Transport in cells

1. Cell Biology

Diffusion is how most small molecules get into and out of cells, and it costs the cell nothing. Whether an organism can rely on diffusion alone depends on how much surface it has compared with its volume - which is why anything bigger than a single cell has to grow specialised exchange surfaces.

Start the quiz All topics

143 questions3 subtopicsFoundation tier1 required practical

What transport in cells covers

Every subtopic below has its own questions, mark schemes and notes.

4.1.3.1Diffusion and exchange surfaces

4.1.3.2Osmosis

Osmosis is just diffusion with the spotlight on water. Water moves through a partially permeable membrane from the more dilute side to the more concentrated side - and that one sentence explains everything the potato practical does.

4.1.3.3Active transport

Sometimes a cell needs to take in a substance that is scarcer outside than inside. Diffusion cannot do that, so the cell spends energy from respiration to drag the substance the wrong way up the concentration gradient.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Diffusion — The spreading out of particles, giving a net movement from a higher to a lower concentration.

Concentration gradient — The difference in concentration between two places.

Net movement — The overall movement once the particles going each way are added up.

Partially permeable — Letting some substances through but not others.

Surface area to volume ratio — How large an object's surface area is compared with its volume.

Exchange surface — A part of an organism adapted for substances to move in and out of it efficiently.

Ventilation — Moving air in and out over a gas exchange surface.

Alveolus — One of the tiny air sacs in the lungs where gas exchange happens.

Villus — One of the tiny projections lining the small intestine that absorb digested food.

Lamellae — The tiny plate-like structures on gill filaments that increase surface area.

Stomata — Small holes, mostly on the underside of a leaf, that gases diffuse through.

Guard cells — The pair of cells either side of a stoma that open and close it.

Osmosis — The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

Dilute solution — A solution with a lot of water and little dissolved solute.

Concentrated solution — A solution with less water and more dissolved solute.

Percentage change in mass — The change in mass as a percentage of the starting mass, used so different-sized samples can be compared.

Independent variable — The thing you deliberately change in an investigation.

Dependent variable — The thing you measure to see the effect.

Active transport — The movement of a substance against a concentration gradient, using energy from respiration.

Against the concentration gradient — From a lower concentration to a higher concentration.

Mineral ions — Dissolved substances such as nitrate that a plant absorbs from the soil and needs for healthy growth.

Root hair cell — A root cell with a long projection into the soil that absorbs water and mineral ions.

Respiration — The reaction in cells that releases energy from glucose.

Where marks get lost

The mistakes that come up again and again in this topic.

Diffusion is a net movement, not a one-way movement. Saying particles only travel one way loses the mark.
Diffusion needs no energy from respiration. Only active transport does.
A large organism does not have a small surface area - it has a small surface area compared with its volume. Say 'compared with' or 'ratio' every time.
When you calculate a surface area to volume ratio, simplify it by dividing both sides by the volume, and check the answer looks sensible: small objects should come out with the bigger first number.
'Good blood supply' on its own is a weak answer. Say what it does: it carries the substance away and keeps the concentration gradient steep.
If a question asks how a structure is adapted, name the feature and then say what it achieves. Feature alone rarely gets both marks.
Alveoli and villi both have a large surface area and a good blood supply, but only alveoli are ventilated - do not credit breathing to the small intestine.
Osmosis is the movement of water only. If your answer says 'particles' or 'molecules' without saying water, it will not score.
Always name the partially permeable membrane - 'through a membrane' is not enough.
Dilute and concentrated refer to the dissolved solute. Water moves from where there is more water to where there is less water, which is the same thing said the other way round.
Do not say the cell 'wants' to even up the concentrations, or that water is 'attracted' or 'sucked' across. Water moves because of random movement.
In the practical, dry the cylinders before reweighing - and if a question asks why, say that surface water would add to the measured mass.
Percentage change can be negative. Do not drop the minus sign.
Where the graph crosses zero, the answer is not 'the potato has no sugar in it' - it is that the sugar solution and the cell contents are at the same concentration.
Say energy 'from respiration'. Just saying 'it uses energy' often only gets you halfway.
Active transport does not make energy - respiration releases it and active transport spends it.
The two examples AQA wants are mineral ions into root hair cells and sugar from the gut into the blood. Learn both; one is not enough if the question asks for a plant and an animal example.
Water goes into roots by osmosis. Writing that roots absorb water by active transport is one of the most common errors on this topic.
Glucose is not always absorbed by active transport - while there is more of it in the gut than in the blood it diffuses. Active transport takes over when the gradient reverses.
If asked for the difference between diffusion and active transport, give both differences: the direction relative to the gradient, and the need for energy.

Ready to test yourself?

143 questions on transport in cells, marked instantly, with everything you get wrong scheduled to come back.

Start the quiz

Retrieve GCSE