Active Transport & Co-transport (AQA A Level Biology): Revision Note

Active transport

• Active transport is defined as:

The movement of molecules or ions through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration

• Active transport requires carrier proteins (each carrier protein being specific for a particular type of molecule or ion)

• The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane

• Energy is provided by the hydrolysis of ATP (adenosine triphosphate) into ADP and inorganic phosphate

Processes that use active transport

• Active transport is important in:

  • Reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules

  • Absorption of some products of digestion from the digestive tract

  • Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant

  • Loading inorganic ions from the soil into root hairs

Co-transport

• Co-transport is the coupled movement of two substances across a membrane via a carrier protein

• One moves down its concentration gradient, allowing the other to move against its gradient

Processes that use co-transport

• In the mammalian ileum, co-transport absorbs glucose and sodium ions:

  1. Active transport moves Na⁺ from the epithelial cell into the blood, creating a Na⁺ gradient.

  2. Na⁺ then diffuses in from the ileum, carrying glucose via a co-transporter.

  3. Glucose moves into the blood by facilitated diffusion