Cambridge International AS & A Level Biology 9700: Cell membranes and transport – Revision Notes

These revision notes condense topic 4, Cell membranes and transport, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. They cover sub-topics 4.1 (Fluid mosaic membranes) and 4.2 (Movement into and out of cells). Everything here is AS Level content, tested on Paper 1 and Paper 2, and assumed knowledge for Paper 4. For full explanations and worked examples, use the cell membranes and transport study guide.

Other pages for this topic: the cell membranes practice questions, the 9700 course hub, the printable 9700 checklist and the free 10-minute 9700 AS diagnostic.

4.1 Fluid mosaic membranes

Key definitions

• Fluid mosaic model: a phospholipid bilayer in which molecules move sideways (fluid), with proteins scattered through it (mosaic).
• Hydrophilic: interacts with water (phosphate heads; polar R groups).
• Hydrophobic: repelled by water (fatty acid tails; non-polar R groups).
• Ligand: a signalling chemical that binds to a specific receptor.
• Cell surface antigen: a glycoprotein or glycolipid that identifies a cell (cell recognition).

Why the structure forms

• Heads face water on both sides; tails face each other, away from water.
• Membrane-spanning proteins have hydrophobic regions in the core and hydrophilic regions at each surface.

Positions to remember

Component	Position
Cholesterol	Between phospholipids, in both layers
Glycolipids	In the outer layer; carbohydrate chain outside the cell
Glycoproteins	Carbohydrate chain on the outer surface only
Channel and carrier proteins	Span the bilayer

Roles in one line each

Role	Components responsible
Stability	Cholesterol; glycolipids and glycoproteins (hydrogen bonds with water)
Fluidity	Phospholipids (fatty acid tails); cholesterol regulates it
Permeability	Hydrophobic core blocks ions and polar molecules; cholesterol reduces permeability further
Transport	Channel proteins (pores) and carrier proteins (change shape)
Cell signalling	Receptors: proteins or glycoproteins with a complementary binding site
Cell recognition	Glycoproteins and glycolipids as antigens (links to topic 11.1)

Method in steps: cell signalling

1. Signalling cell secretes the ligand.
2. Ligand is transported to target cells (for example, in blood).
3. Ligand binds to a complementary cell surface receptor on the target cell.
4. A specific response follows inside that cell. Cells without the receptor do not respond.

4.2 Movement into and out of cells

The six processes compared

	Needs ATP?	Needs membrane protein?	Against gradient?
Simple diffusion	No	No	No
Facilitated diffusion	No	Yes (channel or carrier)	No
Osmosis	No	No	No (down water potential gradient)
Active transport	Yes	Yes (carrier)	Yes
Endocytosis	Yes	No	Bulk movement in
Exocytosis	Yes	No	Bulk movement out

Definitions worth writing word for word

• Diffusion: net movement of molecules or ions from a region of higher concentration to a region of lower concentration, by random movement.
• Osmosis: net movement of water from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
• Active transport: movement of molecules or ions against a concentration gradient through carrier proteins, using energy from ATP.

What decides the route across

• Small and non-polar (O₂, CO₂) → simple diffusion through the core.
• Charged or polar (ions, glucose) → facilitated diffusion through proteins, or active transport if moving against the gradient.
• Very large or bulk (bacteria, secreted proteins) → endocytosis or exocytosis.

Water potential rules

• Pure water = 0 kPa, the highest value.
• Solutes lower ψ, so solutions are negative.
• Water moves from less negative to more negative.
• Solute potential and pressure potential are not required by the syllabus.

Outcome	Animal cell	Plant cell
Placed in higher ψ	Swells, may burst	Turgid; wall stops bursting
Placed in lower ψ	Shrinks, crenated	Plasmolysed; protoplast pulls from wall

SA:V formulas

Shape	Surface area	Volume
Cube, side x	6x²	x³
Cuboid, l × w × h	2(lw + lh + wh)	lwh
Cylinder, radius r, length l	2πr² + 2πrl	πr²l

Write the ratio as “n : 1” (divide SA by V). Bigger objects have a smaller SA:V, so diffusion alone reaches a smaller fraction of their volume.

Worked reminder: SA:V of a cylinder

A cylinder has radius 1 mm and length 10 mm.

SA = 2π(1)² + 2π(1)(10) = 22π = 69.1 mm²
V  = π(1)²(10) = 10π = 31.4 mm³
SA:V = 69.1 / 31.4 = 2.2 : 1

Method in steps: agar block investigation

1. Make agar containing an indicator; cut cubes of different sizes accurately.
2. Put each cube in the same volume and concentration of solution at the same temperature.
3. Record the time for complete colour change, or the distance moved in a fixed time.
4. Calculate SA:V for each cube and plot it against the result.
5. Conclusion: larger SA:V gives faster complete colour change.

Method in steps: water potential of a tissue

1. Cut equal-sized cylinders from one tissue; blot and weigh each.
2. Immerse in a range of sucrose concentrations (include distilled water) for a fixed time and temperature.
3. Blot the same way; reweigh.
4. Percentage change = (final − initial) / initial × 100.
5. Plot percentage change against concentration; read the concentration at 0 % change.
6. Convert that concentration to water potential using the data supplied.

Worked reminder: finding the 0 % point without a graph

Between two readings the change is roughly linear. At 0.30 mol dm⁻³ the change is +3.0 %; at 0.40 mol dm⁻³ it is −1.0 %.

fraction of the gap = 3.0 / (3.0 + 1.0) = 0.75
concentration at 0 % = 0.30 + 0.75 × 0.10 = 0.375 mol dm⁻³

At about 0.38 mol dm⁻³ the tissue gains and loses water equally, so its water potential equals that of the solution. A plotted line of best fit through all the points is more reliable than two readings.

Visking tubing

• Partially permeable: lets water and small molecules (glucose) through, not large ones (starch).
• Use it as a model cell membrane for diffusion and osmosis without living tissue.
• Diffusion: fill with starch and glucose, stand in water; glucose appears in the water, starch does not.
• Osmosis: fill with sucrose solution, stand in water; the tubing gains mass as water enters from the higher water potential.

Must-know distinctions

• Channel vs carrier protein: a channel is a fixed pore; a carrier binds the substance and changes shape.
• Facilitated diffusion vs active transport: both use proteins; only active transport uses ATP and moves against a gradient.
• Diffusion vs osmosis: osmosis is only water, and needs a partially permeable membrane.
• Turgid vs flaccid vs plasmolysed: pushing on the wall; not pushing; protoplast pulled away.
• Glycoprotein vs glycolipid: carbohydrate on a protein vs on a lipid; both face outwards.
• Endocytosis vs exocytosis: into the cell vs out of the cell; both use vesicles and ATP.

Quick self-test

1. Which part of a phospholipid faces the cytoplasm?
2. State where cholesterol is found in a cell surface membrane.
3. Give two roles of cholesterol.
4. Name the three stages of cell signalling in the syllabus.
5. Name the type of protein that changes shape to move a solute across a membrane.
6. Calculate SA:V for a cube of side 3 cm.
7. Calculate SA:V for a cuboid 2 cm × 2 cm × 1 cm.
8. A potato cylinder goes from 3.20 g to 2.96 g. Calculate the percentage change in mass.
9. Cell X has ψ = −450 kPa and is placed in a solution of ψ = −900 kPa. What happens to cell X if it is a plant cell?
10. Why does a red blood cell burst in distilled water when a leaf cell does not?
11. Agar cubes of side 1 cm and 2 cm are placed in the same acid. Which changes colour completely first, and why?
12. Glucose enters a cell faster than simple diffusion through the bilayer allows, but uptake stops rising at high glucose concentrations. Suggest the process involved.

Answers

1. The hydrophilic phosphate head (of the inner layer).
2. Between phospholipid molecules, in both layers of the bilayer.
3. Any two: regulates fluidity; increases stability; reduces permeability to ions and polar molecules.
4. Secretion of a ligand; transport of the ligand to target cells; binding of the ligand to a cell surface receptor.
5. A carrier protein.
6. SA = 54 cm², V = 27 cm³, so 2 : 1.
7. SA = 2(4 + 2 + 2) = 16 cm², V = 4 cm³, so 4 : 1.
8. (2.96 − 3.20) / 3.20 × 100 = −7.5 %.
9. Water leaves by osmosis (the solution has a lower water potential); the cell becomes flaccid and then plasmolysed.
10. Water enters both by osmosis, but the red blood cell has no cell wall. The leaf cell wall resists expansion, so the cell becomes turgid instead.
11. The 1 cm cube: its SA:V is 6 : 1 compared with 3 : 1, and the acid has a shorter distance to diffuse to the centre.
12. Facilitated diffusion through carrier proteins; the rate levels off when all the carriers are in use.

Where marks are usually lost

• Writing “water moves from high to low concentration”. Use water potential, and say “through a partially permeable membrane”.
• Calling −800 kPa a “higher water potential” than −200 kPa.
• Saying cholesterol “increases fluidity” with no condition.
• Placing carbohydrate chains on the cytoplasmic side of the membrane.
• Saying facilitated diffusion needs ATP.
• Naming only “proteins” for transport when the question needs channel or carrier.
• In the signalling outline, skipping the transport stage or not saying the receptor is complementary in shape to the ligand.
• Reporting SA:V without “: 1”, or leaving out faces when adding surface areas.
• Using change in mass instead of percentage change when starting masses differ.
• Saying plant cells “burst” or “shrink” instead of “turgid” and “plasmolysed”.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for 2025, 2026 and 2027 (Version 1), Cambridge International (Cambridge University Press & Assessment). Topic 4, Cell membranes and transport, sub-topics 4.1 and 4.2.