*Cell Transport Questions
If water moves from A to B and A has a water potential of -510kPa. And water moves from B to C and C has a water potential of -550kPa. What are the possible values of B?
-511kPa to -549kPa As water moves down a water potential gradient from A to B and from B to A. Therefore B must be more negative than -510kPa, the value of A, but less negative than -550kPa.
What concentration of sucrose solution is equivalent to the mean water potential of the potato cells? Why?
0.35 mol dm⁻³ (where line of best fit crosses x-axis) there is no net movement of water (as there is no concentration gradient) so volume of water in = volume of water out
Give two ways active transport differs from diffusion?
1. Active transport requires ATP energy as it is an active process, diffuse does not as it is passive 2. Active transport moves substances against their concentration gradient, whereas diffusion moves substances down their concentration gradient (active transport requires the use of carrier proteins, diffusion does not always).
Give two ways in which active transport differs from diffusion?
1. Active transport requires ATP energy so is an active process whereas diffusion is a passive process that does not require ATP 2. Active transport transports substances against a concentration gradient whereas diffusion transports substances down a concentration gradient 3. ATP requires membrane proteins/carriers whereas diffusion does not necessarily require membrane proteins
Give two functions of the protein molecules in the cell membrane?
1. As channel and carrier proteins facilitating the transport of substances into and out of the cell 2. Receptors e.g. for recognition of hormones and other cells
Explain in terms of water potential, why the potato cubes increased in mass when placed in the distilled water?
1. Water moved across a partially permeable membrane from the 2. less negative water potential in the distilled water to the more negative water potential of the potato cubes 3. down a water potential gradient by osmosis. The volume of water moving in was more than the volume of water moving out, therefore mass increased.
Explain the decreasing shape of the curve as the concentration of sucrose decreases from 0.3 mol dm⁻³ to 0.1 mol dm⁻³? (4)
1. Water moves into the potato cells 2. By osmosis, (down a water potential gradient) 3. Increasing mass 4. More water enters at 0.1 mol dm⁻³ (as there is an increased concentration gradient)
What is the percentage increase in mass if the total mass at the start was 52.75g and the increase in mass was 2.56g?
2.56/52.75 x 100 = 4.85%
What is the surface are ate volume ratio of one cube with a volume of 27cm³ and surface area of 54cm²
54:27 2:1
What is the surface are ate volume ratio of one cube with a volume of 1cm³ and surface area of 6cm²?
6:1
Name the process that results in the concentration of nitrate ions becoming greater in the cytoplasm of a plant cell than in the nutrient soil?
Active transport
Describe how ions are transported through a cell surface membrane by active transport?
Active transport uses protein carriers which are transported by the change of shape or carrier protein in which energy is used to attach the ion to the carrier protein
Where would the potato cells be turgid?
Anywhere on the graph where the ratio of initial to final is less than one (as the final is bigger than initial).
Why is it necessary to bubble air through the nutrient through the solution during an investigation into the transport of ions to a greater concentration into the cytoplasm?
As ATP used to transport substances against a concentration gradient is released in aerobic respiration which requires oxygen
Explain why graphs for facilitated diffusion through channel proteins and simple lipid diffusion are similar up to a point?
Because the external concentration is rate limiting; increasing concentration increases rate
Suggest why the cell membrane is known as fluid mosaic?
Fluid: molecules within the membrane are able to move Mosaic: contains a mixture of phospholipids and proteins
Some ions can enter cells faster than others. Use your knowledge of membrane structure to suggest an explanation for this?
If there are more carrier proteins (as the carrier proteins are specific to specific ions) If the ion is less charged If the ion is smaller
Peeled potatoes were cut into two size cubes: 1cm³ and 27cm³. 27 of the 1cm³ cubes were placed in a beaker of distilled water and the 27cm³ in another beaker. An hour later they are removed, dried and reweighed. Why were twenty-seven small cubes used?
In order for the volume of potato to be the same and only surface area is different.
Glucose is taken up by active transport. Explain why, as concentration of glucose increase, the rate of uptake increases, but then levels of?
Initially, concentration of glucose is limiting As the number of glucose molecules increases, more protein carriers become occupied In the second part, the portion carries are working at full capacity as all carriers are occupied with glucose
Cylinders were placed in sucrose solutions and their mass recorded before and after immersion. Explain why the change in mass was given as a percentage change?
Not all the initial masses were the same, so allows for direct comparison between different concentrations of sucrose solutions
What are the part of a phospholipid molecule?
Phosphate group Two fatty acids Glycerol
The initial length of a potato cylinder in 0.1 mol dm⁻³ sucrose solution is 5.0cm. How would you calculate the final length of the cylinder using a graph showing 'ratio of initial length to final length' against 'concentration of sucrose solution'?
Read off the 'ratio of initial to final' at 0.1 mol dm⁻³ sucrose solution the value is 0.9 0.9 = initial: final 0.9 = initial/final 0.9 = 5/final final x 0.9 = 5 final = 5/0.9 = 5.56cm
Explain how hydrophobic areas in the membrane are important to its function?
Selectively permeable Prevents the passage of water soluble/polar substances Allows the passage of lipid soluble/non polar molecules
Describe the role pf phospholipids in controlling the passage of molecules through the cell membrane?
Selectively permeable to allow the passage of non-polar, lipid soluble substances but prevent the passage of polar/water-soluble/ions
What are three factors that result in a high rate of diffusion molecules of a gas such as carbon dioxide through a membrane?
Short diffusion distance Large concentration gradient Large surface area
Give one factor which would effect the uptake of different sugar by red blood cells and why?
Temperature As kinetic energy of the molecules increases resulting with faster movement and more change of colliding with carriers
Suggest why the rate of uptake of glucose is greater tan galactose or xylose?
The different protein molecules have a different receptor site, complementary to different, specific molecules Therefore an increased number of glucose carriers are present
Ions enter the alga cell by active transport. The cell contains 58 mol dm⁻³ of chloride, 93 mol dm⁻³ potassium and 51 mol dm⁻³ of sodium. What does this suggest about the active transport of the three ions?
The ions are transported through different carrier proteins specific to each of the ions As more potassium is transported than other ions
Scientists investigated rate of diffusion: hole diameter volume per hour volume /mm /cm³ per cm² of hole per hour 22.70 0.24 0.06 6.03 0.10 0.09 3.23 0.04 0.48 2.00 0.02 0.76 Give two conclusions?
The larger the diameter of the hole, the larger the volume of oxygen diffused per hour. The smaller the diameter of the hole, the greater the volume of gas diffusing per cm² of hole/unit area
Describe osmosis in terms of water potential?
The movement of water across a partially permeable membrane from a region of high water potential to a region of low water potential down a water potential gradient.
Explain what causes the shapes of the curves to differ after this point?
The number of channels becomes rate limiting in facilitated diffusion as a limited number of molecules can pass through at any one time
The percentage increase in mass of the smaller cubes was greater than the percentage increase in mass of the large cube. Explain the difference?
The smaller cubes had an increased surface area:volume ratio, therefore rate of osmosis was increased. (And a larger volume of water passed into the cubes overall.)
Sodium ions move against a concentration gradient Cyanide ions stop the functioning of the electron carrier system in mitochondria. Explain how the addition of cyanide ions would affect the uptake of sodium ions by a cell?
Transport of ions would decrease As active transport against a concentration gradient requires ATP But cyanide decreases ATP released by stopping function of the electron carrier system in mitochondria
What effect does solute have on the water potential within a cell?
Water potential become more negative
Where would the point of the curve be where the potato cells have the same water potential as the sucrose solution?
Where the ratio of initial to final is one.