What is a Membrane?

Phospholipids

  • Primary component of cell membranes
  • Head - negatively charged, polar, hydrophilic
  • Tails - uncharged, nonpolar, hydrophobic
  • Form a bilayer in an aqueous solution
    • Head groups oriented towards the water
    • Tails oriented away from the water
notion image

Characteristics of a membrane

Fluid

  • Phospholipids constantly moving
    • Temperature dependent
  • Cholesterol provides some stability

Impermeable to changed molecules

  • Water-soluble substances can’t diffuse through the membrane e.g. ions, proteins
  • Small, uncharged polar molecules can diffuse freely e.g. oxygen, CO2
  • Membrane is selectively permeable

Functions of a membrane

  • Hydrophobic interior acts as a barrier
    • Cell can maintain differences in solute composition/concentrations inside and outside the cell
  • Responsible for fluidity of membrane
    • Enables cells to change shape e.g. RBC

Membrane proteins

  • Peripherally associated proteins: adhere to the cytoplasmic or extracellular surfaces of plasma membrane (not embedded)
  • Integral membrane proteins
    • Transmembrane proteins: span lipid bilayer (once or several times)
    • Some are embedded but don’t cross bilayer
    • Some are linked to a lipid component that intercalates the membrane

Functions

  • Ligand binding receptors e.g. hormone receptors
  • Adhesion molecules - form physical contacts with the extracellular matrix or with cellular neighbours e.g. integrins, CAMs
  • Transmembrane movement
    • Pores and channels for water or specific ions
    • Carriers - facilitated transport, or couple transport molecule to other solutes
    • Pumps - use ATP to drive transport in/out of cell
  • Can be enzymes
  • Intracellular signaling - associated with cytoplasmic surface
  • Docking-marker acceptors - exocytosis
    • Located on inner surface membrane

Membrane carbohydrates

  • Glycolax: glycoproteins and glycolipids located on outer surface of cells

Functions

  • Self-identity markers
    • Tissue growth - cells do not overgrow their own territory
  • Cancer cells have abnormal markers

Specialised cell junctions

  • Tight junctions: join lateral edges of epithelial cells near to their luminal (apical) membranes, can be tight or leaky
  • Desmosomes: adhering junctions that anchor cells together, especially in tissues subject to stretching (e.g. skin, heart)
  • Gap junctions: (‘communicating junctions’) allow the movement of charge carrying ions and small molecules between two adjacent cells