Elementary plant Biochemistry and Biotechnology (2+1)

Molecular aggregation of phospholipids

Glycerophospholipids are virtually insoluble in water

• Depending on the precise conditions and the nature of lipids used, three types of lipid aggregates can form when amphipathic lipids are mixed with water

Micelles

• Free fatty acids, lysophospholipids and sodium dodecyl sulphate (SDS) form micelle
• Micelles are relatively small spherical structures involving a few dozen to few thousand molecules arranged so that their hydrophobic regions aggregate in the interior excluding water and their hydrophilic head groups are at the surface in contact with water
• This molecular arrangement eliminates unfavourable contacts between water and the hydrophobic tails

Bilayer

• A second type of lipid aggregate in water is the bilayer in which two lipid monolayers combine to form a two dimensional sheet
• The hydrophobic portions in each monolayer interact excluding water
• The hydrophilic head groups interct with water at the two surfaces of the bilayer lipid bilayers form the structural basis of biological membranes


Liposomes

• The third type of lipid aggregate is formed when a lipid bilayer folds back on itself to form a hollow sphere called a liposome or vesicle
• These bilayer vesicles enclose water creating a separate aqueous compartment

Biological membranes

• Proteins and polar lipids account for mass of biological membranes
• The relative proportions of protein and lipid differ in different membranes, reflecting the diversity of biological roles
• Amphipathic molecules form a lipid bilayer with the non polar region of lipids facing outward
• In this lipid bilayer, globular proteins are embedded at regular intervals held by hydrophobic interactions
• Some proteins protrude from one or other face of the membrane (peripheral proteins); some span its entire width (integral proteins)
• The individual lipid and protein subunits in a membrane form a fluid mosaic
• The membrane is fluid because the interactions among lipids, between lipids and proteins are non covalent, leaving individual lipid and protein molecules free to move laterally
• One of the key functions of a membrane is to control the passage of substances across it
• They are said to be selectively permeable. The different membranes of the cell have different selective permeabilities