Introduction to Molecular and Cell Biology, Biol. 220 Lecture 7: The three roles of rNA.
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FUNCTIONS

 1. Compartmentalization

 2. Provides a selectively permeable membrane barrier
(Fig. 1).

 3. Provides for the transportation of solutes
(Fig. 1).

 4. Ability to respond to external signals
Signal Transduction  (Fig. 1)

 5. Provides for intercellular and intracellular interactions

• cell-cell adhesion
• cell-ECM adhesion
• linkers add strength and shape to membrane and localize other proteins  (Fig. 1).

 6. Serves as a site for biochemical activities
multi-enzyme complexes i.e. electron transport chains.

7. Provides for energy transduction

Fig. 1 Membrane Permeability and Function

 

 STRUCTURAL PROPERTIES OF CELL MEMBRANES

 Formation of cell membranes is based upon the properties of lipids.

 All are bi-layers of phospholipids with associated proteins. As previously mentioned, phospholipids are amphipathic meaning that they possess both a hydrophobic and hydrophilic end (Fig. 2).

Fig. 2 Phospholipids

 

 Polar head groups are in contact with water while fatty acid tails aggregate in the interior of the membrane.

 The four major phospholipids found in cell membranes:

• phosphatidyl choline (Fig. 2)
• phosphatidyl ethanolamine (Fig. 2)
• phosphatidyl serine (Fig. 2)
• sphingomyelin - a non-glycerol phospholipid (Fig. 3)
• Fig. 3 Sphingomyelin
  

 

 Various glycolipids are also found in the outer leaflet of the cell membrane.

 Cholesterol is another important constituent of the animal cell membrane (Fig. 4).

Fig. 4 Steroids

 Lipid composition differs in the different types of cells and in different types of organisms. The average eukaryotic plasma membrane - ~50% of mass is lipid and 50% protein.

 The lipid bilayer behaves as a fluid.

FLUID MOSAIC MODEL

The fluid mosaic model was first proposed by Singer and Nicolson .
Lipids and proteins can readily move laterally and can also undergo rotation (Fig.1). The degree of membrane fluidity is determined by temperature and lipid composition.

 Lipids with shorter fatty acid chains are less rigid and remain fluid at lower temperatures. This is because interactions between shorter chains is weaker than for longer chains.

 Lipids containing unsaturated fatty acids increase membrane fluidity The = bonds introduce kinks, preventing tight packing of the fatty acids (Fig. 2).

 Cholesterol (Fig. 4)with its hydrocarbon ring structure plays a distinct role in determining membrane fluidity.
Polar hydroxyl group positions close to the phosphate head group. Rigid rings interact with regions of fatty acid chain adjacent to phospholipid head groups. This interaction decreases the mobility of the outer portions of the fatty acid chains, making this region of the membrane more rigid, even at higher temperatures.

 On the other hand...
Insertion of cholesterol interferes with interactions between fatty acids, thereby maintaining fluidity at lower temperatures.

 Cholesterol is not present in bacteria or plant cells.

 Plant cell membranes do contain sterols which function in a manner similar to cholesterol.

 In the fluid mosaic model of the membrane ----there are membrane proteins inserted into the lipid bilayer. The lipids provide the basic structure, but proteins carry out the specific functions of the different types of membranes (Fig. 1).

 

Created 2004 by CA Rinehart• email CA Rinehart

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