrounding the cells. What is the approximate osmotic potential of the onion cells? Use information from Figure 4 and Table 2. - -2580 kPa - -1030 kPa - 0 kPa - 0.37 kPa (1 MARK) (b) Suggest two improvements the students could make to find a more accurate value for the osmotic potential of the onion cells. (2 MARKS)

(a) -1030 kPa (b) 1. Repeat at closer intervals of sucrose concentration (around Ys of cells); 2. Count> 20 cells (each time);

Describe channel proteins.

(facilitated) transport small polar (charged) molecules through its pore, e.g. ions (some are gated so can open / close e.g. voltage-gated sodium ion channels)

Q: Explain each of the following processes by which substances pass through a cell surface membrane. (i) diffusion of oxygen (ii) facilitated diffusion of glucose (iii) active transport of sodium ions (9 MARKS)

(i) Random movement of oxygen molecules; (Net) movement from high to lower concentration; Molecules can pass between phospholipid molecules / through bilayer; Rate dependent on concentration gradient. (ii) Movement from high to lower concentration; Use of carrier / channel / intrinsic protein; No energy/ATP needed. (iii) Movement against concentration gradient; Use of carrier/channel/intrinsic/pump proteins; Energy / ATP required; Ions moved through membrane as proteins change shape / position.

Q: Six cylinders of a standard size were cut from a single large potato. One cylinder was placed in distilled water and the others were placed in sucrose solutions of different concentrations. The length of each cylinder was measured every 5 minutes for the next 50 minutes. The graph shows the changes in length at each sucrose concentration. (i) Explain why: the potato cylinder in distilled water increased in length, the potato cylinder in the 1.0 mol dm-3 sucrose solution showed no further decrease in length after 40 minutes. (ii) Describe the difference in the rate of decrease in length during the first 10 minutes between the cylinder in the 0.4 mol dm-3 and the cylinder in the 0.8 mol dm-3 solution (iii) Use your knowledge of water potential to explain this difference.

(i) potato more negative water potential / hypertonic; water enters by osmosis causing cells to become turgid; (i) little / no water remaining in potato / fully plasmolysed / all water has moved out; cell wall prevents further shrinkage / sucrose solution moves in; (ii) faster rate (of decrease) in 0.8 mol dm-3; 1 (iii) bigger water potential gradient / greater difference in water potentials (between potato and surrounding solution);

Describe carrier proteins.

(remember as aircraft carrier; big boat so it carried large molecules) transport large molecules, the protein changes shape when molecule attaches

Q: A student investigated the effect of putting cylinders cut from a potato into sodium chloride solutions of different concentrations. He cut cylinders from a potato and weighed each cylinder. He then placed each cylinder in a test tube. Each test tube contained a different concentration of sodium chloride solution. The tubes were left overnight. He then removed the cylinders from the solution and reweighed them. The student repeated the experiment several times at each concentration of sodium chloride solution. His results are shown in the graph. The student made up all the sodium chloride solutions using a 1.0 / mol dm-3 sodium chloride solution and distilled water. Complete table 1 to show how he made 20cm3 of 0.2 mol dm-3 sodium chloride solution. (1 MARK)

- 4 cm3 of 1.0 mol dm-3 sodium chloride solution - And 16cm3 of distilled water

Q: Figure 1 shows the structure of a cell surface membrane. Identify structures A-C in Figure 1. (3 MARKS)

- A: phospholipid bilayer - B: cholesterol - C: glycoprotein

EQ: Name two other biological molecules that always contain phosphorus. (2 MARKS)

- ATP - ADP - Phospholipids

Describe ATP and its role in biological processes:

- ATP cant be stored so has to be made continuously made by the mitochondria - ATP is used in energy-requiring processes in cells including: - Metabolic processes: (eg starch from glucose, polypeptides from amino acids) - Movement: contraction - Active transport: change shape of carrier proteins for ions to move against concentration gradient - Secretion: ATP needed to form lysosomes - Activation of molecules: phosphate released during ATP hydrolysis can be used to phosphorylate other compounds yo make them more reactive and lower activation energy (eg addition od phosphate to glucose molecules at start of glycolysis)

Q: Some proteins in the cell membrane are involved in active transport and facilitated diffusion. Describe the role of proteins in these cell transport mechanisms. (7 MARKS)

- Active transport: molecule binds into protein, protein changes shape - Molecules move against a concentration gradient - Use of ATP - Facilitated diffusion: proteins as channels - Channels can open or close - For larger / polar molecules to pass through membrane - Molecules move down a concentration gradient

Q: A biologist was investigating how surface area affects osmosis in potato cubes. Step 1: She cut two cubes of potato tissue, each with sides of 4cm in length Step 2: She put one cube into a concentrated sucrose solution Step 3: She cut the other cube into eight equal-sized smaller cubes and put them into a sucrose solution of the same concentration as the solution used for the large cube Step 4: She recorded the masses of the cubes at intervals Her results are shown in the graph in Figure 1. The scientist who collected the results in Figure 1 started with 1.0 mol dm-3 sucrose solution but for the investigation she needed to produce 40cm3 of a 0.2 mol dm-3 sucrose solution. Describe how she could produce this using the original 1.0 mol dm-3 sucrose solution. (2 MARKS)

- Adding 8 cm3 of 1.0 mol dm-3 sucrose solution to 32 cm3 distilled water

Q: A student investigated the effect of putting cylinders cut from a potato into sodium chloride solutions of different concentrations. He cut cylinders from a potato and weighed each cylinder. He then placed each cylinder in a test tube. Each test tube contained a different concentration of sodium chloride solution. The tubes were left overnight. He then removed the cylinders from the solution and reweighed them. The student carried out several repeats at each concentration of sodium chloride solution. Explain why the repeats were important. (2 MARKS)

- Allows anomalies to be identified - Makes the average more reliable / allows concordant results

Q: A student investigated the effect of putting cylinders cut from a potato into sodium chloride solutions of different concentrations. He cut cylinders from a potato and weighed each cylinder. He then placed each cylinder in a test tube. Each test tube contained a different concentration of sodium chloride solution. The tubes were left overnight. He then removed the cylinders from the solution and reweighed them. The student calculated the percentage change in mass rather than the change in mass. Explain the advantage of this. (2 MARKS)

- Allows comparison - As cylinders have different starting masses

Describe how the phospholipid bilayer in the cell membrane is adapted for the entry and exit of molecules in a cell.

- Allows movement of non-polar, small, lipid-soluble molecules e.g. oxygen or water, down a concentration gradient (simple diffusion) - Make the membrane flexible - Restricts the movement of larger / polar / hydrophilic / water soluble molecules (e.g. water is polar so doesn't diffuse through easily → moves into cells through aquaporins; water channels). **this is because the hydrophobic fatty acid tails in the centre of the phopsholipids bilayer

Q: Suggest two features of the structure of cholesterol that make it effective in performing its role in relation to membrane fluidity.

- Amphiphilic / contains hydrophilic and hydrophobic regions so it can enter and remain in the bilayer - flat structures means it can sit easily between layers of bilayer

EQ: Lymphocytes are a type of white blood cell. Scientists investigated the movement of substances in the cell-surface membranes of lymphocytes. Lymphocytes have molecules of a substance called immunoglobulin attached to their cell-surface membranes. The scientists added proteins called antibodies specific to immunoglobulin. They also attached a fluorescent marker molecule to the antibody molecule. Figure 2 shows the scientists' method and their results. Suggest why the fluorescence was able to move at 37 °C but not at 0 to 4 °C. Use your knowledge of the fluid mosaic model of the cell-surface membrane in your answer. (2 MARKS)

- At 37 °C, phospholipid is more fluid / molten; - so allows movement of proteins

Q: A group of students investigated the water potential of potato cells. They cut cubes of potato of equal size and shape, weighed them and placed a single cube into one of four different concentrations of sucrose solution. One cube was placed in pure water. They re-weighed each of the cubes every hour and after 12 hours the mass of all the cubes remained constant. The overall change in mass for each cube is shown in figure 3. If the experiment was repeated with cubes that have a larger surface area you would expect the mass of all the cubes to become constant before 12 hours, at 12 hours or after 12 hours? Explain your answer. (2 MARKS)

- Before 12 hours - Because the rate of osmosis will be faster due to increase in surface area

Q: How might cells be adapted for transport through their internal or external membranes

- By an increase in surface area - Increase in number of protein channels / carriers

Q: Some substance can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer. Describe other ways by which substances cross this membrane

- By osmosis from aa high water potential to a low water potential (down a water potential gradient) - Through aquaporins / water channels - By facilitated diffusion: channel / carrier protein, down a concentration gradient - By active transport: carrier protein against concentration gradient using ATP

Q: Ca2+ ATPases are carrier proteins that transport Ca2+ ions across cell-surface membranes. Each Ca2+ ATPase has one subunit that has an ATP binding site and acts as an enzyme. Explain why Ca2+ ions are always transported across cell-surface membranes via carrier proteins or channel proteins. (2 MARKS)

- Ca2+ ions carry a charge making them polar / hydrophilic - This makes it difficult to them to travel directly through hydrophobic centre of phospholipid bilayer

Name the transport proteins.

- Carrier proteins - Channel proteins.

Describe the direct active transport of single molecule/ion:

- Carrier proteins accept molecule/ion on one side to transport - molecule/ion bind to receptors on the channels of the carrier protein - On the inside of the cell, ATP binds to the protein, causing it to split to ADP and a phosphate molecule - The protein changes shape as a result and poems to the opposite side of the membrane molecule/ions then released to other side of membrane - Phosphate molecule is released from the protein and recombines with ADP to form ATP during respiration - This causes the protein to revert to its original shape, ready for the process to be repeated

Q: The lipid composition of red blood cell membranes is approximately 24% cholesterol. Phagocytes (a kind of white blood cell) contain around 11% cholesterol. Explain this difference.

- Cholesterol increases rigidity of membrane, low cholesterol membranes are more flexible/fluid - Phagocytes change shape substantially (engulfing pathogens) / red blood cells remain relatively fixed in shape - Phagocytes need more membrane fluidity so contain less cholesterol

Q: An onion cell membrane contains less cholesterol than an animal cell membrane. Suggest and explain why this is (2 MARKS)

- Cholesterol is responsible for giving cells rigidity / helping maintain the shape of cells - Onion cells have a cell wall which provides them with rigidity so they dont need as much cholesterol in their membranes as plant cells.

Q: Name the components of the plasma (cell surface) membrane labelled D, E and F. (3 MARKS)

- D: cholesterol - E: protein / glycoprotein / intrinsic protein / protein channel / protein pump / transport protein / carrier protein - F: phospholipid bilayer / phospholipid head

EQ: Describe how oxygen in air in the alveoli enters the blood in capillaries.

- Diffusion; - Across (alveoli) epithelium / (capillary) endothelium;

Q: Scientists investigated the percentages of different types of lipid in plasma membranes from different types of cell. Table 2 shows some of their results. The scientists expressed their results as a percentage of lipid in plasma membrane by mass. Explain how they would find these values. (2 MARKS)

- Divide mass of each lipid by total mass of all lipids - Multiply answer by 100

- Due to high uptake of more water - As higher water concentration outside potato OR idea of largest different in concentrations of solutions

EQ: Contrast the processes of facilitated diffusion and active transport. (3 MARKS).

- Facilitated diffusion involves carrier or channel proteins whereas active transport only involves carrier proteins. - Facilitated diffusion doesn't use ATP / is passive whereas active transport uses ATP - Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient

Q: Figure 1 and 2 show onions cells under a light microscope the cytoplasm appears dark grey. One of the figures shows the onions cells after they have been placed in a weak salt solution. The solution has a lower water potential than the onion cells. Which figure shows the cells after the have been placed in salt solution? Explain your answer. (2 MARKS)

- Figure 2 - Because the lower water potential of the salts solution has caused the water to move out of the cells down the water potential gradient - This has reduced the volume of the cell's cytoplasm

Q: Describe the routes that water molecules take through the cell surface membrane (2 MARKS)

- Fit between phospholipids / through phospholipid bilayer - Via protein channels / protein pores / aquaporins

Q: Complete the following paragraph about cell membranes, using the most appropriate terms. (5 MARKS) The model of cell membrane structure is called the __________ ________ model. Phospholipid bilayers with specific membrane proteins account for the ability of the membrane to allow both passive and ______ transport mechanisms. Ions and most polar molecules are insoluble in the phospholipid bilayer. However, the bilayer allows diffusion of most non-polar molecules such as _______. Protein channels, which may be gated, and ______ proteins enable the cell to control the movement of most polar substances.

- Fluid mosaic - Active - Fats / lipids / oils / cholesterol / oxygen / carbon dioxide / steroid hormones / fat soluble vitamins - Carrier / (co)transport(er)

Q: Some scientists investigated the uptake of magnesium ions in rice plants. They divided the plants into two groups and placed their roots in solutions containing radioactive magnesium ions. Group Y: plants had a substance that inhibited respiration added to the solution Group Z: plants didnt have the respiratory inhibitor added to the solution The scientist calculate the total amount of magnesium ions absorbed by the plants every 5 minutes. Their results are shown in the graph in Figure 1 below. Consider the graph in Figure 1. Explain the results of the student's investigation. (4 MARKS)

- GROUP Y: initial uptake of magnesium ions slower because diffusion only - GROUP Y: uptake of magnesium ions levels off because reaches equilibrium (same conc inside and outside cell) - GROUP Z: uptake of magnesium ions faster because diffusion and active transport - GROUP Z: uptake of magnesium fails to level off because uptake against gradient (no equilibrium to be reached) - GROUP Z: rate of uptake of magnesium ions slows because fewer magnesium ions in external solution used up

Explain the structure of Glycolipids and Glycoproteins in the cell membrane.

- Glycolipids: lipids and attached polysaccharide chain - Glycoproteins: proteins with polysaccharide chain attached (cell recognition)

EQ: Students investigated the uptake of chloride ions in barely plants. They divided the plants into two groups and placed their roots in solutions containing radioactive chloride ions. Group A plants had a substance that inhibited respiration added to the solution Group B plants didn't have the substance added to the solution. The students calculated the total amount of chloride ions absorbed by the plants every 15 minutes. Their results are shown in figure 4. Explain the results shown in Figure 4. (4 MARKS)

- Group A - initial uptake slower because by diffusion only - Group A - levels off because same concentrations inside cells and outside cells / reached equilibrium - Group B - uptake faster because by diffusion plus active transport - Group B - fails to level off because no equilibrium to be reached - Group B - rate slows because fewer chloride ions in external solution / as respiratory substrate used up.

Q: E. coli has no cholesterol in its cell-surface membrane. Despite this, the cell maintains a constant shape. Explain why. (2 MARKS)

- Has cell wall so cell unable to change shape - Wall is rigid / made of peptidoglycan / murein

Describe high water potential.

- Higher % of water molecules - Low % of solute - Low solute concentration - Hypotonic (less dissolved particles)

Q: A group of students investigated the water potential of potato cells. They cut cubes of potato of equal size and shape, weighed them and placed a single cube into one of four different concentrations of sucrose solution. One cube was placed in pure water. They re-weighed each of the cubes every hour and after 12 hours the mass of all the cubes remained constant. The overall change in mass for each cube is shown in figure 3. The students recorded the difference in mass between the cubes at the start and end of the experiment in grams, but plotted the overall change as a percentage. Suggest why the graph was plotted this way. (1 MARK)

- In the case that the cubes didn't all start out with the same mass - To enable a fair comparison between the cubes.

Q: Figure 1 below shows a cell surface membrane and some molecules of a small nonpolar substance known as substance X. State and explain how an increase in temperature would affect the rate of movement of substance Y across a cell membrane

- Increased rate of movement / diffusion - Because increasing temperature increases KE of molecules so causing them to move at faster rate

Q: State two examples of active transport in cells. For each example, you should name the substance that is transported and the cell involved.

- Ions into root hair cell - Hydrogen ions out of companion cells

EQ: Figure 6 shows a carrot. A group of students investigated the effect of sucrose concentration on the length of cylinders cut from a carrot. The students used a cork borer to cut cylinders from the carrot. Describe how the students should cut these cylinders to make sure this would produce reliable results. (2 MARKS)

- Lengthways/down the root; - Through one tissue only / through same part / same proportion of tissues;

Q: Vitamin D deficiency reduces the uptake of calcium ions by epithelial cells lining the small intestine. The diagrams show how calcium ions are transported through normal epithelial cells and those deficient in vitamin D. Use the information in the diagrams to explain how vitamin D deficiency reduces calcium ion uptake through gut epithelial cells. (2 MARKS)

- Less / no calbindin protein; - calcium not transported / moved (across the cytoplasm); - so diffusion gradient reduced at small intestine interface;

Q: A group of students investigated the effect of sucrose concentration on the change in length of cylinders of tissue cut from a young carrot. They measured the initial lengths of the carrot cylinders, then placed one in each of a number of sucrose solutions. After 18 hours, they removed the carrot cylinders and measured their final lengths. Some of the results are shown in the table. Explain how you would use a graph to predict the concentration of sucrose that would result in no change in length of the carrot cylinder. (2 MARKS)

- Line/curve of best fit; - find where it crosses x-axis;

Describe low water potential.

- Lower % water molecules - Higher % solute - Higher solute concentration - Hypertonic (more dissolved particles)

Q: The absorption of glucose into the cell leads to the movement of water into the cell. Explain how. (2 MARKS)

- Lowers water potential of the cell - Water moves in by osmosis

Describe the function of the phospholipid bilayer.

- Maintains a different environment on each side of the cell or compartmentalisation of cell - To form a selectively / partially permeable membrane - Allow non-polar small molecules to pass through Phospholipid bilayer is fluid: Can bend to take up different shapes for phagocytosis / to form vesicles

Q: In an osmosis investigation, a student prepared five pieces of raw potato of equal mass and a range of sucrose solutions of different concentrations. One piece of potato was placed in each sucrose solution. After two hours, the potato pieces were removed and blotted dry and the change in mass of each potato piece was calculated. The results are shown in the table below. The student suggested that there would be no change in the mass of a piece of potato placed in a sucrose solution of 0.75 mol dm-3. Give an explanation for this suggestion. (2 MARKS)

- Mass increased at 0.6 and mass decreased at 0.8 (mol dm-3) OR results were plotted onto a graph - Concentration is closer to 0.8 than 0.6 mol dm-3 as the decrease in mass is greater than the increase in mass - 0.11 is closer to zero than + 0.31

Q: The water potential of a plant cell is -400 kPa. The cell is put in a solution with a water potential of -650 kPa. Describe and explain what will happen to the cell. (3 MARKS)

- Membrane/cytoplasm shrinks/pulls away from cell wall/cell plasmolysed/ goes flaccid; - Water moves down water potential gradient/to lower/more negative water potential; - By osmosis;

Describe how the small intestine is adapted for diffusion in the body.

- Microvilli - Large blood supply - Intestine wall → one cell thick → short diffusion distance

Q: The epithelial cells that line the small intestine are adapted for the absorption of glucose. Describe one way in which epithelial cells are adapted for their purpose (1 MARK)

- Microvilli / large surface area - Many mitochondria produce ATP / release energy for active transport - Carrier proteins for active transport - Channel proteins for facilitated diffusion

EQ: Read the following passage. Human milk contains all the nutrients a young baby needs in exactly the right proportions. It is formed in the mammary glands by small groups of milk-producing cells. These cells absorb substances from the blood and use them to synthesise the lipids, carbohydrates and proteins found in milk. Milk-producing cells are roughly cube-shaped and have a height to breadth ratio of approximately 1.2 : 1. The main carbohydrate in milk is lactose. Lactose is a disaccharide formed by the condensation of two monosaccharides, glucose and galactose. (A molecule of galactose has the same formula as a molecule of glucose - the atoms are just arranged in a different way.) Lactose is synthesised in the Golgi apparatus and transported in vesicles through the cytoplasm. Because lactose is unable to escape from these vesicles, they increase in diameter as they move towards the plasma membrane. The vesicle membranes fuse with the plasma membrane and the vesicles empty their contents out of the cell. Use the information from the passage and your own knowledge to answer the following questions. Suggest one advantage of milk-producing cells containing large numbers of mitochondria. (2 MARKS)

- Mitochondria supply energy/ATP; - For active transport / absorption against concentration gradient / synthesis / anabolism / exocytosis / pinocytosis;

Q: Figure 1 shows a molecule (X) that is about to be transported across a cell membrane. Describe the events that would take place in order for molecule X in Figure 1 to move from the exterior of the cell into the cytoplasm. (3 MARKS)

- Molecule X binds to a specific binding site on the carrier protein - ATP is hydrolysed / broken down to release energy - The protein changes shape and releases molecule X into the cytoplasm

Q: Explain why the rate of diffusion is more rapid at higher temperatures. (2 MARKS)

- More (kinetic) energy; - Molecules are moving faster;

EQ: Give two similarities in the movement of substances by diffusion and osmosis (2 MARKS)

- Movement down a gradient / from high concentration to low concentration - Passive / no active processes OR don't use energy from respiration / from ATP

EQ: A scientist measured the rate of uptake of a monoglyceride and a monosaccharide by epithelial cells of the small intestine of mice. A monoglyceride is a molecule of glycerol with one fatty acid attached. She did this for different concentrations of monoglyceride and monosaccharide. Her results are shown here. Use your knowledge of transport across membranes to explain the shape of the curve in Figure 1 for uptake of monosaccharides between concentrations: (3 MARKS) A and B: C and D:

- Movement through carrier proteins OR facilitated diffusion - Rate of uptake proportional to external concentration - All Channel / carrier proteins in use / saturated / limiting

Q: Glucose is absorbed from the lumen of the small intestine into epithelial cells. Explain how the transport of sodium ions is involved in the absorption of glucose by epthelial cells. (5 MARKS)

- Na+ ions leave the epithelial cell and enter blood - Transport out is by active transport / pump / carrier protein using ATP - So, Na+ concentration in cell is lower than in lumen (of gut); maintains diffusion gradient for Na+ from lumen into cells - Sodium / Na+ ions enter by facilitated diffusion (from high to low concentration through a cotransport protein) - Glucose absorbed with Na+ ions against their concentration / diffusion gradient / glucose absorbed down an electrochemical gradient

EQ: Figure 1 shows the co-transport mechanism for the absorption of amino acids into the blood by a cell lining the ileum. The addition of a respiratory inhibitor stops the absorption of amino acids. Use Figure 1 to explain why. (3 MARKS)

- No ATP produced OR not active transport - Sodium ions not moved (into/out of cell) - No diffusion / concentration gradient for sodium to move into cell with amino acid

Q: the diagram below represents the approximate concentrations of oxygen and carbon dioxide inside an alveolus and a capillary in the lungs. These gases will diffuse at different rates. Using the information in the diagram, explain the difference in the rate of diffusion of these gases. (3 MARKS)

- O2 diffuses more quickly than CO2 - Different concentration gradients - Molecules are different sizes

Complete the passage below (5 MARKS): Membranes have a variety of functions in cells. All membranes are ........... permeable. This means that they allow the pages of certain substances by processes such as active transport or ............ through the membrane. The cell surface membrane, Also known as the ............ membrane, surrounds the cytoplasm. The cell surface membrane consists of a bilayer of ........... . to stabilize the structure of the membrane and keep it fluid, molecules of .............. are also found in this bilayer.

- Partially / selectively (DON'T ACCEPT semi) - (Facilitated) diffusion OR osmosis - Plasma - Phospholipids - Cholesterol

Explain the structure of the phospholipid bilayer (arrangment) within the cell membranes.

- Phosphate heads are hydrophilic (polar) so attracted to water - orientate to the aqueous environment either side of the membrane (face outwards) - Fatty acid tails are hydrophobic (non-polar) so repelled by water - orientate to the inside/interior of the membrane (face inwards)

Q: The cell's surface membranes contain phospholipids. Describe the arrangement of the phospholipids based on the fluid mosaic model of membrane structure. (2 MARKS)

- Phospholipids are arranged in a bilayer - With the hydrophilic heads facing outwards - And the hydrophobic tails facing inwards

Q: Scientists investigated the effect of a drug called a proton pump inhibitor. The drug is given as a tablet to people who produce too much acid in their stomach. It binds to a carrier protein in the surface membrane of cells lining in the stomach. This carrier protein usually moves hydrogen ions into the stomach by active transport. The scientists used two groups of people in their investigation. All the people produced too much acid in their stomach. People in group P were given the drug. Group Q was the control group. The graph shows the results. Suggest how the control group would have been treated. (2 MARKS)

- Placebo - Otherwise treated the same

EQ: A group of students carried out an investigation to find the water potential of potato tissue. The students were each given a potato and 50cm3 of a 1.0 mol dm-3 solution of sucrose. - They used the 1.0 mol dm-3 solution of sucrose to make a series of different concentrations - They cut and weighed discs of potato tissue and left them in the sucrose solution for a set time - They then removed the discs of potato tissue and reweighed them Table 3 shows how one student presented his processed results. Describe how you would use the students' results in table 3 to find the water potential of the potato tissue. (3 MARKS)

- Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis - Find concentration where curve crosses the x-axis / where percentage change is zero - Use another resource to find water potential of sucrose concentration (where curve crosses x-axis)

EQ: Figure 6 shows a carrot. A group of students investigated the effect of sucrose concentration on the length of cylinders cut from a carrot. The students then measured the final lengths of the carrot cylinders. Their results are shown in Table 2. The students used these results to find the concentration of sucrose that has the same water potential as the carrot cylinders. Describe how they could have done this. (2 MARKS)

- Plot data on a graph; - Find (sucrose concentration) from the graph where the ratio is 1;

Q: In an osmosis investigation, a student prepared five pieces of raw potato of equal mass and a range of sucrose solutions of different concentrations. One piece of potato was placed in each sucrose solution. After two hours, the potato pieces were removed and blotted dry and the change in mass of each potato piece was calculated. The results are shown in the table below. A second student wanted to perform this investigation by measuring the change in length of the potato pieces. The student was advised that this method would not be as accurate as weighing the potato pieces. Suggest two reasons why measuring the change in length wouldn't be as accurate as weighing the potato pieces. (2 MARKS)

- Potato pieces are not straight - Widths are different

Q: The graph shows the rate of uptake of a substance by facilitated diffusion into a cell. Using the information in the graph, explain why the rate of uptake changes. (3 MARKS)

- Rate increases when the concentration increases - This increases the concentration gradient - Plateau / leveling off of curve due to channel proteins being saturated with molecules OR idea that no more can be carried / max rate of entry reached

Q: A student investigated the movement of molecules into red blood cells. He placed red blood cells into a solution of molecule W. He then measured the concentration of W inside the cells over a period of 15 minutes. The graph below shows the results of his investigation. Using the information in the graph, describe the uptake of molecule W into the red blood cells during the 15 minutes of this investigation (3 MARKS)

- Rate of uptake is constant for first 5 minutes - Concentration of W reaches a maximum / no more uptake / stays at 0.6 dm-3 from 10 minutes - Suitable manipulation of figures e.g. rate of uptake in first 5 minutes is 0.1 moldm-3 per minute

EQ: Read the following passage. Human milk contains all the nutrients a young baby needs in exactly the right proportions. It is formed in the mammary glands by small groups of milk-producing cells. These cells absorb substances from the blood and use them to synthesise the lipids, carbohydrates and proteins found in milk. Milk-producing cells are roughly cube-shaped and have a height to breadth ratio of approximately 1.2 : 1. The main carbohydrate in milk is lactose. Lactose is a disaccharide formed by the condensation of two monosaccharides, glucose and galactose. (A molecule of galactose has the same formula as a molecule of glucose - the atoms are just arranged in a different way.) Lactose is synthesised in the Golgi apparatus and transported in vesicles through the cytoplasm. Because lactose is unable to escape from these vesicles, they increase in diameter as they move towards the plasma membrane. The vesicle membranes fuse with the plasma membrane and the vesicles empty their contents out of the cell. Use the information from the passage and your own knowledge to answer the following questions. Describe and explain how you would expect the height to breadth ratio of an epithelial cell from a lung alveolus to differ from the height to breadth ratio of a milk-producing cell. (2 MARKS)

- Ratio would be less/smaller; - Cell is thin / has large surface area / (adapted) for diffusion;

Describe how the lungs are adapted for diffusion in the body.

- SA : vol of alveoli (high surface area to volume ratio) - Large blood supply - Alveoli wall: one cell thick

Q: Pieces of phospholipid bilayer were analysed from two different mammalian cell surface membranes. Sample A contained phospholipid molecules at a density of 5.0 x 10^6 molecules micro meters-2, whereas sample B contained phospholipid molecules at a density of 4.1 x 10^6 molecules micrometres-2. one sample was from a hormone-secreting liver call and the other was from a skin cell. Identify which cell type corresponds to samples A and B. Give reasons for your choice. (3 MARKS)

- Sample B is from liver cell - There is a lower density of phospholipids in B meaning B contains more membrane proteins - That are required for receptors / a secretory function

Q: Explain how hydrophobic areas in the membrane are important to its function.

- Selectivity; - stops the loss of water soluble / polar molecules from inside the cell;

Q: Suggest three properties of molecules that enable them to enter a cell by diffusion. (3 MARKS)

- Small - Non-polar - Lipid soluble

Q: In a landmark scientific experiment in the 1920s, two Dutch scientists made a new claim about the structure of membranes. They calculated the area of red blood cell membrane and then extracted the lipids that were present. These were dissolved in petroleum ether and allowed to spread into a tightly-packed layer one molecule thick on a surface of water and the area was measured. Their data is shown in table 1. Predict and explain the consequences to digestion if mammalian intestinal epithelial cells were to stop performing active transport of sodium ions. (4 MARKS)

- Sodium ions are normally actively transported out of the intestinal epithelial cells into the lumen - Which maintains a concentration gradient of sodium ions - Which are co-transported with glucose from the lumen into the cell - no active transport would prevent concentration gradient of sodium forming so there would be no-cotransport of glucose - so glucose is not absorbed / causes diarrhoae

Q: Explain why crop plants dont grow when soil salinity is 2% or higher.

- Soil has lower water potential ; plant have higher water potential - Osmosis from plant; plant take up less / not enough water by osmosis

What do both channel proteins and carrier proteins have?

- Specific: Different carrier and channel proteins facilitate the diffusion of different specific molecules - High → low concentration - Same proteins can be used for active transport (against the concentration gradient using ATP)

What are the factors affecting rate of ATP?

- Speed of carrier proteins - Number of carrier proteins - Rate of respiration and availability of ATP

State and explain the factors that affect the rate of diffusion?

- Surface area: the larger the surface area to volume ratio, the faster the rate of diffusion (e.g. microvilli, alveoli, cristae, thylakoids) - Concentration gradient: the steeper the concentration gradient, the faster the rate of diffusion (e.g. ventilation of air in the lungs (fish gills as well!!) - Thickness of surface/diffusion distance: the shorter / thinner the diffusion distance, the faster the rate of diffusion (e.g. capillaries have thin walls (one cell thick)

REQUIRED PRACTICAL: Investigation of the effect of solute concentration on the uptake or loss of water from plant tissue. Control variables

- Temperature - Source of plant tissue (eg same potato) - Size and shape of plant tissue - Batch of stock solution - Length of time the plant is immersed in the solution

Q: Glucose is a product of digestion. It is also a relatively large polar molecule. Once glucose has been digested. It must be absorbed into the bloodstream from the cells in the ileum. Part of the absorption process happens by active transport. Another stage of the absorption process happens by facilitated diffusion. Suggest why glucose must use facilitated diffusion rather than simple diffusion to cross the cell-surface membranes of the ileum. (3 MARKS)

- The centre of phospholipid bilayer is hydrophobic - It forms a barrier to the diffusion of water-soluble substances including most polar molecules - Glucose is a polar molecule that cant diffuse directly across the membrane

Q: A scientist was investigating how the rate of diffusion of potassium ions across the cell surface membrane (as seen in Figure 1) is affected by the concentration of potassium ions within the cell. Their results can be seen in the graph in Figure 2. Describe the limiting factor on the rate of diffusion of potassium ions across the membrane between W and X on the graph in Figure 2, and state how the graph provides evidence for this. (2 MARKS)

- The concentration of potassium ions inside cell is limiting factor - As the concentration fo potassium ions inside cell increases as the independent variable increases so does the rate of diffusion.

Q: Figure 1 below shows a group of onion cells viewed under an optical microscope. Explain the cytoplasm that can be seen in Figure 1. (3 MARKS)

- The onion cells have been placed in a solution with a lower water potential - water has moved out of the cells by osmosis - causing cytoplasm / protoplast to shrink / decrease in volume

Q: Define the term 'water potential' and explain why values of water potential are expressed in kilopascals (kPa) (2 MARKS)

- The tendency of water to diffuse across a partially permeable membrane - kPa is a unit of pressure because moving water creates pressure changes

Q: A group of students investigated the water potential of potato cells. They cut cubes of potato of equal size and shape, weighed them and placed a single cube into one of four different concentrations of sucrose solution. One cube was placed in pure water. They re-weighed each of the cubes every hour and after 12 hours the mass of all the cubes remained constant. The overall change in mass for each cube is shown in figure 3. Explain why the cubes in -500,m -750 and -1000 kPa solutions lost mass (2 MARKS)

- The water potential in these three solutions must have been lower than the water potential in the potato cells - So water moved out of the cells by osmosis

Q: Describe the routes that water molecules take through the cell surface membrane. (2 MARKS)

- Through phospholipid bilayer - Via protein channels / protein pores / aquaporins

EQ: Figure 6 shows a carrot. A group of students investigated the effect of sucrose concentration on the length of cylinders cut from a carrot. They measured the initial length of each cylinder then placed the cylinders into test tubes containing different concentrations of sucrose solution. Stoppers were placed in the tubes and the tubes were left overnight. Explain why the stoppers were placed in the tubes. (2 MARKS)

- To prevent the water from evaporating / prevent evaporation; - Changing the concentrations / water potential (of solution);

Q: A student investigated the movement of molecules into red blood cells. He placed red blood cells into a solution of molecule W. He then measured the concentration of W inside the cells over a period of 15 minutes. The graph below shows the results of his investigation. From his results, the student concluded that molecule W was taken up by diffusion. Using the information in the graph, give an explanation for his conclusion. (2 MARKS)

- Uptake slows down / is less / stops - Because of small concentration / diffusion gradient

Q: Give two ways in which active transport is different from facilitated diffusion

- Uses ATP - Against concentration gradient - Does not use channel proteins (only carrier proteins)

Describe co-transport.

- Via a carrier protein - It moves 2 molecules at once - One goes down its concentration gradient - The other molecules goes against it's concentration gradient - No ATP used (piggy backs, with the molecule going down the concentration gradient)

- Volume of vacuole is reduced (so) pigment is more concentrated; - Water loss by osmosis/ diffusion; - High to low water potential/ Y sucrose soln < cells;

Q: A student wanted to sweeten some strawberries, so she sprinkled some sugar on top of them, one hour before eating them. The student noticed that the sugar that she had sprinkled on them was no longer visibles and that there was some juice at the bottom of the bowl. The student thought that the juice was the sugar dissolved in water and that the water had come from the fruit. Using your knowledge of cell transport mechanisms and the properties of water, explain how the juice is formed from the water that came from the fruit. (3 MARKS)

- Water potential gradient (between sugar and strawberries) - Osmosis (of water from inside the strawberry to outside) - Idea that water is found in cytoplasm

EQ: Species of tubifex worm that live in ponds, lakes and rivers cannot survive in seawater. Use your knowledge of water potential to explain why they cannot survive in seawater (2 MARKS).

- Water potential higher in worm / lower water potential in sweater - Water leaves by osmosis (and worm dies)

EQ: Read the following passage. Human milk contains all the nutrients a young baby needs in exactly the right proportions. It is formed in the mammary glands by small groups of milk-producing cells. These cells absorb substances from the blood and use them to synthesise the lipids, carbohydrates and proteins found in milk. Milk-producing cells are roughly cube-shaped and have a height to breadth ratio of approximately 1.2 : 1. The main carbohydrate in milk is lactose. Lactose is a disaccharide formed by the condensation of two monosaccharides, glucose and galactose. (A molecule of galactose has the same formula as a molecule of glucose - the atoms are just arranged in a different way.) Lactose is synthesised in the Golgi apparatus and transported in vesicles through the cytoplasm. Because lactose is unable to escape from these vesicles, they increase in diameter as they move towards the plasma membrane. The vesicle membranes fuse with the plasma membrane and the vesicles empty their contents out of the cell. Use the information from the passage and your own knowledge to answer the following questions. The lactose-containing vesicles increase in diameter as they move towards the plasma membrane of the milk-producing cell (lines 11-12). Use your knowledge of water potential to explain why. (2 MARKS)

- Water potential inside vesicle more negative/lower; - Water moves into vesicle by osmosis/diffusion;

EQ: A group of students carried out an investigation to find the water potential of potato tissue. The students were each given a potato and 50cm3 of a 1.0 mol dm-3 solution of sucrose. - They used the 1.0 mol dm-3 solution of sucrose to make a series of different concentrations - They cut and weighed discs of potato tissue and left them in the sucrose solution for a set time - They then removed the discs of potato tissue and reweighed them Table 3 shows how one student presented his processed results. Explain the change in mass of potato tissue in the 0.4 mol dm-3 solution of sucrose. (2 MARKS)

- Water potential of solution is less than / more negative than that of potato tissue - Tissue loses water by osmosis

- Water potential of sucrose solution is less than that of potato tissue - This means the potato loses water by osmosis

- Water will affect the mass / only want to measure the water taken up or lost - Amount of water on cylinders varies / ensure the same amount of water on outside

Q: The diagram in Figure 1 shows part of a plasma membrane. Name the part of the membrane that contains: - Carbon and hydrogen only: - Surface / extrinsic protein: (2 MARKS)

- X - W

How does the bilayer formed by phospholipids affect entry and exit of substances into and out of a cell?

- allows movement of lipid soluble / non-polar molecules - prevents movement of water soluble molecules - membrane partially / selectively permeable - large, charged molecules dont move through, small non-polar molecules do

Q: Epithelial cells in the mammalian ileum absorb nutrients from a mammal's food. Suggest and explain two ways in which the surface membranes of these cells might be adapted to their function. (4 MARKS)

- cell-surface membranes likely to have high proportion of carrier/channel proteins - in order to carry nutrients via facilitated diffusion / active transport - cell-surface membrane is likely to have a large surface area to maximise rate of absorption

Q: Distinguish between the features of channel proteins and carrier proteins and their roles in membrane transport. (4 MARKS)

- channel proteins are an open 'pore' that allow polar molecules to pass through (eg aquaporins) - carrier proteins change shape, channel proteins dont - active transport requires carrier proteins - site of ATP hydrolysis - carrier proteins can be active or passive, channel proteins always passive - carrier proteins can transport up or down conc gradient, channel always transport down the gradient

Q: Explain the effect that a higher percentage of cholesterol would have on a cell membrane (2 MARKS)

- cholesterol molecules restrict the movement of phospholipids - making the structure more rigid

Q: In a landmark scientific experiment in the 1920s, two Dutch scientists made a new claim about the structure of membranes. They calculated the area of red blood cell membrane and then extracted the lipids that were present. These were dissolved in petroleum ether and allowed to spread into a tightly-packed layer one molecule thick on a surface of water and the area was measured. Their data is shown in table 1. Suggest three refinements of the Dutch scientists' claim that have arisen from subsequent research by other scientists, which have led us towards the currently-accepted model of the structure of membranes. (3 MARKS)

- discovery of presence of proteins - different types of membrane protein - glycoproteins / glycolipids / cholesterol - fluid mosaic model

Q: Active transport via a carrier protein is not the only active mechanism by which substances can cross cell boundaries against their concentration gradients. Describe and explain one other of such mechanisms. (2 MARKS)

- endocytosis - utilises vesicles to release content across membrane

Q: Give four functions of proteins that occupy the plasma membrane. (4 MARKS)

- enzymes - antigens / cell recognition proteins - receptors for hormones - carriers for active transport - channel proteins for facilitated diffusion

Q: Researchers have discovered that an individual phospholipid molecule can exchange places with its neighbouring phospholipid molecule in a monolayer as frequently at 10^7 times per second. By contrast, phospholipid molecules almost never exchange places with each other from one monolayer to the other within a bilayer. referred to as a 'flip-flop' exchange. The 'flip-flop' takes place around once a month for a typical phospholipid molecule. use your knowledge of membrane structure to explain this difference in molecular behaviour (2 MARKS)

- exchange within monolayer is easy / swapping polar heads with other polar heads - a flip-flop required polar heads to cross the non-polar centre of the membrane (molecular repulsion makes this difficult)

Q: Figure 1 shows two configurations of a voltage-gated membrane protein that is found in the cell surface membrane of nerve cells. This protein allows the transport of ions when open. Its role is in the generation and transmission of nerve impulses. Suggest the mode of transport ions that this protein imploys, give reasons for your answer. (3 MARKS)

- facilitated diffusion - ions are polar molecules / need channel to pass through bilayer - not active transport because ions diffuse down the concentration gradient (passive)

Q: Figure 1 below shows a cell surface membrane and some molecules of a small nonpolar substance known as substance X. Substance Y is a large molecule and is not lipid-soluble. Describe how substance Y would travel across a cell membrane from an area of high to low concentration. (2 MARKS)

- facilitated diffusion - via a specific channel protein / carrier protein

Q: The diagram in Figure 1 shows part of a plasma membrane. Give a function of the structure labelled W in Figure 1. (1 MARK)

- for cell recognition - for identification - for binding to hormones

Describe two functions of the hydrophobic fatty acids in a phospholipid.

- form(water) impermeable barrier to water-soluble substances - selectively permeable - allows non-polar molecules to pass through; - allows cell to maintain different concentrations either side;

Q: The diagram shows part of a plasma membrane. Describe two functions of the structure made from the parts labelled X (2 MARKS)

- form(water) impermeable barrier to water-soluble substances / selectively permeable / - allows non-polar molecules to pass through; allows cell to maintain different concentrations either side; - able to form vesicles / gives flexibility / fluidity;

EQ: Suggest a change a student could make in his procedure so that 10cm3 of oxygen would be produced in less than 6 seconds. (1 MARK0

- increase surface area of potato chips - use bigger chips - increase temp - change pH

Q: The part labelled Z is involved in facilitated diffusion of substances across the membrane. (i) Give one similarity in the way in which active transport and facilitated diffusion transport substances across the membrane. (ii) Give one way in which active transport differs from facilitated diffusion. (2 MARKS)

- involves carrier / transmembrane / transport proteins; - requires energy / requires use of ATP

EQ: Following digestion and absorption of food, the undigested remains are processed to form faeces in the parts of the intestine below the ileum. The faeces of people with consitipation are very dry and hard. Constipiation can be treated by drinking lactulose. lactulose is soluble, but isnt digested or absorbed by the human intestine. Use your knowledge of water potential to suggest why lactulose can be used to help people suffering form cobsitipation. (2 MARKS)

- lactulose lowers the water potential of the faeces - water enters due to osmosis and softens the faeces

Q: The function of a neurone cell relies upon the rapid movement of cations across its cell membrane. Suggest and explain a adaptation you might expect to observe in the cell membrane of a neurone cell. (2 MARKS)

- large number of carrier / channel proteins - to allow cations to cross the cell membrane quickly

Q: Figure 2 below shows the absorption of certain nutrients from the lumen of the small intestine into the blood. state the purpose of the process taking place at the position labelled C in Figure 2.

- maintains / creates concentration gradient for sodium

Q: Give one advantage of the fluid mosaic membrane structure being fluid. (1 MARK)

- membrane proteins can migrate diffuse to areas where they are needed - membrane bound organelles can fuse together OR be separated (eg vesicles forming / fusing with cell membrane via exo/endocytosis)

Q: Using the information in Figure 2, describe the direction of movement of water across the cell membrane. Give a reason for your answer. (2 MARKS)

- movement of water from exterior to interior by osmosis - because water potential of exterior is higher than water potential interior

Describe what happens when a plant cell is placed in a hypotonic solution.

- net movement of water into cell - protoplast swells - cell is turgid

Describe what happens when a plant cell is placed in a hypertonic solution.

- net movement of water out of cell - protoplast shrinks - cell is plasmolysed

Describe what happens when a plant cell is placed in a isotonic solution.

- no overall net movement of water - no change in protoplast - incipient plasmolysis

Q: State what would happen if onion cells were placed into pure water. (2 MARKS)

- onion cell wouldnt burst - because cell wall provides additional strength / can withstand increased pressure

Q: Explain why structure A in Figure 1 forms in the aqueous watery environment of the cell cytoplasm. (2 MARKS)

- phospholipid molecules have hydrophillic head and hydrophobic tail - the molecules arrange themselves os that contact between the hydrophobic tails and surrounding water is minimised.

What are the components of the cell membrane?

- phospholipids - embedded transport proteins, glycoproteins / glycolipids, - cholesterol

Q: When phospholipids come together to form a cell membrane, a bilayer is always formed. Explain why. (3 MARKS)

- phospholipids have hydrophobic tail and hydrophilic head - hydrophilic head attracted to water molecules in cytoplasm and cell surroundings - hydrophobic tails repelled by water surroundings so bilayer formed

EQ: This fat substitute is a lipid. Despite it being a lipid, it cannot cross the cell-surface membranes of cells lining the gut. Suggest why it cannot cross the cell-surface membranes (1 MARK)

- polar - too large

Q: Malabsorption of glucose in the small intestine may lead to diarrhoea. Explain why. (3 MARKS)

- poor absorption leaves glucose in intestinal tract which lowers the water potential - water diffuses out of surrounding tissue into lumen by osmosis - large intestine cannot absorb all the water, so watery stool / diarrhoea is the result

Q: A scientist was investigating how the rate of diffusion of potassium ions across the cell surface membrane (as seen in Figure 1) is affected by the concentration of potassium ions within the cell. Their results can be seen in the graph in Figure 2. Describe why the graph plateaus between point Y and Z. (2 MARKS)

- potassium ions are passing through the channels/pores at their maximum rate - the rate of diffusion is now limited by the number of potassium channels / another limiting factor

Q: Cell membranes vary in structure due to the adaptation fo specialised cells to their function. Figure 1 models the arrangement of molecules in a typical cell membrane, observed from above. Describe the model illustrated in Figure 1. (2 MARKS)

- proteins scattered amongst phospholipid like tiles in mosaic - phospholipids are constantly moving so structure is fluid

Q: Explain why an aqueous solution has a lower water potential than pure water (2 MARKS)

- pure water has no solute molecules - therefore all water molecules are free to diffuse freely across a membrane

Describe the role of cholesterol in the cell membrane

- regulates fluidity / increases stability / strength - Makes membrane more rigid / table / flexible by restricting lateral movement of molecules making up the membrane eg phospholipids; binds to fatty acid tails causing them to pack more closely together - This adds strength + support - Not present in bacterial cell membranes

Q: State two functions of membranes in living cells (2 MARKS)

- separate the internal cell environment from the external environment - separate the internal cell organelles - enable cellular conditions to be different between one enclosed space and another. - regulate movement / exchange of substances - displaying antigens / receptors

EQ: Tubifex worms are small, thin animals that live in water. They have no specialised gas exchange or circulatory system. Figure 2 shows a tubifex worm. (i) Name the process by which oxygen reaches the cells inside the body of a tubifex worm. (1 MARK) (ii) Using the information provided, explain how two features of the body of the tubifex worm allow efficient gas exchange. (2 MARKS)

- simple diffusion - thin small; short diffusion path - flat / long; so lathe surface area to vol ratio

Q: When pieces of carrot are placed in water, chloride ions are released from the cell vacuoles. Identical pieces of carrot were placed in water at different temperatures. The concentration of chloride ions in the water was measured after a set period of time. The graph shows the results. Describe and explain the shape of the curve. (3 MARKS)

- slow rise, sharp rise, levelling off (reject 'becomes constant'); - sharp rise / above 50C proteins are denatured; - levelling off due to concentration of chloride ions in water becoming equal / maximum loss of Cl- ions;

Q: Explain how co-transport is used to transport sodium ions and glucose into cells in the mammalian ileum, across their cell membranes. (5 MARKS)

- sodium ions are actively transported out of the ileum epithelial cells into the blood by the sodium pump - this creates concentration gradient of sodium ions between lumen and ileum and interior of epithelial cells - sodium ions diffuse down concentration gradient to epithelial cells - via sodium-glucose co-transporter proteins - co-transporter proteins transport glucose into cell along with sodium ions

Q: There is a higher concentration gradient between the cytoplasm and the vacuole than between the cytoplasm and the intercellular fluid. Suggest how the vacuole membrane maintains this higher concentration gradient.

- sodium ions transported more into vacuole (than to outside); - because more sodium carrier proteins / pumps in vacuole membrane;

Q: Explain how three different factors can affect the fluidity of membranes. (6 MARKS)

- temperature - high temp makes molecules vibrate more / more fluid - proportion of saturated fatty acids in phospholipid tails - higher proportion of saturated fatty acids increases molecular forces / makes membrane less fluid - cholesterol content of bilayer - cholesterol inserts itself between phospholipid tails, / can stabilise the membrane at high temperatures - Restricts the movement of phospholipids / other molecules making up the membrane; - length of fatty acid tails - longer fatty acid tail length causes less fluidity (More rigidty) - greater intermolecular interactions

Q: Outline the main factors that govern the rate of diffusion across a phospholipid bilayer. (4 MARKS)

- temperature - steepness of concentration gradient - surface area - properties of molecules or ions that are diffusing

Q: The graph shows the relationship between the concentration of a substance outside a cell and the rate of entry of this substance into the cell. Explain the evidence from the graph that this substance is entering the cell by facilitated diffusion and not by simple diffusion. (2 MARKS)

- the curve levels off above a certain external concentration of substance; - as channel proteins are saturated with molecules (and no more can be carried), becoming limiting;

Explain the types of embedded transport proteins in the cell membrane.

- they can be intrinsic of extrinsic - channel proteins and carrier proteins (intrinsic)

Q: A protein was found to be a channel protein. It had a hydrophobic region and a hydrophilic region. Describe the function of a channel protein and explain how its structure allows it to carry out this function. (3 MARKS)

- transport molecules and ions across cell membranes - hydrophobic regions are repelled by water and hydrophilic regions are attracted to it - causes protein to fold up and form a channel through the membrane (through which water soluble molecules can pass)

Q: After 45 minutes the potato cylinder in the 0.8 mol dm-3 solution was removed and blue dye added to this solution. Some of this blue-stained solution was drawn into a syringe. A drop was then released, slowly, halfway down a test tube of fresh 0.8 mol dm-3 sucrose solution as shown in the diagram. The blue drop quickly moved to the surface of the liquid in the test tube. (I) The density of a solution depends on its concentration. The more concentrated the solution the greater its density. Explain why the blue drop had a lower density and therefore moved up. (2 MARKS) (ii) A sucrose solution of concentration 0.3 mol dm-3 has a water potential which is equivalent to that of the potato cells. Describe and explain what would happen to the blue drop from this solution. (2 MARKS)

- water moved into the solution from the potato; - solution diluted/becomes less concentrated; - no net movement of water (in or out); - drop would not move/densities the same;

Q: Figure 1 below shows a group of onion cells viewed under an optical microscope. Predict the appearance of a herbaceous (non-woody) plant that contained cells looking like those seen in Figure 1.

- wilted / no longer able to stand up straight

Q: A group of students investigated the water potential of potato cells. They cut cubes of potato of equal size and shape, weighed them and placed a single cube into one of four different concentrations of sucrose solution. One cube was placed in pure water. They re-weighed each of the cubes every hour and after 12 hours the mass of all the cubes remained constant. The overall change in mass for each cube is shown in figure 3. Use Figure 3 to estimate the water potential of the potato cells. (1 MARK)

-425 kPa

What is the water potential of pure water?

0 kPa

EQ: A student investigates the uptake of potassium ions by pea plants. The student: • uses two flasks, A and B • adds equal volumes of a solution containing radioactive potassium ions to each flask • adds a sample of the roots of a pea plant to each flask • adds a solution of a respiratory inhibitor to flask B and the same volume of water to flask A • measures, at regular intervals, the concentration of potassium ions remaining in the solution in each flask. Figure 2 shows the results from the investigation. Calculate the mean rate of uptake of potassium ions by the roots of the pea plant in Flask A from 0 to 30 minutes. Give your answer to three significant figures. (2 MARKS)

0.133 / 0.130;; Arbitrary units minute−1

Q: A student investigated the effect of putting cylinders cut from a potato into sodium chloride solutions of different concentrations. He cut cylinders from a potato and weighed each cylinder. He then placed each cylinder in a test tube. Each test tube contained a different concentration of sodium chloride solution. The tubes were left overnight. He then removed the cylinders from the solution and reweighed them. Use the graph to find the concentration of sodium chloride solution that has the same water potential as the potato cylinders.

0.35 mol dm-3

Q: Some substances pass through the plasma membrane of a milk-producing cell by diffusion. Describe the structure of a plasma membrane and explain how different substances are able to pass through the membrane by diffusion. (6 MARKS)

1 Phospholipids forming bilayer 2. Details of arrangement with "heads" on the outside; 3 Two types of protein specified; e.g. extrinsic in outer layer only. or intrinsic extended through bilayer - channel proteins and carrier proteins 4 Reference to other molecule e.g. cholesterol or glycoprotein; 5 Substances move down concentration gradient/from high to low concentration; 6 Water/ions through channel proteins/pores; 7 Small/lipid soluble molecules/examples pass between phospholipids/through phospholipid layer; 8 Carrier proteins involved with facilitated diffusion;

EQ: A student uses potato cylinders to investigate the effect of sucrose concentration on the uptake and loss of water from plant tissue. The student: • makes five dilutions of sucrose solution using a 1.0 mol dm−3 sucrose solution and distilled water • cuts five cylinders from a potato, blots them dry and weighs them • puts each cylinder in a different concentration of sucrose solution for 24 hours • removes the cylinders from the sucrose solutions, blots them dry and weighs them. Table 3 shows the student's results. Describe how the student could use the results in Table 3 to find the sucrose concentration that is in equilibrium with the solution in the cells of the potato tissue. (2 MARKS)

1 Plot a graph with concentration (of sucrose) of on the x-axis and percentage change (in mass) on the y-axis; 2 Find concentration where percentage change in mass is 0 / find concentration where line crosses x-axis / find x-axis intercept;

EQ: A student uses potato cylinders to investigate the effect of sucrose concentration on the uptake and loss of water from plant tissue. The student: • makes five dilutions of sucrose solution using a 1.0 mol dm−3 sucrose solution and distilled water • cuts five cylinders from a potato, blots them dry and weighs them • puts each cylinder in a different concentration of sucrose solution for 24 hours • removes the cylinders from the sucrose solutions, blots them dry and weighs them. Table 3 shows the student's results. Explain the change in mass of the potato tissue in the 0.2 mol dm−3 sucrose solution. (2 MARKS)

1 Water has entered (potato tissue) by osmosis / diffusion / (potato tissue) has gained water by osmosis / diffusion; 2 Water potential of solution is higher than that of potato tissue / water potential of solution is less negative than that of potato tissue;

Q: Explain how three features of a plasma membrane adapt it for its functions. (6 MARKS)

1 phospholipid bilayer (as a barrier); 2 forms a barrier to water soluble / charged substances / allows non-polar substances to pass 3 bilayer is fluid; 4 can bend to take up different shapes for phagocytosis / form vesicles / self repair; 5 channel proteins (through the bilayer) 6 let water soluble / charged substances through / facilitated diffusion; 7 carrier proteins (through the bilayer); 8 allow facilitated diffusion / active transport; 9 surface proteins / extrinsic proteins, glycoproteins / glycolipids; 10 cell recognition / act as antigens / receptors; 11 cholesterol; Regulates fluidity / increases stability;

EQ: A student investigates the uptake of potassium ions by pea plants. The student: • uses two flasks, A and B • adds equal volumes of a solution containing radioactive potassium ions to each flask • adds a sample of the roots of a pea plant to each flask • adds a solution of a respiratory inhibitor to flask B and the same volume of water to flask A • measures, at regular intervals, the concentration of potassium ions remaining in the solution in each flask. Figure 2 shows the results from the investigation. The curve for Flask B levelled off after 60 minutes. Explain why. (2 MARKS)

1. (Absorption by) diffusion no longer occurs; 2. As no concentration / diffusion gradient;

EQ: Read the following passage. Low-density lipoprotein (LDL) is a substance found in blood. A high concentration of LDL in a person's blood can increase the risk of atheroma formation. Liver cells have a receptor on their cell-surface membranes that LDL binds to. This leads to LDL entering the cell. A regulator protein, also found in blood, can bind to the same receptor as LDL. This prevents LDL entering the liver cell. People who have a high concentration of this regulator protein in their blood will have a high concentration of LDL in their blood. Scientists have made a monoclonal antibody that prevents this regulator protein working. They have suggested that these antibodies could be used to reduce the risk of coronary heart disease. A trial was carried out on a small number of healthy volunteers, divided into two groups. The scientists injected one group with the monoclonal antibody in salt solution. The other group was a control group. They measured the concentration of LDL in the blood of each volunteer at the start and after 3 months. They found that the mean LDL concentration in the volunteers injected with the antibody was 64% lower than in the control group. Use the information in the passage and your own knowledge to answer the following questions. Explain how the monoclonal antibody would prevent the regulator protein from working (lines 7−8). (2 MARKS)

1. (Monoclonal antibody) has a specific tertiary structure / is complementary to regulator protein 2. Binds to / forms complex with (regulator protein) 3. (So regulator protein) would not fit / bind to the receptor / is not complementary to receptor

EQ: A student investigates the uptake of potassium ions by pea plants. The student: • uses two flasks, A and B • adds equal volumes of a solution containing radioactive potassium ions to each flask • adds a sample of the roots of a pea plant to each flask • adds a solution of a respiratory inhibitor to flask B and the same volume of water to flask A • measures, at regular intervals, the concentration of potassium ions remaining in the solution in each flask. Figure 2 shows the results from the investigation. Explain the decrease in the concentrations of potassium ions in each of the two solutions from 0 to 30 minutes. (3 MARKS)

1. (Potassium ions) absorbed by diffusion in B; 2. (Cells have) no energy / ATP available in B or (cells have) energy /ATP available in A; 3. (Potassium ions) absorbed by active transport in A or no active transport in B;

Q: Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substances can cross a cell surface membrane.

1. (Simple / facilitated) diffusion from high to low concentration / down concentration gradient; 2. Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer; 3. Water moves by osmosis 4. Active transport is movement from low to high concentration / against concentration gradient; 5. Active transport / facilitated diffusion involves proteins / carriers; 6. Active transport requires energy / ATP;

Q: Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein. Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not. (5 MARKS)

1. Chloride ions water soluble/charged/polar; 2. Cannot cross (lipid) bilayer (of membrane); 3. Chloride ions transported by facilitated diffusion 4. Oxygen not charged/non-polar; 5. (Oxygen) soluble in/can diffuse across (lipid) bilayer;

EQ: A student uses potato cylinders to investigate the effect of sucrose concentration on the uptake and loss of water from plant tissue. The student: • makes five dilutions of sucrose solution using a 1.0 mol dm−3 sucrose solution and distilled water • cuts five cylinders from a potato, blots them dry and weighs them • puts each cylinder in a different concentration of sucrose solution for 24 hours • removes the cylinders from the sucrose solutions, blots them dry and weighs them. Table 3 shows the student's results. The student left the potato cylinders in the sucrose solution for the same length of time. Suggest two other variables the student controlled. (2 MARKS)

1. Cylinders same length; 2. Cylinders cut from same potato; 3. (Cylinders and solutions kept at same) temperature; 4. (Cylinders kept in same) volume of sucrose solution;

EQ: Read the following passage. Low-density lipoprotein (LDL) is a substance found in blood. A high concentration of LDL in a person's blood can increase the risk of atheroma formation. Liver cells have a receptor on their cell-surface membranes that LDL binds to. This leads to LDL entering the cell. A regulator protein, also found in blood, can bind to the same receptor as LDL. This prevents LDL entering the liver cell. People who have a high concentration of this regulator protein in their blood will have a high concentration of LDL in their blood. Scientists have made a monoclonal antibody that prevents this regulator protein working. They have suggested that these antibodies could be used to reduce the risk of coronary heart disease. A trial was carried out on a small number of healthy volunteers, divided into two groups. The scientists injected one group with the monoclonal antibody in salt solution. The other group was a control group. They measured the concentration of LDL in the blood of each volunteer at the start and after 3 months. They found that the mean LDL concentration in the volunteers injected with the antibody was 64% lower than in the control group. Use the information in the passage and your own knowledge to answer the following questions. Describe how the control group should have been treated. (2 MARKS)

1. Injection with salt solution 2. Otherwise treated the same

Describe how you would make serial dilutions of sucrose solution starting with initial sucrose concentration of 2 M and diluting each solution by a factor of 2

1. Line up five test tubes in rack 2. Add 10cm3 of the initial 2 M sucrose solution to the first test tube and add 5cm3 of distilled water to the other four test tubes 3. Using a pipette draw 5cm3 of the solution from first test tube, add to distilled water in second test tube and mix solution thoroughly. Now you have 10cm3 solution half as concentrated as the solution in the first test tube (1 M) 4. Repeat the process three more time to create solutions 0.5 M, 0.25 M and 0.125 M You don't have to dilute solutions by a factor of 2. To dilute by a factor of 10, taken 1ccm3 from original sample and add to 9cm3 water

EQ: Read the following passage. Low-density lipoprotein (LDL) is a substance found in blood. A high concentration of LDL in a person's blood can increase the risk of atheroma formation. Liver cells have a receptor on their cell-surface membranes that LDL binds to. This leads to LDL entering the cell. A regulator protein, also found in blood, can bind to the same receptor as LDL. This prevents LDL entering the liver cell. People who have a high concentration of this regulator protein in their blood will have a high concentration of LDL in their blood. Scientists have made a monoclonal antibody that prevents this regulator protein working. They have suggested that these antibodies could be used to reduce the risk of coronary heart disease. A trial was carried out on a small number of healthy volunteers, divided into two groups. The scientists injected one group with the monoclonal antibody in salt solution. The other group was a control group. They measured the concentration of LDL in the blood of each volunteer at the start and after 3 months. They found that the mean LDL concentration in the volunteers injected with the antibody was 64% lower than in the control group. Use the information in the passage and your own knowledge to answer the following questions. LDL enters the liver cells (lines 3−4). Using your knowledge of the structure of the cell-surface membrane, suggest how LDL enters the cell. (2 MARKS)

1. Lipid soluble / hydrophobic 2. Enters through (phospholipid) bilayer

EQ: Beetroots are the swollen roots of some species of beet plant. The vacuoles in the cells of beetroots contain a red, water-soluble pigment called betanin. Figure 2 shows the structure of betanin. Betanin does not usually cross the membrane of the vacuole. Suggest two possible reasons why. Use information from Figure 2. (2 MARKS)

1. Molecule too large (to pass between membrane/phospholipid molecules); 2. Betanin charged/polar/hydrophilic/non-lipid soluble; 3. Shape does not fit through channel proteins/pores;

EQ: The movement of substances across cell membranes are affected by membrane structure. Describe how. (5 MARKS)

1. Phospholipid (bilayer) allows movement/diffusion of nonpolar/lipid-soluble substances; 2. Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances OR (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer; 3. Carrier proteins allow active transport; 4. Channel/carrier proteins allow facilitated diffusion/co-transport; 5. Shape/charge of channel / carrier determines which substances move; 6. Number of channels/carriers determines how much movement; 7. Membrane surface area determines how much diffusion/movement; 8. Cholesterol affects fluidity/rigidity/permeability;

Describe the movement across membranes by co-transport, illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum.

1. Sodium - Potassium pump: Na+ ions actively transported out of epithelial cells lining to ileum, into the blood, by the sodium-potassium pump. Creating a concentration gradient of sodium (higher conc. of sodium in lumen than epithelial cell). K+ pumped into cell. Active transport (uses ATP) as low concentration of Na+ in cell. 2. Co-transport: Sodium ions down concentration gradient and glucose move by facilitated diffusion against concentration gradient into the epithelial cell from the lumen, via a co-transporter protein 3. Creating a concentration gradient of glucose - higher conc. Of glucose un epithelial cell than blood 4. Facilitated diffusion: Glucose moves out of cell and absorbed into blood by facilitated diffusion through a protein / carrier channel

EQ: A scientist investigated the uptake of sodium ions by animal tissue. To do this, he: • used two flasks, F and G • put equal masses of animal tissue into each flask • added equal volumes of a solution containing sodium ions to each flask • added to flask F a solution of a substance that prevents the formation of ATP by cells • measured the concentration of sodium ions remaining in the solution in each flask. Figure 2 shows his results. The scientist concluded that the cells in flask G took up sodium ions by active transport. Explain how the information given supports this conclusion (4 MARKS)

1. Uptake in flask G much greater than Flask F 2. = Showing use of ATP in flask G 3. Sodium ion concentration in flask G falls to zero 4. = Showing uptake against a concentration gradient

EQ: A scientist investigated the uptake of sodium ions by animal tissue. To do this, he: • used two flasks, F and G • put equal masses of animal tissue into each flask • added equal volumes of a solution containing sodium ions to each flask • added to flask F a solution of a substance that prevents the formation of ATP by cells • measured the concentration of sodium ions remaining in the solution in each flask. Figure 2 shows his results. The curve for flask F levelled off after 20 minutes. Explain why (2 MARKS)

1. Uptake of sodium occurring by facilitated diffusion - Equilibrium reached / sodium ion concentrations in solution and in cell the same

EQ: The water potential of the blood plasma is more negative at the venule end of the capillary than at the arteriole end of the capillary. Explain why.

1. Water has left the capillary; 2. Proteins (in blood) too large to leave capillary; 3. Increasing / giving higher concentration of blood proteins

EQ: Putting bee honey on a cut kills bacteria. Honey contains a high concentration of sugar. Use your knowledge of water potential to suggest how putting honey on a cut kills bacteria. (3 MARKS)

1. Water potential in (bacterial) cells higher (than in honey) 2. Water leaves bacteria / cells by osmosis; 3. (Loss of water) stops (metabolic) reactions.

Q: A group of students investigated the water potential of potato cells. They cut cubes of potato of equal size and shape, weighed them and placed a single cube into one of four different concentrations of sucrose solution. One cube was placed in pure water. They re-weighed each of the cubes every hour and after 12 hours the mass of all the cubes remained constant. The overall change in mass for each cube is shown in figure 3. What was the change in mass for the potato cube placed in pure water? (1 MARK)

16%

3.3:1

EQ: A student uses potato cylinders to investigate the effect of sucrose concentration on the uptake and loss of water from plant tissue. The student: • makes five dilutions of sucrose solution using a 1.0 mol dm−3 sucrose solution and distilled water • cuts five cylinders from a potato, blots them dry and weighs them • puts each cylinder in a different concentration of sucrose solution for 24 hours • removes the cylinders from the sucrose solutions, blots them dry and weighs them. Table 3 shows the student's results. Describe how the student could use the 1.0 mol dm−3 sucrose solution to make 25 cm3 of 0.2 mol dm−3 sucrose solution. (1 MARK)

5 cm3 (1.0 mol dm-3) sucrose solution and 20 cm3 (distilled) water;

EQ: Figure 1 shows the fluid-mosaic model of the cell-surface membrane. In Figure 1, A, B and C are different types of molecule. Name molecules A, B and C. (3 MARKS)

A = Carbohydrate / sugar; B = Phospholipid; C = Protein;

Q: Vitamin D deficiency reduces the uptake of calcium ions by epithelial cells lining the small intestine. The diagrams show how calcium ions are transported through normal epithelial cells and those deficient in vitamin D. Membrane proteins A and B transport calcium ions through cell surface membranes. Explain how each type of membrane protein transports calcium ions. (4 MARKS)

A is channel / pore protein (for calcium ions); passage by facilitated diffusion down diffusion / concentration gradient; B is carrier protein(for calcium ions); passage by active transport against concentration gradient / requires energy / ATP;

Q: The diagram shows the structure of the cell-surface membrane of a cell. Name A and B. (2 MARKS)

A: phospholipid layer B: pore / channel / pump / carrier / transmembrane / intrinsic / transport protein (ignore unqualified reference to protein).

Q: Ca2+ ATPases are carrier proteins that transport Ca2+ ions across cell-surface membranes. Each Ca2+ ATPase has one subunit that has an ATP binding site and acts as an enzyme. Ca2+ ATPase spans the width of the cell-surface membrane. The ATP binding site is always on the cytoplasm side of the membrane. Suggest why. (1 MARK)

ATP made inside cell rather than outside, so ATP binding site has to face inside

Q: By what transport mechanism is the sodium ion concentration gradient created in an epithelial cell? (1 MARK)

Active transport

Read the following passage Cholera is a water-borne disease. It is caused by a bacterium. The bacterium produces a toxin which acts on the epithelial cells of the small intestine and causes changes in membrane permeability. The cholera toxin affects the movement of ions through the intestinal wall. It causes the loss of chloride ions from the blood into the lumen of the small intestine. This prevents the movement of sodium ions from the lumen of the small intestine into the blood. The resulting high concentration of ions causes diarrhoea. Vaccination can produce immunity to cholera. A new vaccine appears to provide better immunity and has fewer side effects than previously available vaccines. This vaccine is taken orally. For long-term immunity, a booster dose is required after two years. Sodium ions normally enter the blood from cells of the intestinal wall against a concentration gradient. Describe how. The high concentration of ions in the small intestine of a person with cholera causes diarrhoea. (lines 6-7). Explain why.

Active transport; Using ATP; Carrier proteins; Water potential lowered in small intestine; Osmotic loss of water;

EQ: A group of students carried out an investigation to find the water potential of potato tissue. The students were each given a potato and 50cm3 of a 1.0 mol dm-3 solution of sucrose. - They used the 1.0 mol dm-3 solution of sucrose to make a series of different concentrations - They cut and weighed discs of potato tissue and left them in the sucrose solution for a set time - They then removed the discs of potato tissue and reweighed them Table 3 shows how one student presented his processed results. Describe how you would use a 1.0 mol dm-3 solution of sucrose to produce 30 cm3 of a 0.15 mol dm-3 solution of sucrose. (2 MARKS)

Add 4.5 cm3 of 1.0 mol dm-3 solution to 25.5 cm3 distilled water

Describe how ATP acts as a source of energy.

Adenosine triphosphate (ATP0 is a nucleotide, has 3 phosphate groups. The bonds between these phosphate groups are unstable so have low activation energy, so can be easily broken. When the bonds break they release energy. - Conversion of ATP to ADP is a reversible reaction - This reaction is catalysed by the enzyme ATP synthase

EQ: A group of students carried out an investigation to find the water potential of potato tissue. The students were each given a potato and 50cm3 of a 1.0 mol dm-3 solution of sucrose. - They used the 1.0 mol dm-3 solution of sucrose to make a series of different concentrations - They cut and weighed discs of potato tissue and left them in the sucrose solution for a set time - They then removed the discs of potato tissue and reweighed them Table 3 shows how one student presented his processed results. Explain why the data in Table 3 are described as processed results. (1 MARK)

Calculations made from raw data / raw data would have recorded initial and final masses

Q: Figure 1 and 2 show onions cells under a light microscope the cytoplasm appears dark grey. One of the figures shows the onions cells after they have been placed in a weak salt solution. The solution has a lower water potential than the onion cells. Look at the cells labelled A and B on Figure 2. By comparing the size of their membranes, suggest which of these cells is most likely to experience the fastest transport of water molecules into and out of it cytoplasm. Explain your answer. (1 MARK)

Cell A because its membrane has the largest surface area, which increases the rate of osmosis

Describe how surface proteins / extrinsic / glycoproteins / glycolipids are adapted for other functions.

Cell recognition / act as antigens / receptors

EQ: Figure 3 shows how a plant cell produces its cell wall. What is the evidence in Figure 3 that the phospholipid bilayer shown is part of the cell-surface membrane? (1 MARK)

Cell wall forms outside cell-surface membrane / has cellulose on it (on the outside);

Q: State one function for each of the components of D, E and F (3 MARKS)

D = stabilize the membrane OR maintain / affect / control fluidity OR reduces permeability E = allow communication across membrane OR allow, polar / charged, particles to pass through membrane / ACCEPT cell recognition / cell signaling F = act as barrier (to, polar / charged particles) / select what enters or leaves cell / ACCEPT (acts as) selectively permeable or partially permeable membrane

Q: Scientists investigated the effect of a drug called a proton pump inhibitor. The drug is given as a tablet to people who produce too much acid in their stomach. It binds to a carrier protein in the surface membrane of cells lining in the stomach. This carrier protein usually moves hydrogen ions into the stomach by active transport. The scientists used two groups of people in their investigation. All the people produced too much acid in their stomach. People in group P were given the drug. Group Q was the control group. The graph shows the results. Describe the effect that taking the drug had on acid secretion. (1 MARK)

Decrease for 3 hours

Q: Scientists investigated the effect of a drug called a proton pump inhibitor. The drug is given as a tablet to people who produce too much acid in their stomach. It binds to a carrier protein in the surface membrane of cells lining in the stomach. This carrier protein usually moves hydrogen ions into the stomach by active transport. The scientists used two groups of people in their investigation. All the people produced too much acid in their stomach. People in group P were given the drug. Group Q was the control group. The graph shows the results. Calculate the percentage decrease in acid secretion of group P compared to group Q after 8 hours. (2 MARKS)

Difference from graph = 200 - 90 = 110 / 200 x 100 = 55% decrease

Q: Name the process by which oxygen passes from an alveolus in the lungs to the blood.

Diffusion

EQ: The letters P, Q, R, S and T represent ways in which substances can move across membranes. - P: diffusion through phospholipid bilayer - Q: facilitated diffusion - R: active transport - S: co-transport - T: osmosis For each of the following examples of transport across membranes, select the letter that represents the way in which the substance moves across the membrane. Write the appropriate letter in each box produced. (3 MARKS) - transport through a channel protein - Transport of small, non-polar molecules - transport of glucose withs sodium ions.

Facilitated diffusion, diffusion through the phospholipid bilayer. Co-transport

Q: An artificial membrane was made. It consisted only of a bilayer of phospholipid molecules. In an investigation, the permeability of this artificial membrane was compared with the permeability of a plasma membrane from a cell. Explain why: only the plasma membrane allowed glucose to pass through. (2 MARKS)

Glucose unable to pass through artificial membrane as not lipid soluble; Glucose transported by proteins; (Proteins) found in plasma membrane

Q: Glucose is a product of digestion. It is also a relatively large polar molecule. Once glucose has been digested. It must be absorbed into the bloodstream from the cells in the ileum. Part of the absorption process happens by active transport. State the type of molecule that actively transports glucose across the cell-surface membranes of the ileum and briefly describe how it does. (2 MARKS)

It is a carrier protein / co-transporter Which binds glucose and sodium ions at the same time

Q: Diarrhoea involves the production of large amounts of watery faeces. Explain the link between the presence of lactose in the intestine and diarrhoea. (3 MARKS)

Lactose produces a lower / more negative water potential; So water moves into the intestine / less water absorbed; By osmosis / diffusion / down concentration gradient;

Q: Some scientists investigated the uptake of magnesium ions in rice plants. They divided the plants into two groups and placed their roots in solutions containing radioactive magnesium ions. Group Y: plants had a substance that inhibited respiration added to the solution Group Z: plants didnt have the respiratory inhibitor added to the solution The scientist calculate the total amount of magnesium ions absorbed by the plants every 5 minutes. Their results are shown in the graph in Figure 1 below. Using the graph in Figure 1, calculate the rate of uptake of magnesium iosn for group Y plants during the investigation. Give suitable units. (2 MARKS)

Look at image

EQ: A student uses potato cylinders to investigate the effect of sucrose concentration on the uptake and loss of water from plant tissue. The student: • makes five dilutions of sucrose solution using a 1.0 mol dm−3 sucrose solution and distilled water • cuts five cylinders from a potato, blots them dry and weighs them • puts each cylinder in a different concentration of sucrose solution for 24 hours • removes the cylinders from the sucrose solutions, blots them dry and weighs them. Table 3 shows the student's results. Describe how the student calculated the percentage change in mass of a potato cylinder. (1 MARK)

Look at image.

Q: A biologist was investigating how surface area affects osmosis in potato cubes. Step 1: She cut two cubes of potato tissue, each with sides of 4cm in length Step 2: She put one cube into a concentrated sucrose solution Step 3: She cut the other cube into eight equal-sized smaller cubes and put them into a sucrose solution of the same concentration as the solution used for the large cube Step 4: She recorded the masses of the cubes at intervals Her results are shown in the graph in Figure 1. The loss in mass is shown in the graph in Figure 1 in the first 20 minutes is faster in the eight small cubes than in the single large cubes. At 20 minutes, the total mass of the right small cubes was 50 grams and the total mass of the single large cube was 55 grams. Calculate the rate of loss in mass per cm2 per minute for the single large cube and the eight small cubes during the first 20 minutes. Give your answers in grams per cm2 per minute.

Look at image.

Q: In a landmark scientific experiment in the 1920s, two Dutch scientists made a new claim about the structure of membranes. They calculated the area of red blood cell membrane and then extracted the lipids that were present. These were dissolved in petroleum ether and allowed to spread into a tightly-packed layer one molecule thick on a surface of water and the area was measured. Their data is shown in table 1. Explain what the data in table 1 reveals about the alignment of lipids in the cell surface membranes of red blood cells. Reinforce your explanation with suitable calculations. (4 MARKS)

Look at image.

Q: Some scientists investigated the uptake of magnesium ions in rice plants. They divided the plants into two groups and placed their roots in solutions containing radioactive magnesium ions. Group Y: plants had a substance that inhibited respiration added to the solution Group Z: plants didnt have the respiratory inhibitor added to the solution The scientist calculate the total amount of magnesium ions absorbed by the plants every 5 minutes. Their results are shown in the graph in Figure 1 below. calculate the ratio of the mean uptake of magnesium ions in the first 20 minutes to the men of uptake of magnesium ions in the second 20 minutes for group Z plants. Show your working. (2 MARKS)

Look at image.

REQUIRED PRACTICAL: Investigation of the effect of solute concentration on the uptake or loss of water from plant tissue.

METHOD: - Prepare a serial dilution of solute from a known (stock) concentration (o, 0.2, 0.4, 0.8 and 1 M) using distilled water - Prepare cylinders of plant tissue eg potato using cork borer to cut potatoes into identically sized chips (1 cm in diameter) - Cut potato chips using knife so they all have the same length - Use scalpel to remove any skin on the ends - Now they all have the same length and diameter → similar surface area to ensure only variable affecting rate of diffusion is concentration of solute - Weigh mass of each cylinder - Submerge cylinders in the range of solute concentrations for 1 hour (solute volume is 5cm3) - Remove chips and gently pat / blot dry with paper towel and reWeigh cylinders - Calculate percentage change in mass → allows comparison of cylinders of different sizes ((Difference in mass (g) / start mass (g))) x 100 - Plot solute concentration against percentage change (mass) to form a calibration curve - Water potential on the x axis and the percentage change on the y axis - Use calibration curve to estimate the water potential of plant tissue → % change = zero → solute + plant are isotonic

Define diffusion.

Net movement of small, non-polar molecules (e.g. oxygen or carbon dioxide), across a selectively permeable membrane, down a concentration gradient from high to low concentration. - Passive / no ATP / energy required

What is facilitated diffusion?

Net passive movement of larger / polar / water soluble molecules (e.g. glucose) across a selectively permeable membrane, down a concentration gradient through a channel / carrier protein (specific transmembrane) - Passive / no ATP / energy required

EQ: Figure 6 shows a carrot. A group of students investigated the effect of sucrose concentration on the length of cylinders cut from a carrot. The students then measured the final lengths of the carrot cylinders. Their results are shown in Table 2. Was it important in this investigation that the carrot cylinders had the same initial length? Explain your answer. (1 MARK)

No, because the results are given as a ratio/as a proportion of initial length;

Q: The structure of cell membranes can be described as 'proteins floating in a sea of lipids'. This membrane structure allows certain substances to pass through freely whereas other substances cannot. State the term used to describe a membrane through which some substances can pass freely but others cannot. (1 MARK)

Partially permeable

Q: Figure 1 below shows a cell surface membrane and some molecules of a small nonpolar substance known as substance X. Predict and explain what will happen to substance X in Figure 1. (4 MARKS)

Substance X will: - move from the outside of the cell to the inside - Directly through the cell membrane by diffusion Because: - There will be a higher concentration of substance X outside the cell than inside the cell / diffusion takes place down a concentration gradient. - The molecules are small and non-polar so can move between the phospholipid

What is water potential represented and measured by?

The greek letter psi and is measured using kilopascals (kPa).

What is water potential?

The likelihood (potential) of water molecules to diffuse out of or into a solution; pure water has the highest water potential and adding solutes to a solution lowers the water potential (more negative)

What happens if a cell is placed in a hypertonic solution?

There will be net flow of water out of the cell, and the cell will lose volume and shrink. A solution will be hypertonic to a cell if its solute concentration is higher than that inside of the cell, and the solutes cannot cross the membrane

Q: A group of students investigated the water potential of potato cells. They cut cubes of potato of equal size and shape, weighed them and placed a single cube into one of four different concentrations of sucrose solution. One cube was placed in pure water. They re-weighed each of the cubes every hour and after 12 hours the mass of all the cubes remained constant. The overall change in mass for each cube is shown in figure 3. Suggest how the students could make their results more precise. (1 MARK)

They could do repeats of the experiment for each concentration of sucrose solution and calculate a mean percentage change in mass

EQ: A student uses potato cylinders to investigate the effect of sucrose concentration on the uptake and loss of water from plant tissue. The student: • makes five dilutions of sucrose solution using a 1.0 mol dm−3 sucrose solution and distilled water • cuts five cylinders from a potato, blots them dry and weighs them • puts each cylinder in a different concentration of sucrose solution for 24 hours • removes the cylinders from the sucrose solutions, blots them dry and weighs them. Table 3 shows the student's results. Why did the student calculate the percentage change in mass of the potato cylinders instead of just the change in mass? (1 MARK)

To allow comparison (of potato cylinders) / (Potato cylinders had) different starting masses;

Q: Ca2+ ATPases are carrier proteins that transport Ca2+ ions across cell-surface membranes. Each Ca2+ ATPase has one subunit that has an ATP binding site and acts as an enzyme. Suggest and explain why Ca2+ ATPase has a subunit that acts as an enzyme.

To catalyse the hydrolysis of ATP in order to release energy for active transport of the calcium ions

Q: Scientists investigated the effect of a drug called a proton pump inhibitor. The drug is given as a tablet to people who produce too much acid in their stomach. It binds to a carrier protein in the surface membrane of cells lining in the stomach. This carrier protein usually moves hydrogen ions into the stomach by active transport. The scientists used two groups of people in their investigation. All the people produced too much acid in their stomach. People in group P were given the drug. Group Q was the control group. The graph shows the results. The scientists used a control group in this trial. Explain why (1 MARK)

To see the effect of the drug

EQ: A student uses potato cylinders to investigate the effect of sucrose concentration on the uptake and loss of water from plant tissue. The student: • makes five dilutions of sucrose solution using a 1.0 mol dm−3 sucrose solution and distilled water • cuts five cylinders from a potato, blots them dry and weighs them • puts each cylinder in a different concentration of sucrose solution for 24 hours • removes the cylinders from the sucrose solutions, blots them dry and weighs them. Table 3 shows the student's results. Why did the student blot the potato cylinders dry before weighing them? (1 MARK)

Water / solution on the outside would increase the mass / weight of the potato cylinders;

EQ: Figure 3 shows how a plant cell produces its cell wall. Y is a protein. One function of Y is to transport cellulose molecules across the phospholipid bilayer. Using teh information from Figure 3, describe the function of Y. (2 MARKS)

Y is an enzyme to catalyse the polymerisation of beta-glucose into cellulose by condensation reaction

Q: Explain how sodium ions are transported through the membranes.

active transport; by specific carrier proteins / pumps;

Water will move by osmosis from a region of _______ (less negative) water potential to one of __________ (more negative) water potential

higher, lower

Give one similarity in the way in which active transport and facilitated diffusion transport substances across the membrane.

involves carrier/transmembrane/transport proteins;

Exam question pointers for osmosis:

→ Change in water potential → movement of H2O (osmosis) → Effect on cell (eg shrivel / burst)

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3.1.4 TRANSPORT INTO AND OUT OF CELLS

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