UNIT 2.4 MOVEMENT IN AND OUT OF CELLS

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  • UNIT 2.4 MOVEMENT IN AND OUT OF CELLS
    • DIFFUSION
      • DIFFUSION is the result of random movement of particles from an area of high concentration to an area of low concentration.
      • It is a passive process.
      • The rate of diffusion depends on the concentration gradient and the temperature.
        • Concentration gradient is the difference the concentration of each area.
      • Does not require energy to be expended.
      • Occurs with many different molecules but mostly GLUCOSE, WATER, PROTEIN, OXYGEN and CARBON-DIOXIDE.
      • The cells in which diffusion occurs have PARTIALLY PERMEABLE MEMBRANES which allow some but not all particles in or out of the cell. The particles that cant go across the membrane may be too big, have the wrong electrical charge or they are incompatible with the chemical composition of the cells membrane.
      • Occurs down a concentration gradient.
    • OSMOSIS
      • Same concept as DIFFUSION but with WATER ONLY!
      • Occurs down a water potential gradient (high to low)
        • High water potential gradient = solution with many water molecules, Low water potential gradient = solution with few water molecules.
      • It is a passive process.
      • Requires a semi permeable membrane.
      • The water molecules travel from an area of high concentration (dilute solution) to an area of low concentration (concentrated solution).
      • It is through osmosis that plant cells are kept turgid
      • When in a solution with a high water concentration the plant cell will become firm and when in a solution of low water concentration the cell becomes floppy (flaccid).
      • For animal cells (red blood cells) if there is a low water potential in the surrounding solution then the cell shrinks and becomes crenated, if the water potential is high then the cell takes in water, swells and possibly bursts.
        • In a solution where the water potential is equal to that inside the cell, the cell will remain in equilibrium.
      • Is used in the process of PHOTOSYN-THESIS for the uptake of water by the plant which is then combined with CO2 to produce glucose and oxygen.
    • ACTIVE TRANSPORT
      • Is the movement of molecules AGAINST a concentration gradient but it requires energy (ATP) to do so and involves protein carriers in membranes.
      • Occurs if an equilibrium has been reached by diffusion and allows useful particles to continue travelling in and out of the cell.
      • The energy used in active transport is provided by respiration
      • The cell membrane contains protein molecules referred to as carriers which "pick up and carry" particles across the cell membrane.
      • Is used in the uptake of ions by plant root hairs and glucose by epithelial cells in the villi.
      • The protien carriers work in a similar way to enzymes. They recognise particular molecules and selct them from a mixture. This makes active transport a more specific process than osmosis or diffusion because the cell can effectivly choose which particles it wants.

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