1.18 Active transport

What is active transport?

Diffusion and osmosis move substances down a concentration gradient and need no energy. Sometimes a cell needs to take in a substance when its concentration outside is lower than inside. Diffusion would move it the wrong way, so the cell uses active transport.

• Active transport moves substances from a low concentration to a high concentration: against the concentration gradient.
• It needs energy from respiration. The energy is released by mitochondria, so cells that do a lot of active transport have many mitochondria.
• It uses carrier proteins in the cell membrane. The substance attaches to a carrier protein, and energy is used to move it across the membrane.

Examples

Mineral ions in root hair cells. The concentration of mineral ions, such as nitrate, in the soil water is very low. It is higher inside the root hair cells. Active transport lets the root hair cells take up the ions against the concentration gradient. Plants need these ions for healthy growth: for example, nitrate to make amino acids and proteins, and magnesium to make chlorophyll.

Glucose in the gut. After a meal, glucose moves from the small intestine into the blood by diffusion. When the concentration of glucose in the gut becomes lower than in the blood, active transport moves the rest of the glucose into the blood. The cells then use the glucose for respiration.

What affects active transport?

Active transport depends on respiration, so anything that slows respiration slows active transport. For example, if the soil is waterlogged, there is little oxygen. The root cells cannot respire aerobically, so they cannot release enough energy to take in mineral ions, and the plant may grow poorly or die.

Comparing the three processes