lip·id, [ˈlɪp ɪd, ˈlaɪ pɪd]
A fatty or waxy organic compound that is readily soluble in nonpolar solvent (e.g. ether) but not in polar solvent (e.g water), and whose major biological functions involve energy storage, structural component of cell membrane, and cell signaling
A biomolecule refers to any molecule that is produced by living organisms. As such, most of them are organic molecules. The four major groups of biomolecules include amino acids and proteins, carbohydrates (especially, polysaccharides), lipids, and nucleic acids. A lipid refers to any of the group of fatty or waxy organic compounds readily soluble in nonpolar solvent but not in polar solvent. Lipids are involved mainly in energy storage, structural component, and cell signaling. Examples of lipids are waxes, oils, sterols, cholesterol, fat-soluble vitamins, monoglycerides, diglycerides, triglycerides (fats), and phospholipids.
Lipids are organic compounds that are readily soluble in nonpolar solvent (e.g. ether) but not in polar solvent (e.g water). It is usually made up of glycerol or fatty acid units, with or without other molecules. Many lipids are amphiphilic, meaning they have both hydrophobic and hydrophilic components.
Lipids vs. Fats
Fatty acids (including fats) are a subgroup of lipids. In particular, a fat is a triglyceride (lipid) that is usually solid at room temperature. Hence, it will be inaccurate to consider the two terms synonymous.
A fatty acid is a subunit of fats, oils, and waxes. It pertains to any long chain of hydrocarbon, with a single carboxylic group at the beginning and a methyl end, and aliphatic tail. It is produced by the breakdown of fats (usually triglycerides or phospholipids) through a process called hydrolysis. It is represented by R-COOH, where R stands for the aliphatic moiety and COOH as the carboxylic group (making the molecule an acid). Fatty acids may be classified into (1) unsaturated fatty acid and (2) saturated fatty acids. The unsaturated fatty acids may be further grouped into monounsaturated fatty acid and polyunsaturated fatty acid. Examples of unsaturated fats are monounsaturated fatty acid, polyunsaturated fat, omega fatty acids, etc. Saturated fatty acids are fatty acids that lack unsaturated linkages between carbon atoms. Examples include lauric acid, palmitic acid, etc.
A glycerol is a type of lipid made up of a glycerol linked esterically to a fatty acid. One of the most commonly known glycerolipids is the triglyceride (also referred to as triacylglycerol). The triglyceride is an energy-rich compound consisting of a glycerol and three fatty acids (thus, the name). The three hydroxyl groups of glycerol in triglyceride are all esterified. The triglyceride is a major component of animal and plant oils and fats. In plants, they are typically found in plant cell membrane where the fatty acids are mostly unsaturated. In animals, the fatty acid component is largely saturated.
A glycerophospholipid, or simply phospholipid, is a type of lipid that is an essential component of many biological membranes, particularly the lipid bilayer of cells. It is also involved in metabolism and cell signaling. Phospholipids are amphipathic compounds in a way that the 'head' is hydrophilic and the lipophilic 'tail' is hydrophobic. Examples of phospholipids include phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, lecithin, plasmalogens and sphingomyelins.
A sphingolipid is a lipid made up of a sphingoid base (e.g. sphingosine and ceramides) backbone and sugar residue(s) linked by a glycosidic bond. Examples are cerebrosides and gangliosides.
A sterol lipid (e.g. cholesterol) is another type of lipid that serves as an essential component of biological membrane. Many of them act as hormones and signaling molecules.
A prenol lipid is a type of lipid that is synthesized from isopentenyl diphosphate and dimethylallul diphosphate via the mevalonic acid pathway. Simple isoprenoids (e.g. carotenoids) are prenol lipids.
Other types of lipids are saccharolipids (fatty acids linked to a sugar backbone) and polyketides (compounds formed by polymerization of acetyl and proprionyl subunits).
Common biological reactions
Lipids are biosynthesized by a process called lipogenesis. Lipogenesis is the process of producing lipid or fat. In biology, lipogenesis is a biochemical process, e.g. acetyl-CoA is converted to triglyceride. It is to store biochemical energy for later metabolic use. Lipogenesis includes (1) fatty acid synthesis and (2) triglyceride synthesis. Fatty acid synthesis occurs in the cytoplasm and characterized by the repeated addition of two-carbon units to acetyl-CoA. In triglyceride synthesis, three fatty acids are esterified to a glycerol in the endoplasmic reticulum. The cells that carry out lipogenesis are mostly adipocytes and liver cells. The liver cells, though, release triglycerides in the form of very-low-density lipoproteins (VLDL) into the bloodstream.
Apart from biosynthesis, lipids may also be obtained from the diet. Furthermore, animals that consume carbohydrates in excess tend to convert the carbohydrates into trigylcerides.
The process wherein lipid is broken down to extract energy is called lipolysis. Lipolysis is activated when the circulating insulin level is low whereas the circulating epinephrine is high. Fatty acids may be further degraded in the mitochondria or in the peroxisome, producing acetyl-CoA. This metabolic process in which fatty acids are degraded resulting in the formation of acetyl-coA is called beta oxidation. The acetyl-coA, in turn, may enter the citric acid cycle and degraded ultimately into CO2 and water molecules, with the concomitant generation of ATP.
The major biological functions of lipids involve energy storage, structural component of cell membrane, and cell signaling. In biological membranes, the lipid component has a hydrophilic head that may be a glycolipid, a phospholipid, or a sterol (e.g. cholesterol) and a hydrophobic tail.
- French lipide » Greek lipos ("fat")
- annular lipid
- lipid a
- lipid a 4-phosphatase
- lipid a disaccharide synthase
- lipid bilayer
- lipid granulomatosis
- lipid peroxidation
- lipid peroxides
- lipid pneumonia
- lipid profile
- ' lipidic (adjective)
- neutral lipid storage disease
© Biology Online. Content provided and moderated by Biology Online Editors