1.2.6 Active Transport

1.2.6 Active Transport: Explain how active transport moves substances against a concentration gradient using energy from respiration, and give examples in plants and animals.

Unlike diffusion and osmosis, this process is not passive, because the cell must do work.

Against the gradient

Active transport moves substances from a region of lower concentration to a region of higher concentration.

Needs energy

The energy comes from respiration, which makes ATP, so the process stops if respiration is stopped.

Carrier proteins

Specific protein carriers in the cell membrane pick up the substance on one side and release it on the other.

Selective

Each carrier handles particular substances, so the cell chooses which ions or molecules to accumulate.

Examples

Root hair cells

Mineral ions such as nitrate are taken up from soil water even though their concentration is higher inside the cell.

Small intestine

Glucose can be absorbed into the lining cells when its concentration in the gut is lower than in the cell.

Many mitochondria

Cells that carry out lots of active transport contain many mitochondria to supply the energy.

Large surface

Root hair cells have long extensions that give a large surface area for absorbing ions.

Feature	Diffusion	Active transport
Direction	Down concentration gradient	Against concentration gradient
Energy needed	No	Yes, from respiration
Proteins	Not always	Always carrier proteins

If a cell is supplied with a respiration poison, its uptake of ions falls sharply, showing that the process depends on energy.

Cells in well-oxygenated conditions take up more ions than cells with little oxygen, because aerobic respiration releases more energy.

Waterlogged soil contains little oxygen, so root cells respire less and absorb fewer mineral ions, which can harm plant growth.

1.2.6 – Key Points for the Exam

Active transport: The movement of substances against a concentration gradient using energy from respiration.
Carrier protein: A membrane protein that moves a specific substance across the membrane.
Concentration gradient: A difference in concentration between two regions.
ATP: The molecule that transfers energy released by respiration to cell processes.
Mitochondria: Organelles where most of aerobic respiration takes place.

Common Mistakes

Saying that active transport moves substances down a concentration gradient; that is diffusion.
Stating that energy comes from the carrier protein; the energy comes from respiration.
Forgetting that cells with high active transport rates have many mitochondria.
Describing active transport as always moving water; it moves ions and molecules.