4.2.34 · Cell Membranes and Transport

Active Transport — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 10 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

4.2.3 Active Transport — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe and explain active transport, including the role of ATP and carrier proteins, and compare it with passive transport.

Energy source
Without it
Specificity
Carriers, not channels
Number of carrier proteins
ATP supply

Part 1 of 8What active transport is

Active transport

Active transport movement of molecules or ions across a membrane against their concentration gradient,

from lower to higher concentration, using energy from ATP hydrolysis and specific carrier proteins.

Energy source

ATP is produced in respiration and is hydrolysed to supply the energy.

Purpose

Purpose it takes up scarce substances, removes substances that would accumulate and keeps ion concentrations different from the surroundings.

Without it

Without it passive processes would bring the inside and outside of every cell to the same composition.

Part 2 of 8Mechanism of a carrier protein

The molecule or ion binds to a specific binding site on the carrier protein on one side of the membrane.

ATP binds to the protein and is hydrolysed to ADP and phosphate, so the protein is phosphorylated.

The protein changes shape, moving the substance across and releasing it on the other side.

The phosphate is released and the protein returns to its original shape, ready to bind again.

Specificity

Specificity each carrier is specific because its binding site matches only one substance.

Carriers, not channels

Carriers, not channels active transport needs carrier proteins that change shape, not channel proteins.

Part 3 of 8Factors affecting the rate

1Number of carrier proteins

Number of carrier proteins more carriers give a faster rate.

2ATP supply

ATP supply it depends on the rate of respiration, so on oxygen concentration and temperature.

3Concentration of the substance

Concentration of the substance the rate rises up to a limit, when the carriers are saturated.

4Evidence that uptake is active

Evidence that uptake is active it stops with respiratory poisons such as cyanide, falls at low temperature and low oxygen,
- and cells contain many mitochondria.

Part 4 of 8The sodium–potassium pump

Pumping ratio

Pumping ratio each ATP hydrolysed moves three sodium ions out of the cell and two potassium ions in.

Result

Result the inside of the cell has a low Na⁺ and a high K⁺ concentration.

Uses of the gradients

Uses of the gradients nerve impulse transmission and absorption of glucose in the small intestine.

Osmotic role

Osmotic role pumping sodium out helps stop too much water entering the cell by osmosis.

Part 5 of 8Co-transport

The Na⁺/K⁺ pump on the basal membrane of an epithelial cell keeps the sodium concentration inside low.

Sodium ions diffuse back into the cell down their gradient through a co-transporter protein.

The co-transporter carries glucose in with the sodium, so glucose accumulates against its own gradient.

Glucose leaves the cell on the blood side by facilitated diffusion.

Secondary active transport
- glucose is moved by indirect active transport, because the energy comes from the sodium gradient.

Part 6 of 8Active transport compared with facilitated diffusion

Feature	Facilitated diffusion	Active transport
Direction	Down the gradient	Against the gradient
ATP needed	No	Yes
Proteins	Channel or carrier	Carrier (pump) only
Effect of respiratory inhibitor	None	Rate falls
Saturation	Yes	Yes

Part 7 of 8Where active transport matters

Root hair cells

Root hair cells uptake of mineral ions from the soil.

Kidney

Kidney reabsorption of glucose.

Phloem

Phloem loading of sucrose into companion cells.

Nerve and muscle

Nerve and muscle maintenance of ion gradients.

Adapted cells

Adapted cells epithelial cells have many mitochondria to supply ATP and microvilli to give surface area for carriers.

Part 8 of 8More from the specification

Active Transport Across Selectively Permeable Membranes

Active transport uses transmembrane carrier protein pumps and metabolic energy from ATP hydrolysis to transport mineral ions or solutes against their concentration gradient across selectively permeable cell membranes.

Investigating Osmosis in Plant Tissue Cylinders

When potato cylinders are placed into a series of sucrose solutions of differing water potential, water enters or leaves cells by osmosis, altering the tissue mass and volume.

Determining Incipient Plasmolysis and Isotonic Water Potential — The solute concentration at which potato tissue experiences zero net change in mass indicates the point where the external solution has a water potential equal to that of the plant cell sap.

Surface Area to Volume Ratio and Diffusion Rate

As the volume of a biological structure increases, its surface area to volume ratio decreases,
making simple passive diffusion across outer membranes insufficient and requiring specialized active transport systems.

Active Transport Across Selectively Permeable Membranes

Active transport uses transmembrane carrier protein pumps and metabolic energy from ATP hydrolysis to transport mineral ions or solutes against their concentration gradient across selectively permeable cell membranes.

Investigating Osmosis in Plant Tissue Cylinders

When potato cylinders are placed into a series of sucrose solutions of differing water potential, water enters or leaves cells by osmosis, altering the tissue mass and volume.

Surface Area to Volume Ratio and Diffusion Rate

As the volume of a biological structure increases, its surface area to volume ratio decreases,
making simple passive diffusion across outer membranes insufficient and requiring specialized active transport systems.

Bilayer

A double layer of phospholipids with tails facing inwards.

Water potential

A measure of the tendency of water to move; pure water has the highest.

Selectively permeable

Allowing only some substances through.

Phospholipid bilayer

Two layers of phospholipids with hydrophobic tails inwards

Dilute solution

Solution with a high water concentration and low solute concentration.

Animal cell

A cell with a nucleus, cytoplasm, cell membrane and mitochondria but no cell wall, chloroplasts or large vacuole.

Volume

The amount of cytoplasm needing supplies and producing waste.

Permeable

Allowing substances to pass through.

Indicator

A chemical that changes colour depending on pH.

Endocytosis

Bulk transport into a cell using a membrane vesicle.

Key terms

Term	Meaning
Active transport	Movement of substances across a membrane against their concentration gradient using ATP and carrier proteins.
ATP	Adenosine triphosphate, the immediate energy source for cell processes.
Carrier protein	Membrane protein that binds a specific substance and changes shape to move it.
Sodium–potassium pump	Carrier protein moving 3 Na⁺ out and 2 K⁺ into the cell per ATP hydrolysed.
Co-transport	Movement of two substances at once, one down its gradient driving the other against it.
Hydrolysis	Breaking of a bond using water, as in ATP to ADP and Pi.

Avoid these mistakes

Saying active transport uses energy without naming ATP and the carrier protein.

Thinking channel proteins carry out active transport — carrier proteins that change shape are needed.

Claiming active transport never moves a substance down a gradient — co-transport couples a downhill ion to an uphill molecule.

Forgetting that ATP is hydrolysed, not made, during active transport.

Last updated: 15 October 2026