1.1 Cellular Foundations within a common chemical framework For the sake of Nucleus(eukaryotes) clarity, in this book we sometimes risk certain general ucleoid (bacteria) ntains genetic material-DNA and izations, which, though not perfect, remain useful; we ssociated proteins. Nucleus is also frequently point out the exceptions that illuminate scientific generalizations Biochemistry describes in molecular terms the struc- Plasma membrane Tough, flexible lipid bilayer. tures, mechanisms, and chemical processes shared by all organisms and provides organizing principles that polar substances Includes underlie life in all its diverse forms, principles we refer nembrane proteins that function in transport, to collectively as the molecular logic of life. Although biochemistry provides important insights and practical and as enzymes. applications in medicine, agriculture, nutrition, and industry, its ultimate concern is with the wonder of life In this introductory chapter, then, we describe (briefly!) the cellular, chemical, physical(thermody namic), and genetic backgrounds to biochemistry and the overarching principle of evolution-the develop ment over generations of the properties of living cells As you read through the book, you may find it helpful to refer back to this chapter at intervals to refresh your suspended parti memory of this background material and organelles. 1.1 Cellular Foundations ntrifuge at 150, 000 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 of enzymes, RNA, monomeric subunits 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 Pellet: particles and organelles Ribosomes, storage granules, properties, which can be seen at the biochemical level mitochondria, chloroplasts, Cells Are the Structural and Functional units of all FIGURE 1-3 The universal features of living cells. All cells have a Living Organisms nucleus or nucleoid, a plasma membrane, and cytoplasm. The cytosol Cells of all kinds share certain structural features (Fig. is defined as that portion of the cytoplasm that remains in the super 1-3). The plasma membrane defines the periphery of natant after centrifugation of a cell extract at 150,000 g for 1 hour. the cell, separating its contents from the surroundings It is composed of lipid and protein molecules that form The internal volume bounded by the plasma mem a thin, tough, pliable, hydrophobic barrier around the brane, the cytoplasm(Fig. 1-3), is composed of an cell. The membrane is a barrier to the free passage of aqueous solution, the cytosol, and a variety of sus inorganic ions and most other charged or polar com- pended particles with specific functions. The cytosol is pounds. Transport proteins in the plasma membrane al- a highly concentrated solution containing enzymes and low the passage of certain ions and molecules; receptor the rNa molecules that encode them; the components proteins transmit signals into the cell; and membrane (amino acids and nucleotides) from which these macro- enzymes participate in some reaction pathways. Be- molecules are assembled; hundreds of small organic cause the individual lipids and proteins of the plasma molecules called metabolites, intermediates in biosyn membrane are not covalently linked, the entire struc thetic and degradative pathways: coenzymes, com ture is remarkably flexible, allowing changes in the pounds essential to many enzyme-catalyzed reactions shape and size of the cell. As a cell grows, newly made inorganic ions; and ribosomes, small particles(com- ipid and protein molecules are inserted into its plasma posed of protein and rNa molecules) that are the sites membrane; cell division produces two cells, each with its of protein synthesis own membrane. This growth and cell division(fission) All cells have, for at least some part of their life, ei- occurs without loss of membrane integrity ther a nucleus or a nucleoid, in which the genome-within a common chemical framework. For the sake of clarity, in this book we sometimes risk certain general￾izations, which, though not perfect, remain useful; we also frequently point out the exceptions that illuminate scientific generalizations. Biochemistry describes in molecular terms the struc￾tures, mechanisms, and chemical processes shared by all organisms and provides organizing principles that underlie life in all its diverse forms, principles we refer to collectively as the molecular logic of life. Although biochemistry provides important insights and practical applications in medicine, agriculture, nutrition, and industry, its ultimate concern is with the wonder of life itself. In this introductory chapter, then, we describe (briefly!) the cellular, chemical, physical (thermody￾namic), and genetic backgrounds to biochemistry and the overarching principle of evolution—the develop￾ment over generations of the properties of living cells. As you read through the book, you may find it helpful to refer back to this chapter at intervals to refresh your memory of this background material. 1.1 Cellular Foundations 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 (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 com￾pounds. Transport proteins in the plasma membrane al￾low the passage of certain ions and molecules; receptor proteins transmit signals into the cell; and membrane enzymes participate in some reaction pathways. Be￾cause the individual lipids and proteins of the plasma membrane are not covalently linked, the entire struc￾ture 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. The internal volume bounded by the plasma mem￾brane, the cytoplasm (Fig. 1–3), 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 genome— 1.1 Cellular Foundations 3 Nucleus (eukaryotes) or nucleoid (bacteria) Contains genetic material–DNA and associated proteins. Nucleus is membrane-bounded. Plasma membrane Tough, flexible lipid bilayer. Selectively permeable to polar substances. Includes membrane proteins that function in transport, in signal reception, and as enzymes. Cytoplasm Aqueous cell contents and suspended particles and organelles. Supernatant: cytosol Concentrated solution of enzymes, RNA, monomeric subunits, metabolites, inorganic ions. Pellet: particles and organelles Ribosomes, storage granules, mitochondria, chloroplasts, lysosomes, endoplasmic reticulum. centrifuge at 150,000 g 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 super￾natant after centrifugation of a cell extract at 150,000 g for 1 hour. 8885d_c01_003 12/20/03 7:03 AM Page 3 mac76 mac76:385_reb: