Macromolecules Found in Living Organisms

• All living organisms, from bacteria to humans to elephants, contain complex chemical compounds called macromolecules. These macromolecules can contain hundreds of thousands of atoms, including carbon, hydrogen, nitrogen and oxygen, as well as other, less common elements. Macromolecules form from smaller molecules called monomers, which enzymes help to polymerize (i.e., combine into long chains).

Nucleic Acids

• Genes contain deoxyribonucleic acid, or DNA, which is famous for its double helix configuration. Ribonucleic acid, or RNA, transcribes information encoded in DNA and is also essential to protein synthesis. Monomers called nucleotides are responsible for assembling DNA and RNA. A nucleotide consists of a sugar unit, a phosphate unit, and a chemical ring structure called a heterocyclic base. DNA contains four kinds of nucleotides: adenine, cytosine, guanine and thymine. RNA contains the first three, but has uracil instead of thymine.

Proteins

• Through the action of monomers called amino acids, proteins polymerize into short chains called peptides, or longer chains called polypeptides. Twenty amino acids occur in nature. A peptide comprising only eight amino acids has more than 25 billion possible amino-acid combinations. This huge number of possible sequences accounts for the astounding variety of functions that proteins play in practically every process of life. Proteins are structural components of cell membranes, and of skin, bone and muscle. They are the enzymes that catalyze almost every biochemical reaction. They also enable signaling within and among cells, including nerve cells, cells involved in hormone synthesis and suppression, cells that regulate growth and differentiation, and cells involved in immune response.

Carbohydrates

• Carbohydrates, which derive from plant photosynthesis, form from monosaccharides (i.e., simple sugars like glucose and fructose). Cells get energy from monosaccharides, or through the breakdown of disaccharides like lactose (milk sugar) and sucrose (cane sugar). Plants store these compounds by polymerizing them into long chains called polysaccharides, or starches. Animals store them in the liver as glycogen. Other polysaccharides comprise the cellulose structures of plants, and the chitin-based structures of fungi, insects and crustaceans. Part-carbohydrate compounds called glycoproteins and glycolipids carry signals, including immune system responses, among cells.

Lipids

• Although lipid compounds are highly diverse, they share an inability to mix with water. The three main classes of lipids are steroids, triglycerides (or triacylglycerols), and phospholipids. Steroids include hormones like estrogen and cortisol, as well as cholesterol, which serves as a hormone precursor and helps to keep animal cell membranes flexible. Triglycerides store energy more efficiently than starch, and comprise the myelin that surrounds nerve cells. Phospholipids are essential to cell membrane structure.

Tetrapyrroles

• While not as large as many macromolecules, tetrapyrroles are indispensible to life. Each tetrapyrrole molecule contains four chemical ring structures called pyrroles. The four pyrrolic subunits coordinate spatially around a metal atom. In the case of chlorophyll, which colors plants green and makes photosynthesis possible, the metal is magnesium. In heme, which ferries oxygen to animal cells and colors blood red, it is iron. In Vitamin B12, it is cobalt.