The complex organization and interaction of the chemicals within a cell confer the unique
characteristics of life. The key chemicals of life are water, certain small inorganic solutes such as
salts, and four categories of organic molecules: lipids, carbohydrates, amino acids and proteins,
and nucleotides and nucleic acids.
•Large organic complexes and macromolecules—membranes, polysaccharides, proteins, and
nucleic acids—are responsible for the main structures and functions of cells. Functions often
require that these complexes and macromolecules be flexible and dynamic, but in turn, they are
susceptible to disruption by some environmental factors such as temperature. Thus, homeostasis
of the interior environment in organisms in part evolved to protect the macromolecules,
Cells are the living building blocks of multicellular bodies. Body cells, which are too small to be
seen by the unaided eye, have been shown by microscopic techniques to consist of three major
subdivisions: (1) the plasma membrane, which encloses the cell and separates the intracellular
and extracellular fluid; (2) the nucleus, which contains deoxyribonucleic acid (DNA), the cell’s
genetic material; and (3) the cytoplasm, the portion of the cell’s interior not occupied by the
•The cytoplasm consists of cytosol, a complex gelatin-like mass, and organelles, which are highly
organized, membrane-enclosed structures dispersed within the cytosol.
NUCLEUS, CHROMOSOMES AND GENES
•The eukaryotic nucleus contains deoxyribonucleic acid, DNA, in linear strings called
chromosomes. DNA consists of a varying 4 nucleotides designated A, G, T, and C. The sequence
of a particular stretch of DNA, called a gene, contains a code for making proteins, and a
regulatory sequence for controlling the coding region of the gene.