AQA GCSE Biology Foundation (8461), Foundation tier · Cell biology › Transport in cells
Practise Diffusion. 8 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How substances move into and out of cells by diffusion, what affects the rate of diffusion, why surface area to volume ratio matters, and how exchange surfaces are adapted. Expect definitions, surface area to volume calculations and 'explain how this exchange surface is adapted' questions.
Key facts
Diffusion: net movement of particles from higher to lower concentration
Diffusion is passive: no energy from respiration needed
Rate increases with: a bigger concentration gradient, a higher temperature, a larger surface area
Cube: surface area = 6 × side2; volume = side3
Bigger organism → smaller surface area to volume ratio
Good exchange surface: large surface area, thin, good blood supply, ventilated
Diffusion is the spreading out of the particles of a gas or of a substance in solution, resulting in a net movement from an area of higher concentration to an area of lower concentration.
Diffusion: net movement from a higher to a lower concentration, down the concentration gradient.
Diffusion is passive: it does not need energy from respiration.
Oxygen diffuses from the air in the lungs into the blood, and carbon dioxide diffuses from the blood into the lungs (gas exchange).
Urea (a waste product) diffuses from cells into the blood plasma, to be removed by the kidneys.
Factors that affect the rate
Concentration gradient: the bigger the difference in concentration, the faster the rate.
Temperature: the higher the temperature, the faster the particles move, so the faster the rate.
Surface area of the membrane: the larger the area, the faster the rate, because more particles can cross at once.
Surface area to volume ratio
diagram
For a cube: surface area = 6 × side2; volume = side3.
Example: a 2 cm cube has a surface area of 24 cm2 and a volume of 8 cm3, so SA : V = 24 : 8 = 3 : 1.
The bigger the object, the smaller its surface area to volume ratio.
The bigger the organism, the smaller its surface area to volume ratio.
Exchange surfaces
diagram
Exchange surfaces are more effective with: a large surface area; a thin membrane (short diffusion path); an efficient blood supply (animals); ventilation (animals, for gas exchange).
Blood flow and ventilation keep the concentration gradient steep.
Lungs: millions of alveoli (large surface area), walls one cell thick, surrounded by capillaries, ventilated by breathing.
Gas exchange in an alveolus: a large surface area, a thin wall and a good blood supply.
Small intestine: millions of villi (large surface area), a thin lining and a good blood supply.
Fish gills: many thin gill filaments (large surface area), a good blood supply and a constant flow of water.
Plants: root hairs give roots a large surface area; leaves are flat and thin, with stomata and air spaces for gas exchange.
How to answer each type of question
Define diffusion and give examples
1 to 2 marksGrade 3
Include 'net movement', 'particles' and 'from higher to lower concentration'.
For examples, use the ones in the specification: oxygen, carbon dioxide and urea.
Example. (a) Define diffusion. (b) Name one waste product that diffuses from body cells into the blood plasma.
Show the model answerHide the model answer
(a) The net movement of particles from an area of higher concentration to an area of lower concentration (1) (b) urea / carbon dioxide (1)
Explain the effect of a factor on the rate of diffusion
2 marksGrade 5
Say what the factor does to the particles or the membrane.
Link it to the rate: more particles cross per second.
Example. A student put cubes of agar containing an indicator into acid at 20 °C and at 40 °C. The colour of the cubes changed faster at 40 °C. Explain why.
Show the model answerHide the model answer
At a higher temperature the acid particles have more kinetic energy / move faster (1), so they diffuse into the cubes faster (1).
Don’t lose marks
Leaving 'net' or 'concentration' out of the definition, or not saying what is at a higher or lower concentration.
Saying diffusion needs energy. That is active transport.
Forgetting that a cube has 6 faces when working out its surface area.
Saying larger organisms have a larger SA : V ratio. They have a larger surface area but a smaller ratio.
Saying a thin wall means 'less distance for the blood'. Say it gives a short diffusion distance.
More tips
Memory tricks
Diffusion goes 'downhill', from high concentration to low, like a ball rolling down a slope: no energy needed.
Exchange surface checklist: big area, thin, good blood supply, ventilated.
For a cube, SA : V = 6 ÷ side length. A 3 cm cube gives 6 ÷ 3 = 2, so 2 : 1.
Double the side of a cube and its SA : V ratio halves.
Exam technique
Link each adaptation to its effect: 'thin wall, so a short diffusion distance'; 'large surface area, so more diffusion at once'.
When you explain why blood supply or ventilation helps, use the phrase 'concentration gradient'.
Show the surface area and volume as separate steps, with units, before giving the ratio.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define diffusionThe net movement of particles from an area of higher concentration to an area of lower concentration.
Grade 4
Give examples of diffusion in living thingsOxygen and carbon dioxide in gas exchange; urea from cells into the blood plasma.
Grade 5
Explain the factors that affect the rateA bigger concentration gradient, a higher temperature and a larger surface area all increase the rate of diffusion.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Substances move into and out of cells by diffusion.
(a) What is meant by diffusion?[2]
(b) Which substance diffuses out of body cells into the blood plasma so that it can be excreted by the kidneys? Tick (✓) one box.[1]
Glucose
Oxygen
Starch
Urea
(c) Give two factors that affect the rate of diffusion.[2]
Show the answer and mark scheme
(a)
the (net) movement / spreading out of particles (of a gas or a substance in solution)
from an area of higher concentration to an area of lower concentration
(b)Answer: Urea
(c)
difference in concentration / concentration gradient
temperature
surface area (of the membrane)
Question 2Medium7 marks
Fish take in oxygen and remove carbon dioxide at their gills. Each gill is made of many thin filaments. The filaments have a good blood supply. Water flows over the filaments all the time.
(a) Explain how the gills are adapted for efficient gas exchange.[3]
(b) Explain why oxygen moves from the water into the blood in the gills.[2]
(c) Out of water, the gill filaments stick together. Suggest why a fish soon dies when it is taken out of water, even though air contains more oxygen than water.[2]
Show the answer and mark scheme
(a)
many filaments give a large surface area
thin filaments / membranes give a short diffusion path
good blood supply carries oxygen away
water flow brings more oxygen, so a steep concentration gradient is maintained
(b)
the concentration of oxygen is higher in the water than in the blood
(so oxygen) diffuses from a higher to a lower concentration
(c)
the surface area for gas exchange is greatly reduced
so not enough oxygen diffuses into the blood (for respiration)