algae simple, green organisms with cell walls, but without the complex structure of plants. amino acid organic chemicals with an acid group (-COOH) at one end and an amino group (-NH2) at the other; the constituent molecules of proteins. catalyst a substance that increases the rate of a chemical reaction without being used up in the reaction. DNA deoxyribose nucleic acid, the substance of the genes. fermentation an enzyme-controlled reaction that takes place in the absence of oxygen, such as the conversion of sugar to alcohol by the action of yeast. fertilization 1. the process of applying nutrients to the soil to stimulate plant growth. 2. the union of a sperm and an egg cell to form a zygote. ovary 1. in animals, an organ that produces egg cells. 2. the part of the stigma of a flower that contains the ovules. sedimentary rock rock formed by the consolidation of sediments such as sand or mud, or by precipitation from solution. semen the body secretion that carries sperms out. sperm a male gamete, typically self-propelled. zygote a cell formed by the fusion of two gametes.