Lipids General Characteristics Classification Biological function Lipids and detergents (HW)

Characteristics (use in conjunction with Interactive Concepts in Biology) Composed of a hydrocarbon chain (R or alkyl group), hydrogen groups and carboxyl group at the end of the chain As the size of the alkyl group increases, the relative contribution of the carboxyl group to the physical properties of the molecule decreases Insoluble in water, soluble in organic solvents, fat-soluble

What are lipids? Are fat molecules or formed by dehydration or condensation reactions of fatty acids (an even # of carbons, typically 10-24 carbons in the chain) and glycerol Are involved in: energy storage (triglycerols in fat cells) and utilization (some fatty acids are used as building blocks, most are used for energy production structural elements of membranes, hormones (steroids, sex hormones), some aid in digestion and nutrient transport, may be part of proteins and carbohydrates1 1 Campbell, N. et al 2002 Biology Fifth Edition. Benjamin Cummings Publishing Company. United States

Classification of Lipids Lipids are classified: according to solubility Fats and oils (triglycerides), waxes, phospholipids, sterols Formation of triglycerides: Glycerol + 3 Fatty Acids TRIGLYCERIDE Ester Linkage Fatty acid chain could be saturated or unsaturated

Unsaturated fatty acids Saturated with H Contain single carbon- carbon covalent bonds Examples: 16:0 Palmitic acid CH3(CH2)14COOH 18:0 Stearic acid CH3(CH2)16COOH Unsaturated fatty acids Contain 1 or more C-C double bonds (mono-poly) Example: 18:1 Oleic acid CH3(CH2)7 CH=CH (CH2) 7COOH

…or according to behavior in basic environments Lipids are often classified as saponifiable and nosaponifiable. Saponifiable: are hydrolyzed by bases (strong bases NaOH, KOH) to give salts and alcohol. Examples include tryglycerols, glycolipids, phospholipids, and some waxes Nonsaponifiable: not hydrolyzed by base. Examples: steroids, prostaglandins, some waxes. Saponification = base catalyzed hydrolysis

Lipids and Detergents Soaps are salts of fatty acids (Na or K salts) The hydrocarbon portion (the tail) of the fatty acid salt is water-insoluble but the ionic is water soluble, even more than alcohol!! “Grease (and dirt) have the hydrocarbon character and so it dissolves in the hydrocarbon chains of the soap molecules

Hydrophilic and Hydrophobic and lipophilic (tail) Water Hydrocarbons Hydrophilic and Hydrophobic and lipophilic (tail) Lipophobic portion of soap portion of the soap Soluble in hydrocarbon part of fatty acid

Then how does it help “dissolve” lipids? As a consequence oil (and dirt) bits are collected in the center of a micelle. Micelles stay in basic solutions as a colloid and do not come together to precipitate because of the ion repulsion

Biological Function of Lipids Triglycerides and energy storage High in energy and water-insoluble In general, the less oxygen the molecule contains, the more energy it contains, then fats and oils have more energy than sugars and proteins (contain numerous hydroxyl atoms)2 Fats and oils have about 9 kcal (Cal) of energy per gram, twice as effective as compared to dry carbohydrates and proteins “Fats and oils are stored “dry” in fat cells, but glycogen is hydrated by numerous water molecules”3 2 Bradford, M. et al. 1993 Organic and Biological Chemistry. Saunders College Publishing, United States. p. 301 3 Ibid

Biological functions… Fatty tissue serves a protective role Help to cushion and protect organs from injury Large amounts of fatty acids are required in animals These are used in insulation against cold temperatures and energy source in animals that hibernate Subcutaneous fat in humans also has an insulating role