transport in cells

• Substances may move into and out of cells across the cell membranes via diffusion.
• Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
• Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney
• Factors which affect the rate of diffusion are:

  • the difference in concentrations (concentration gradient)
  • the temperature
  • the surface area of the membrane.
• The greater the difference in concentration, the quicker the rate of diffusion.
• The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly.
• The greater the surface area, the faster the rate of diffusion.
• A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.
• Multicellular organisms require exchange surfaces and a transport system in order for substances to pass in and out of the cells. The larger the surface area is to the volume, the quicker and more substances can pass through the membranes. Multicellular organisms have specialised exchange surfaces - such as lungs, gills or villi in the intestines so that there is a larger surface area for the transport of substances to take place.
• Cells must remove waste substances and to take in useful substances in order for the cell to function
• adaption for exchanging materials:

  • Having a large surface area
  • A membrane that is thin, to provide a short diffusion path
  • (in animals) Having an efficient blood supply
  • (in animals, for gaseous exchange) Being ventilated
• Water may move across cell membranes via osmosis.
• Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
• investigate the effect of a range of concentrations if salt or sugar solutions on the mass of plant tissue

  1. prepare a range of sucrose solutions. the concentration of a solution is measured in mol dm-3
  2. prepare boiling tubes with different sucrose solutions with distilled water as 0.0 mole dm-3 (control)
  3. label the tube with the concentration
  4. weigh the potato before and after
  5. calculate mass increase or decrease
• Active transport moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration.
• Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth.
• It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for cell respiration.