The body can be studied from its simplest to its most complex level, beginning with the cell, the basic unit of living organisms. Cells carry out metabolism, the sum of all of the physical and chemical activities that occur in the body. Providing the energy for metabolic reactions is the chemical ATP (adenosine triphosphate), commonly described as the energy compound of the cell. The main categories of organic compounds in the body are:

• Proteins, which include the enzymes, some hormones, and structural materials.

• Carbohydrates, which include sugars and starches. The main carbohydrate is the sugar glucose, which circulates in the blood to provide energy for the cells.

• Lipids, which include fats. Some hormones are derived from lipids, and adipose (fat) tissue is designed to store lipids.

Within the cytoplasm that fills the cell are subunits called organelles, each with a specific function. The main cell structures are named.

All body functions derive from the activities of billions of specialized cells. The nucleus is the control region of the cell. It contains the chromosomes, which carry genetic information. Each human cell, except for the sex cells, contains 46 chromosomes. The chromosomes are composed of a complex organic substance, DNA (deoxyribonucleic acid), which is organized into separate units called genes. Genes control the formation of enzymes, the catalysts needed for metabolic reactions. To help manufacture enzymes, the cells use a compound called RNA (ribonucleic acid), which is chemically related to DNA.

When a body cell divides, by the process of mitosis, the chromosomes are doubled and then equally distributed to the two daughter cells. Sex cells (egg and sperm) divide by another process (meiosis) that halves the chromosomes in preparation for fertilization.

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