Cell Membranes


Passive Transport

  1. Uses Diffusion
    1. Movement of molecules is down the concentration gradient.
    2. Result of thermal motion of molecules.
    3. Osmosis
      1. Special case of diffusion across a differentially permeable membrane.
      2. Water (solvent) can cross the membrane but not the solute.
      3. Water tends to diffuse across the membrane so that the solution concentrations are the same on both sides of the membrane.
      4. The side with more solute is hypertonic.
      5. The side with less solute is hypotonic.
      6. Isotonic is when solute concentrations on both sides are equal.
      7. Water balance in cells is crucial.
    4. Facilitated Diffusion
      1. Movement across membrane following the concentration gradient.
      2. Channel protein opens a "hole" for water soluble substances.
      3. Carrier proteins move specific molecules through membrane.

Active Transport

  1. Movement across membrane against the concentration gradient.
  2. Transport proteins use energy (ATP) to move molecules or ions.
  3. Example: Sodium (Na+) - potassium (K+) pumps

Exocytosis and Endocytosis

  1. Exocytosis - vesicles fuse with cell membrane to release contents.
  2. Endocytosis
    1. Three types based upon what is taken in:
      1. pinocytosis: liquids and whatever solutes might be in it.
      2. phagocytosis: vacuoles formed around ingested particles.
      3. receptor-mediated: clustered proteins in membrane that bind specific macromolecules which are then engulfed.