Carbohydrates
These molecules consist of carbon, hydrogen, and oxygen in a ratio of roughly 1:2:1. If a test question involves identifying a compound as a carbohydrate, count the atoms and see if they fit that ratio. Carbohydrates are formed by the chemical reaction process of condensation, or dehydration synthesis, and broken apart by hydrolysis, the cleavage of a chemical by a reaction that adds water. There are several subcategories of carbohydrates:
Monosaccharides, also called monomers or simple sugars, are the building blocks of larger carbohydrate molecules and are a source of stored energy. Key monomers include glucose (also known as blood sugar), fructose, and galactose.
These three have the same numbers of carbon (6), hydrogen (12), and oxygen (6) atoms in each molecule — formally written as C6H12O6 — but the bonding arrangements are different. Molecules with this kind of relationship are called isomers. Two important five-carbon monosaccharides (pentoses) are ribose, a component of ribonucleic acids (RNA), and deoxyribose, a component of deoxyribonucleic acids (DNA).
Disaccharides, or dimers, are sugars formed by the bonding of two monosaccharides, including sucrose (table sugar), lactose, and maltose.
Oligosaccharides (from the Greek oligo, a few, and sacchar, sugar) contain three to nine simple sugars that serve many functions. They are found on plasma membranes of cells where they function in cell-to-cell recognition.
Polysaccharides, or polymers, are formed when many monomers bond into long, chainlike molecules. Glycogen is the primary polymer in the body; it breaks down to individual monomers of glucose, an immediate source of energy for cells.
Lipids
Commonly known as fats, these molecules contain carbon, hydrogen, and oxygen, and sometimes nitrogen and phosphorous. Insoluble in water because they contain a preponderance of nonpolar bonds, lipid molecules have six times more stored energy than carbohydrate molecules. Upon hydrolysis, however, most fats form glycerol and fatty acids.
A fatty acid is a long, straight chain of carbon atoms with hydrogen atoms attached. If the carbon chain has its full number of hydrogen atoms, the fatty acid is saturated (examples include butter and lard). If the carbon chain has less than its full number of hydrogen atoms, the fatty acid is unsaturated (examples include margarine and vegetable oils). All fatty acids contain a carboxyl or acid group, –COOH, at the end of the carbon chain.
Phospholipids, as the name suggests, contain phosphorus and often nitrogen and form a bilayer in the cell membrane. Steroids are fat-soluble compounds such as vitamins A or D and hormones that often serve to regulate metabolic processes.
Proteins
Among the largest molecules, proteins can reach molecular weights of some 40 million atomic units. Proteins always contain the four HONC elements — hydrogen, oxygen, nitrogen, and carbon — and sometimes contain phosphorus and sulfur. The human body builds protein molecules using 20 different kinds of smaller molecules called amino acids.
Each amino acid molecule is composed of an amino group, –NH2, and a carboxyl group, –COOH, with a carbon chain between them. Amino acids link together by peptide bonds to form long molecules called polypeptides, which then assemble into proteins. These bonds form when the carboxyl group of one molecule reacts with the amino group of another molecule, releasing a molecule of water (dehydration synthesis reaction).
Examples of proteins in the body include antibodies, hemoglobin (the red pigment in red blood cells), and enzymes (catalysts that accelerate reactions in the body).
Nucleic acids
These long molecules, found primarily in the cell’s nucleus, act as the body’s genetic blueprint. They’re comprised of smaller building blocks called nucleotides. Each nucleotide, in turn, is composed of a five-carbon sugar (deoxyribose or ribose), a phosphate group, and a nitrogenous base.
The nitrogenous bases in DNA (deoxyribonucleic acid) are adenine, thymine, cytosine, and guanine; they always pair off A-T and C-G. In RNA (ribonucleic acid), which occurs in a single strand, thymine is replaced by uracil, so the nucleotides pair off A-U and C-G.
In 1953, James Watson and Francis Crick published their discovery of the three-dimensional structure of DNA — a polymer that looks like a ladder twisted into a coil. They called this structure the double-stranded helix.
