The unity and diversity of organisms become apparent even at the cellular level. The smallest organisms consist of single cells and are microscopic. Larger, multicellular organisms contain many different types of cells, which vary in size, shape, and specialized function. Despite these obvious differences, all cells of the simplest and most complex organisms share certain fundamental properties, which can be seen at the biochemical level. Cells Are the Structural and Functional Units of All Living Organisms Cells of all kinds share certain structural features. The plasma membrane defines the periphery of the cell and separates its contents from the environment. It is made up of lipid and protein molecules that form a thin, tough, flexible hydrophobic barrier around the cell. The membrane is a barrier to the free passage of inorganic ions and most other charged or polar compounds. Transport proteins in the plasma membrane allow the passage of certain ions and molecules; Receptor proteins transmit signals into the cell; and membrane enzymes participate in several pathways. Since individual plasma membrane lipids and proteins are not covalently bound, the entire structure is remarkably flexible and allows for changes in cell size and shape. As a cell grows, newly formed lipid and protein molecules insert into its plasma membrane; Cell division creates two cells, each with its own membrane. This cell growth and division occurs without loss of membrane integrity.
The internal volume bounded by the plasma mem brane, the cytoplasm, is composed of an aqueous solution, the cytosol, and a variety of sus pended particles with specific functions. The cytosol is a highly concentrated solution containing enzymes and the RNA molecules that encode them; the components (amino acids and nucleotides) from which these macro molecules are assembled; hundreds of small organic molecules called metabolites, intermediates in biosyn thetic and degradative pathways; coenzymes, com pounds essential to many enzyme-catalyzed reactions; inorganic ions; and ribosomes, small particles (com posed of protein and RNA molecules) that are the sites of protein synthesis.
All cells have, for at least some part of their life, ei ther a nucleus or a nucleoid, in which the genomehe complete set of genes, composed of DNA-is stored and replicated. The nucleoid, in bacteria, is not sepa rated from the cytoplasm by a membrane; the nucleus, in higher organisms, consists of nuclear material en closed within a double membrane, the nuclear envelope. Cells with nuclear envelopes are called eukaryotes (Greek eu, “true,” and karyon, “nucleus”); those with out nuclear envelopes-bacterial cells are prokary otes (Greek pro, “before”).
Cellular Dimensions Are Limited by Oxygen Diffusion
Most cells are microscopic, invisible to the unaided eye. Animal and plant cells are typically 5 to 100 um in di ameter, and many bacteria are only 1 to 2 um long (see the inside back cover for information on units and their abbreviations). What limits the dimensions of a cell? The lower limit is probably set by the minimum number of each type of biomolecule required by the cell. The smallest cells, certain bacteria known as mycoplasmas, are 300 nm in diameter and have a volume of about 10-14 mL. A single bacterial ribosome is about 20 nm in its longest dimension, so a few ribosomes take up a sub stantial fraction of the volume in a mycoplasmal cell.