Questions on active transport, co-transport and bulk transport

Question 1

Describe how a carrier protein moves a substance across a membrane by active transport, from the substance binding to the protein returning to its original shape.

Accepted Answer

B1 the substance binds to a specific binding site on the carrier protein, on the side it is starting from B1 ATP binds to the protein and is hydrolysed to ADP and inorganic phosphate B1 the phosphate group attaches to the protein and changes its tertiary structure, so the binding site now faces the other side of the membrane B1 the substance is released, the phosphate detaches, and the protein returns to its original shape ready to repeat the cycle

Question 2

Explain why the absorption of glucose from the lumen of the ileum is described as active, even though the co-transporter protein that carries the glucose hydrolyses no ATP.

Accepted Answer

B1 glucose is moved into the epithelial cell against its own concentration gradient, which diffusion alone could never achieve B1 sodium-potassium pumps in the membrane facing the blood hydrolyse ATP to move sodium ions out of the cell, keeping the concentration of sodium inside the cell low B1 sodium ions then diffuse in from the lumen down the gradient this creates, through the co-transporter protein, which carries glucose at the same time B1 the ATP was therefore spent earlier, at a different protein and on a different ion; the energy is held in the sodium gradient rather than used at the co-transporter

Question 3

Compare endocytosis with exocytosis, referring to the direction of movement, what the membrane does, and the energy required.

Accepted Answer

B1 in endocytosis material is brought into the cell, whereas in exocytosis material is released out of the cell B1 in endocytosis the cell surface membrane invaginates around the material and pinches off to form a vesicle, whereas in exocytosis a vesicle fuses with the cell surface membrane B1 both move material in bulk, with no channel or carrier protein involved B1 both require ATP, for moving the vesicle along the cytoskeleton and for the pinching off or fusion of the membrane

Question 4

A tissue takes up two substances, X and Y. When a respiratory inhibitor is added, uptake of X falls almost to zero within minutes while uptake of Y continues unchanged. Suggest what this shows about how each substance crosses the membrane, and suggest one further observation that would support your conclusion about X.

Accepted Answer

B1 X is taken up by active transport, because its uptake depends on ATP from aerobic respiration B1 the inhibitor stops ATP production by aerobic respiration, so the carrier proteins cannot be phosphorylated and the shape change that carries X across cannot happen B1 Y crosses by a passive process, simple or facilitated diffusion, because it requires no ATP and is therefore unaffected by the inhibitor B1 any one supporting observation: X accumulating inside the cell to a higher concentration than outside, or uptake of X falling when the oxygen concentration is lowered

Question 5

Compare primary active transport with co-transport, referring to where ATP is used and to what actually moves the transported substance against its gradient.

Accepted Answer

B1 in primary active transport, ATP is hydrolysed directly at the carrier protein moving the substance, such as the sodium-potassium pump B1 in co-transport, ATP is not hydrolysed at the co-transporter itself; instead a separate pump uses ATP to build up a concentration gradient of one ion, such as sodium B1 in co-transport the substance moves against its own gradient because it is carried alongside an ion that is itself moving down the gradient the pump created B1 both are still classed as active transport, because both ultimately depend on ATP and both move a substance against its own concentration gradient

Question 6

A root hair cell takes up nitrate ions from soil water in which the nitrate concentration is lower than inside the cell. Suggest which transport mechanism must be responsible, giving your reasoning, and suggest two structural features the cell should have and why.

Accepted Answer

B1 nitrate is more concentrated inside the cell than outside, so it must be moving against its own concentration gradient B1 no passive process, simple or facilitated diffusion, can move a substance against its own gradient, so this must be active transport, using a carrier protein and ATP B1 the cell should contain many mitochondria, to supply enough ATP for this active transport by aerobic respiration B1 the root hair gives the cell a long thin extension, increasing the surface area available for nitrate carrier proteins and so increasing the possible rate of uptake

Question 7

A carrier protein in a root hair cell moves nitrate ions into the cell at a rate of 4 × 10⁴ ions per second, using one ATP molecule for every two nitrate ions moved. Calculate the number of ATP molecules hydrolysed per second by one such carrier, and calculate how many ATP molecules it would use in one hour.

Accepted Answer

M1 ATP hydrolysed per second = 4 × 10⁴ ÷ 2 A1 2 × 10⁴ ATP molecules per second M1 ATP used in one hour = 2 × 10⁴ × 3600 A1 7.2 × 10⁷ ATP molecules

Question 8

An epithelial cell from a kidney tubule is packed with mitochondria and its surface is folded into microvilli. Explain how each feature suits the cell to active transport.

Accepted Answer

B1 active transport uses energy from the hydrolysis of ATP at carrier proteins, and its rate depends on the ATP supply B1 mitochondria are the site of aerobic respiration, so a large number of them keeps the cell supplied with the ATP the carriers spend B1 microvilli give the membrane a large surface area, with room for many more carrier proteins, so more substance can be moved at once

Question 9

Explain why the sodium-potassium pump is described as electrogenic, and explain the contribution this makes to a neurone's resting potential.

Accepted Answer

B1 for every ATP hydrolysed the pump moves three sodium ions out but only two potassium ions in, so net positive charge is moved out of the cell B1 the pump itself therefore makes the inside of the cell more negative relative to the outside, which is what electrogenic means B1 this pump-driven charge movement contributes to a neurone's resting potential, alongside the membrane's much greater permeability to potassium than to sodium, which is the larger contribution

Question 10

Describe how receptor-mediated endocytosis allows a cell to take in LDL cholesterol specifically, rather than taking in extracellular fluid indiscriminately.

Accepted Answer

B1 a receptor on the cell surface membrane binds specifically to LDL, because the receptor's binding site is complementary to it B1 the receptors, with LDL bound, cluster in pits in the membrane, which invaginate and pinch off to form a vesicle carrying LDL at a far higher concentration than in the surrounding fluid B1 the cell therefore takes in LDL in bulk while other extracellular molecules not bound to a receptor enter only in the small volume of fluid inside the vesicle

Question 11

Explain why an epithelial cell reabsorbing glucose in the kidney would show reduced glucose uptake if its sodium-potassium pumps were inhibited, even though the pumps themselves do not carry glucose.

Accepted Answer

B1 the sodium-potassium pumps normally keep the sodium ion concentration inside the cell low, by actively transporting sodium out B1 if the pumps are inhibited, sodium is no longer removed from the cell, so the sodium concentration gradient between the filtrate and the cell is lost or greatly reduced B1 the co-transporter that brings glucose into the cell depends on sodium diffusing in down that gradient to provide the energy for glucose uptake, so with the gradient gone glucose uptake would fall

Question 12

State how many sodium ions and how many potassium ions the sodium-potassium pump moves for each molecule of ATP it hydrolyses, and state the direction of each movement.

Accepted Answer

B1 three sodium ions are moved out of the cell per ATP hydrolysed B1 two potassium ions are moved into the cell per ATP hydrolysed

Question 13

The sodium-potassium pumps in a resting neurone's membrane hydrolyse 2 × 10⁶ molecules of ATP each second. Calculate how many sodium ions the pumps move out of the cell each second, and how many potassium ions they move in.

Accepted Answer

M1 each ATP hydrolysed moves three sodium ions out and two potassium ions in, so multiply the ATP count by 3 and by 2 A1 6 × 10⁶ sodium ions out and 4 × 10⁶ potassium ions in, each second

Question 14

Endocytosis takes two forms. In phagocytosis the cell surface membrane extends around a solid particle, such as a bacterium, and encloses it in a vesicle. In pinocytosis the membrane folds inwards around a droplet of the surrounding fluid, taking in whatever is dissolved in it. A cell takes up a dye that is dissolved in its culture medium and cannot cross the membrane by any other route, and doubling the concentration of the dye doubles its rate of uptake. Identify which form of endocytosis is responsible, and explain why the rate of uptake is proportional to the concentration of the dye.

Accepted Answer

B1 pinocytosis, because the dye is dissolved in the fluid rather than present as solid particles B1 each vesicle takes in a volume of fluid together with whatever it contains, and there is no carrier or binding site to become saturated, so doubling the concentration doubles the amount of dye in each vesicle

Question 15

State what is meant by active transport, and state the energy source it uses.

Accepted Answer

B1 active transport is the movement of a substance across a membrane against its concentration gradient, using a carrier protein B1 the energy source is ATP, hydrolysed to ADP and inorganic phosphate