Lesson 1.10 · AQA Combined Science

Active transport and exchange surfaces

Move against the gradient

Active transport moves dissolved substances from a lower concentration to a higher concentration, against their concentration gradient.

Membrane transport proteins move the substances. This requires energy from respiration.

Two important examples

Root hair cells

Mineral ions can enter root hair cells from very dilute soil solution, even when the ion concentration is higher inside the root. Energy is needed.

The small intestine

Sugars such as glucose can be absorbed from a lower concentration in the gut into blood with a higher concentration. Active transport helps absorption continue.

Compare the three processes

Process	Net movement	Energy from respiration?
Diffusion	Particles: high → low concentration	No
Osmosis	Water: dilute → concentrated solution, through a partially permeable membrane	No
Active transport	Substances: low → high concentration	Yes

Watch the wording: concentrated solution means more solute, but less water per unit volume. Osmosis follows the water concentration gradient.

Why big organisms need exchange surfaces

As an organism gets bigger, its surface area to volume ratio decreases. There is less outer surface for each unit of living material, and diffusion distances are longer.

A single-celled organism has a relatively large ratio and short distances. Diffusion across its surface can supply its needs.

Multicellular organisms need specialised exchange surfaces and transport systems to supply cells fast enough.

The larger cube has more surface area in total, but less surface area per unit volume.

Calculate surface area to volume ratio

For a cube of side length a: surface area = 6a²; volume = a³.

Side	Surface area	Volume	SA : V
1 cm	6 cm²	1 cm³	6 : 1
2 cm	24 cm²	8 cm³	3 : 1

For the larger cube, 24 : 8 simplifies to 3 : 1. A smaller ratio makes exchange through the outside alone less effective.

Features of effective exchange

Large surface area: more material crosses at once.
Thin surface: short diffusion distance.
Good blood supply in animals: carries substances to and from the surface, maintaining gradients.
Ventilation for gas exchange: renews air or water at the surface.

Recognise the examples

Alveoli in lungs

Many tiny air sacs give a large surface area. Thin walls, a rich blood supply and ventilation allow rapid gas exchange.

Villi in the small intestine

Many projections give a large area for absorption. Thin surfaces and a good blood supply support rapid exchange.

Fish gills

Many filaments with thin lamellae give a large surface area. Water flow and blood flow maintain concentration gradients.

Plant roots

Root hairs give a large area for water and mineral ion absorption. The extension brings the surface into contact with soil solution.

Plant leaves

Broad leaves have a large surface area. They are thin, and stomata and internal air spaces let gases reach cells over short distances.

Challenge · link the ideas

An actively transporting cell needs a supply of energy. If respiration is reduced, active transport may slow down. Cells carrying out lots of active transport often have many mitochondria.

Explain exchange with a chain: thin wall → short diffusion path → faster diffusion.

Remember

Active transport: low → high concentration, using energy from respiration. Efficient exchange surfaces are large and thin, with gradients maintained by flow.