This section details the properties and biological/industrial roles of four major classes of lipids: acylglycerols (fats and oils), phospholipids, terpenes, and waxes.
Acylglycerols are esters formed from glycerol and fatty acids. Triacylglycerols (triglycerides) are the most common type.
- Density: They have a lower density than water (i.e., they float).
- Solubility: Insoluble in water but soluble in organic solvents like chloroform, ether, and hot alcohol.
- Melting Point: The melting point is determined by the fatty acid chains.
- Acylglycerols with saturated and long-chain fatty acids have higher melting points (e.g., tristearin melts at 71°C).
- Those with unsaturated and short-chain fatty acids have lower melting points (e.g., triolein melts at -17°C).
- Emulsification: They possess great emulsifying power, allowing them to form stable mixtures with immiscible liquids like water.
- Hydrolysis: When heated in an acidic aqueous medium, they break down into their constituent parts: glycerol and fatty acids.
- Oxidation: Can undergo oxidation upon decay, producing compounds with unpleasant odors and flavors (rancidity).
- Energy Storage: Triacylglycerols are the major lipids in fat deposits and serve as a primary long-term energy reserve in the body. They yield twice the energy compared to carbohydrates.
- Food Source: They are a major component of lipids found in food.
- Health Implications: High levels in the body can lead to diseases such as obesity, diabetes, and hyperlipoproteinemia.
- Industrial Use: The hydrogenation (addition of hydrogen) of unsaturated acylglycerols is used to produce margarine and butter.
Phospholipids are a class of lipids whose molecule has a "head" containing a phosphate group, and two "tails" derived from fatty acids, joined by a glycerol molecule.
- Amphipathic Nature: Phospholipids are amphipathic, meaning they have a hydrophilic (polar, water-attracting) head and a hydrophobic (non-polar, water-repelling) tail. This unique property is fundamental to their function.
- Membrane Structure: This dual nature allows them to form lipid bilayers, which are the basic structure of all cell membranes. They help anchor proteins within the membrane.
- Biological Fluids: They are important components of bile (which aids in fat digestion) and lipoproteins (which transport fats in the blood).
- Structural: They are primary structural components of the cell membrane. For more on membrane architecture, see Cell Wall and Plasma Membrane→.
- Permeability Regulation: They control the passage of substances into and out of the cell by regulating membrane permeability.
- Metabolism & Transport:
- Involved in the absorption of fat from the intestine.
- Play a role in the electron transport chain (ETC) in mitochondria for ATP production.
- Help transport and remove cholesterol from cells.
- Inhibit the accumulation of fats in the liver.
- Other Roles:
- Act as surfactants in the lungs, preventing alveolar collapse.
- Aid in the coagulation of blood.
- Serve as precursors for the synthesis of molecules like lipoproteins and prostaglandins.
Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers. They are constructed from repeating units of isoprene (C5H8).
Terpenes are known for a wide range of medicinal and biological properties:
- Anti-inflammatory and analgesic (pain-relieving)
- Antioxidant, anticonvulsive, and depressive (calming)
- Anticancer, antitumor, and anti-mutagenic (protect against cancer and DNA damage)
- Neuroprotective (protect nerve cells)
- Antiallergic and antibiotic
- Anti-diabetic activities
- Natural Rubber: A well-known terpene polymer.
Waxes are esters of long-chain fatty acids with long-chain monohydric alcohols. They are extremely hydrophobic and solid at a wide range of temperatures.
Waxes are primarily used for their protective, water-repellent properties in both nature (e.g., coating on leaves) and industry.
- Protective Coatings: Used in polishing, waterproofing, and anti-fouling applications. They prevent water loss in plants and provide a barrier against pathogens.
- Commercial Products: A key ingredient in cosmetics, crayons, candles, and car waxes.
- Industrial Applications: Used in the production of paints, inks, adhesives, and tyres.
| Lipid Type | Primary Function/Role | Key Property |
|---|
| Acylglycerol | Long-term energy storage | Insolubility in water, high energy density |
| Phospholipid | Structural component of cell membranes | Amphipathic nature |
| Terpene | Diverse medicinal and biological activities | Often aromatic, biologically active |
| Wax | Waterproofing, protection, industrial applications | Highly hydrophobic, solid at room temp. |