AQA A Level Biology复习笔记2.4.10 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

• 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.