Cell transport

• Diffusion:

  • The spreading out of particles, resulting in a net movement from an area of HIGHER concentration to an area of LOWER concentration
  • Particles move DOWN the concentration gradient
  • No energy required - passive process
• Osmosis:

  • The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
  • Water moves from an area of LOWER solute concentration to an area of HIGHER solute concentration
  • No energy required - passive process
• Active transport:

  • The movement of particles from a more dilute solution to a more concentrated solution using energy from respiration
  • Particles move against the concentration gradient, from an area of low to high concentration
  • Energy is required - using energy released during respiration
• Examples of diffusion:

  • Humans - nutrients in the small intestine diffuse into the blood in the capillaries through the villi. oxygen diffused from the air in the alveoli into the blood in the capillaries, carbon dioxide diffuses from the blood in the capillaries into the air in the alveoli. Urea diffuses from cells into the blood for excretion by the kidney
  • Fish - oxygen from water passing over the gills diffuses into the blood in the gill filaments. Carbon dioxide diffuses from the blood in the gill filaments into the water
  • Plants - carbon dioxide used for photosynthesis diffuses into leaves through the stomata. Oxygen produced during photosynthesis diffuses out of the leaves through the stomata
• Examples of osmosis:

  • Plants - water moves by osmosis from a dilute solution in the soil to a concentrated solution in the root hair cell
• Examples of active transport:

  • Humans - active transport allows sugar molecules to be absorbed from the small intestine when the sugar concentration is higher in the blood than in the small intestine
  • Plants - active transport is used to absorb mineral ions into the root hair cells from more dilute solutions in the soil
• Model of osmosis, diffusion and active transport:

  
• Factors that affect the rate of diffusion:

  • Difference in concentration - the steeper the concentration gradient, the faster the rate of diffusion
  • Temperature - the higher the temperature, the faster the rate of diffusion
  • Surface area of the membrane - the larger the membrane surface area, the faster the rate of diffusion
• The villi are adapted for exchanging substances (absorbing nutrients) in the small intestine by having a network of capillaries, large surface area due to folding, thin walls (only one cell thick) and a good blood supply
• Alveoli in the lungs are adapted for gas exchange by having a network of capillaries for providing a good blood supply, and the rate of diffusion is increased because the membrane of the alveoli has a large surface area, is most and is only one cell thick and therefore has a short diffusion pathway
• Fish gills are adapted for gas exchange as they’re made up of stacks of thin filaments with a large surface area to increase diffusion and a network of capillaries for a food supply
• Root hair cells are adapted for uptake of water and minerals by having lots of mitochondria to take in mineral ions by active transport and having a large surface area to help efficient absorption of water and mineral ions