Lipid Structure of Biological Membranes

Lipid Structure of Biological Membranes

• Phospholipids are diesters of phosphoric acid, which can form monoesters, diesters, and triesters.
• Phosphatidic acids are the parent compound for glycerol-based phospholipids, consisting of sn-glycerol-3-phosphate with fatty acids esterified at the 1- and 2-positions.

Phosphoglycerides

• Contain a glycerol backbone linked by ester bonds to two fatty acids and phosphoric acid.
• Fatty acid residues have 12-20 carbon atoms.
• The phosphate group at C3 has an ester link to an amino alcohol.
• Comprise the major lipid component of cell membranes.
• Nonpolar tails aggregate in the center of a bilayer, while the ionic head is exposed to the solvent.

Sphingolipids

• The other major group of phospholipids, containing a sphingosine or dihydroxyamine backbone.
• Constituents of plant and animal cell membranes.
• Abundant in brain and nerve tissue, forming a coating around nerve fibers.

Prostaglandins

• Lipid autacoids derived from arachidonic acid, catalyzed by cyclooxygenase (COX).
• C-20 lipids containing a five-membered ring with two long side chains.
• Present in small amounts in all body tissues and fluids, with many physiological effects.

Glycosphingolipids

• Derivatives of ceramide, lacking in phosphorus and nitrogen base.
• Abundant in the myelin sheath of nerves.
• Galactocerebrosides are present in all adult brains, while glucocerebrosides are present in small amounts in certain pathological conditions and in immature brains.

Gangliosides

• Acidic glycosphingolipids with polar heads that are oligosaccharides with one or more residues of sialic acid (N-acetylneuraminic acid or NANA).
• Found in nerve endings and believed to function in the transmission of nerve impulses.

Terpenoids

• Found in plants and account for the flavor and scents of plant structures (flowers and fruits).
• Chemically related to compounds in turpentine (from pine sap) called terpenes, which are hydrocarbons that do not contain esters.
• Derived from isoprene units connected head-to-tail (Isoprene Rule).

Classification of Terpenoids

• According to the number of di-isoprenoid (C10) units they contain.
• Examples include the triterpene lanosterol, the precursor of steroid hormones, and the tetraterpene beta-carotene, a source of vitamin A.

Steroids

• A class of non-saponifiable lipid, defined by structure, with four fused rings A, B, C, and D.
• Carbon atoms are numbered beginning in the A ring, and the six-membered rings are in fixed chair conformations.
• As hormones, steroids are chemical messengers secreted by glands and carried through the bloodstream to target tissues.
• Widely distributed as cholesterol.

Learn about the structure and composition of phospholipids and phosphatidic acids in biological membranes, including their esterification and importance in natural systems.