2.4.10 Active Transport & Co-transport

Active transport

• Active transport is the movement of molecules and 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)
• Although facilitated diffusion also uses carrier protein, active transport is different as it requires energy
• The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane
• The energy required is provided by ATP (adenosine triphosphate) produced during respiration. The ATP is hydrolysed to release energy

Active transport diagram

A carrier protein changing shape during 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 substances across a cell membrane via a carrier protein
  • This means that two types of molecule are moved across the membrane at the same time; the movement of one is dependent on the movement of the other
  • It involves a combination of facilitated diffusion and active transport
• A well-known example of a co-transporter protein can be found on the cell surface membrane of the epithelial cells lining the mammalian ileum
  • This specific co-transport protein is involved in the absorption of glucose and sodium ions as follows:
    • The active transport of sodium ions from the epithelial cell into the blood lowers the sodium ion concentration inside the cell and generates a sodium ion concentration gradient between the ileum and the epithelial cell
    • Sodium ions move into the cell from the ileum by facilitated diffusion, carrying glucose molecules along with them via a cotransport protein
    • The glucose concentration inside the epithelial cell increases, and glucose molecules enter the blood via facilitated diffusion
  • Note that it can help to explain cotransport by beginning with the active part of the process (as above); this may seem a bit backwards, but active transport generates the concentration gradient needed for cotransport to occur, so this is a logical starting point

Cotransport of sodium and glucose diagram

Both facilitated diffusion and active transport occur during co-transport. Glucose molecules can only enter the epithelial cell when sodium ions are present.