Active Transport and Surface Area

Active transport moves particles against a concentration gradient (from low to high concentration) using energy from respiration and carrier proteins in the membrane. Root hair cells use it to absorb mineral ions from dilute soil water; gut cells use it to absorb glucose. Larger organisms have a smaller surface area to volume ratio, so they need specialised exchange surfaces and transport systems.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 30 minRevision 1 · updated 2026-10-30

Section 1 of 14

Learning objectives

Define active transport and give examples.
Compare diffusion, osmosis and active transport.
Calculate surface area to volume ratios and explain their significance.

Pause and think

Soil water contains far fewer nitrate ions than root cells do. How do roots still absorb nitrates?

Section 2 of 14

Core idea

Start herePumping uphill

Root cells contain mineral ions at concentrations hundreds of times higher than soil water. They get them by pumping ions in against the gradient, like pushing water uphill.

The big idea

Active transport uses energy from respiration and carrier proteins to move substances against a concentration gradient. Big organisms need exchange surfaces because their surface area to volume ratio is small.

1. Three ways to cross a membrane

Comparing transport

Drawing diagram…

Only active transport needs energy.

2. Surface area to volume ratio

Worked exampleCubes of different sizes

Compare a 1 cm cube and a 4 cm cube.

Goal: SA:V ratios.
Strategy: $6L^2$ and $L^3$ .

$1\text{ cm}: 6:1$
SA 6, V 1.
$4\text{ cm}: 96:64=1.5:1$
SA $6\times16$ , V $64$ .

AnswerThe larger cube has a quarter of the ratio.

✓ CheckRatio

=6/L

✓.

Key takeawayAdaptations Exchange surfaces maximise area and minimise distance: alveoli, villi, gills, root hairs, leaves.

Pause and think

Why are cells so small?

Summary

Active: against gradient, ATP.
SA:V falls as size rises.
Exchange surfaces compensate.

Active transport: Movement against a concentration gradient using energy.
Carrier protein: Membrane protein that moves specific particles across.
SA:V ratio: Surface area divided by volume.

HistoryThe sodium pump. Jens Skou discovered the sodium–potassium pump, the carrier protein behind much active transport, in 1957, and won the 1997 Nobel Prize in Chemistry.

Fun factEnergy hungry. Your brain spends a large share of its energy on active transport, pumping ions across neurone membranes to keep them ready to fire.

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