Cambridge International AS & A Level Biology 9700: Cell membranes and transport – Study Guide

This study guide teaches topic 4, Cell membranes and transport, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers sub-topics 4.1 (Fluid mosaic membranes) and 4.2 (Movement into and out of cells). All of it is AS Level content, so it is tested on Paper 1 (multiple choice) and Paper 2 (AS Level structured questions), and Paper 4 can draw on it because A Level questions require AS knowledge. The investigations in 4.2 also build the practical skills assessed in Paper 3 and Paper 5.

Use it with the 9700 course hub and the printable 9700 checklist. When you have worked through it, condense it with the cell membranes and transport revision notes, then test yourself with the cell membranes practice questions and the free 10-minute 9700 AS diagnostic.

What this topic covers

9700 outcome	What you must be able to do
4.1.1	Describe the fluid mosaic model, using hydrophobic and hydrophilic interactions to explain the bilayer and the position of proteins
4.1.2	Describe where cholesterol, glycolipids and glycoproteins sit in the cell surface membrane
4.1.3	Describe the roles of each component: stability, fluidity, permeability, transport, cell signalling and cell recognition
4.1.4	Outline the stages of cell signalling: ligand secreted, transported, binds to a receptor
4.2.1	Describe and explain simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis
4.2.2	Investigate diffusion and osmosis with plant tissue, Visking tubing and agar
4.2.3	Calculate surface areas and volumes of simple 3-D shapes to show SA:V falls as size increases
4.2.4	Investigate the effect of SA:V on diffusion using agar blocks
4.2.5	Estimate the water potential of a plant tissue from immersion results
4.2.6	Explain water movement in terms of water potential, and its effects on plant and animal cells

The topic builds on phospholipid and protein structure from Biological molecules and on cell organelles from Cell structure.

4.1 The fluid mosaic model

Why a bilayer forms

A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails. In water, the heads interact with water molecules and the tails are repelled by them. The most stable arrangement is a bilayer: heads face the watery cytoplasm on one side and the watery tissue fluid on the other, while the tails point inwards and form a hydrophobic core.

It is fluid because phospholipids and many proteins move sideways within their layer, and a mosaic because proteins are scattered through it in an irregular pattern.

Where the proteins sit

Proteins are positioned by the same interactions. A protein that spans the membrane has hydrophobic R groups on the part in contact with the fatty acid tails and hydrophilic R groups on the parts exposed to water on either side. Some span the bilayer; others sit in one layer or on the surface.

Cholesterol, glycolipids and glycoproteins

• Cholesterol sits between the phospholipid molecules, within both layers of the bilayer.
• Glycolipids are lipids with a carbohydrate chain attached. The chain projects outwards from the outer surface.
• Glycoproteins are proteins with carbohydrate chains attached, and again the chains project outwards from the outer surface.

The carbohydrate chains are only on the outer face of the cell surface membrane, never the cytoplasmic face.

Roles of each component

Component	Role
Phospholipids	Form the bilayer; the hydrophobic core is a barrier to ions and polar molecules, so it controls permeability; the bilayer is fluid
Cholesterol	Regulates fluidity (stops the membrane becoming too fluid when warm or too rigid when cold); adds stability; reduces permeability to ions and polar molecules
Channel proteins	Water-filled pores that let specific ions and polar molecules cross by facilitated diffusion (transport)
Carrier proteins	Bind specific molecules or ions and change shape to move them across, in facilitated diffusion or active transport (transport)
Receptors (proteins or glycoproteins)	Have a binding site with a shape complementary to a specific ligand (cell signalling)
Glycoproteins and glycolipids	Act as cell surface antigens for cell recognition; also stabilise the membrane by forming hydrogen bonds with water

Cell surface antigens return in topic 11 (Immunity), where you use them to explain self and non-self.

Cell signalling

The syllabus asks for three stages that lead to a specific response:

1. A cell secretes a specific chemical, the ligand (for example, a hormone).
2. The ligand is transported to target cells, for example in the blood.
3. The ligand binds to a cell surface receptor on the target cell. Only cells with a receptor whose binding site is complementary in shape to the ligand respond.

Binding triggers a response inside the target cell, which is why a hormone carried to every cell affects only some of them.

4.2 Movement into and out of cells

The six processes

Process	Direction	Energy from ATP?	Membrane protein?	Example
Simple diffusion	Down a concentration gradient	No	No	Oxygen, carbon dioxide
Facilitated diffusion	Down a concentration gradient	No	Yes: channel or carrier	Glucose, many ions
Osmosis	Water, from higher to lower water potential across a partially permeable membrane	No	Not required	Water entering root hair cells
Active transport	Against a concentration gradient	Yes	Yes: carrier protein	Mineral ion uptake
Endocytosis	Bulk material into the cell in vesicles	Yes	No	Phagocytes taking in bacteria
Exocytosis	Bulk material out of the cell in vesicles	Yes	No	Secretion of digestive enzymes

Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, as a result of their random motion. Small, non-polar molecules pass straight through the hydrophobic core. Ions and larger polar molecules cannot, so they need facilitated diffusion through channel or carrier proteins.

Active transport uses energy from ATP, released by respiration, to move molecules or ions against their concentration gradient. A carrier protein binds the substance, ATP is hydrolysed, and the protein changes shape to release it on the other side.

In endocytosis, the cell surface membrane folds around material and pinches off as a vesicle (phagocytosis for solids, pinocytosis for liquids). In exocytosis, vesicles fuse with the membrane and release their contents. Both need ATP.

Water potential and osmosis

Water potential (ψ) measures the tendency of water to move out of a solution. Pure water has the highest possible value, 0 kPa. Dissolving solutes lowers it, so every solution has a negative water potential. Water moves by osmosis from a higher (less negative) to a lower (more negative) water potential across a partially permeable membrane. The syllabus says knowledge of solute potential and pressure potential is not expected.

Solution water potential compared with the cell	Animal cell (red blood cell)	Plant cell
Higher	Water enters; the cell swells and may burst (haemolysis), because it has no wall	Water enters; the protoplast pushes on the cell wall and the cell becomes turgid; the wall stops it bursting
Equal	No net movement	No net movement
Lower	Water leaves; the cell shrinks and the membrane crinkles	Water leaves; the protoplast shrinks and pulls away from the wall (plasmolysis); the cell is flaccid

Surface area to volume ratio

As an object gets larger, its volume increases faster than its surface area, so SA:V falls. The syllabus expects you to calculate surface areas and volumes of cuboids and cylinders.

Worked example 1: cubes of agar.

Side / cm	Surface area / cm²	Volume / cm³	SA:V
0.5	6 × 0.5² = 1.5	0.5³ = 0.125	12 : 1
1	6	1	6 : 1
2	24	8	3 : 1
4	96	64	1.5 : 1

Each doubling of side length halves SA:V.

Worked example 2: same volume, different shape. A cuboid 4 cm × 1 cm × 1 cm has volume 4 cm³.

SA = 2(lw + lh + wh) = 2(4 + 4 + 1) = 18 cm²
SA:V = 18 / 4 = 4.5 : 1

A cube of side 2 cm has a larger volume (8 cm³) and SA:V of only 3 : 1. Long, thin shapes keep a large SA:V. A cylinder of radius 0.5 cm and length 4 cm gives the same ratio:

SA = 2πr² + 2πrl = 2π(0.25) + 2π(0.5)(4) = 14.1 cm²
V  = πr²l = π(0.25)(4) = 3.14 cm³
SA:V = 4.5 : 1

Investigating SA:V with agar blocks

Agar containing an indicator is cut into cubes of different sizes and placed in a solution that changes the indicator colour as it diffuses in (for example, agar made with dilute alkali and phenolphthalein turns from pink to colourless as acid diffuses in). You can time how long each block takes to change colour completely, or measure how far the colour change has moved in a fixed time.

Worked example 3. After a fixed time, acid has diffused 0.25 cm in from every face of three cubes. What percentage of each cube has changed colour?

1 cm cube: unchanged core = (1 − 0.5)³ = 0.125 cm³; changed = 0.875 cm³ → 87.5 %
2 cm cube: core = 1.5³ = 3.375 cm³; changed = 4.625 cm³ → 57.8 %
3 cm cube: core = 2.5³ = 15.625 cm³; changed = 11.375 cm³ → 42.1 %

The smallest cube, with the largest SA:V, has the largest proportion reached by diffusion. Control the temperature, the acid concentration and volume, and the time.

Visking tubing and plant tissue

Visking (dialysis) tubing is partially permeable: water and small solute molecules pass, but large molecules such as starch do not. Filling tubing with a starch and glucose mixture, standing it in water and testing the water shows that glucose diffuses out and starch stays in.

Estimating the water potential of a tissue

Cut cylinders of the same size from one potato, blot, weigh, and leave each in a different sucrose solution for the same time at the same temperature. Blot and reweigh. Percentage change in mass allows for different starting masses.

Worked example 4.

Sucrose / mol dm⁻³	Start mass / g	End mass / g	Change / %
0.0	2.50	2.78	+11.2
0.1	2.46	2.63	+6.9
0.2	2.52	2.58	+2.4
0.3	2.48	2.43	−2.0
0.4	2.55	2.40	−5.9
0.5	2.50	2.29	−8.4

For 0.2 mol dm⁻³: (2.58 − 2.52) / 2.52 × 100 = +2.4 %.

Plot percentage change against concentration. The line crosses 0 % at about 0.25 mol dm⁻³. At that concentration there is no net water movement, so the tissue has the same water potential as the solution. Using supplied values of −540 kPa for 0.2 mol dm⁻³ and −820 kPa for 0.3 mol dm⁻³, 0.25 mol dm⁻³ lies halfway, so the tissue’s water potential is about −680 kPa. The two conversion values are supplied data for this example, not figures to learn.

Common errors

• Saying phospholipid tails are “hydrophilic inside the membrane”. The core is hydrophobic.
• Putting glycoproteins or glycolipids on the inner face of the membrane.
• Writing that cholesterol “makes the membrane more fluid” without a condition. It regulates fluidity.
• Saying osmosis moves water “from high to low concentration” without stating water potential or naming the membrane.
• Describing water potential values the wrong way round: −300 kPa is higher than −700 kPa.
• Giving SA:V as a single number without “: 1”, or calculating only one face of a cube.
• Using raw change in mass instead of percentage change when starting masses differ.

Next steps

Condense this page with the revision notes, then try the cell membranes practice questions, which cover transport evidence and exocytosis in exam-style contexts. Take the 9700 AS diagnostic to check where you stand across the AS topics, and continue to topic 5 with the mitotic cell cycle study guide.

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.