AS · Practice questions · Diffusion, active transport and bulk transport

Passive or active? Check the gradient.

Six original Cambridge-style questions on movement across membranes, including SA : V calculations and planning the agar block investigation.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Route, gradient, energy.

Simple diffusion: small, non-polar, through the bilayer.
Facilitated: channel or carrier protein, still down the gradient, no ATP.
Active: carrier protein, against the gradient, ATP.
SA : V falls as size rises; give it as a ratio to 1.

Recall

[2 marks]

Define diffusion.

The net movement of molecules or ions from a region of higher concentration to a region of lower concentration (down a concentration gradient). ✓
As a result of their random movement; it is passive and needs no energy from the cell. ✓

Calculation

[3 marks]

Calculate the surface area to volume ratio of an agar block measuring 4 cm × 1 cm × 1 cm, and compare it with that of a 2 cm cube.

Surface area = 2(4 × 1) + 2(4 × 1) + 2(1 × 1) = 8 + 8 + 2 = 18 cm²; volume = 4 cm³. ✓
SA : V = 18 ÷ 4 = 4.5 : 1. ✓
The 2 cm cube has 24 ÷ 8 = 3 : 1, so the long thin block has the larger ratio and would clear faster. ✓

Analysis

[4 marks]

Plan an investigation into the effect of surface area to volume ratio on the rate of diffusion, using agar blocks containing an indicator.

Make agar containing an alkali and phenolphthalein (pink); cut cubes of side, for example, 0.5, 1, 1.5 and 2 cm with a sharp knife and ruler. ✓
Place each cube in the same volume and concentration of dilute hydrochloric acid at the same temperature. ✓
Time how long each cube takes to become completely colourless; repeat each size at least three times and calculate a mean. ✓
Calculate SA : V for each cube and plot time (or 1 ÷ time as a rate) against SA : V. ✓

Explain

[3 marks]

Explain why the rate of facilitated diffusion of glucose into a cell stops increasing at high external glucose concentrations, while the rate of simple diffusion of oxygen does not level off in the same way.

Glucose is polar, so it can only cross through specific carrier proteins. ✓
At high concentrations all the carrier proteins are occupied (saturated), so the number of carriers limits the rate. ✓
Oxygen diffuses through the phospholipid bilayer itself, so its rate keeps rising in proportion to the concentration gradient. ✓

Explain

[3 marks]

The sodium-potassium pump moves 3 Na⁺ out of a cell and 2 K⁺ into it for each molecule of ATP used. Explain why this is described as active transport.

The ions are moved against their concentration gradients: Na⁺ out to where its concentration is already higher, K⁺ in to where its concentration is already higher. ✓
A specific carrier protein is involved, which changes shape when it is phosphorylated. ✓
Energy is supplied by the hydrolysis of ATP produced in respiration. ✓

Data

[3 marks]

Root tissue was kept in a solution of potassium ions. With oxygen bubbled through, uptake was 12 arbitrary units per hour. With nitrogen bubbled through instead, uptake fell to 2 units per hour. Explain these results.

With oxygen, aerobic respiration makes plenty of ATP, so K⁺ is taken up by active transport, which is fast. ✓
With nitrogen there is no oxygen, so little ATP is made and active transport almost stops. ✓
The small remaining uptake is probably by facilitated diffusion through channel proteins (or diffusion into cell walls), which does not need ATP. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.