Exchange Across Plasma Membranes

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  • Exchange Across Plasma Membranes
    • Diffusion
      • Passive movement of particles
        • Passive means no energy is needed
      • From a high to a low concerntration
      • Molecules will diffuse both ways but the net movement will always be a low concerntration
      • The rate of diffusion depends on many things.
        • Concentration gradient - the higher it is, the faster the rate
        • Thickness of exchange surface - thinner the exchange surface, shorter the distance there is to travel, faster the rate
        • Surface area - larger the surface area (of the membrane) faster the rate
    • Osmosis
      • Diffusion of water molecules
        • Across a partially permeable membrane
        • From a high water potential to a low water potential
          • Water potential is basically the same thing as concentration of particles
          • Pure water has the highest water potential
    • Facilitated Diffusion
      • Some larger molecules and charged atoms cant directly diffuse the membrane
        • Uses carrier proteins for large molecules
          • Large molecule attaches itself to the carrier protein
            • Protein changes shape
              • This releases the molecule on the opposite side of the membrane
        • Uses Protein channels in the membrane for charged particles
          • Protein channels form pores in the membrane for charged particles to diffuse through
      • High to low concerntration
      • Passive process
    • Active Transport
      • Moves against the concentration gradient
      • Uses carrier proteins
        • Same process as facilitated diffusion
          • Except it  uses energy from ATP to move the substance against its concentration gradient
      • Also uses co-transporters
        • Type of carrier protein
        • Attach two molecules at a time
          • The concentration of one of the molecules is used to move the other against its own concentration gradient
            • So one molecule is going down its gradient and the other is going against its concentration gradient
  • The rate of diffusion depends on many things.
    • Concentration gradient - the higher it is, the faster the rate
    • Thickness of exchange surface - thinner the exchange surface, shorter the distance there is to travel, faster the rate
    • Surface area - larger the surface area (of the membrane) faster the rate

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