Biomolecules - Lipids

Biological substances that are insoluble in water are classified as lipids. This characteristic physical property of lipids makes them very different from other biomolecules like carbohydrates, proteins, and nucleic acids. The structure of lipids are extremely varied. In popular culture, fats are synonymous with lipids, giving lipids a negative role in diet and health. However, lipids play vital roles in many cellular processes including energy storage, structural support, protection, and communication. Common lipid groups include waxes, steroids, triglycerides, and phospholipids.

The function of lipids are very diverse. Some lipids are used to store energy. In addition to serving as energy sources, some lipids accumulate in tissues around to act as insulation and protection. Other lipids carry chemical messages and help regulate cell activity from one part of the body to another. Another group forms cellular barriers that selectively allow molecules in and out of cells.

One type of lipid building block is a fatty acid. Fatty acids consist of an carboxyl acid group (-COOH) at the end of a chain of at least 4 carbon atoms. These carbon chains in fatty acids are hydrophobic, meaning they are insoluble in water. The acid group at the end of the fatty acid is hydrophilic which means it likes to interact with water molecules and in soluble in water. Unlike other biomolecule groups, fatty acid monomers are not directly bonded to each other to form long chains. Also, not all lipid groups contain fatty acids.

Fats and oils are types of triglycerides. Fats are solid, are found primarily in animal tissues. Fats can be used by animals to store energy, provide insulation, protect sensitive organs inside their bodies. Oils are liquids, obtained usually from plants. Plants first store energy in the form of carbohydrates but another option for plants is to store excess energy in the forms of oils. This is particularly done in plant seeds. A plant is able to store more energy per molecule in lipids than in carbohydrates.

Waxes

Waxes are a class of lipids that contain two monomers, one fatty acid bonded through an ester linkage to one alcohol (a hydrocarbon containing a hydroxyl group). The hydrocarbon chain in the alcohol monomer of waxes varies from a short linear chain to complex carbon ring structures. Waxes provide protective barriers to prevent water loss and protect cells. Waxes protect seeds and nutrients inside plant fruits and coat the surface of plant leaves, forming a cuticle to prevent water loss. Bees synthesize beeswax honeycombs for storing food and protecting offspring. Waxes prevent dehydration from body surfaces of many insects and repel water on the surface of bird feathers and some animal furs.

Steroids

Steroids are a class of lipids containing four connected carbon rings. Although steroids can bond to fatty acids, steroid molecules do not contain a fatty acid chain, and the monomer of a steroid biomolecule is difficult to define. Steroid rings usually contain a few small functional groups including hydroxyls (-OH) or carboxyls (-COO). Cholesterol and other steroids containing a hydroxyl group are called sterols. Cholesterol and related sterols are present in animal cell membranes and are precursors for the synthesis of many vital steroids and other sterol derivatives.

Many steroids and their derivatives perform vital cellular functions. Steroid hormones such as estrogen and testosterone control reproductive processes and development. Bile salts and fat-soluble vitamins are lipids derived from cholesterol and related lipid molecules. Scientists modify steroids in laboratories, synthesizing medical drugs that work by mimicking natural compounds in the human body. Anabolic steroids, a specific class of artificially manufactured steroid drugs, stimulate muscle growth and increased development of secondary sex characteristics. In individuals with metabolic diseases, anabolic steroids can improve health by restoring normal signals, but anabolic steroid use by otherwise healthy individuals can be extremely harmful to internal organ function.

Phospholipids

One last kind of lipids is a phospholipid. This type of lipids has some similarities to triglycerides. Phospholipids have a glycerol backbone which is bonded to fatty acids. The difference is that in a phospholipid there are only 2 fatty acid chains not three like in the triglyceride. Phospholipids like triglyceride have hydrophobic carbon chains and a hydrophillic "head".