|Lecture Notes: 22 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 ]:
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) 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).
Assembly occurs on scaffolding of previous membrane
Membrane lipids are synthesized in/on membrane (Eukaryotes synthesis on cytosolic face of ER, transport by budding and Phospholipid exchange protein. Signal hypothesis for targeting of many membrane and exported proteins:
Proteins are transported in coated vesicles: membranous sacs encased in polyhedral frameworks of clathrin. (Figure 22.29)
Catabolism: degradation of molecules to provide energy.
Anabolism: reactions using energy to synthesize new molecules for growth etc.
(overhead - Interactions of Metabolic Pathways; Figures 10.5 and 10.6) sequences of consecutive enzyme catalysed reactions which are readily studied and traced. A more rational definition is that of Newsholme and Leach (Biochemistry for the Medical Sciences, Wiley, 1983: pg.42)
"[A] metabolic pathway is a series of enzyme-catalyzed reactions, initiated by a flux-generating step and ending with either the loss of products to the environment, to a stored product (a metabolic 'sink') or in a reaction that precedes another flux-generating step (that is, the beginning of the next pathway)." Where a flux generating step is a non-equilibrium reaction that generates the flux going through the pathway and to whose rate all other reactions of the pathway conform. Note that by this definition some pathways may be inter-organ while others may take place in single compartment. We will explore this definition/concept as we look at metabolism.
Last modified 22 March 2010