Transport in cells

• Diffusion: The movement of molecules from an area of high concentration to an area of lower concentration
• Example of diffusion: In a leaf

  
• Example of diffusion: In the lungs

  
• Example of diffusion: In the liver

  
• Osmosis: (Diffusion for water) The movement of water molecules from an area of high concentration to an area of lower concentration through a partially permeable membrane
• Osmosis ONLY refers to the movement of water molecules
• When the concentration of water is the same on both sides of the membrane, the movement of water molecules will be the same in both directions.
• Cells contain dilute solutions of ions, sugars andamino acids.

  The cell membrane is partially permeable.

  Water will move into and out of cells by osmosis.
• Animal cells also take in and lose water by osmosis. They do not have a cell wall, so will change size and shape when put into solutions that are at a different concentration to the cell contents
• In animals, the concentration of body fluids – blood plasma and tissue fluid – must be kept within strict limits – if cells lose or gain too much water by osmosis, they do not function efficiently
• Substances are transported passively down concentration gradients. Often, substances have to be moved from a low to a high concentration - against a concentration gradient
• Active transport is a process that is required to move molecules against a concentration gradient.
• Active transport: The movement of molecules from an area of low concentration to an area of high concentration against a concentration gradient
• For plants to take up mineral ions, ions are moved into root hairs, where they are in a higher concentration than in the dilute solutions in the soil. Active transport then occurs across the root so that the plant takes in the ions it needs from the soil around it
• In animals, glucose molecules have to be moved across the gut wall into the bloodstream
• All the glucose in the gut needs to be absorbed. When the glucose concentration in the intestine is lower than in the intestinal cells, movement of glucose involves active transport. The process requires energy produced by respiration.