A cell is a pool of controlled chemical composition bounded by an outer membrane. It is the main structure of undifferentiated single-celled organisms and the essential building block of highly complex multicellular animals composed of many different cell types. A cell (1) sequesters biological resources relative to the outside world, and also internally; (2) maintains the necessary concentrations of chemical components, pH, and so forth; (3) localizes, transports, exports, and imports select molecules; (4) uses selected, controlled, context-dependent subsets of its genes and controls whether gene products are kept local, for use by this cell, or are sent outside the cell; (5) allows differentiation from the surrounding medium, and (6) provides a building-block mechanism by which life can evolve more complex traits.
Although there is extensive variation in cells found in the biosphere, generalizations can be made about them and hence about life, a fact basic to our understanding of how life works and evolved. These generalizations have placed at least some constraints on what has evolved and on what can or will evolve in the future. All known organisms (except very primitive "life" forms like viruses and prions, which, although not cells themselves, depend on cells to replicate and to continue to exist) are composed of one of two basic cell types, prokaryote and eukaryote, shown schematically in Figure 6-1. Figure 6-2 provides a detail of the cell membrane. See Table 6-1 for details of the structure of prokaryotes and eukaryotes.
All cells use the DNA-RNA coding system for replication and for coding proteins. All cells are bounded by lipid membranes, although the composition of the membrane varies across cell types. All cells use ATP (adenine 5'-triphosphate) as their source of energy for carrying out metabolic processes. They do this by glycol-ysis, the anaerobic (in the absence of oxygen) breakdown of glucose into lactic acid with a net gain of ATP molecules, or by photosynthesis, the conversion of sunlight into energy either nonoxidatively or oxidatively with a net gain of many more ATP molecules than glycolysis yields. Actually, as we currently understand early life, the evolution of these processes both depended on and drastically altered the Earth's atmosphere, and knowledge about how particular cells make ATP and carry out metabolic processes such as movement, synthesis of various cellular constituents, and the like tells us something about the evolution of life itself. Because much of cellular physiology ultimately rests on basic energy metabolism and that is highly constrained at the molecular level, some of the basic nature or constraints may be predictable solely on chemical grounds (Morowitz et al. 2000).
Beyond these generalities, prokaryotes and eukaryotes are quite different. Prokaryotes, the simpler of the two, are generally much smaller. Two distinct groups of prokaryotes are known: bacteria and archaea. Bacteria are single spherical or rod-shaped cells, found living in almost every known niche—in the water and on, above, and deep below the surface of the earth. Often found protruding from the cell wall are flagella, which allow the bacterium to move, and pili, which are flagella-like, but are involved in the transfer of genetic information between cells or in anchoring the cell to other cells. Archaea have some features in common with Eukarya and some with bacteria. They are often found in extreme environments— intense heat, cold, salinity, alkalinity, acidity, and the like.
Nuclear membrane Mitochondrion
Nuclear membrane Mitochondrion
Prokaryotes typically are bounded by a tough cell wall. However, among eukary-otes, plants, fungi, and algae do have cell walls, whereas animal and protozoan cells lack them. Bacterial cell walls are complex, but generally are composed of a type of petidoglycan called murein, a complex polysaccharide, whereas archaeal cell walls are made of protein, glycoprotein, and carbohydrate or pseudomurein, but never petidoglycan. Plant cell walls are made of cellulose and other polymers, and fungal cell walls are different still. The common characteristic of these cell walls, even if they differ in composition, is that they provide rigid structural support for the organism.
Within the cell wall is a plasma membrane, composed of lipids, mainly phospho-lipids and cholesterol, and proteins (see Figure 6-2). In bacteria, the lipids are saturated or monounsaturated. The lipids in archaeal membranes are quite different from those of either bacteria or eukaryotes. Among other things, they are polyunsaturated. The specific components of these membranes are probably what allow archaea to live in the extreme conditions in which they are found. In all cells,
Ion-gated channel ■
Seven transmembrane receptor
Figure 6-2. Schematic of constituents of prokaryote and eukaryote cell membrane/wall. The elaboration of the original bilipid membrane now enables cells to undertake a complex variety of functions, and provides sophisticated levels of sequestration within the cell itself.
no matter what their composition, the plasma membrane encloses the genomic material, proteins, and small molecules, all of which float free in the cytoplasm of prokaryotic cells, but are encased in a variety of organelles in eukaryotes.
In prokaryotes, the genome consists of one circular molecule wound tightly to form the nucleoid. Also in the cytoplasm are ribosomes, which are composed of ribosomal RNA and associated proteins and which translate RNA into proteins; inclusion bodies, which are globules of starch, glycogen or lipid, and stored nutrients; and metachromatic granules, which are storage sites for phosphate. Some bacteria also have magnetosomes in their cytoplasm, which help them orient in their environment.
Many of the metabolic processes of the prokaryotic cell take place on the cell membrane, including energy metabolism and waste removal. Other processes, such as protein synthesis and glycolysis, occur in the cytoplasm.
An important class of substructures in prokaryotic cells, and occasionally in lower eukaryotes, is plasmids, small circular extra-chromosomal DNA molecules found in many bacterial and archaeal cells. They contain genes for proteins that a bacterium can generally function and replicate without, although plasmids are also replicated when the cell divides. They provide functions like virulence factors, which are important for pathogenesis in some bacteria, and they can code for antibiotic resistance,
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