IGCSE · Practice questions · Diffusion

Down the gradient, for free.

Six original Cambridge-style questions on the definition of diffusion, kinetic energy, gas exchange and the factors that affect the rate, including agar cube data.

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

Keep these straight

Net, random, passive.

Definition: net movement from higher to lower concentration, down a concentration gradient, by random movement.
Energy: kinetic energy of the particles; no respiration needed.
Faster with: larger surface area, higher temperature, steeper gradient, shorter distance.
In organisms: say which side supplies the substance and which side removes it.

Recall

[3 marks]

Define diffusion and state where the energy for diffusion comes from.

The net movement of particles from a region of their higher concentration to a region of their lower concentration. ✓
Down a concentration gradient, as a result of their random movement. ✓
The energy comes from the kinetic energy of the random movement of the molecules and ions. ✓

Explain

[3 marks]

Explain why oxygen diffuses from the air in an alveolus into the blood, and why this continues for as long as a person is alive.

The oxygen concentration in the alveolus is higher than in the blood arriving in the capillary, so oxygen diffuses down the gradient into the blood. ✓
Breathing replaces the air, keeping the oxygen concentration in the alveolus high. ✓
Blood flow carries oxygenated blood away, keeping the concentration in the capillary low, so the gradient is maintained. ✓

Data

[4 marks]

Cubes of agar containing a pink indicator were put into acid. The indicator turns colourless in acid. The table shows the time for each cube to turn completely colourless.

Side of cube / cm	Surface area / cm²	Volume / cm³	Time / s
1	6	1	240
2	24	8	590
3	54	27	1100

(a) Calculate the surface area to volume ratio of each cube. (b) Describe and explain the pattern in the results.

(a) 1 cm: 6 : 1; 2 cm: 3 : 1; 3 cm: 2 : 1. ✓
(b) As the cube gets bigger, the time to turn colourless increases. ✓
Larger cubes have a smaller surface area to volume ratio, so less surface for acid to enter per unit volume. ✓
The acid also has a greater distance to diffuse to reach the centre. ✓

Analysis

[4 marks]

Plan an investigation to find the effect of temperature on the rate of diffusion of a dye in water. Include the variables you would control.

Set up beakers of water at several temperatures, for example 10, 20, 30, 40 and 50 °C, using water baths and a thermometer. ✓
Add the same volume of the same dye to each beaker and start a stop-clock. ✓
Record the time for the colour to spread evenly (or measure the distance spread in a fixed time). ✓
Control: volume of water, volume and concentration of dye, size and shape of beaker; do not stir; repeat and calculate a mean. ✓

Explain

[2 marks]

Glucose diffuses from the blood into respiring muscle cells. Explain why the concentration gradient for glucose is maintained.

Respiration inside the muscle cells uses up glucose, keeping its concentration in the cells low. ✓
Blood flow keeps bringing glucose, keeping its concentration in the blood high. ✓

Recall

[3 marks]

Name three substances that move into or out of living cells by diffusion and state the direction of movement for each.

Oxygen: into respiring cells (or out of photosynthesising leaf cells). ✓
Carbon dioxide: out of respiring cells (or into leaf cells in the light). ✓
Glucose / amino acids: from the small intestine into the blood, or from the blood into cells. (Also accept water vapour out of a leaf, urea from cells into the blood.) ✓

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.