Lecture 5 – Reading
Membrane Structure and Transport
One of the keys to the evolution of life was the development of the cell or plasma
Plasma Membrane: selectively permeable barrier, allows for the uptake of key
nutrients and elimination of waste products while still maintaining a protected
environment for cellular processes to occur.
Development of this membrane allowed for compartmentalization of processes and
Current view of membrane based on: Fluid mosaic model.
- Membranes are not rigid but consists of proteins in a mixture of lipids the
consistency of olive oil.
- Lipid molecules are double layered (bilayer).
- Lipid molecules vibrate, flex back and forth, very rarely does a lipid go
between the two layers.
- There is much exchange in molecules within a layer.
- Maintaining membrane in fluid state is very important to its function.
- Most membranes contain an assortment of different types of proteins.
- Proteins much larger than lipid molecules, therefore move much slower
- Some proteins anchor the cytoskeleton and don’t move
- Some membranes have more proteins and some have more lipids, depending
on their function.
- The proteins and other components of one half of the lipid bilayer are
different from those that make up the other half of the bilayer. (Membrane
Evidence in support in fluid mosaic model:
- Membranes are fluid: were able to mix the membranes of human and mouse
- Membrane asymmetry: cells frozen then fractured, fracture slipts bilayers in
half. It was clearly seen that the particles on either side of the membrane
differ in size, number and shape.
Lipid molecules are underlying fabric of all membranes.
Lipid: diverse group of water-insoluble molecules.
- Fats (phospholipids) are dominant lipids in membranes.
- Types of lipids can be adjusted so that the membrane doesn’t become too
liquid or too solid.
Lipid bilayer is formed of phospholipids. Each phospholipid has a head attached to two lond chains og carbon and hydrogen
Head group is made of glycerol linked to one of several types of alcohols or amino by
a phosphate group.
All phospholipids are amphipathic: the molecules contains a part that’s hydrophobic
and a part that’s hydrophilic.
Fatty acid chains of a lipid are nonpolar and the phosphate containing head group is
Polar molecules tend to be hydrophilic and n