MEMBRANE LIPIDS BASIC INFORMATION AND TUTORIALS

WHAT ARE MEMBRANE LIPIDS?
The Importance of Membrane Lipids

Membranes form boundaries around the cell and around distinct subcellular compartments. They act as selectively permeable barriers and are involved in signaling processes.

All membranes contain varying amounts of lipid and protein and some contain small amounts of carbohydrate.

In membranes the three major classes of lipids are the glycerophospholipids, the sphingolipids and the sterols. The glycerophospholipids have a glycerol backbone that is attached to two fatty acid hydrocarbon chains and a phosphorylated headgroup.

These include phosphatidate, phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol and phosphatidylserine. The sphingolipids are based on sphingosine to which a single fatty acid chain is attached and either a phosphorylated headgroup (sphingomyelin) or one or more sugar residues (cerebrosides and gangliosides, the glycosphingolipids).

The major sterol in animal plasma membranes is cholesterol, while the structurally related stigmasterol and β-sitosterol are found in plants.

The fatty acid chains of glycerophospholipids and sphingolipids consist of long chains of carbon atoms which are usually unbranched and have an even number of carbon atoms (e.g. palmitate C16, stearate C18).

The chains are either fully saturated with hydrogen atoms or have one or more unsaturated double bonds that are in the cis configuration (e.g. oleate C18:1 with one double bond).
Membrane lipids are amphipathic since they contain both hydrophilic and hydrophobic regions. In the glycerophospholipids and the sphingolipids the fatty acid hydrocarbon chains are hydrophobic whereas the polar headgroups are hydrophilic.

In cholesterol the entire molecule except for the hydroxyl group on carbon-3 is hydrophobic. In aqueous solution the amphipathic lipids arrange themselves into either micelles or more extensive bimolecular sheets (bilayers) in order to prevent the hydrophobic regions from coming into contact with the surrounding water molecules.

The structure of the bilayer is maintained by multiple noncovalent interactions between neighboring fatty acid chains and between the polar headgroups of the lipids. In biological membranes there is an asymmetrical distribution of lipids between the inner and outer leaflets of the bilayer.
Lipids are relatively free to move within the plane of the bilayer by either rotational or lateral motion, but do not readily flip from one side of the bilayer to the other (transverse motion). Increasing the length of the fatty acid chains or decreasing the number of unsaturated double bonds in the fatty acid chains leads to a decrease in the fluidity of the membrane.

In animal membranes, increasing the amount of cholesterol also decreases the fluidity of the membrane.

The fluid mosaic model describes the structure of biological membranes, in which the membranes are considered as two-dimensional solutions of orientated lipids and globular proteins.
Within biological membranes lipids and proteins cluster together in discrete domains. Lipid rafts are domains of the plasma membrane that are enriched in cholesterol, sphingomyelin and glycosphingolipids, as well as lipid modified proteins.

THE IMMUNE SYSTEM CHEMISTRY BASIC INFORMATION AND TUTORIALS

WHAT IS THE CHEMISTRY OF IMMUNE SYSTEM?
Immune System Chemistry Basics

The immune system has two main functions; to recognize invading pathogens and then to trigger pathways that will destroy them. The humoral immune system relies on B lymphocytes to produce soluble antibodies that will bind the foreign antigens.

The cellular immune system uses killer T lymphocytes that recognize and destroy invading cells directly.

The primary immune response occurs on initial contact with a foreign antigen and results in production of immunoglobulin M (IgM) and then immunoglobulin G (IgG). If the same antigen is encountered again, immunological memory leads to a secondary immune response that produces a much more rapid and larger increase in specific IgG production.

A large number of antibody-producing cells exist in an animal even before it encounters a foreign antigen, each cell producing only one specific antibody and displaying this on its cell surface. An antigen binds to cells that display antibodies with appropriate binding sites and causes proliferation of those cells to form clones of cells secreting the same antibody in high concentration.
Cells that produce antibody that reacts with normal body components are killed early in fetal life so that the adult animal normally is unable tomake antibodies against self, a condition called self tolerance.

Antibodies bound to an invading microorganism activate the complement system via the classical pathway. This consists of a cascade of proteolytic reactions leading to the formation of membrane attack complexes on the plasma membrane of the microorganism that cause its lysis.

Polysaccharides on the surface of infecting microorganisms can also activate complement directly in the absence of antibody via the alternative pathway.