AQA GCSE Biology (8461)

4.1.3 Transport in cells

Cells need to get materials in (oxygen, glucose, mineral ions, water) and get waste out (CO₂, urea). This page covers AQA GCSE Biology 4.1.3: the three transport processes — diffusion, osmosis and active transport — plus surface area to volume ratio (why small things move stuff easily and big things need transport systems). The exam tests the same three scenarios every year: oxygen across the alveolus, water across a cell membrane, mineral ions into a root hair cell. Once you can distinguish 'down the gradient with no energy' (diffusion + osmosis) from 'up the gradient with energy from respiration' (active transport), you can answer every question on this topic.

Why this matters

Every cell is a partially-permeable bag. Substances need to get in and out — but the cell membrane doesn't just let everything cross freely. Small uncharged molecules (oxygen, CO₂, water) can move across by themselves, driven by concentration differences. Larger or charged molecules (glucose, ions, amino acids) need help — they cross via specific transport proteins, and sometimes the cell has to pump them against their natural flow using energy from respiration. This single piece of biology underlies almost every later topic: how your lungs absorb oxygen, how your gut absorbs glucose, how a plant takes up minerals, how a kidney concentrates urine, how a nerve impulse propagates. Master diffusion / osmosis / active transport and the rest of GCSE Biology suddenly makes mechanistic sense.

How to learn this topic

Build on what you already know

GCSE 4.1.1: cell membrane is the boundary that controls what enters and leaves the cell.
KS3: particles in liquids and gases move randomly; substances spread from where they're concentrated to where they're not.
GCSE 4.4.2: respiration in mitochondria releases the energy (as ATP) cells need.

Start with diffusion — random movement of particles, net flow high to low.
Then osmosis — special case of diffusion, but only of WATER across a partially permeable membrane.
Then active transport — the exception that proves the rule: against the gradient, needs energy.
Compare all three side by side so you never confuse them in an exam.
Finish with surface area to volume ratio — the reason small organisms can diffuse, but big organisms need lungs / gills / circulation.

Key terms

diffusion: The net movement of particles from an area of higher concentration to an area of lower concentration. A passive process — no energy required. (Always include 'net movement' and 'higher to lower concentration'. Stating just 'particles move' loses the mark.)
osmosis: The movement of water across a partially permeable membrane from a dilute solution (high water concentration) to a more concentrated solution (lower water concentration). (Three required elements: 'water', 'partially permeable membrane', 'dilute → concentrated'. Missing any one usually loses the mark.)
active transport: The movement of a substance against a concentration gradient (from low to high) using energy from respiration (ATP) and a carrier protein. (Examiners want THREE features: 'against the gradient', 'requires energy from respiration', and (usually) 'via a carrier protein'.)
concentration gradient: A difference in concentration of a substance between two regions — the driver of diffusion and (in reverse) of osmosis. (If asked 'in which direction does diffusion happen', the marking phrase is 'down the concentration gradient' (high to low).)
partially permeable membrane: A membrane that lets some substances through but not others — typically lets small molecules (like water) pass but blocks larger or charged solutes (like sugar, salt ions). Cell membranes are partially permeable.
turgid: The firm state of a plant cell when full of water — the cell membrane is pushed against the cell wall, creating turgor pressure that keeps the plant upright. ('Turgid' is the marking phrase for a healthy, water-filled plant cell. Don't write 'inflated' or 'puffed up'.)
plasmolysis: The state of a plant cell after losing water by osmosis — the cytoplasm shrinks away from the cell wall and the plant wilts. (Distinguish 'plasmolysed' (plant cell, has wall) from 'crenated' (animal cell, shrivelled).)
lysis: Bursting of an animal cell that has taken up too much water by osmosis. The cell membrane cannot withstand the pressure (no cell wall to support it). (Animal cells lyse in pure water. Plant cells don't — the cell wall prevents bursting.)
exchange surface: A region of an organism specially adapted for transferring substances between the inside of the body and the outside (or between body compartments). Examples: alveoli, intestinal villi, root hairs, gills. (Three standard adaptations: large surface area, thin membrane, maintained concentration gradient (e.g. by blood supply).)
surface area to volume ratio (SA:V): The ratio of an object's surface area to its volume. Small objects have a high SA:V; large objects have a low SA:V. Determines whether diffusion alone is enough. (As size increases, volume grows proportionally faster than surface area, so SA:V decreases — that's the marking-phrase reason large organisms need transport systems.)
root hair cell: A specialised plant cell at the root tip with a long thin extension that increases surface area for absorbing water and mineral ions from the soil. Contains many mitochondria for active transport of ions. (Examiners want 'many mitochondria → ATP → active transport of mineral ions' as the explicit chain.)
alveolus: A tiny air sac in the lung where gas exchange happens. Walls are one cell thick, total surface area is huge, and capillaries deliver/remove blood right next to the wall.

Notes

Diffusion — high to low, no energy needed

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. The particles are moving randomly all the time — but if there are more of them on one side than the other, the net result is a flow from where there are more to where there are fewer.

Key features:

Happens to particles in a gas OR particles dissolved in a liquid.
Driven by the concentration gradient (difference in concentration).
No energy from respiration is needed — it's a passive process.
Continues until the concentrations are equal (equilibrium).

Factors that increase the rate of diffusion: steeper concentration gradient, larger surface area, shorter diffusion distance, higher temperature (particles move faster).

Exam examples:

Oxygen diffuses from the air in the alveoli (high concentration) into the blood (low concentration).
Carbon dioxide diffuses out of the blood (high) into the alveoli (low) — the reverse direction.
Glucose diffuses from the small intestine into the blood — at least when intestinal concentration is higher.

Osmosis — diffusion of water, with one rule

Osmosis is the movement of water molecules across a partially permeable membrane, from a dilute solution (high water concentration) to a concentrated solution (lower water concentration).

Why 'partially permeable' matters: a normal partially-permeable cell membrane lets water through but not the dissolved solutes (sugar, salt). So water moves to balance the concentrations, but the solute can't follow.

Three scenarios you need to know:

Animal cell in pure water — water rushes in by osmosis. The cell swells. With no cell wall to stop it, the cell eventually bursts (called lysis). Red blood cells placed in pure water famously do this.
Animal cell in concentrated solution — water rushes out. The cell shrinks and shrivels (crenation).
Plant cell in pure water — water rushes in, but the cell wall stops the cell bursting. The cell becomes firm (turgid) — this is what holds plants up.
Plant cell in concentrated solution — water rushes out, the cytoplasm shrinks away from the cell wall (plasmolysis) and the plant wilts.

Osmosis still needs no energy from respiration — it's passive, driven by water concentration differences.

Active transport — against the gradient, with energy

Sometimes a cell needs to move substances AGAINST their concentration gradient — from where there's less of them to where there's more. Diffusion can't do this. Instead the cell uses active transport:

Specific carrier proteins in the membrane bind the substance.
Energy from respiration (in the form of ATP, made by mitochondria) is used to power the carrier protein.
The carrier protein moves the substance across the membrane against the concentration gradient.

Key exam examples:

Mineral ions (nitrates, phosphates) are absorbed from the soil into root hair cells. The soil has a low concentration of these ions; the root hair cell already has a high concentration. So the plant uses active transport — which is why root hair cells have lots of mitochondria (to make ATP for this).
Glucose absorption in the small intestine: most glucose moves into the blood by diffusion, but when blood glucose is already higher than gut glucose (after a previous meal has been absorbed), active transport is used so the cell can still take up every last molecule.
The sodium-potassium pump in nerve cells (a higher-level example) — pumps Na⁺ out and K⁺ in against their gradients to set up the resting potential.

Comparing the three — the exam table

|---|---|---|---|

| Energy from respiration? | no | no | yes (ATP) |

The single most important question to answer in any exam scenario: 'Is the substance moving high→low or low→high?' If high→low, it's diffusion (or osmosis if it's water). If low→high, it must be active transport — which means energy from respiration AND a carrier protein.

Surface area to volume ratio — why size matters

A tiny organism (a single bacterium) has a huge surface area compared to its volume — every cell is close to the outside, so diffusion alone is fast enough to get oxygen in and waste out.

As an organism gets bigger, volume grows faster than surface area. Quickly the surface becomes too small (relative to all the cells inside it) for diffusion alone to keep them supplied. That's why large organisms have:

Specialised exchange surfaces (lungs, gills, intestinal villi, root hairs) — large surface area, thin membranes, good blood supply.
Transport systems (blood, xylem, phloem) — to carry materials over long distances from the exchange surface to every cell.

A worked example: a 1 cm cube has a surface area of 6 cm² and a volume of 1 cm³ — ratio 6:1. A 2 cm cube has a surface area of 24 cm² and a volume of 8 cm³ — ratio 3:1. The same logic applies to whole organisms: as size doubles, the SA:V ratio roughly halves.

How exchange surfaces are adapted

Three adaptations always increase the rate of diffusion. Every exchange surface in the body uses at least two:

Large surface area — folds, villi, alveoli, root hairs.
Thin membrane / short diffusion path — alveoli walls are one cell thick.
Maintained concentration gradient — good blood supply takes substances away, breathing brings fresh air in.

Exam tips

Always state the direction explicitly: diffusion is 'from a higher concentration to a lower concentration'. Just saying 'down the gradient' usually scores but the full phrase is safer.
Osmosis questions: name the membrane as 'partially permeable' and the movement as 'water from dilute solution to more concentrated solution'. All three elements are required for the mark.
Active transport questions: name ALL three features — against gradient + needs energy from respiration + carrier protein.
On root hair cell questions, the marking chain is: 'many mitochondria → make ATP → for active transport of mineral ions against the gradient'. Skipping any step loses marks.
Animal cell in pure water → BURSTS (lysis). Plant cell in pure water → TURGID (firm, cell wall stops bursting). Don't muddle them.
For surface area to volume ratio, the marking phrase is 'volume increases proportionally faster than surface area' — that's the reason, not 'small things have more surface area'.
Exchange surface adaptations: name at least TWO of large surface area / thin membrane / maintained concentration gradient via blood supply.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

What's the difference between diffusion and osmosis?

Diffusion is the net movement of particles (gas or dissolved) from high to low concentration. Osmosis is specifically the movement of WATER molecules across a PARTIALLY PERMEABLE MEMBRANE from a dilute solution to a more concentrated one. Both are passive (no energy needed). Think of osmosis as a special case of diffusion that only applies to water and only when there's a membrane in the way.

Why does active transport need energy when diffusion doesn't?

Diffusion just lets particles wander randomly down a concentration gradient — the energy comes from the particles' own kinetic energy. Active transport works AGAINST the gradient: it pumps particles from low to high concentration, which is the opposite of what they'd do naturally. Pushing something the 'wrong' way requires work, which costs energy. The cell pays that cost using ATP made by respiration in mitochondria — which is why cells doing lots of active transport (root hair cells, gut lining cells, kidney cells) are packed with mitochondria.

Why doesn't a plant cell burst in pure water like an animal cell does?

Both cells take up water by osmosis. The difference is the cell wall. Animal cells have only a flexible cell membrane, so when too much water enters, the membrane stretches and eventually ruptures (lysis). Plant cells have a tough cellulose cell wall outside the membrane. As water enters, the cell becomes turgid — the membrane presses against the cell wall, but the wall is rigid enough to hold its shape. This turgor pressure is exactly what keeps non-woody plants upright. Lose the water and the cells become flaccid → the plant wilts.

How is a root hair cell adapted for its function?

Three adaptations: (1) Long thin extension — increases the surface area in contact with the soil for water and mineral absorption. (2) Many mitochondria — provide ATP for active transport of mineral ions against the concentration gradient (the soil has fewer ions than the cell already does). (3) Thin cell wall — speeds up water uptake. The exam-marking chain you want is 'extension → surface area → faster absorption' and 'mitochondria → ATP → active transport → minerals taken up against gradient'.

Why does an organism's size affect how it exchanges gases?

Tiny organisms like amoebae or single-celled bacteria have a high surface area to volume ratio — their surface is large relative to the small amount of cytoplasm inside, so diffusion across the surface is fast enough to supply every part of the cell. As organisms get bigger, volume grows proportionally faster than surface area — so the SA:V ratio decreases. Eventually the inner cells are too far from the surface for diffusion alone to deliver oxygen quickly enough. That's why bigger organisms have specialised exchange surfaces (lungs, gills) AND a transport system (blood) to carry substances over longer distances.

What's the difference between turgid, flaccid and plasmolysed plant cells?

TURGID: the cell is full of water, cytoplasm pressed against the cell wall, the plant is upright and firm. This is the healthy state — the marking phrase is 'turgid'. FLACCID: the cell has lost some water, the cytoplasm is no longer pressed firmly against the wall, the plant starts to wilt. PLASMOLYSED: the cell has lost so much water that the cytoplasm and cell membrane have pulled away from the cell wall — the cell is severely dehydrated. All three states involve water movement by osmosis — only the direction and amount differ.