Cell Membranes

 1. Compartmentalization
Membranes enclose compartments --- plasma membrane encloses entire cell----nuclear and cytoplasmic membranes enclose various internal cellular spaces in which specialized activities take place.

 2. Provides a selectively permeable membrane barrier
Movement of molecules either into or out of cell is regulated . Only very small uncharged molecules can readily diffuse i.e.. O2 and CO2 (Fig. 1).

 3. Provides for the transportation of solutes
The membrane contains the machinery for physically transporting substances from one side of the membrane to another --- even against a concentration gradient (Fig. 1).

 4. Ability to respond to external signals
Receptors on surface have a particular ligand. When ligand binds to the receptor the signal must be transduced to the nucleus.
Signal Transduction  (Fig. 1)
Hormones, growth factors, neurotransmitters can all serve as ligands.

 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
Organizes cellular activities by the association of multi-enzyme complexes i.e. electron transport chains.

7. Provides for energy transduction
Transduction refers to the conversion of one type of energy into a different form of energy.
Ex: the energy of sunlight is absorbed by membrane-bound photosynthetic pigments and converted to chemical energy contained in carbohydrates.

 Conversely- the chemical energy in carbohydrates and fats can be converted to ATP.

(Text: p. 77-84 & 467-476)

 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).

 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)

 

 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).

 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.

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).