Biological Membranes

Biological Membranes

Describe the structure and function of the biological membrane

Structure

  • Biological membranes are composed of lipid, protein and carbohydrate
  • Fluid state
  • Define and control composition of enclosed space
  • Outer and intracellular membrane that serve distinct functions in the formation of various intracellular organelles (nucleus and mitochondria)
  • Carbohydrates are either attached to lipids (glycolipids) or proteins (glycoproteins)
  • Lipid and protein composition of membranes vary between cell type
  • Three main groups of lipids
    • Phospholipids
    • Sphingolipids
    • Cholesterol
  • Lipid bilayer

Function

  • Boundary between internal and external; maintains cell shape
  • Transport of substances in and out of cell; prevents passage of hydrophilic ions and molecules
  • Cell-cell interaction
  • Signal transduction

Function of membrane proteins

  • Localization of enzyme activity
  • Energy transduction
  • Facilitated transport
  • Extracellular signal receptors

Understand the fluidity of membranes and factors that affect it

  • Fluidity is the degree of motion of the hydrocarbon chain within the bilayer
  • Affected by:
    • Long chain saturated fatty acids decrease fluidity of membrane; straight tails allow tight packing
    • Unsaturated increase fluidity (cis); kinked chains allows less dense packing

Discuss the fluid mosaic model

  • The membrane is a dynamic structure (constant motion) in which both proteins and lipids rapidly diffuse
  • Fluid because individual phospholipids and proteins move around freely within the layer
  • Mosaic because the pattern produced by the scattered protein molecules when viewed from above

Discuss the ways in which molecules are transported across biological membranes

Passive transport

  • Does not require energy
  • Moves down concentration gradient
    • Osmosis
    • Simple diffusion
    • Facilitated diffusion

Primary active transport

  • Requires ATP
  • Uses carrier proteins
    • Bulk transport- large quantities
      • Endocytosis
      • Exocytosis

Secondary active transport

  • Does not directly use energy in the form of ATP but requires energy
  • Carrier proteins transport one molecule in favor of their concentration gradient (generating energy) and one against the gradient (using energy) for simultaneous transport.

Donnan Membrane Equilibrium

  • Unequal distribution of ions across a semipermeable membrane
    • Due to presence on non-diffusible anions
  • At equilibrium
    • Diffusible cation concentration higher on the side with the non-diffusible anion
    • Diffusible anion concentration will be higher on the side without the non-diffusible anion
  • In the body proteins are present inside the cells
    • Due to the Donnan effect, more osmotically active articles are present in cells.
    • Na+/K+ pump counter balances the effect
  • Due to concentration difference, an electrical difference is formed
    • Balanced by opposite charges on the membrane
  • There are more proteins in the blood
    • Ionic movement occurs across capillary walls