Lipids: Fats, Waxes, and Complex Lipids

Lipids: Fats and Waxes

• Lipids include complex lipids (diacylglycerols, fatty acids, alcohols, sterols, vitamins D, E, K, carotenoids, terpenes, eicosanoid compounds) and neutral lipids (acylglycerols, cholesterol, cholesteryl esters)

Fatty Acids

• Fatty acids are aliphatic mono-carboxylic acids, mainly occurring in triglycerides
• They have a hydrocarbon chain (mostly even-numbered, 4-20 carbons long) with a carboxyl group at the end
• Naturally occurring saturated fatty acids are volatile liquids at room temperature, below 8 carbons
• Waxes have 34 carbon atoms, while bacterial waxes can have up to 90 carbon atoms

Saturated Fatty Acids

• Saturated fatty acids have no double bonds in their hydrocarbon chain
• Palmitic and stearic acids are the most abundant fatty acids in humans
• Lignoceric acid is a component of cerebrosides
• Systemic names for saturated fatty acids include palmitic acid (n-hexadecanoic acid), stearic acid (n-octadecanoic acid), and arachidic acid (n-eicosanoic acid)

Important Saturated Fatty Acids

• Acetic acid (2:0), butyric acid (4:0), caproic acid (6:0), palmitic acid (16:0), stearic acid (18:0), arachidic acid (20:0), and lignoceric acid (24:0)

Unsaturated Fatty Acids

• Unsaturated fatty acids possess one or more double bonds in their hydrocarbon chain
• They are more reactive than saturated fatty acids
• Monounsaturated fatty acids have one double bond, while polyunsaturated fatty acids have more than one double bond
• Oleic and palmitoleic acids are abundant in humans
• Dienoic, trienoic, and tetraenoic fatty acids have 2, 3, and 4 double bonds, respectively

Important Unsaturated Fatty Acids

• Palmitoleic acid (16:1 ω-7), oleic acid (18:1 ω-9), linoleic acid (18:2 ω-6,9), alpha-linolenic acid (18:3 ω-3,6,9), gamma-linolenic acid (18:3 ω-6,9,12), and arachidonic acid (20:4 ω-5,8,11,14)

Melting Point of Fatty Acids

• Melting points of saturated fatty acids increase with increasing hydrocarbon chain length
• Melting points of unsaturated fatty acids decrease with increasing number of double bonds
• Examples: butyric acid (8°C), palmitic acid (62°C), stearic acid (70°C), oleic acid (14°C), linoleic acid (-5°C), and arachidonic acid (-50°C)

Solubility of Fatty Acids

• Terminal COOH ionizes to become anionic (-COO¯), increasing water solubility
• Hydrophilicity increases solubility, while hydrophobicity decreases solubility
• Solubility decreases with increasing hydrocarbon chain length
• Presence of double bonds increases solubility

Isomerism in Fatty Acids

• In saturated fatty acids, they can be straight chains or branched
• In unsaturated fatty acids, presence of double bonds causes geometric isomerism (cis or trans)
• In mammals, all unsaturated fatty acids are cis
• Cis configuration causes a rigid bend in the hydrocarbon chain

Nutritionally Essential Fatty Acids

• Certain fatty acids must be taken in by humans, as they cannot be synthesized by the body
• Examples: linoleic acid and alpha-linolenic acid (polyunsaturated fatty acids)

Chemical Properties of Fatty Acids

• Formation of salts (soaps) with alkali or alkaline earth metals
• Sodium and potassium soaps are soluble, while calcium and magnesium soaps are insoluble