Functional roles of lipids in membranes

Introduction and overview:-

Lipids as a class of molecules display a wide diversity in both structure and biological function. A primary role of lipids in cellular function is in the formation of the permeability barrier of cells and subcellular organelles in the form of a lipid bilayer. Although the major lipid type defining this bilayer in almost all membranes is glycerol-based phospholipid, other lipids are important components and vary in their presence and amounts across the spectrum of organisms, Sterols are present in all eukaryotic cytoplasmic membranes and in a few bacterial membranes.

The ceramidebased sphingolipids are also present in the membranes of all eukaryotes. Neutral glycerol-based glycolipids are major membrane-forming components in many Grampositive bacteria and in the membranes of plants while Gram-negative bacteria utilize a glucosamine-based phospholipid  as a major structural component of the outer membrane. Additional diversity results in the variety of the hydrophobic domains of lipids. In eukaryotes and eubacteria these domains are usually long chain fatty acids or alkyl alcohols with varying numbers and positions of double bonds.

In the case of archaebacteria, the phospholipids have long chain reduced polyisoprene moieties, rather than fatty acids, in ether linkage to glycerol. If one considers a simple organism such as Escherichia coli with three major phospholipids and several different fatty acids along with many minor precursors and modified products, the number of individual phospholipid species ranges in the hundreds. In more complex eukaryotic organisms with greater diversity in both the phospholipids and fatty acids, the number of  individual species is in the thousands.

If one or two phospholipids are sufficient to form a stable bilayer structure, why is the above diversity in lipid structures present in biological membranes. The adaptability and flexibility in membrane structure necessitated by environment is possible only  with a broad spectrum of lipid mixtures. The membrane is also the supporting matrix for a wide spectrum of proteins involved in many cellular processes. Approximately 20-35% of all proteins are integral membrane proteins, and probably half of the remaining proteins function at or near a membrane surface.

Therefore, the physical and chemical properties of the membrane directly affect most cellular processes making the role of lipids dynamic with respect to cell function rather than simply defining a static barrier. In this chapter, the diversity in structure, chemical properties, and physical properties of lipids will be outlined. Next, the various genetic approaches available to study lipid function in vivo will be summarized. Finally, how the physical and chemical properties of lipids relate to their multiple functions in living systems will be reviewed.