|Lecture Notes: 1 March||
Lipid Properties: An important consideration for lipids of all sorts is their fluidity. Thus membranes must be fluid enough to allow the diffusion of proteins, transport processes etc. but not so fluid as to weaken the membranes structure. For storage want fat to be fluid enough to flow to fill out body shape at normal operating temperatures. A number of strategies are used by organisms to adjust lipid fluidity:
Detergents & Micelles: Polar heads of detergents and soaps (such as long chain fatty acids) tend to associate with polar solvents such as water, while non-polar "tails" are excluded by water and are forced to associate with themselves making globules known as micelles.
Lipid Bilayer: Figures 9.20 [overhead 11-12, V&V; 12-11]:
The lipid bilayer forms the core for the lipid bilayer membrane as seen in the Fluid Mosaic Model of biological membranes.
Fluid Mosaic Model: (Figure 9.21) [overhead 11-21 V&V] This model has as its core element lipid bilayer (predominantly glycero-phospholipid). This bilayer makes a very effective barrier for the flow of charged and polar species between aqueous compartments. Within the bilayer itself, however, flow occurs readily - it is a two- dimensional liquid with a viscosity similar to olive oil. Thus we see rapid exchange between adjacent phospholipid molecules on a face of the bilayer, but very rare exchange between faces (the polar "head" groups would have to cross the non-polar bilayer interior). A lipid bilayer membrane thus separates the interior of the cell from the outside.
Of course a cell also needs to communicate with the outside world - doors and windows are needed. Such communication occurs largely through proteins acting as pores, gates, and shuttles. Note that these proteins "float" in the bilayer. They have unconstrained movement in the two-dimensions of the sheet. Changes in protein conformation can also cause them to "sink" into the hydrophobic interior of the bilayer etc. Protein movement can be constrained by linkage to protein networks (cytoskeleton) within the cell as is exemplified by red blood cells (RBC's).
Last modified 1 March 2007