Plasma membrane

The plasma membrane delimits the internal environment of the cell from the extracellular fluid. It is about 10 nanometres in diameter. Its basic structure is that of a lipid bilayer with hydrophilic external surfaces and a hydrophobic core. Proteins sit within the membrane and carbohydrate is attached to its outer surface. The components are thought to behave like a fluid-mosaic. Electron microscopy on freeze-fracture samples has largely revealed the membrane structure.

It is vital to cell function:

• provides physical protection
• it actively works to transport and exclude certain molecules from the cell, so:
  • maintaining a favourable environment for cell metabolism
  • keeping ions such as calcium low, so permitting its use as an intracellular signalling molecule
  • establishing an electrical potential across the membrane - making cells 'excitable' - which allows specialization for function e.g. myocytes, neurons
• it has receptors for a wealth of transmitters, hormones and chemical mediators with adaptations to transduce the signal internally
• it is involved in intercellular and cell-matrix interaction
• it has surface molecules involved in immune recognition