Cell Membranes and Transport — A-Level Biology Study Guide

For: A-Level Biology candidates sitting A-Level Biology.

Covers: Fluid mosaic membrane structure, simple and facilitated diffusion, osmosis and water potential, active transport with ATP and pumps, and endocytosis/exocytosis bulk transport mechanisms.

You should already know: IGCSE Biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the A-Level Biology style for educational use. They are not reproductions of past Cambridge International examination papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official Cambridge mark schemes for grading conventions.

1. What Is Cell Membranes and Transport?

Cell membranes are selectively permeable phospholipid bilayers that enclose all living cells and organelles, controlling the movement of substances into and out of the cell; transport refers to the set of passive and active mechanisms that move ions, molecules, and larger particles across these membranes. This is a core A-Level Biology topic tested in both AS and A2 papers, often linked to enzyme activity, cell signalling, homeostasis, and practical assessment tasks. Common synonyms include plasma membrane transport and cell surface membrane transport.

2. Membrane structure — fluid mosaic

Proposed by Singer and Nicholson in 1972, the fluid mosaic model is the universally accepted structure of cell membranes. The name breaks down into two key properties:

• Fluid: Phospholipids and most embedded proteins can move laterally within the bilayer, rather than being fixed in place.
• Mosaic: Scattered proteins, glycoproteins, glycolipids, and cholesterol molecules create a mosaic-like pattern when the membrane is viewed from above.

Key components of the membrane:

1. Phospholipid bilayer: Hydrophilic phosphate heads face the aqueous extracellular and cytoplasmic environments, while hydrophobic fatty acid tails point inward, forming a 7 nm thick hydrophobic core that blocks the passage of most charged or large polar molecules.
2. Intrinsic (integral) proteins: Span the full width of the bilayer, including channel proteins, carrier proteins, and active transport pumps.
3. Extrinsic (peripheral) proteins: Bound to the inner or outer surface of the bilayer, often involved in cell signalling or acting as membrane-bound enzymes.
4. Glycoproteins/glycolipids: Have short carbohydrate chains attached only to the extracellular surface of the membrane, functioning in cell recognition, immune response, and cell adhesion.
5. Cholesterol: Found exclusively in animal cell membranes, it reduces fluidity at high temperatures and prevents the bilayer from solidifying at low temperatures, maintaining structural stability.

Worked example: Arctic fish have unusually high cholesterol content in their cell membranes. Explain why this is an adaptation to their environment. Answer: Cold temperatures would normally make phospholipid bilayers rigid and impermeable. Cholesterol disrupts tight packing of fatty acid tails, preventing the membrane from freezing and retaining its fluidity at sub-zero temperatures.

Exam tip: Examiners frequently ask you to label fluid mosaic diagrams; you will lose marks if you draw carbohydrate chains on the cytoplasmic side of the membrane, as they only exist on the extracellular surface.

3. Diffusion and facilitated diffusion

Diffusion is the passive net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, with no ATP required. It uses the inherent kinetic energy of moving particles, so rate increases with temperature.

Simple diffusion

Occurs directly through the phospholipid bilayer, only for small, non-polar, lipid-soluble molecules (e.g., oxygen, carbon dioxide, steroid hormones) or very small uncharged polar molecules (e.g., urea, small amounts of water). Rate is determined by concentration gradient, surface area, exchange surface thickness, and molecular size.

Facilitated diffusion

Passive movement of larger polar or charged molecules that cannot cross the hydrophobic bilayer core, via intrinsic transport proteins, with no ATP required. There are two types of transport protein:

1. Channel proteins: Water-filled pores that open or close in response to stimuli (e.g., voltage-gated sodium channels in neurones), highly specific to their target ion or molecule.
2. Carrier proteins: Bind to a specific molecule on one side of the membrane, undergo a conformational (shape) change, and release the molecule on the other side of the bilayer.

Worked example: Calculate the rate of facilitated diffusion of glucose across a red blood cell membrane if $1.8\times 10^6$ glucose molecules cross a $2\mu m^2$ area of membrane in 3 seconds. Rate = $\frac{\text{Number of molecules}}{\text{Area}\times \text{Time}}=\frac{1.8\times 10^6}{2\times 3}=3\times 10^5$ molecules $\mu m^{-2}{s}^{-1}$

Exam tip: If a question refers to transport of charged particles (e.g., $N{a}^{+}$, $K^{+}$) or large polar molecules (e.g., glucose, amino acids), it is always facilitated diffusion (or active transport, if against a gradient) — never simple diffusion, as these particles cannot cross the hydrophobic bilayer core.

4. Osmosis — water potential

Osmosis is the net passive movement of water molecules from a region of higher water potential ($\Psi$, pronounced psi) to a region of lower water potential, across a selectively permeable membrane, down a water potential gradient, with no ATP required.

Key definitions

• Water potential: The tendency of water molecules to move out of a solution, measured in kilopascals (kPa). Pure water at standard temperature and pressure has a $\Psi$ of 0 kPa. Adding solutes reduces water potential, so all aqueous solutions have negative $\Psi$ values.
• Solute potential (${\Psi }_s$): The component of water potential due to dissolved solutes, always a negative value. Higher solute concentration = more negative ${\Psi }_s$.
• Pressure potential (${\Psi }_p$): The component of water potential due to hydrostatic pressure acting on the solution, usually positive in turgid plant cells (due to the cell wall pushing back on swollen cytoplasm).

The formula for total water potential in plant cells is: $$\Psi ={\Psi }_s+{\Psi }_p$$

Tonicity effects

• Isotonic solution: $\Psi$ of solution = $\Psi$ of cell cytoplasm, no net water movement: animal cells retain normal shape, plant cells are flaccid.
• Hypotonic solution: $\Psi$ of solution > $\Psi$ of cell, water moves into the cell: animal cells burst (haemolysis for red blood cells), plant cells become turgid (the normal supported state for plants).
• Hypertonic solution: $\Psi$ of solution < $\Psi$ of cell, water moves out of the cell: animal cells crenate (shrivel), plant cells plasmolyse (cell membrane pulls away from the cell wall).

Worked example: A plant cell has a solute potential of -1100 kPa and a pressure potential of 350 kPa. It is placed in a sucrose solution with a water potential of -900 kPa. Predict the net direction of water movement.

1. Calculate cell water potential: $\Psi =-1100+350=-750$ kPa
2. Compare to solution $\Psi$: -750 kPa > -900 kPa, so water moves out of the cell into the sucrose solution, down the water potential gradient.

Exam tip: Examiners often test the inverse relationship between solute concentration and water potential. Never confuse the two: higher solute concentration = lower (more negative) water potential.

5. Active transport — pumps and ATP

Active transport is the net movement of molecules or ions from a region of lower concentration to a region of higher concentration, against a concentration gradient, requiring energy from ATP hydrolysis and specific carrier protein pumps.

ATP role

ATP (adenosine triphosphate) is hydrolysed to ADP (adenosine diphosphate) and inorganic phosphate ($P_i$), releasing energy that causes a conformational change in the carrier pump protein, allowing it to move the target substance across the membrane against its gradient.

The most well-documented example is the sodium-potassium ($N{a}^{+}/K^{+}$) ATPase pump, found in all animal cell membranes:

• For every 1 ATP molecule hydrolysed, 3 $N{a}^{+}$ ions are pumped out of the cell, and 2 $K^{+}$ ions are pumped into the cell.
• This creates an electrochemical gradient used for nerve impulse transmission, glucose co-transport in the intestines, and maintenance of cell osmotic balance.

Rate of active transport is determined by the number of pump proteins in the membrane, and the rate of ATP production (affected by oxygen concentration, temperature, glucose availability, and presence of respiratory poisons like cyanide).

Worked example: A sample of intestinal epithelial cells is treated with cyanide, which stops aerobic respiration and ATP production. Explain why glucose uptake by the cells drops by 80% but does not stop entirely. Answer: 80% of glucose uptake is via active transport against the glucose concentration gradient, which stops when ATP is unavailable. The remaining 20% is via facilitated diffusion down the glucose concentration gradient, which is passive and does not require ATP, so it continues until equilibrium is reached.

Exam tip: If a question states that transport is inhibited by low oxygen or respiratory poisons, it must be active transport (or bulk transport), as passive transport mechanisms do not require ATP.

6. Endocytosis and exocytosis

Endocytosis and exocytosis are forms of bulk transport, used to move large particles (e.g., proteins, bacteria, whole cells) that are too large to pass through transport proteins. Both processes use membrane-bound vesicles and require ATP, so they are classified as active transport mechanisms.

Endocytosis

The cell surface membrane invaginates (folds inward) around a target particle, pinches off to form a vesicle inside the cytoplasm containing the ingested material. There are three main types:

1. Phagocytosis ("cell eating"): Ingestion of large solid particles, e.g., white blood cells phagocytosing pathogenic bacteria, amoeba ingesting food particles.
2. Pinocytosis ("cell drinking"): Ingestion of small liquid droplets containing dissolved solutes, e.g., intestinal epithelial cells taking up dissolved nutrients from the gut lumen.
3. Receptor-mediated endocytosis: Specific form where target molecules bind to complementary receptors on the cell surface before endocytosis occurs, e.g., uptake of cholesterol bound to low-density lipoprotein (LDL) receptors.

Exocytosis

A vesicle containing secretory material (formed by the Golgi apparatus) moves to the cell surface membrane, fuses with it, and releases its contents to the extracellular environment. Common examples include secretion of digestive enzymes from pancreatic cells, secretion of hormones from endocrine cells, and release of neurotransmitters from neurone synapses.

Worked example: Explain why pancreatic exocrine cells (which secrete large amounts of digestive enzymes) have very high numbers of Golgi bodies and mitochondria. Answer: Golgi bodies modify and package secretory proteins into vesicles for exocytosis. Mitochondria produce the ATP required for vesicle movement through the cytoplasm and fusion with the cell surface membrane during exocytosis.

7. Common Pitfalls (and how to avoid them)

• Wrong move: Stating that channel proteins are used in active transport. Why: Students confuse channel and carrier proteins, assuming all transport proteins are used for all transport types. Correct move: Only carrier proteins can act as pumps for active transport; channel proteins are only used for facilitated diffusion, as they cannot use ATP to move substances against a gradient.
• Wrong move: Claiming water moves from lower to higher water potential. Why: Students mix up water potential with solute concentration, assuming "higher concentration" is the direction of movement. Correct move: Water always moves down its water potential gradient, from higher (less negative) $\Psi$ to lower (more negative) $\Psi$.
• Wrong move: Saying cholesterol is present in plant and prokaryotic cell membranes. Why: Students assume all cell membranes have identical components. Correct move: Cholesterol is only found in animal cell membranes; plant and prokaryotic membranes use other sterols for stability, so you will lose marks for stating otherwise in A-Level Biology exams.
• Wrong move: Describing facilitated diffusion as requiring ATP. Why: Students associate protein-mediated transport with energy use. Correct move: Facilitated diffusion is passive, no ATP required, as it moves substances down a concentration gradient; only active transport and bulk transport use ATP.
• Wrong move: Describing the fluid mosaic model as a solid layer with proteins only on the surface. Why: Students misinterpret the "mosaic" label and forget the "fluid" property. Correct move: Explicitly state that the bilayer is fluid (components move laterally) and proteins are embedded both inside and on the surface of the bilayer, not just on top.

8. Practice Questions (A-Level Biology Style)

Question 1

(a) Name the component of the cell surface membrane that: (i) Acts as an antigen for cell recognition [1] (ii) Reduces membrane fluidity at high temperatures [1] (iii) Allows charged sodium ions to pass via facilitated diffusion [1] (b) Explain why the cell membrane is described as a "fluid mosaic" structure [2]

Solution 1

(a) (i) Glycoprotein or glycolipid [1], (ii) Cholesterol [1], (iii) Channel protein / intrinsic carrier protein [1] (b) "Fluid" refers to the ability of phospholipids and embedded proteins to move laterally within the bilayer [1]; "mosaic" refers to the scattered arrangement of proteins, glycoproteins, and other components across the bilayer surface, resembling a mosaic pattern [1]

Question 2

A plant cell has a solute potential of -900 kPa and a pressure potential of 280 kPa. It is placed in a sucrose solution with a solute potential of -750 kPa (assume the solution has 0 pressure potential). (a) Calculate the water potential of the plant cell. Show your working. [2] (b) Predict and explain the net movement of water between the cell and the sucrose solution. [3]

Solution 2

(a) $$\Psi ={\Psi }_s+{\Psi }_p=-900+280=-620\text{ kPa}$$ 1 mark for correct formula, 1 mark for correct final answer. (b) Net movement of water is into the sucrose solution [1]. The water potential of the cell (-620 kPa) is higher than the water potential of the sucrose solution (-750 kPa) [1]. Water moves down a water potential gradient across the selectively permeable cell membrane [1].

Question 3

An experiment measured uptake of two substances, A and B, by human cells, with and without cyanide (a respiratory poison that stops ATP production). Results are shown below:

Solution 3

(a) Substance B [1]. Its uptake rate drops drastically when ATP production is inhibited by cyanide, indicating it requires energy from ATP, a defining feature of active transport [1]. (b) Substance A is taken up by passive transport (simple or facilitated diffusion) [1]. Its uptake rate is almost unchanged when ATP production is stopped, so it does not require energy, which is characteristic of passive transport mechanisms [1].

9. Quick Reference Cheatsheet

Quick exam tips:

1. Carbohydrate chains are only found on the extracellular side of membranes
2. Cholesterol is exclusive to animal cell membranes
3. Transport inhibited by low oxygen/respiratory poisons = active transport / bulk transport

10. What's Next

This topic forms the foundation for multiple higher-level A-Level Biology topics moving forward. The $N{a}^{+}/K^{+}$ pump you learned here is critical for understanding nerve impulse transmission in the nervous system topic, osmosis links directly to plant water transport and mammalian kidney function, and membrane receptor proteins are core to cell signalling and hormonal control. You will also see membrane transport tested in practical assessments, where you may be asked to calculate water potential from potato cylinder mass change experiments, or investigate factors affecting diffusion rate.

If you struggle with any part of this topic, from calculating water potential to distinguishing between different transport mechanisms, you can ask Ollie for personalised explanations, extra practice questions, or walkthroughs of past paper questions any time on the homepage. Make sure to test your understanding by completing official A-Level Biology past papers on cell membranes and transport, and cross-check your answers against the official mark schemes to learn exactly what examiners are looking for to score full marks.

Aligned with the Cambridge International AS & A Level Biology 9700 syllabus. OwlsAi is not affiliated with Cambridge Assessment International Education.