Membrane Functions

Membranes and Their Functions

Boundary and Permeability Barrier

  • Membranes act as boundaries and permeability barriers.
  • Analogy: Walls of a house.
    • The membrane is the wall.
    • The inside of the cell is the house.
  • Regulate the movement of substances in and out of the cell with "doors and windows".
  • Antimicrobial proteins can form holes in membranes.
    • Humans produce 20 different proteins that can do this.
    • These proteins poke holes in bacteria membranes, causing them to die and protecting the body.
    • This process must be tightly regulated to avoid damaging our own membranes.

Organization and Localization of Function

  • Membranes act as sites for biological functions.
  • Proteins embedded in membranes perform specific functions.
  • The function of a membrane depends on the proteins it contains.
  • Example: Glucose-6-phosphatase in the ER membrane.
    • This protein removes a phosphate from carbon 6 of glucose so it can leave the cell.
    • Glucose can only leave the cell if it does not have a phosphate attached.
    • This is the last step in allowing glucose to leave a cell into the bloodstream for use by other cells.
    • This function is associated with the ER because of the protein found in its membrane.

Transport Proteins

  • Membranes have transport proteins that regulate the movement of substances.
  • These proteins allow ions to flow in and out.
  • Example: Cardiomyocytes (heart muscle cells).
    • Specific transporters allow sodium, potassium, and calcium ions to flow.
    • Sodium Channels: Sodium channels let sodium flow in, which makes the cell more positive.
    • Calcium Channels: Calcium channels let calcium flow in, increasing the charge and positivity.
    • Potassium Channels: Potassium channels let potassium flow out, to reduce the charge and negativity.
  • Na^+
  • K^+
  • Ca^{2+}
  • This changes the membrane potential and triggers contraction.
  • Analogy: Windows in the walls of a house regulating airflow.

Signal Detection

  • Hormones can only function if they bind to their receptors.
  • Protein hormones cannot enter cells, so they bind to receptors on the cell membrane.
  • Receptor binding transmits a signal into the cell.
  • The presence of receptors in the membrane gives the membrane the function of signal detection.

Cell-to-Cell Interactions

  • Epithelial cells need to hold together tightly.
  • Proteins within membranes connect cells.
  • Examples:
    • Adhesive junctions
    • Tight junctions
  • Gap Junctions: Cardiomyocytes communicate through protein channels that connect the inside of one cell to the next, allowing ions to move through.
  • Desmosomes: Zipper-like structures that prevent cells from pulling apart during contraction.
  • These structures consist of proteins embedded in the membrane.
  • Overall, proteins slide against each other which keeps the cells from pulling apart.