Cell Membranes and Transport: Biology 9700 (Cambridge International AS & A Level)
Syllabus 4.1, 4.2 · Strand 1 Cells and biological molecules
- Questions
- 10
- Total marks
- 47
- Tier mix
- 10 Core
0 of 10 questions completed
Syllabus coverage
- 4.1 3 questions completed
- 4.2 7 questions completed
The membrane surrounding every cell is far more than a simple boundary, and this topic (syllabus 4.1 and 4.2) explains both its structure and how substances cross it. The fluid mosaic model describes a bilayer of phospholipids, with hydrophilic heads facing the watery surroundings and hydrophobic tails pointing inward, studded with a shifting mosaic of proteins. You should be able to describe the roles of cholesterol, glycolipids, glycoproteins and both carrier and channel proteins in giving the membrane its stability, fluidity, selective permeability and its part in cell recognition. The membrane is also the site of cell signalling, where a ligand released from one cell binds to a receptor on a target cell and triggers a specific response.
The second half of the topic covers movement across the membrane. Simple and facilitated diffusion and osmosis are passive, needing no energy from the cell, while active transport moves substances against their concentration gradient using ATP, and bulk transport happens by endocytosis and exocytosis. Water movement is explained using water potential, the tendency of water to move from a less negative to a more negative potential. Applying this idea explains why an animal cell may burst or shrink and why a plant cell becomes turgid or plasmolysed, depending on the solution around it.
The exam-style questions below are original, written to match these objectives, each with a full worked solution so you can check your reasoning step by step.
Question 1
Cholesterol molecules sit between the phospholipids of an animal cell-surface membrane, with their hydroxyl group close to the phospholipid heads and their hydrocarbon ring system among the fatty acid tails.
Which statement correctly describes the effect of cholesterol on the fluidity of the membrane?
Question 2
A student is studying a section of a cell-surface membrane. It contains: a phospholipid bilayer; a protein that spans the full width of the bilayer and forms a water-filled channel; a short, branching carbohydrate chain attached to a protein on the outer surface of the membrane; and a second protein that is attached only to the inner (cytoplasmic) surface of the membrane and does not cross the bilayer.
(a) Describe the arrangement of the phospholipid molecules within the bilayer, and explain how the structure of a phospholipid produces this arrangement. [2]
(b) Distinguish between an intrinsic (integral) membrane protein and an extrinsic (peripheral) membrane protein, referring to the channel protein and to the protein attached only to the inner surface in your answer. [2]
(c) The carbohydrate chain attached to the protein on the outer surface forms a glycoprotein. Explain two roles that glycoproteins carry out at the cell surface. [2]
Question 3
A skeletal muscle cell obtains glucose from the surrounding tissue fluid using a glucose carrier protein in its cell-surface membrane. The same cell also accumulates calcium ions inside an internal membrane sac against a steep concentration gradient, using a different type of carrier protein, and occasionally removes a small, damaged patch of its own cell-surface membrane by pinching it inward to form a vesicle in the cytoplasm.
(a) Glucose enters the muscle cell down its concentration gradient using the glucose carrier protein. Explain how this process, facilitated diffusion, differs from simple diffusion. [2]
(b) Calcium ions are moved against their concentration gradient using the second carrier protein. Explain how this process, active transport, differs from facilitated diffusion in terms of its energy requirement and the direction of net movement. [3]
(c) Suggest why a cell that carries out large amounts of active transport, such as this muscle cell, typically contains an unusually large number of mitochondria. [2]
(d) Explain how the removal of the damaged patch of membrane, described above, involves the cell-surface membrane, and describe how this process, endocytosis, differs from exocytosis. [2]
Question 4
A cell taken from the petiole (leaf stalk) of a rhubarb plant, cell Q, has a solute potential (ψs) of −950 kPa. At the moment it is examined, cell Q is not fully turgid, and has a pressure potential (ψp) of +350 kPa.
(a) Using the equation ψ = ψs + ψp, calculate the water potential of cell Q. Show your working. [2]
(b) Cell Q is then placed into an external sucrose solution with a water potential of −1250 kPa. Compare this value with your answer to part (a), predict the direction of net water movement between cell Q and the solution, and explain your reasoning. [3]
(c) Describe how cell Q would appear if examined under a microscope after being left in this solution for some time, and explain, referring to the cell wall and the cell-surface membrane, why it would look this way. [2]
Question 5
A sample of human red blood cells, which have no cell wall, is placed into a solution with a water potential higher (less negative) than the water potential of the cytoplasm inside the cells.
Which statement correctly describes what happens to the red blood cells, and why?
Question 6
Oxygen diffuses across the cell-surface membrane of a cell by simple diffusion, moving from a region of higher oxygen concentration to a region of lower oxygen concentration.
Which change would decrease the rate at which oxygen diffuses across the membrane?
Question 7
A student cuts equal-sized discs of beetroot tissue, whose vacuoles contain a red pigment, and rinses each disc thoroughly in distilled water to remove any pigment released by the cutting. Each disc is then placed into its own tube of distilled water, and the tubes are held at a different temperature: 10 °C, 30 °C, 50 °C and 70 °C. After 20 minutes, the student compares the colour intensity of the water in each tube.
The water in the 10 °C and 30 °C tubes stays almost colourless, the water in the 50 °C tube develops a faint red colour, and the water in the 70 °C tube develops a strong, deep red colour.
(a) Explain, in terms of the fluid mosaic model, why the pigment leaks out of the beetroot cells more at 70 °C than at 30 °C. [3]
(b) The pigment molecules are too large to cross the phospholipid bilayer by simple diffusion. Suggest how they are able to leave the cell in large amounts at 70 °C. [2]
(c) Predict and explain what would happen to the colour intensity of the water if the experiment were repeated with beetroot discs held at 95 °C instead of 70 °C. [2]
Question 8
A disc of potato tuber tissue, cell P, is flaccid (has no pressure potential) before an experiment. Cell P has a solute potential (ψs) of −700 kPa and a pressure potential (ψp) of 0 kPa. Cell P is then placed into a beaker of distilled water, which has a water potential of 0 kPa.
(a) Using the equation ψ = ψs + ψp, calculate the water potential of cell P before it is placed in the distilled water. Show your working. [2]
(b) Compare this value with the water potential of the distilled water, and predict and explain the direction of net water movement between cell P and the distilled water immediately after cell P is placed in it. [3]
(c) As cell P takes in water, its pressure potential rises because the cell wall resists the expansion of the protoplast. Explain why the net movement of water into cell P eventually stops, and state the water potential of cell P at this point. [3]
Question 9
The soil solution surrounding a root hair cell contains nitrate ions at a much lower concentration than the concentration of nitrate ions already inside the root hair cell's cytoplasm. Despite this, the root hair cell continues to take up nitrate ions from the soil solution, using a specific carrier protein in its cell-surface membrane.
(a) Explain why the root hair cell cannot take up these nitrate ions by facilitated diffusion, and state the type of transport it must use instead. [2]
(b) Describe, with reference to changes in the shape of the carrier protein, how this type of transport moves a nitrate ion across the membrane. [3]
(c) State one structural difference between the carrier protein described above and a channel protein used in facilitated diffusion. [1]
Question 10
A macrophage (a type of white blood cell) engulfs a bacterium by extending folds of its cell-surface membrane around the bacterium and pinching off a vesicle, containing the bacterium, into its cytoplasm. This process is called phagocytosis, a form of endocytosis.
Which statement about this process is correct?