Chapter 1 The Foundations of Biochemistry the complete set of genes, composed of DNA--is stored molecular oxygen by diffusion from the surrounding and replicated. The nucleoid, in bacteria, is not sepa- medium through its plasma membrane. The cell is so rated from the cytoplasm by a membrane; the nucleus, small, and the ratio of its surface area to its volume is in higher organisms, consists of nuclear material en- so large, that every part of its cytoplasm is easily reached closed within a double membrane the nuclear envelope. by O2 diffusing into the cell. As cell size increases, how- Cells with nuclear envelopes are called eukaryotes ever, surface-to-volume ratio decreases, until metabo- (Greek eu, " true, and karyon,"nucleus ); those with- lism consumes O2 faster than diffusion can supply it out nuclear envelopes-bacterial cells-are prokary Metabolism that requires O, thus becomes impossible otes (Greek pro, "before") as cell size increases beyond a certain point, placing a theoretical upper limit on the size of the cell Cellular Dimensions Are Limited by Oxygen Diffusion Most cells are microscopic, invisible to the unaided eye There Are Three Distinct Domains of life Animal and plant cells are typically 5 to 100 um in di- All living organisms fall into one of three large groups ameter, and many bacteria are only I to 2 um long (see (kingdoms, or domains)that define three branches of the inside back cover for information on units and their evolution from a common progenitor (Fig. 1-4). Two abbreviations). What limits the dimensions of a cell? The large groups of prokaryotes can be distinguished on bio lower limit is probably set by the minimum number of chemical grounds: archaebacteria(Greek arche, "ori- each type of biomolecule required by the cell. The gin") and eubacteria (again, from Greek eu," true") smallest cells, certain bacteria known as mycoplasmas, Eubacteria inhabit soils, surface waters, and the tissues re 300 nm in diameter and have a volume of abo of other living or decaying organisms. Most of the well 10mL. A single bacterial ribosome is about 20 nm in studied bacteria, including Escherichia coli, are eu- its longest dimension, so a few ribosomes take up a sub- bacteria. The archaebacteria, more recently discovered stantial fraction of the volume in a mycoplasmal cell. are less well characterized biochemically, most inhabit rate The upper limit of cell size is probably set by the extreme environments-salt lakes, hot springs, highly rate of diffusion of solute molecules in aqueous systems. acidic bogs, and the ocean depths. The available evi- For example, a bacterial cell that depends upon oxygen- consuming reactions for energy production must obtain diverged early in evolution and constitute two separate Eubacteria Eukaryotes Animals Ciliates Gram- nonsulfur Purple bacteria bacteria bacteria Plants Flagellates acte 0a Flavobacteria Microsporic Thermotos Extreme halophiles Methanogen Extreme thermophiles Archaebacteria 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 betweer the closer is their evolutionary relationshipthe 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
m in di￾ameter, 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 sub￾stantial 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, how￾ever, surface-to-volume ratio decreases, until metabo￾lism 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. There Are Three Distinct Domains of Life 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 bio￾chemical grounds: archaebacteria (Greek arche- , “ori￾gin”) 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 evi￾dence suggests that the archaebacteria and eubacteria diverged early in evolution and constitute two separate 4 Chapter 1 The Foundations of Biochemistry Purple bacteria Cyanobacteria Flavobacteria Thermotoga Extreme halophiles Methanogens Extreme thermophiles Microsporidia Flagellates Plants Fungi Animals Ciliates Archaebacteria Gram￾positive bacteria Eubacteria Eukaryotes Green nonsulfur bacteria 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. 8885d_c01_01-46 10/27/03 7:48 AM Page 4 mac76 mac76:385_reb: