The Cell Membrane

Describe the cell membrane and cellular organelles and their properties.

Cell membranes are:

• Formed by a phospholipid bilayer
  Separates the intracellular and extracellular fluid.
• Semi-permeable
  Leads to different ionic concentrations (and therefore electrical charge) on either side of the membrane.
  • Alteration in charge means the membrane acts as a capacitor, with most cells having a resting potential 70-80mV lower than extracellular fluid

Ion Permeability

At rest, the cell is:

• Permeable to potassium
  • Potassium flows out down its concentration gradient
    This makes the resting potential becomes more negative.
    • This negative charge opposes the further movement of potassium and so an equilibrium is established between opposing electrical and chemical gradients
• Impermeable to other cations
  The membrane is not perfectly impermeable to sodium, and Na⁺ will leak in down its concentration gradient.
  • The 3Na⁺-2K⁺ ATPase pumps three sodium ions outside in exchange for two potassium ions in order to maintain these gradients
    As there is an unequal exchange of charge, this pump is electrogenic.

References

1. Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong's Review of Medical Physiology. 24th Ed. McGraw Hill. 2012.
2. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.