Carbon is an element that is essential to living things. Due to its valence shell electron configuration , carbon can form 4 bonds. Additionally, carbon has an intermediate electronegativity, and therefore is most likely to form covalent bonds with other atoms.

There are different names for the various subsets of molecules that contain carbon. Substances that contain carbon and are synthesized by cells are referred to as organic compounds. Substances that contain only carbon and hydrogen are referred to as hydrocarbons. Finally, long chains of carbon-carbon bonds in organic molecules are referred to as carbon skeletons.

Although two molecules may contain the same number of carbon and hydrogen molecules, they may look very different. Molecules with the same chemical formula, but different chemical structures are called isomers. For example, consider C4H8:

Macromolecules are very large molecules that may consist of thousands of covalently connected atoms. There are four important classes of macromolecules: proteins, nucleic acids (including DNA), carbohydrates, and lipids (including fats). Most macromolecules are made by joining many identical or smaller molecular units, called monomers, together to form chains called polymers. A vast number of unique polymers can be made from a relatively small number of different monomers.

Monomers are joined together by cells to form polymers during a process called dehydration synthesis. During dehydration synthesis, a hydroxyl (OH) group is removed from one monomer and a hydrogen is removed from the other to join them together to form a polymer. During this process, water is produced (see below).

A disaccharide is a molecule consisting of two monosaccharides. It is formed by dehydration synthesis. A polysaccharide is a polymer consisting of a few hundred to a few thousand monosaccharides. Like disaccharides, polysaccharides are formed by dehydration synthesis. Starch is a common polysaccharide that plants use to store sugar energy. When the stored energy is needed, starch is broken down into glucose via the process of hydrolysis.

In animals, excess sugar is stored in the form of the polysaccharide glycogen. Glycogen is very similar to starch, although it contains many more branches. Glycogen stores are hydrolyzed when energy is needed.

Other polysaccharides such as cellulose can provide structural support for organisms. Cellulose fibrils help make wood strong enough to support trees that are hundreds of feet high. Most animals cannot hydrolyze cellulose, and therefore it passes through our digestive tract unchanged. Cellulose is commonly known as fiber.

Lipids are molecules composed mainly of carbon and hydrogen atoms. Bonds between carbon and hydrogen atoms are nonpolar. Therefore, lipids are hydrophobic (water-fearing) and do not mix with water. A fat is a type of large lipid molecules that consists of a combination of glycerol and fatty acids. Dehydration synthesis links three fatty acids to a glycerol molecule to form a triglyceride, or fat molecule.

In addition to fats, there are three other important types of lipids: phospholipids, waxes, and steroids. Although phospholipids are very similar to fats, they only have two fatty acids and contain the element phosphorous. Phospholipids are a major component of cell membranes.

Waxes are molecules that consist of one fatty acid connected to an alcohol. Waxes are even more hydrophobic than fats, and often form a coat over fruits and even insects to help keep them from drying out.

Steroids are lipids with four fused carbon rings. Cholesterol is an example of a steroid, and is used by the body as the starting material for making female and male sex hormones such as testosterone. We often hear about athletes and steroids. The steroids that we hear about associated with athletics are called anabolic steroids. Anabolic steroids are synthetic variants of testosterone. By taking these steroids, an athlete can quickly build muscle mass with little work. However, the health hazards of these steroids are numerous. Possible side effects include:

• Reduced sperm count
• Development of breasts in men
• Shrinking of the testicles
• Facial hair growth in women
• Deepened voice in women
• Acne
• Rapid weight gain
• Liver Damage