A-Level Biology cell membranes and transport: fluid mosaic, osmosis and active transport

Where membranes and transport sit in each specification

The key ideas

The phospholipid bilayer forms because the hydrophilic phosphate heads face the watery cytoplasm and tissue fluid while the hydrophobic fatty acid tails face inwards. Small non-polar molecules such as oxygen and carbon dioxide diffuse straight through. Ions and polar molecules such as glucose need channel or carrier proteins. 'Fluid' refers to the phospholipids and proteins moving within the layer; 'mosaic' to the scattered proteins.

Diffusion and facilitated diffusion are passive: net movement down a concentration gradient, with no ATP used. Osmosis is the net movement of water molecules from a region of higher water potential to lower water potential through a partially permeable membrane. Pure water has the highest water potential, zero; adding solute makes it negative. Active transport moves substances against a concentration gradient using carrier proteins and ATP from respiration.

Surface area to volume ratio falls as an organism or block gets bigger, so diffusion alone cannot supply the centre of a large organism. That is why large organisms have specialised exchange surfaces and transport systems. Fick's law, in the Edexcel specification, summarises this: rate of diffusion is proportional to surface area × concentration difference ÷ thickness of the exchange surface.

Worked example 1: surface area to volume ratio of agar cubes

1. A cube of side 1 cm: surface area = 6 × 1 × 1 = 6 cm^2, volume = 1 cm^3, ratio 6:1.
2. A cube of side 2 cm: surface area = 6 × 2 × 2 = 24 cm^2, volume = 8 cm^3, ratio 24:8 = 3:1.
3. A cube of side 4 cm: surface area = 96 cm^2, volume = 64 cm^3, ratio 1.5:1.
4. Doubling the side halves the ratio. In the agar experiment, coloured dye or acid reaches the centre of the smallest cube first, because it has the most surface per unit volume and the shortest distance to the centre.

Worked example 2: estimating the water potential of potato tissue

1. Potato cylinders are weighed, left in sucrose solutions for a set time, blotted and reweighed. A cylinder going from 2.50 g to 2.65 g has a percentage change of (0.15 ÷ 2.50) × 100 = +6.0%.
2. Results: 0.0 mol dm^-3, +12.0%; 0.2 mol dm^-3, +5.0%; 0.4 mol dm^-3, -2.0%; 0.6 mol dm^-3, -8.0%.
3. Plot percentage change in mass against concentration. Where the line crosses zero there is no net movement of water, so the solution and the tissue have the same water potential.
4. Interpolating between 0.2 and 0.4: 0.2 + 0.2 × (5.0 ÷ 7.0) = 0.34 mol dm^-3.
5. The tissue has the same water potential as a 0.34 mol dm^-3 sucrose solution. Using percentage change, not mass change, allows for cylinders of different starting masses.

Common mistakes that cost marks

• Saying water moves from a 'high concentration of water'. Use water potential.
• Saying facilitated diffusion uses energy. It is passive; only active transport uses ATP.
• Calling channel proteins and carrier proteins the same thing. Carriers change shape; channels form a water-filled pore.
• Saying animal cells become 'plasmolysed'. Animal cells shrink or burst (lyse); plant cells become plasmolysed or turgid.
• Describing the effect of temperature on membranes without mentioning increased fluidity, or protein denaturation at high temperatures.
• Forgetting to calculate percentage change before plotting osmosis results.

Exam technique and how a tutor helps

Questions often give unfamiliar data, such as uptake of a substance with and without a respiratory inhibitor. Use the evidence: if uptake falls when respiration is inhibited, ATP is needed, so the process is active transport. If uptake levels off at high concentrations, the carrier proteins are saturated. Quote figures from the data in your answer.

For the beetroot practical (Edexcel Core Practical 3, also a common Cambridge Paper 3 or Paper 5 context), know why pigment leaks: higher temperature increases fluidity and then denatures membrane proteins, and alcohol dissolves the phospholipid bilayer. Name the dependent variable (absorbance or percentage transmission measured with a colorimeter) and the controls.

In one-to-one lessons a tutor rehearses the precise language with the student, because transport answers lose marks on wording more than knowledge, then sets data-analysis questions from past papers and marks them against the scheme.

Self-check: can you do these?

• Define osmosis in one sentence using water potential.
• What is the water potential of pure water? (Answer: zero, the highest possible)
• Find the SA:V ratio of a 3 cm cube. (Answer: 54:27 = 2:1)
• Why does cyanide stop active transport? (Answer: it stops aerobic respiration, so no ATP is made)
• Which component of the membrane reduces fluidity at higher temperatures in animal cells? (Answer: cholesterol)