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BiologyGCSE, IGCSE & A Level

What is active transport?

Active transport moves substances against a concentration gradient using carrier proteins and energy from ATP.

Active transport explained

Because it needs ATP, cells that carry out a lot of active transport — root hair cells, intestinal epithelium, kidney tubules — are packed with mitochondria.
Any factor that stops respiration, such as a lack of oxygen or a metabolic poison, halts active transport but leaves diffusion unaffected.

Key formula

low concentration → high concentration, requires ATP

Worked example

Root hair cells absorb nitrate ions from dilute soil water into a more concentrated cytoplasm.

Examiner tip

Mention both 'against the concentration gradient' and 'using ATP from respiration' — most mark schemes require both phrases.

Active transport: common questions

How is active transport different from facilitated diffusion?
Both use carrier proteins, but only active transport moves substances against the gradient and requires ATP.
Why do cells doing active transport have many mitochondria?
Mitochondria supply the ATP that the carrier proteins need to change shape and pump substances across.

Related terms

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Osmosis

Osmosis is the net movement of water molecules from a dilute solution to a concentrated one through a partially permeable membrane.

Respiration

Respiration is the enzyme-controlled release of energy from glucose in every living cell, transferring it to ATP.

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