Pellet: particles and organelles Ribosomes, storage granules, mitochondria, chloroplasts, lysosomes, endoplasmic reticulum.
Cells Are the Structural and Functional Units of All Living Organisms
Cells of all kinds share certain structural features (Fig. 1-3). The plasma membrane defines the periphery of the cell, separating its contents from the surroundings. It is composed of lipid and protein molecules that form a thin, tough, pliable, 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 some reaction pathways. Because the individual lipids and proteins of the plasma membrane are not covalently linked, the entire structure is remarkably flexible, allowing changes in the shape and size of the cell. As a cell grows, newly made lipid and protein molecules are inserted into its plasma membrane; cell division produces two cells, each with its own membrane. This growth and cell division (fission) occurs without loss of membrane integrity.
FIGURE 1-3 The universal features of living cells. All cells have a nucleus or nucleoid, a plasma membrane, and cytoplasm. The cytosol is defined as that portion of the cytoplasm that remains in the supernatant after centrifugation of a cell extract at 150,000 g for 1 hour.
The internal volume bounded by the plasma membrane, the cytoplasm (Fig. 1-3), is composed of an aqueous solution, the cytosol, and a variety of suspended 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, compounds essential to many enzyme-catalyzed reactions; inorganic ions; and ribosomes, small particles (composed of protein and RNA molecules) that are the sites of protein synthesis.
All cells have, for at least some part of their life, either a nucleus or a nucleoid, in which the genome—
the complete set of genes, composed of DNA—is stored and replicated. The nucleoid, in bacteria, is not separated from the cytoplasm by a membrane; the nucleus, in higher organisms, consists of nuclear material enclosed within a double membrane, the nuclear envelope. Cells with nuclear envelopes are called eukaryotes (Greek eu, "true," and karyon, "nucleus"); those without nuclear envelopes—bacterial cells—are prokary-otes (Greek pro, "before").
Most cells are microscopic, invisible to the unaided eye. Animal and plant cells are typically 5 to 100 ^m in diameter, and many bacteria are only 1 to 2 ^m 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 substantial fraction of the volume in a mycoplasmal cell.
The upper limit of cell size is probably set by the rate of diffusion of solute molecules in aqueous systems. For example, a bacterial cell that depends upon oxygen-consuming reactions for energy production must obtain molecular oxygen by diffusion from the surrounding medium through its plasma membrane. The cell is so small, and the ratio of its surface area to its volume is so large, that every part of its cytoplasm is easily reached by O2 diffusing into the cell. As cell size increases, however, surface-to-volume ratio decreases, until metabolism consumes O2 faster than diffusion can supply it. Metabolism that requires O2 thus becomes impossible as cell size increases beyond a certain point, placing a theoretical upper limit on the size of the cell.
All living organisms fall into one of three large groups (kingdoms, or domains) that define three branches of evolution from a common progenitor (Fig. 1-4). Two large groups of prokaryotes can be distinguished on biochemical grounds: archaebacteria (Greek arche, "origin") and eubacteria (again, from Greek eu, "true"). Eubacteria inhabit soils, surface waters, and the tissues of other living or decaying organisms. Most of the well-studied bacteria, including Escherichia coli, are eu-bacteria. The archaebacteria, more recently discovered, are less well characterized biochemically; most inhabit extreme environments—salt lakes, hot springs, highly acidic bogs, and the ocean depths. The available evidence suggests that the archaebacteria and eubacteria diverged early in evolution and constitute two separate
FIGURE 1-4 Phylogeny of the three domains of life. Phylogenetic relationships are often illustrated by a "family tree" of this type. The fewer the branch points between any two organisms, the closer is their evolutionary relationship.
(carbon from CO2)
Examples: •Cyanobacteria •Plants
(carbon from organic compounds)
•Purple bacteria •Green bacteria
(energy from chemical compounds)
Was this article helpful?