AQA A Level Biology复习笔记2.4.10 Active Transport & Co-transport

The Process of Active Transport & Co-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


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 is the coupled movement of substances across a cell membrane via a carrier protein
    • 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
    • Sodium ions and glucose molecules are transported into the epithelial cells via facilitated diffusion
    • The facilitated diffusion can only continue if a concentration gradient is maintained
    • The active transport of sodium ions out of the cell into the blood helps to maintain this gradient
    • The glucose molecules exit the epithelial cell and enter the blood via facilitated diffusion



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


Exam Tip

Be careful not to get carrier proteins and channel proteins confused when answering questions on active transport. Active transport requires carrier proteins (transmembrane transport proteins that undergo conformational change) not channel proteins.