Lipids Structure and Function

Biological Lipids

• Are a chemically diverse group of organic compounds that are insoluble in water
• Soluble in organic solvents
• Related to fatty acids and utilized by living cells

Structure of Lipids

• Formed mainly from alcohol and fatty acids combined by ester linkage
• Amphipathic compounds

Major Biological Functions Of Lipids

• Structural elements of cell membrane (phospholipids, sphingoglycolipids, and cholesterol)
• Energy sources (fatty acids)
• Energy storage (triacylglycerols)
• Protection and heat insulation around subcutaneous tissues and organs
• Electrical insulator in the propagation of depolarization waves along myelinated nerves
• Emulsifiers (bile acids)
• Cell surface component for cell recognition and tissue immunity
• Hydrophobic barrier in the division of intracellular structures
• Involved in the function of fat-soluble vitamins (A, D, E, K) as regulatory and coenzyme
• Roles in prostaglandin and steroid hormones in body homeostasis

Classification of Lipids

• Divided into simple and complex lipids
• Simple lipids: fatty acids and esters of fatty acids with various alcohols (neutral oils, waxes)
• Complex lipids: fatty acid esters that contain other groups in addition to fatty acids and alcohols
• Phospholipids and glycolipids are mainly complex lipids
• Lipoproteins may also be included in this class
• Precursor and derivative lipids: alcohols, fatty aldehydes and ketone bodies, fat-soluble vitamins and hormones, in addition to fatty acids, glycerol, steroids, glycerol, and sterols

Fatty Acids

• Present in very small amounts as free fatty acids in cells and tissues
• Generally found as building blocks of complex lipids
• Can be oxidized by many tissues to produce energy
• More than 100 types of fatty acids have been isolated from various cells and tissues

Structure of Fatty Acids

• Have a long hydrocarbon chain, a methyl group at one end, and a carboxyl group at the other
• Amphipathic compounds
• Fatty acids found in natural oils are generally in straight chain form and have an even number of carbon atoms
• Hydrocarbon chain may be saturated or unsaturated

Nomenclature of Fatty Acids

• Named by adding the suffix -oic to the end of the hydrocarbon chain name (saturated –anoic, unsaturated –enoic)
• Carbon atoms of fatty acids are numbered from the carboxyl carbon
• Carbon adjacent to the carboxyl carbon is α, carbon 3 is the β carbon, and the last methyl carbon is the ω (omega) carbon
• Location of the double bond is indicated by the numbers written above the ∆ symbol

Properties of Fatty Acids

• Chain lengths of fatty acids found in higher plants and animals range from 14 to 22
• Majority of these are fatty acids with 16 and 18 carbons
• Saturated fatty acids are relatively resistant to oxidation outside the body
• Unsaturated fatty acids oxidize slowly and spontaneously in air
• Melting points of fatty acids increase as the chain lengthens and decrease as the number of double bonds increases
• Double bond of the membrane fatty acids allows the lipids here to remain in liquid form

Unsaturated Fatty Acids

• Most unsaturated fatty acids in higher organisms have double bonds between carbon atoms 9 and 10
• The most abundant unsaturated fatty acids in higher organisms are oleic, linoleic, linolenic, and arachidonic acids
• Linoleic and linolenic acids cannot be synthesized in mammals and are therefore essential fatty acids
• Dietary linoleic and linolenic acid act as precursors for the synthesis of many other unsaturated fatty acids