Transport across cell membranes

State the factors which affect the rate of facilitated diffusion of a substance through a membrane.

Temperature, the number of channel proteins, concentration gradient.

What does it mean if the solution surround the cell is hypotonic?

The fluid surrounding the cell has a higher water potential than the fluid in the cell so water moves into the cell by osmosis.

What is the potential of water?

0 ( all other values are negative)

Students investigated the effect of different concentrations of sodium chloride solution on discs cut from an apple. They weighed each disc and then put one disc into each of a range of sodium chloride solutions of different concentrations. They left the discs in the solutions for 24 hours and then weighed them again. The students were advised that they could improve the reliability of their results by taking additional readings at the same concentrations of sodium chloride. Explain how

Allows anomalies to be identified a mean to be calculated.

Whats the difference between diffusion and facilitated diffusion?

Although both are passive processes, facilitated diffusion relies on the kinetic energy of diffusing molecules and uses carrier proteins whilst simple diffusion does not.

apply knowledge of the fluid mosaic model to suggest how temperature/alcohol affects membrane permeability

As the temperature increases the phospholipids have more energy and this increase the permeability of the membrane

Factors that affect the rate of facilitated diffusion

Number of channel or carrier proteins, concentration gradient

Define osmosis in terms of water potential

Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential

Describe and explain how the plasma membrane is adapted for its function

Phospholipid bilayer. Hydrophobic tail and hydrophilic head means that only small lipid-soluble substances can diffuse in and out of the membrane.

Define what is meant by facilitated diffusion

This is when large or charged molecules diffuse through the membrane using carrier proteins or protein channels, down its concentration gradient.

What is the unit of pressure?

kPa

Explain the role of extrinsic proteins

receptor proteins

What is haemolysis?

The rupture or destruction of red blood cells.

Explain the process of facilitated diffusion to move large molecules across membranes.

A large molecule attaches to the carrier protein in the membrane. The protein undergoes a conformational change.This releases the molecule on the opposite side of the membrane

What is the fluid mosaic model?

A model that describes the structure of cell membranes. Whilst the fluid represents the molecules that can move freely within the phospholipid bilayer, the mosaic illustrates how the proteins are different shapes and sizes, much like a mosaic.

Compare and contrast active transport and facilitated diffusion

Both require carrier proteins. Active transport requires ATP, facilitated diffusion does not. Active transport is an active process, facilitated diffusion is a passive process.

Function of cell membranes

Cell membranes control the passage of substances into and out of the cells.

Scientists investigated the percentages of different types of lipid in plasma membranes from different types of cell. Table 2 shows some of their results. Cholesterol increases the stability of plasma membranes. Cholesterol does this by making membranes less flexible. E. coli has no cholesterol in its cell-surface membrane. Despite this, the cell maintains a constant shape. Explain why.

Cell unable to change shape because cell has a rigid cell wall made of peptidoglycan (murein)

Dissolved substances can move into and out of cells by diffusion. The greater the difference in concentration, the greater the rate of diffusion.

Dissolved substances can move into and out of cells by diffusion. The greater the difference in concentration, the greater the rate of diffusion.

What's the difference between endocyotosis and pinocytosis?

Endocytosis is when a cell engulfs a bacterium whilst pinocytosis is when a cell engulfs a liquid

Describe the functions of membrane proteins

Enzymes, channel protein, protein carriers, receptors for hormones, cell recognition

What is the difference between extrinisic and intrinsic protein.

Extrinsic protiens occur either on the surface of the bilayer or are partly embeddded in. They never penetrate completely across the memebrane. Extrnisic protiens provide mechanical suppport or act as cell receptors for molecules such as hormones. Intrinsics proteins are completelye emebdded in the bilayer from one side to the other. Some act as enzymes whilst others act as carriers to transport water soluble materials.

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

Facilitated diffusion down concentration gradient. Small lipid-soluble molecules pass via phospholipid bilayer whilst large water-soluble molecules go through proteins. Water moves by osmosis from high water potential to low water potential . Active transport is movement against concentration gradient. Active transport and facilitated diffusion involves proteins carriers. Active transport requires energy from ATP. Ref. to Na+ / glucose co-transport.

Organochlorines are insoluble in water, but soluble in lipids. One common use of organochlorines was in sheep dips, and some washed into rivers and lakes. The gills of fish were found to absorb organochlorines, even though they are insoluble in water. Suggest and explain why fish absorb large amounts of organochlorines through the gills.

Gills have no protective covering so cells are exposed. Cells have phospholipids in membrane. Organochlorides can diffuse through membranes. Gills have large surface area exposed to water;

In osmosis, what direction do water molecules move?

Higher water potential to a lower water potential ( less negative to more negative)

Role of glycoproteins

Identify chemical signals and prevent the cell from succumbing to infection

Plasmolysis

In a hypertonic solution the cell membrane shrinks within the cell but the cell wall remains intact. This is called plasmolysis

Pernicious anaemia is another type of anaemia. One method of identifying pernicious anaemia is to measure the diameter of the red blood cells in a sample of blood that has been diluted with an isotonic salt solution. Explain why an isotonic salt solution is used to dilute the blood sample..

Isotonic solution has the same water potential so net movement of water (by osmosis). This means cells will not swell and burst.(No osmotic lysis)

Describe the adaptations of small intestine epithelial cells to absorption

Its folded to form Villus (large surface area). cells lining SI have Microvilli (large surface area).wall of SI is thin (short diffusion distance). rich blood supply (maintains concentration gradient). cells lining SI have transport proteins and mitochondria.

Identify which substances rely on facilitated diffusion and why they cannot enter or leave cells by diffusion

Large, water soluble molecules require facilitated diffusion to cross the membrane

Describe the movement of molecules in solution

Molecules dissolved in a solution are in CONSTANT RANDOM motion due to their kinetic energy.

Describe and explain what happens to a plant cell when its put in pure water.

Plant cell swells but does not burst because the cell wall prevents it from bursting a sit is made of cellulose which is a strong material and gives the cell wall turgidity.

Students investigated the effect of different concentrations of sodium chloride solution on discs cut from an apple. They weighed each disc and then put one disc into each of a range of sodium chloride solutions of different concentrations. They left the discs in the solutions for 24 hours and then weighed them again. The students used a graph of their results to find the sodium chloride solution with the same water potential as the apple tissue. Describe how they did this.

Plot sodium chloride concentration against ratio. Draw a line of best fit. Find sodium chloride concentration from the graph where there is no change in mass.

What is the function of the plasma membrane?

Protects the cell's contents from its surroundings. Regulates movement of substances in and out of the cell. Has receptor molecules enabling it to respond to chemicals.

Scientists investigated the percentages of different types of lipid in plasma membranes from different types of cell. Table 2 shows some of their results. Cholesterol increases the stability of plasma membranes. Cholesterol does this by making membranes less flexible. Suggest one advantage of the percentage of cholesterol in red blood cells being higher than in the cells lining the ileum.

Red blood cells free in blood and not supported by other cells so cholesterol helps to maintain shape. cell from ileum is supported by others in endothelium so cholesterol has less effect on maintaining shape.

Why do poisons which inhibit respiration, result in active transport stopping?

Respiration produces ATP. Active transport requires ATP

Name the 6 main ways in which substances cross this membrane.

Simple diffusion, Facilitated Diffusion, osmosis, active transport, phagocytosis, exocytosis

Describe and explain how molecular size affects the rate of diffusion

Small molecules diffuse faster because they have higher kinetic energy.

Explain the process of co-transport in the context of glucose.

Sodium ions are actively transported out of the ileum epithelial cells into the blood by the sodium potassium pump. This creates a concentration as there are now more sodium ions in the blood and less in the cell. Tihs causes sodium ions to diffuse from the lumen of the ileum into the epithelial cells down their concentration gradient via sodium glucose transporter proteins. The cotransporter carries glucose into the cell with the sodium and as a result the concentration of glucose inside the cells increase. Glucose diffuse out of the cell inot the blood down it concentration gradient through a protein channel by facilitated diffusion.

What does it mean if a cell is hypotonic?

The fluid surrounding the cell has a lower water potential than the fluid in the cell so water moves out of the cell by osmosis.

Diffusion

The movement of molecules from a region of high concentration to a region of lower concentration (passive process).

Active transport

The movement of molecules or ions from a region of low concentration to a region of high concentration against its concentration gradients

Osmosis

The movement of water molecules from a region of high water potential to a region of lower water potential, through a partially permeable membrane.

Define what is meant by diffusion

The net movement of particles from an area of higher concentration to an area of lower concentration.

Describe how phospholipids are arranged in a plasma membrane.

The phospholipid is arranged in a bilayer with the hydrophobic tail facing inside and the hydrophilic head facing outside.

What is water potential

The potential (liklihood) of water molecules to diffuse out of or into a solution.

What is turgo pressure?

The pressure exerted by water inside the cell against the cell wall

Factors that affect the rate of active transport

The rate of respiration, the number of carrier proteins present, the speed of individual carrier proteins.

Explain the term isotonic

The solutions have the same water potential so there is no net movement of water.

The terms, hypotonic, isotonic, and hypertonic, are used to compare the osmolarity of a cell to the osmolarity of the extracellular fluid around it.

The terms, hypotonic, isotonic, and hypertonic, are used to compare the osmolarity of a cell to the osmolarity of the extracellular fluid around it.

Suggest why overwatering of plants can kill the plant.

Too much water in the soil can deplete the oxygen in the soil pores.A lack of oxygen in a waterlogged soil damages or kills plant roots. Plant roots require oxygen for cellular respirationn. Also anaerobic microbes in a waterlogged soil produce toxic waste products that can harm plant roots.

A scientist investigated the uptake of sodium ions by animal tissue. To do this, he: used two flasks, F and G and added to flask F a solution of a substance that prevents the formation of ATP by cells 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.

Uptake in flask G much greater than in flask F thus showing use of ATP in flask G. Sodium ion concentration in flask G falls to zero showing uptake against a concentration gradient.

A scientist investigated the uptake of sodium ions by animal tissue. To do this, he: used two flasks, F and G and added to flask F a solution of a substance that prevents the formation of ATP by cells Figure 2 shows his results.The scientist concluded that the cells in flask G took up sodium ions by active transport. The curve for flask F levelled off after 20 minutes. Explain why.

Uptake of sodium ions occurring by facilitated diffusion. Equilibrium reached.

Explain why water soluble molecules cannot pass across the bilayer by diffusion.

Water-soluble molecules cannot pass through the hydrophobic middle portion of the phospholipid bilayer.

Explain the role of intrinsic proteins.

channel proteins

Define what is meant by co-transport

A process in which two substances are simultaneously transported across a membrane by one protein, or protein complex which does not have ATPase activity

Relate the structure of the membrane to its role around and inside cells.

cell signalling

Explain what a dilution series is,

A serial dilution is the stepwise dilution of a substance in solution. Usually the dilution factor at each step is constant, resulting in a geometric progression of the concentration in a logarithmic fashion.

Some substances 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 a high water potential to a low water potential down a water potential gradient through aquaporins. By facilitated diffusion using channel proteins down a concentration gradient. By active transport active concentration gradient through carrier proteins using ATP from respiration. By phagocytosis. Engulfing by cell surface membrane to form vesicle. By exocytosis which involves the fusion of vesicle with cell surface membrane.

Explain the process of facilitated diffusion to move charged molecules across membranes.

Channel proteins form pores in the membrane for charged particles to diffuse through

Explain role of cholesterol

Cholesterol is a lipid found in all cell membranes except bacterial membranes. They bind to hydrophobic tails of the phospholipid causing them to pack more closely together. This restricts the movement of the phospholipid making the membranes less fluid and more rigid. Cholesterol helps to maintain the shape of animals cells which is importatn for cells that arent supported by other cells e.g. red blood cells It provides stability.

The scientists' hypothesis was that adding the inhibitor of amylase to the food would lead to a lower blood glucose concentration. Use your knowledge of digestion to suggest how the addition of the inhibitor could lead to a lower blood glucose concentration.

Less starch digested to maltose so less glucose from maltose so less absorption of glucose from the gut.

Students investigated the effect of different concentrations of sodium chloride solution on discs cut from an apple. They weighed each disc and then put one disc into each of a range of sodium chloride solutions of different concentrations. They left the discs in the solutions for 24 hours and then weighed them again. The students were advised that they could improve their graph by taking additional readings. Explain how.

Line of best fit is more reliable. Point where line crosses axis, is more reliable. Can plot standard deviation values to show variability about the mean.

Explain the process of active transport

Molecules in area of low concentration bind to carrier protein. ATP breaks down to ADP, inorganic phosphate and energy. The Pi and Energy cause the carrier protein to change shape. The carrier protein releases molecules on opposite side in the area of high concentration. The carrier protein releases the attached Pi to return to its original shape.

A scientist investigated the effect of cyanide on the uptake of sodium ions by animal tissue. He set up two beakers, J and K. He put equal volumes of a solution containing sodium ions and equal masses of an animal tissue in each beaker. He added cyanide to beaker J. He did not add cyanide to beaker K. Cyanide is a substance which affects respiration. Use information in the question to explain the effect of cyanide on the uptake of sodium ions by the tissue.

Reduces respiration less ATP produced. ATP needed for active transport.

State the factors that affect diffusion.

Surface area Concentration gradient Diffusion pathway Mass molecules diffusion

Factors that affect the rate of diffusion

Surface area, concentration gradient, thickness of the exchange surface

Factors that affect the rate of osmosis

Surface area, water potential gradient, thickness of the exchange surface

Identify key variables which affect membrane permeability

Temperature

Describe and explain what happens to a plant cell when its put in concentrated sugar solution.

Water leaves by osmosis. Cell wall prevents cell from shrinking, keeps it rigid. The protoplast shrink. The cell is Plasmolysed.

What happens to a plant cell when placed in hypotonic solution?

When placed in a hypotonic solution, cells surrounding by cell walls swell but do not burst because the cell wall is strong enough to withstand the turgo presssure.

For the following examples states what direction if any, the water molcules will move by osmosis a) -600kPa -400kPA b) -200kPa -600kPa c) -300kpa -300kPa

a)← b) → c) no movement

State Fick's law

ficks law