Actin -- A protein found in muscle tissue that acts together with myosin as a factor in muscle contraction.

Actomyosin -- The system of actin and myosin that, with other substances, constitutes muscle fiber and is responsible for muscular contraction.

Addison's disease -- A disease caused by partial or total failure of adrenocortical function, which is characterized by a bronzelike pigmentation of the skin and mucous membranes, anemia, weakness, and low blood pressure.

Adrenal gland, or the suprarenal gland -- Either of two small endocrine glands, one located above each kidney, consisting of the cortex, which secretes several steroid hormones, and the medulla, which secretes epinephrine.

ATP, or Adenosine triphosphate -- Nucleoside triphosphate composed of adenine, ribose, and three phosphate groups that is the primary carrier of chemical energy in cells. The terminal phosphate groups are highly reactive in the sense that their hydrolysis, or transfer to another molecule, takes place with the release of a large amount of free energy.

Benzidine -- A yellowish, white, or reddish-gray crystalline powder used in dyes and to detect blood stains.

Bioflavonoids -- Any of a group of biologically active substances found in plants and functioning in the maintenance of the walls of small blood vessels in mammals.

Carbohydrate -- Any of a group of organic compounds that includes sugars, starches, celluloses, and gums and serves as a major energy source in the diet of animals. These compounds are produced by photosynthetic plants and contain only carbon, hydrogen, and oxygen, with twice as many hydrogen atoms as oxygen atoms.

Catalyst -- A substance, typically used in small amounts relative to the reactants, that modifies and increases the rate of a reaction without being consumed in the process.

Cell respiration -- The metabolic processes by which certain organisms obtain energy from organic molecules. These processes take place in the cells and tissues, release energy, and produce carbon dioxide that is absorbed by the blood and transported to the lungs.

Covalent bond -- A chemical bond formed by the sharing of one or more electrons, especially pairs of electrons, between atoms.

De-localized electrons -- Electrons that do not reside along a single bond, but move from bond to bond, as in an aromatic carbon bond.

Electron transfer, or charge transfer -- Movement of electrons from substrates to oxygen via the carriers of the respiratory chain.

Enzyme -- A protein molecule that catalyzes chemical reactions of other substances without itself being destroyed or altered by the reactions. Made up of a complex of amino-acids, enzymes are part of every chemical reaction in living things. They aid in digestion, the growth and building of cells, and all reactions involving transformation of energy. Inside the cell, enzymes create RNA and DNA by facilitating the reaction of ribose with adenosine. They also specify the sites for linking to build RNA along a DNA template. Each enzyme works only on one specific substance (called the substrate). Enzymes are usually designated by the suffix -ase.

Free radical -- An atom or group of atoms that has at least one unpaired electron and is therefore unstable and highly reactive. In animal tissues, free radicals can damage cells and are believed to accelerate the progression of cancer, cardiovascular disease, and age-related diseases.

Glycerol -- A small molecule with three alcohol groups. It is a basic building block of fats and oils.

Krebs cycle, or citric acid cycle -- A series of enzymatic reactions in aerobic organisms involving the oxidative metabolism of acetyl units and producing high-energy phosphate compounds, which serve as the main source of cellular energy.

Oxidation -- Oxidation is the loss of one or more electrons by an atom, molecule, or ion. Oxidation is accompanied by an increase in oxidation number on the atoms, molecules, or ions that lose electrons.

Peroxidase -- Any of a group of enzymes that occur especially in plant cells and catalyze the oxidation of a substance by a peroxide.

Redox reaction -- A reaction involving transfer of electrons from one substance to another. Redox reactions always involve a change in oxidation number for at least two elements in the reactants.

Reduction -- The gain of one or more electrons by an atom, molecule, or ion. Reduction is accompanied by a decrease in oxidation number.

Scurvy -- A disease caused by deficiency of vitamin C. It is characterized by spongy and bleeding gums, bleeding under the skin, and extreme weakness.

Vitamin C, or ascorbic acid -- A white, crystalline vitamin found in citrus fruits, tomatoes, potatoes, and leafy green vegetables. Vitamin C is vital to the production of collagen, the most common component of all connective tissue, and used to prevent scurvy.