IB Biology B2.1: Membranes and membrane transport worked solutions

The diagram shows part of a plasma membrane with four proteins, P, Q, R and S. Which statement is correct? [Graph: Part of a membrane: P is a small protein lying on the outer surface; Q is a large protein spanning both halves of the bilayer; R is embedded in the inner half only; S lies on the inner surface.] (A) P is a peripheral protein and R is an integral protein. (B) Q is an integral protein and S is an integral protein. (C) P is an integral protein and Q is a peripheral protein. (D) R is a peripheral protein because it does not span the bilayer.

Potassium ions move into a cell down their concentration gradient. Which statement is correct? (A) They diffuse between the phospholipids because they are small. (B) They pass through a channel protein by facilitated diffusion. (C) They are pumped through a protein using ATP. (D) They cross the membrane by osmosis.

The concentration of ion X is 0.40\,\mathrm{mol\,dm^{-3}} inside a root cell and 0.02\,\mathrm{mol\,dm^{-3}} in the soil water. Uptake of X stops when the root is placed in nitrogen gas, which excludes oxygen. What does this show? (A) X enters by simple diffusion. (B) X enters through channel proteins by facilitated diffusion. (C) X enters by active transport using ATP from aerobic respiration. (D) X enters by osmosis.

Cells P and Q have the same size and the same solute concentration inside. P has many aquaporins and Q has few. Both are placed in the same dilute solution. What is the expected result? (A) Water enters P faster, and both reach the same final volume. (B) Water enters P more slowly because aquaporins use ATP. (C) Water enters P faster and P reaches a larger final volume. (D) Water enters P only, because Q is impermeable to water.

Phospholipids placed in water form a bilayer. Which statement explains the arrangement? (A) The hydrophilic heads face the water and the hydrophobic tails are shielded in the middle. (B) The hydrophobic tails face the water on both sides and the heads meet in the middle. (C) The heads face the water on one side and the tails face the water on the other. (D) The hydrophobic tails form hydrogen bonds with the water on both sides.

Four membranes have the same amount of phospholipid. Which will stay fluid at the lowest temperature? (A) Tails that are all saturated and short. (B) Tails with many cis double bonds. (C) Tails that are all saturated and long. (D) Tails that are all saturated, with extra peripheral proteins.

Which row correctly pairs each channel in a neuron with the event that opens it? (A) Neurotransmitter-gated channel: the neurotransmitter binds. Voltage-gated channel: the membrane potential changes. (B) Neurotransmitter-gated channel: the membrane potential changes. Voltage-gated channel: the neurotransmitter binds. (C) Both channels open when ATP binds. (D) Both channels open when a neurotransmitter binds.

Cells lining the gut stay attached to one another as food passes. Which description best fits the molecules that hold the cells together? (A) Integral membrane proteins whose outer parts bind to matching proteins on the neighbouring cell. (B) Peripheral proteins that pump ions from one cell to the next. (C) Phospholipids that form a bridge between the two cells. (D) Vesicles that fuse the two plasma membranes into one.

Sketch and label a plasma membrane to show the fluid mosaic model, marking its outside and inside.

Facilitated diffusion and active transport both move particles across the plasma membrane. (a) State two features that the two processes share. [2] (b) Distinguish between the two processes in terms of the direction of movement and the source of energy. [2] (c) Name one particle moved by each process. [1]

Cells were placed in solutions of two substances, P and Q. The rate of uptake was measured at different concentrations.\begin{array}{lcccccc}\hline \text{Concentration / mmol\,dm}^{-3}&\text{0}&\text{2}&\text{4}&\text{8}&\text{16}&\text{32}\\ \hline \text{Uptake of P / units}&\text{0}&\text{3}&\text{6}&\text{12}&\text{24}&\text{48}\\ \hline \text{Uptake of Q / units}&\text{0}&\text{14}&\text{25}&\text{38}&\text{46}&\text{47}\\ \hline\end{array} (a) Describe the relationship between concentration and uptake of P. [1] (b) Calculate the percentage increase in the uptake of Q when the concentration doubles from 16 to 32 mmol dm^{-3}. [1] (c) Explain the plateau in the uptake of Q. [2] (d) Predict, with a reason, the effect on the uptake of Q of doubling the number of channel proteins. [1] (e) Suggest why P shows no plateau. [1]

Protein T may carry substance Y into root cells. Uptake of Y was measured (mean \pm SD, six samples).\begin{array}{lc}\hline \text{Treatment}&\text{Uptake of Y / }\mu\text{mol\,g}^{-1}\text{\,h}^{-1}\\ \hline \text{Control}&\text{8.0 }\pm\text{ 0.5}\\ \hline \text{Compound Z, which binds protein T}&\text{2.0 }\pm\text{ 0.4}\\ \hline \text{Compound Z}'\text{, similar shape, does not bind T}&\text{7.6 }\pm\text{ 0.6}\\ \hline \text{Cyanide, which stops respiration}&\text{1.5 }\pm\text{ 0.3}\\ \hline\end{array} (a) Calculate the percentage reduction in uptake caused by compound Z. [1] (b) Using the standard deviations, state whether Z' differs from the control. [1] (c) Deduce what the cyanide result suggests about the way Y enters. [1] (d) Evaluate the claim that these results prove that protein T carries Y, including the purpose of treatment Z'. [3]

Frog egg cells were placed in a solution much more dilute than their cytoplasm. Some cells had been given the gene for an aquaporin and control cells had not. Volumes were measured at the start and after 10 minutes.\begin{array}{lcc}\hline \text{Group}&\text{Start volume / nl}&\text{Volume after 10 min / nl}\\ \hline \text{Control}&\text{940}&\text{970}\\ \hline \text{Aquaporin}&\text{1010}&\text{1210}\\ \hline\end{array} (a) Calculate the percentage increase in volume for each group. [2] (b) Explain why percentage change is a better comparison than final volume. [1] (c) Explain the result for the aquaporin cells. [2] (d) Predict the change in volume if the aquaporin cells were placed in a solution with the same solute concentration as their cytoplasm. [1]

Dialysis tubing was filled with a mixture of starch and glucose solutions, sealed and placed in water. After 30 minutes the water outside was tested. Glucose was present and starch was absent. (a) Explain why starch was not found in the water. [1] (b) Explain how glucose reached the water. [2] (c) Identify two ways in which dialysis tubing is a limited model of a plasma membrane. [2]

Equal cubes of beetroot were placed in water at different temperatures for 10 minutes. The absorbance of the water, which measures the red pigment that leaked out, was recorded. Each value is the mean of three tubes.\begin{array}{lccccc}\hline \text{Temperature / }^\circ\text{C}&\text{20}&\text{30}&\text{40}&\text{50}&\text{60}\\ \hline \text{Absorbance}&\text{0.04}&\text{0.05}&\text{0.09}&\text{0.31}&\text{0.78}\\ \hline\end{array} (a) State two variables that should be kept constant. [2] (b) Describe the trend in the results, using values. [1] (c) Explain the result at 60 ^\circC. [2] (d) Suggest one way to make the conclusion more reliable. [1]

Part of a transmembrane protein forms a channel for ions. The table shows the main amino acids in three regions of the chain. Regions X and Y lie within the thickness of the bilayer.\begin{array}{ll}\hline \text{Region}&\text{Main R groups}\\ \hline \text{X}&\text{leucine, valine, phenylalanine}\\ \hline \text{Y}&\text{serine, glutamate, asparagine}\\ \hline\end{array} (a) Deduce which of X and Y faces the phospholipid tails and which lines the pore. [2] (b) Explain why the lining of the pore must be polar. [2] (c) Predict the effect of replacing the lining of the pore with leucine. [1]

Oxygen crosses the plasma membrane without a protein, but sodium ions need one. (a) Explain why oxygen can cross the phospholipid bilayer unaided. [2] (b) Explain why sodium ions cannot. [2]

A secretory cell has a surface membrane area of 1200\,\mu\mathrm{m}^2. Secretory vesicles fuse with the surface at 60 per minute, and each has a membrane area of 0.50\,\mu\mathrm{m}^2. Endocytic vesicles each have a membrane area of 0.20\,\mu\mathrm{m}^2. (a) Calculate the membrane area added to the surface per minute by exocytosis. [1] (b) Express this as a percentage of the surface area. [1] (c) The surface area of the cell stays constant. Calculate the number of endocytic vesicles that must form per minute. [2] (d) Describe how endocytosis forms a vesicle and why fluidity is needed. [2] (e) Explain why the contents of a new endocytic vesicle are not yet in the cytoplasm. [1] (f) State one use of ATP in exocytosis. [1]

An intestinal epithelial cell has 2.0\times10^{5} sodium-potassium pumps in its basal membrane. Each pump completes 50 cycles per second. (a) Calculate the number of sodium ions pumped out of the cell per second. [2] (b) Calculate the net number of positive charges removed from the cell per second. [2] (c) Explain how the pump allows the cell to take up glucose from the gut against its concentration gradient. [3] (d) The drug ouabain blocks the pump. Predict the effect on glucose uptake over the next hour, with a reason. [1]

Fluidity was measured in two artificial membranes at three temperatures. Membrane M contains phospholipids only. Membrane N contains the same phospholipids with cholesterol. Higher values mean more fluid.\begin{array}{lccc}\hline \text{Temperature / }^\circ\text{C}&\text{10}&\text{25}&\text{40}\\ \hline \text{Fluidity of M}&\text{12}&\text{45}&\text{90}\\ \hline \text{Fluidity of N}&\text{30}&\text{42}&\text{55}\\ \hline\end{array} (a) Calculate the change in fluidity between 10 and 40 ^\circC for each membrane. [2] (b) Compare the effect of cholesterol at 10 ^\circC and at 40 ^\circC. [2] (c) Explain these two effects. [2] (d) Suggest one change in the phospholipids that helps a cold-water fish keep its membranes fluid. [1]

Lipids were extracted from red blood cells and spread in a single layer on water. The lipid from 1.0\times10^{8} cells formed a film of area 2.0\times10^{10}\,\mu\mathrm{m}^2. The mean surface area of one cell is 100\,\mu\mathrm{m}^2. (a) Calculate the ratio of the film area to the total surface area of the cells. [2] (b) One model says a membrane has a single layer of lipid and another says it has two layers. State the ratio each predicts. [1] (c) State which model the data support. [1] (d) Explain why this evidence cannot show where the proteins are. [1] (e) Suggest one source of error in the investigation. [1]

B2.1 + C2.2. In a resting neuron the concentration of \ce{K+} is 140 mmol dm^{-3} inside and 5 outside. The concentration of \ce{Na+} is 12 inside and 145 outside. (a) State the direction of net passive movement of each ion if its channels open. [2] (b) Explain how the sodium-potassium pump maintains these concentrations. [2] (c) A toxin blocks voltage-gated sodium channels. Predict the effect on an action potential and explain it. [2]

B2.1 + D3.3. Glucose is filtered from blood into the nephron and is normally reabsorbed completely by the cells of the proximal convoluted tubule, even where the glucose concentration in the filtrate is lower than in the blood. (a) Explain how the tubule cells reabsorb glucose against its concentration gradient. [3] (b) In diabetes mellitus the blood glucose is very high and glucose appears in the urine. Suggest why reabsorption is incomplete. [2] (c) Explain why the tubule cells contain many mitochondria. [1]

B2.1 + B2.2 + B1.2. A pancreatic cell makes a digestive enzyme, which is a protein, and secretes it into a duct. (a) Outline the pathway of the enzyme from synthesis to release. [3] (b) Explain what happens to the membrane of the secretory vesicle during release. [2] (c) Suggest why the cell contains many mitochondria. [1]