(1) A lipid with one or more phosphate groups attached to it.
(2) A lipid consisting of a glycerol bound to two fatty acids and a phosphate group.
Phospholipids are a subgroup of lipids. Other major types of lipids are fatty acids, sphingolipids, sterol lipids, and prenol lipids. Lipids are organic compounds that are readily soluble in nonpolar solvent (e.g. ether) but not in polar solvent (e.g. water).
A phospholipid is a type of lipid that is an essential component of many biological membranes, particularly the lipid bilayer of cells. This is because phospholipids are amphipathic compounds in a way that the ‘head’ is hydrophilic and the lipophilic ‘tail’ is hydrophobic. The ‘tail’ is usually made up of two long fatty acid chains. The ‘head’ has a glycerol constituent and a negatively charged phosphate group. The phosphate group is attached to one of the three carbons of the glycerol backbone whereas the remaining two carbons are bound to two fatty acid chains (mostly a saturated fatty acid on C-1 and an unsaturated fatty acid on C-2). The phosphate may further be bound to: hydrogen, choline, serine, ethanolamine, inositol, etc. The hydrophilic component determines the type of phospholipid: phosphatidic acid, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, etc., respectively. Each of these phospholipids goes through a particular biosynthetic pathway. The phosphatidic acid is the most fundamental of these phospholipids as it serves as the precursor to many phospholipids. The biosynthesis of phospholipids usually starts at Gro3P.
Being amphipathic, the phospholipid tails tend to avoid interacting with water. In contrast, the phospholipid heads may interact with water. Thus, when placed in water or an aqueous solution, the phospholipids tend to aggregate by orienting their tails towards each other. Thus, the ‘heads’ tend to face the water or the aqueous solution. As a result, the phospholipid component of biological membranes enables its distinctive “lipid bilayer”. The phospholipid tails line up internally while the phospholipid heads are on both sides facing the exterior. The phospholipids in biological membranes are prevented from packing together by the presence of sterols (another group of lipids).
Phospholipids are amphipathic compounds in a way that the ‘head’ is hydrophilic and the lipophilic ‘tail’ is hydrophobic. This is essential in the formation of a lipid bilayer feature of biological membranes. Phospholipids serve as a major structural component of most biological membranes, e.g. cell membrane. The phospholipids are vital to the function of the cell membrane. Being amphipathic, their presence creates an effective barrier preventing the entry of all molecules. Not all molecules would be able to enter the cell. Only those that are small enough (e.g. oxygen and carbon dioxide) and those that are non-polar can be allowed passage across the lipid bilayer. Other molecules (especially polar molecules) would need carriers or transport systems in order to enter the cell through the cell membrane.
Phospholipids are also involved in metabolism and cell signaling. For instance, phosphatidylinositol (4,5)-bisphosphate is split into inositol triphosphate and diacylglycerol (by the action of the enzyme phospholipase C). Together, they act as second messenger molecules used in signal transduction in biological cells.
- phosphor– » from phosphorus + –lipid » from Greek lipos, fat.
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