Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
114 PANEL 2–5: Fatty Acids and Other Lipids COMMON FATTY ACIDS TRIACYLGLYCEROLS O These are carboxylic acids with long hydrocarbon tails. COOH COOH COOH CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH CH2 CH2 CH CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 CH2 CH2 palmitic acid (C16) CH2 CH3 stearic acid (C18) CH3 oleic acid (C18) H2C O HC O H2C O Fatty acids are stored as an energy reserve (fats and oils) through an ester linkage to glycerol to form triacylglycerols, also known as triglycerides. C O H 2C OH C HC OH O H 2C OH C glycerol 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. –O –O O O C C 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. oleic acid space-filling model carbon skeleton UNSATURATED CARBOXYL GROUP SATURATED PHOSPHOLIPIDS Phospholipids are the major constituents of cell membranes. If free, the carboxyl group of a fatty acid will be ionized. hydrophilic group O choline O C O stearic acid _ O _ O P O CH2 CH CH2 But more usually it is linked to other groups to form either esters O C O C hydrophobic fatty acid tails or amides. space-filling model of the phospholipid phosphatidylcholine O C N H general structure of a phospholipid 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 (alcohols). CHAPTER 2 PANELS 115 POLYISOPRENOIDS LIPID AGGREGATES long-chain polymers of isoprene Fatty acids have a hydrophilic head and a hydrophobic tail. O– micelle O In water they can form a surface film or form small micelles. P O– O Their derivatives can form larger aggregates held together by hydrophobic forces: 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 4 nm OTHER LIPIDS STEROIDS Lipids are defined 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. CH3 C CH2 CH CH2 isoprene Steroids have a common multiple-ring structure. OH HO O cholesterol—found in many membranes testosterone—male steroid hormone GLYCOLIPIDS Like phospholipids, these compounds are composed of a hydrophobic region, containing two long hydrocarbon tails, and a polar region, which, however, contains one or more sugars and no phosphate. galactose H OH C C C H H H C C NH hydrocarbon tails O O CH2 sugar a simple glycolipid dolichol phosphate—used to carry activated sugars in the membrane-associated synthesis of glycoproteins and some polysaccharides