Lipids: Fatty Acids & Phospholipids

Lipids

Note: Fat and its derivatives are collectively called lipids. The term lipid was coined by Bloor.

General properties of lipids

• Lipids are organic compounds composed mainly of carbon (C), hydrogen (H) and oxygen (O), but the hydrogen-to-oxygen ratio is not 2:1 as in carbohydrates; the oxygen content is relatively low.
• They are insoluble in water and soluble in organic solvents such as acetone, chloroform, benzene, ether and hot alcohol.
• In cells, lipids occur as minute globules in the protoplasm and in specialised storage tissues.
• Lipids do not form true polymers like proteins or polysaccharides; many are small to medium-sized molecules or aggregates.
• Lipids provide more than twice the energy per gram compared to carbohydrates and proteins when oxidised.
• In animals, fat is stored in the subcutaneous layer and functions as a food reserve, thermal insulator and shock absorber.
• Because lipid molecules are hydrophobic and compact, they require less space for storage than carbohydrates.
• Oxidation of lipids yields a large amount of metabolic water, more than carbohydrates or proteins.
• Lipids are not strictly macromolecules, but they often occur as aggregates or complex molecules important for cell structure and function.

Classification of lipids (overview)

• Simple lipids (neutral fats and waxes)
• Conjugated lipids (contain additional groups such as phosphate or carbohydrate; e.g., phospholipids, glycolipids)
• Derived lipids (substances derived from simple or conjugated lipids; e.g., steroids, terpenes, prostaglandins)

Simple lipids (Neutral fats)

Simple lipids are mainly esters of long-chain fatty acids with alcohol. The most common simple lipids in living systems are triglycerides (triacylglycerols), which are esters of three fatty acid molecules with the triol glycerol. Formation of a triglyceride involves three ester bonds and produces three molecules of water (dehydration synthesis).

• Triglyceride structure: glycerol + three fatty acids linked by ester bonds.
• Fatty acids in a triglyceride may be identical or different; thus fats are mixtures of different molecular species.

Fatty acids

Fatty acids are long-chain carboxylic acids with a general formula R-COOH, where R is a hydrocarbon chain. They vary in chain length and in the number and position of double bonds.

• Saturated fatty acids: hydrocarbon chain contains only single bonds and is saturated with hydrogen atoms. Examples:
  • Palmitic acid: CH₃(CH₂)₁₄COOH (C16)
  • Stearic acid: CH₃(CH₂)₁₆COOH (C18)
• Unsaturated fatty acids: contain one or more C=C double bonds. They are classified as:
  • Monounsaturated (one double bond) - e.g. oleic acid: CH₃(CH₂)₇CH=CH(CH₂)₇COOH
  • Polyunsaturated (more than one double bond) - e.g. linoleic acid, linolenic acid; arachidonic acid also is polyunsaturated
• Essential fatty acids: certain polyunsaturated fatty acids such as linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3) cannot be synthesised by many animals and must be obtained in the diet.
• Physical state: triglycerides containing mainly saturated fatty acids are usually solid at room temperature (called fats), while those containing unsaturated fatty acids are usually liquid (called oils).
• Health note: diets high in certain saturated fats are associated with raised blood cholesterol and greater risk of cardiovascular disease; increasing polyunsaturated fatty acids (while controlling total fat intake) may lower blood cholesterol levels.

Waxes

Waxes are esters of long-chain fatty acids with long-chain monohydroxy alcohols (not glycerol). They are more resistant to hydrolysis than triglycerides and function mainly in protection.

• Waxes form water-insoluble coatings on plant leaves, stems and fruits, and on animal skin, feathers and hair.
• Examples:
  • Beeswax (hexacosyl palmitate)
  • Carnauba wax (myricyl cerotate) - on leaves and fruits
  • Spermaceti - found in some marine mammals
  • Cerumen (ear wax) - in the external auditory canal
  • Lanolin - a wool-derived lipid (sometimes described as cholesterol ester components) used as a skin lubricant

Conjugated (compound) lipids

Conjugated lipids contain fatty acids plus other chemical groups such as phosphate, carbohydrates or proteins. They are important structural and functional components of cells.

Phospholipids (phosphatides)

Phospholipids are esters composed of two fatty acid molecules, glycerol, phosphoric acid (H₃PO₄) and a nitrogenous compound (such as choline or ethanolamine). They are major constituents of cell membranes.

• Phospholipids are amphipathic: they have a polar hydrophilic head (phosphate + nitrogenous group) and two non-polar hydrophobic tails (fatty acid chains).
• Because of this property, phospholipids spontaneously arrange into bimolecular layers (bilayers) in aqueous environments and form the basic structure of biological membranes.
• Important phospholipids include:
  • Lecithin (phosphatidylcholine): the nitrogenous group is choline. Lecithin occurs in egg yolk, oil seeds and blood. In blood it helps transport other lipids.
  • Cephalin (phosphatidylethanolamine): contains ethanolamine as the nitrogenous group. Cephalin is abundant in nervous tissue, egg yolk and blood platelets.
  • Sphingolipids (sphingomyelins): similar to phospholipids but contain sphingosine (an amino alcohol) instead of glycerol. They are important components of the myelin sheath around nerve fibres.
  • Other examples: phosphatidylserine, phosphatidylinositol and plasmalogens.

Glycolipids

Glycolipids contain fatty acids linked to a backbone (often sphingosine) and one or more carbohydrate residues. They occur especially in membranes of nerve cells.

• Cerebrosides: contain a single sugar (e.g. galactose) and are found in the white matter of the brain.
• Gangliosides: complex glycolipids found in nerve ganglia and spleen; they may contain N-acetylneuraminic acid (sialic acid) and other sugars.

Derived lipids

Derived lipids are substances derived from simple or conjugated lipids by hydrolysis or other reactions. They are chemically diverse and include steroids, terpenes (chromolipids), and eicosanoids (such as prostaglandins).

Steroids

Steroids share a common tetracyclic skeleton known as the cyclopentanoperhydrophenanthrene nucleus. They differ in the functional groups attached to this ring system.

• Sterols are alcoholic steroids. Example: cholesterol, a major sterol found in brain, nervous tissue, adrenal glands and skin. Cholesterol is a precursor for many biologically important steroids.
• 7-dehydrocholesterol in skin is a provitamin that, on exposure to ultraviolet radiation, is converted to cholecalciferol (vitamin D₃).
• Other steroid-related compounds:
  • Ergosterol: occurs in fungi and some plants; precursor of ergocalciferol (a form of vitamin D).
  • Coprosterol: formed by bacterial action on cholesterol and found in faecal matter.
  • Bile acids (e.g. cholic acid, lithocholic acid) aid in the emulsification and digestion of fats.
• Sterones (ketonic steroids) include sex hormones and adrenocorticoids. Some plant steroids (e.g. diosgenin from yam, Dioscorea) are used industrially in the synthesis of steroid drugs including antifertility agents.

Chromolipids (Terpenes)

Chromolipids, also called terpenes, are lipids built from repeating isoprene (C5) units. They form a wide range of biologically important molecules.

• Examples include carotenoids, vitamins A, E and K, and natural rubber (polyterpene).

Prostaglandins and other eicosanoids

• Prostaglandins are derived lipids synthesised from polyunsaturated fatty acids (PUFAs), especially arachidonic acid.
• They act as local signalling molecules and have roles in uterine and fallopian tube contraction, blood clotting, smooth muscle contraction, inflammation, and other physiological processes.

Biological significance and applications

• Structural: phospholipids and glycolipids are essential components of biological membranes and determine membrane fluidity and permeability.
• Energy storage: triglycerides are efficient long-term energy stores in adipose tissue.
• Insulation and protection: subcutaneous fat insulates against heat loss and acts as a mechanical cushion.
• Signalling: steroid hormones, prostaglandins and other lipid-derived molecules act as hormones or local mediators.
• Vitamins and pigments: lipid-related compounds include fat-soluble vitamins (A, D, E, K) and pigments (carotenoids).
• Industrial and medicinal uses: sterols and terpenes serve as raw materials for drug synthesis; waxes and oils have many commercial uses.

Summary

Lipids are a broad group of hydrophobic biomolecules that include triglycerides, phospholipids, glycolipids, waxes and derived compounds such as steroids, terpenes and prostaglandins. Fatty acids-classified as saturated or unsaturated-are the building blocks of many lipids. Phospholipids are amphipathic molecules that form the structural basis of cell membranes. Lipids serve multiple roles: energy storage, structural components of cells, insulation, protection and signalling molecules essential for normal physiology.