Lipids
Lipids are biological compounds that are soluble in inorganic solvents abut insoluble in water. The hydrocarbon back chain confers the hydrophobic characteristics in the compounds. Lipids exist in various forms in organisms varying in structure and function. Triglycerides are used as energy suppliers in the body, while phospholipids and glycolipids form part of the cell membranes. Steroids derived from cholesterol make hormones such as estrogen and testosterone, vitamin D, and bile salts. Inflammatory responses are initiated by signaling molecules derived from eicosanoids. Lipids are long-chain straight-chain hydrocarbons with a carboxylic acid in one end. They are classified by the presence or absence of double bonds, with saturated fatty acids have no carbon-carbon double bonds, while unsaturated forms have one or more double bonds. Monounsaturated fatty acids, such as Oleic acid, are monounsaturated fatty acids. Polyunsaturated fatty acids have more than one double bond, such as linoleic acid.
The shape of fatty acids is determined by the nature of the double carbon- carbon bond. Cis double bonds create a kink in the otherwise linear configuration seen in saturated and trans unsaturated fatty acids. The shape of the fatty acid determines the physical properties. The melting point is influenced by the number of carbon particles in the chain with fatty acids with more carbons having greater dispersion forces. More heat is required to break the intermolecular forces of attraction. The higher the number of cis bonds in a fatty acid, the lower the melting point. Cis bonds create kinks in the chain, reducing the dispersion forces, consequently reducing the meting point. The body cannot synthesize essential fatty acids from metabolic intermediates. They include linoleic and linolenic acids found in fish and walnuts.
Triglycerides are the most abundant lipids in the diet. They are chemically made of three fatty acids esterified to one glycerol molecule forming three ester groups. They are non-polar and insoluble in water. Triglycerides derived from saturated fatty acids are solids at room temperature, referred to as fats. In contrast, those derived from unsaturated fatty acids are liquid at room temperature, referred to as oils. Fats have tighter intermolecular packing with more vital intermolecular forces increasing their melting points. Fatty acid metabolism is the central energy pathway in many cells producing 80% of heart and liver cells’ energy. Fatty acids contain the highest form of energy when compared to carbohydrates and proteins. Digestion starts in the small intestines, where emulsification of triglycerides occurs. Bile sails released from the gall bladder break down the fat globules into small round micelles, increasing the fat exposed to water-soluble lipase enzymes, which catalyze the breakdown into glycerol and fatty acids. The products cross the intestinal mucosa and are esterified to form triglycerides, packed into chylomicrons, finally entering the lymphatic system to be recirculated to the liver through the hepatic portal system. They are stored in adipocytes for use when needed. The transport of triglycerides relies on lipoproteins, spherical assemblies of proteins and lipids with hydrophobic centers and hydrophilic exteriors. They ferry cholesterol in high density and low-density form to target organs.
Cell membranes are composed of glycolipids and phospholipids. They are amphipathic, displaying both hydrophobic and hydrophilic characteristics. The polar head is the phosphate group and the positively charged nitrogen in a phospholipid or the carbohydrate in a glycolipid. The non-polar tails are derived from fatty acids and, in the case of a sphingolipid, fatty acid and sphingosine. Phospholipids are characterized by a polar phosphate group linked to the terminal hydroxyl group of glycerol to sphingosine in the cell membrane. A second phosphate ester bond is formed between the hydroxyl group and the amine group of a small amino alcohol, mostly serine, choline, or ethanolamine. This results in either glycerophospholipid or sphingophospholipid. Glycolipids contain a carbohydrate joined between the anomeric carbon and the terminal hydroxyl group of sphingosines. The carbohydrate groups can be mono, di, or oligosaccharides. Glycosphingolipids containing monosaccharides are referred to as cerebrosides. The cell membrane layer is 7nm wide and is described as the fluid mosaic because the glycolipid and phospholipid layers interact non-covalently through forces of attraction. Cell membranes with more saturated phospholipids are less permeable to the passage of ions than those with more unsaturated fatty acids.
Cholesterol is an essential steroid in the body playing structural and precursor roles in body cells. Bile salts such as cholate used in the emulsification of fats and steroid hormones are synthesized from cholesterol. Glucocorticoids are steroid hormones synthesized from the adrenal cortex. Thy regulates glucose metabolism and has anti-inflammatory activity treating chronic conditions such as asthma and rheumatoid arthritis. Mineralocorticoids are synthesized from the adrenal cortex and regulate electrolyte balance in the body. Aldosterone promotes retention of water and reabsorption of sodium ions in the kidney regulating blood pressure. Sex hormones, including progestins, estrogen, and androgens, influence the reproductive system coordinating maturation and development of sexual characteristics.