Exchange Across Cell Membranes

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  • Exchange Across Cell Membranes
    • Diffusion
      • Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration
        • Molecules will diffuse both ways but the net movement will be to the area of lower concentration. This continues until particles are evenly distributed thorough the liquid of gas
        • The concentration gradient is the path from an areas of high concentration to an area of lower concentration. Particles diffuse down a concentration gradient
        • Diffusion is a passive process - no energy is needed for it to happen
        • Particles can diffuse across cell membranes as long as they can move freely through the membrane
      • The rate of diffusion depends on several factors
        • The concentration gradient - the higher it is the faster the rate of diffusion
        • The thickness of the exchange surface - the thinner the exchange surface the faster the rate of diffusion
        • The surface area - the larger the surface area the faster the rate of diffusion
      • Facilitated Diffusion
        • Carrier proteins move large molecules into or out of the cell, down their concentration gradient. Different carrier proteins facilitate diffusion of different molecules
          • First, a large molecule attaches to a carrier protein in the membrane
            • Then the protein changes shape
              • This releases the molecyle on the opposite side of the membrane
        • Channel proteins from pores in the membrane for charged particles to diffuse through. Different channel proteins facilitate the diffusion of different charged particles
    • Osmosis
      • Osmosis is Diffusion of water molecules
      • Osmosis is the diffusion of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential
        • Water potential is the potential of water molecules to diffuse out of or into a solution
          • Pure water has the highest water potential of zero. All solutions have a lower water potential than pure water
    • Active transport
      • Active transport uses energy to move molecules and ions across plasma membranes, against a concentration gradient
      • Carrier proteins are involved in active transport
        • A molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side
        • Co-transporter are a type of carrier protein
          • They bind two molecules at a time
            • The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient


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