CaTalYsis anD The Use of eneRGY BY Cells
One property of living things above all makes them seem almost miraculously different from nonliving matter: they create and maintain order, in a universe that is tending always to greater disorder (Figure 2–12). To create this order, the cells in a living organism must perform a never-ending stream of chemical reactions. In some of these reactions, small organic molecules—amino acids, sugars, nucleotides, and lipids—are being taken apart or modified to supply the many other small molecules that the cell requires. In other reactions, small molecules are being used to construct an enormously diverse range of proteins, nucleic acids, and other macromolecules that endow living systems with all of their most distinctive properties. Each cell can be viewed as a tiny chemical factory, performing many millions of reactions every second.
Cell metabolism is organized by enzymes
The chemical reactions that a cell carries out would normally occur only at much higher temperatures than those existing inside cells. For this reason, each reaction requires a specific boost in chemical reactivity. This requirement is crucial, because it allows the cell to control its chemistry. The control is exerted through specialized biological catalysts. These are almost always proteins called enzymes, although RNA catalysts also exist, called ribozymes. Each enzyme accelerates, or catalyzes, just one of the many possible kinds of reactions that a particular molecule might undergo. Enzyme-catalyzed reactions are connected in series, so that the product of one reaction becomes the starting material, or substrate, for the next (Figure 2–13). Long linear reaction pathways are in turn linked to one another, forming a maze of interconnected reactions that enable the cell to survive, grow, and reproduce.
Two opposing streams of chemical reactions occur in cells: (1) the catabolic pathways break down foodstuffs into smaller molecules, thereby generating both a useful form of energy for the cell and some of the small molecules that the cell needs as building blocks, and (2) the anabolic, or biosynthetic, pathways use the 20 nm 50 nm 10 µm 0.5 mm 20 mm
Figure 2–12 biological structures are highly ordered. Well-defined, ornate, and beautiful spatial patterns can be found at every level of organization in living organisms. in order of increasing size: (a) protein molecules in the coat of a virus (a parasite that, although not technically alive, contains the same types of molecules as those found in living cells); (B) the regular array of microtubules seen in a cross section of a sperm tail; (C) surface contours of a pollen grain (a single cell); (D) cross section of a fern stem, showing the patterned arrangement of cells; and (e) a spiral arrangement of leaves in a succulent plant. (a, courtesy of Robert Grant, stéphane Crainic, and James m. hogle; B, courtesy of lewis Tilney;  C, courtesy of Colin macfarlane and Chris Jeffree; D, courtesy of Jim haseloff.)
Figure 2–13 How a set of enzyme-catalyzed reactions generates a metabolic pathway. each enzyme catalyzes a particular chemical reaction, leaving the enzyme unchanged. in this example, a set of enzymes acting in series converts molecule a to molecule f, forming a metabolic pathway. (for a diagram of many of the reactions in a human cell, abbreviated as shown, see figure 2–63.) small molecules and the energy harnessed by catabolism to drive the synthesis of the many other molecules that form the cell. Together these two sets of reactions constitute the metabolism of the cell (Figure 2–14).
The details of cell metabolism form the traditional subject of biochemistry and most of them need not concern us here. But the general principles by which cells obtain energy from their environment and use it to create order are central to cell biology. We begin with a discussion of why a constant input of energy is needed to sustain all living things.
Biological order is made possible by the Release of heat energy from Cells