Fatty acids are stored as an energy reserve (fats and oils) through an ester linkage to glycerol to form triacylglycerols, also known as triglycerides. H2COCOHCOCOH2COCOH2COHHCOHH2COHglycerol TRIACYLGLYCEROLS These are carboxylic acids with long hydrocarbon tails. COOHCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3COOHCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COOHCH2CH2CH2CH2CH2CH2CH2CHCHCH2CH2CH2CH2CH2CH2CH2CH3CH3stearic acid (C18) palmitic acid (C16) oleic acid (C18) COMMON FATTY ACIDS OOCstearic acid This double bond is rigid and creates a kink in the chain. The rest of the chain is free to rotate about the other C–C bonds. –OCARBOXYL GROUP O_OCOOCNOCCHIf free, the carboxyl group of a fatty acid will be ionized. But more usually it is linked to other groups to form either esters or amides. PHOSPHOLIPIDS CH2CHCH2PO_OOOhydrophobic fatty acid tails general structure of a phospholipid –OOColeic acid space-flling model carbon skeleton hydrophilic head choline In phospholipids, two of the –OH groups in glycerol are linked to fatty acids, while the third –OH group is linked to phosphoric acid. The phosphate is further linked to one of a variety of small polar groups, such as choline. space-flling model of the phospholipid phosphatidylcholine Hundreds of different kinds of fatty acids exist. Some have one or more double bonds in their hydrocarbon tail and are said to be unsaturated. Fatty acids with no double bonds are saturated. Phospholipids are the major constituents of cell membranes. UNSATURATED SATURATED 98 PaNel 2–5: Fatty acids and Other lipids
Fatty acids have a hydrophilic head and a hydrophobic tail. In water they can form a surface flm or form small micelles. Their derivatives can form larger aggregates held together by hydrophobic forces: Triacylglycerols (triglycerides) can form large spherical fat droplets in the cell cytoplasm. Phospholipids and glycolipids form self-sealing lipid bilayers that are the basis for all cell membranes. 200 nm or more micelle long-chain polymers of isoprene O
OTHER LIPIDS Lipids are defned as the water-insoluble molecules in cells that are soluble in organic solvents. Two other common types of lipids are steroids and polyisoprenoids. Both are made from isoprene units. CH3CCHCH2CH2isoprene
STEROIDS Steroids have a common multiple-ring structure.
GLYCOLIPIDS Like phospholipids, these compounds are composed of a hydrophobic region, containing two long hydrocarbon tails and a polar region, which contains one or more sugars and, unlike phospholipids, no phosphate. CCCH2HNHOHHOCOsugar a simple glycolipid Hdolichol phosphate—used to carry activated sugars in the membrane-associated synthesis of glycoproteins and some polysaccharides galactose