osmosis, active transport and diffusion

• Diffusion is the movement of particles from an area of high concentration to an area of low concentration down a concentration gradient through a selectively permeable membrane

  • Temperature speeds up diffusion
  • The bigger the concentration gradient the faster diffusion will be
• Osmosis is the diffusion of water through a selectively permeable membrane. The water moves from a solution of low solute concentration to a solution of high solute concentration.
• Active transport is the movement of a substance from an area of low concentration to an area of high concentration against a concentration gradient. Requires energy.
• osmosis in plant cells:

  plant cells have a strong cellulose cell wall on the outside of the cell membrane.

  • supports the cell and stops it bursting when it gains water by osmosis
  • the cytoplasm pushes against the cell wall and the cell becomes turgid
  • when a plant is in a solution where the water concentration is lower than cells contents it becomes flaccid (plamolysed)
  • water leaves the cell and the cytoplasm pulls away from the cell wall and the plant wilts

  A) flaccid (plamolysed)
• osmosis in animal cells:

  Animal cells do not have a cell wall. They change size and shape when put into solutions that are at a different concentration to the cell contents.

  For example, red blood cells:

  • gain water, swell and burst in a more dilute solution (this is called haemolysis)
  • lose water and shrink in a more concentrated solution (they become crenated or wrinkled)
• osmosis in potatoes:

  Cylinders or discs of fresh potato are often used to investigate osmosis in living cells. To carry out this type of experiment, you need to:

  1. cut equal-sized pieces of potato
  2. blot with tissue paper and weigh
  3. put pieces into different concentrations of sucrose solution for a few hours
  4. remove, blot with tissue paper and reweigh
  5. calculate the change in mass (end mass - start mass) x 100
  6. start mass
• 1. Rub a clean cotton bud gently on the inside of your cheek.
  2. Smear the sample across a clean glass slide.
  3. Cells are transparent, place a few drops of a dye called methylene blue onto the smear so the cells will be visible under the microscope.
  4. Use a mounted needle to gently lower a glass coverslip onto the sample on the slide, take care not to form air bubbles under the coverslip.
• microscopy:

  • to make image clearer adjust focusing wheel
  • objective lens enables you to adjust the magnification
  • eyepiece lens allows you observe the image
  • the stage is where the microslide is place into view
  • magnification = image size/actual size