Membranes Functions

Membranes and Their Functions

Boundary and Permeability Barrier

  • Membranes act as walls, regulating what enters and leaves the cell (the 'house').
  • They have 'doors' and 'windows' to allow transport.
  • Antimicrobial Proteins: Some proteins create holes in bacterial membranes, leading to cell death and preventing infection.
    • Humans produce around 20 of these proteins.
    • Example: On skin, these proteins can puncture bacterial membranes.
    • Regulation is critical to prevent damage to our own cell membranes.

Organization and Localization of Function

  • Membranes are sites for specific biological functions due to embedded proteins.
  • Proteins within the membrane dictate the membrane's purpose.
  • Example: Glucose-6-phosphatase in the ER membrane.
    • This protein removes a phosphate from carbon 6 of glucose.
    • Glucose \rightarrow Glucose-6-Phosphate
    • Glucose can only exit the cell without a phosphate group.
    • Thus, the ER helps prepare glucose to be released into the bloodstream

Transport Proteins

  • Membranes possess transport proteins that regulate substance movement.
  • These proteins act as channels for specific ions or molecules.
  • Example: Cardiomyocytes (heart muscle cells).
    • Cardiomyocytes have sodium (Na^{+}), potassium (K^{+}), and calcium (Ca^{2+}) channels.
    • To initiate muscle contraction, the cell needs to become more positive.
    • The normal membrane potential of a muscle cell is very negative.
    • Sodium channels open, and Na^{+} flows in.
    • Calcium channels then open, and Ca^{2+} flows in, further increasing positivity.
    • This depolarization triggers contraction.
    • To repolarize, potassium channels open, and K^{+} flows out, restoring the negative charge.

Signal Detection

  • Membranes facilitate signal detection via receptors.
  • Protein hormones cannot enter cells, so they bind to receptors on the cell membrane.
  • Receptor binding transmits a signal into the cell, initiating a response based on the hormone's presence.

Cell to Cell Interactions

  • Membranes enable cell-to-cell interactions through various junctions.
  • Epithelial Cells: Require tight connections for structural integrity (e.g., skin).
    • Adhesive junctions and tight junctions hold cells together.
  • Cardiomyocytes: Require rapid communication.
    • Gap junctions: Protein channels connect cell interiors, facilitating ion and molecule movement.
    • This ensures coordinated contraction.
  • Desmosomes: Zipper-like structures that prevent cells from pulling apart during contraction, maintaining tissue strength.
    • Proteins slide against each other, preventing cell separation.