Movement in and out of cells

Diffusion
1. Explanation
  1. Diffusion is passive transport, which refers to the natural, inbuilt motion of particles.
    1. All particles are constantly in motion due to kinetic energy they possess.
    2. The motion is random
    3. Particles are constantly bouncing off one another and other objects
      1. If all particles are at one side, they will bounce and move to the other side. There will be no movement in the reverse direction initially, so there will be a net overall movement from an area of high concentration to an area of lower concentration
2. Definition
  1. The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower
3. Rate of diffusion
  1. Concentration gradient
    1. Greater the difference in concentration of molecules or ions on either side of an exchange surface, the faster the rate of diffusion
    2. The path from an area of higher concentration to an area of lower concentration
      1. Particles diffuse down the concentration gradient
  2. Area over which diffusion takes place
    1. The larger the area of an exchange surface, the faster the rate of diffusion
  3. Thickness of exchange surface
    1. The thinner the exchange surface, the faster the rate of diffusion
  4. Surface area x difference in concentration/length of diffusion path
Facilitated Diffusion
1. Explanation
  1. Like diffusion, it occurs down a concentration gradient, but it occurs at specificpoints on the plasma membrane where there are special protein molecules.
    1. These proteins form water-filled channels (protein channels) across the membrane.
      1. The channels are selective, each openin only in the presence of a specific ion.
      2. These channels allow water soluable ions to pass through. Such molecules would usually diffuse only very slowly through the phospholipid bilayer of the plasma membrane.
  2. An alternative form of facilitated diffusion involves carrier proteins that span the plasma membrane. When a molecule, eg. glucose, that is specific to the protein present, it binds with the protein. This causes it to change shape in such a way that the molecule is released to the inside of the membrane. No external energy is required for this. The molecules move from a region where they are highly concentrated to one of lower concentration, using only kinetic energy
Osmosis
1. Definition
  1. The passage of water from a region where it has a higher water potential to a region where it has a lower water potential
2. Solutes and water potential
  1. Water potential
    1. The pressure created by water molecules. Pure water is said to have a water potential of 0
  2. The addition of a solute to pure water will lower its WP
  3. WP of a solute must always be <0
  4. The more solute added, the lower its WP
  5. Water will move by osmosis from a region of higher (less negative) WP to one of lower (more negative) WP
Active Transport
1. Definition
  1. The movement of molecules or ions into or out of a cell from a region of lower concentration to an area od higher concentration
2. Explanation
  1. ATP is needed
    1. Used in 1 of 2 ways
      1. To directly move molecules
      2. Using a concentration gradient that has already been set up by direct active transport. Also known as co-transport
  2. Against the concentration gradient
  3. Carrier protein molecules are involved
  4. Process is selective, specific substances are transported
3. Process
  1. Carrier proteins span cell surface membrance and accept molecules or ions to be transported
    1. Molecules or ions bind to receptors on the channels of the carrier protein
      1. Inside the cell, ATP binds to the protein, causing it to split into ADP and a phosphate molecule, casuing the protein molecule to change shape and open to the opposite side of the membrane
        The molecules or ions are then released to the other side of the membrane
        The phosphate molecule is released from the protein and recombines with the ADP to form ATP during respiration
        The protein reverts to its original shape

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Movement in and out of cells

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	Created by Beth Ritchie over 10 years ago