Figure 2–63 Glycolysis and the citric acid cycle are at the center of an elaborate set of metabolic pathways in human cells. some 2000 metabolic reactions are shown schematically with the reactions of glycolysis and the citric acid cycle in red. many other reactions either lead into these two central pathways—delivering small molecules to be catabolized with production of energy—or they lead outward and thereby supply carbon compounds for the purpose of biosynthesis. (adapted with permission from Kanehisa laboratories.) supply of glucose from the bloodstream. In contrast, liver cells supply glucose to WhaT We Don’T KnoW actively contracting muscle cells and recycle the lactic acid produced by muscle cells back into glucose. All types of cells have their distinctive metabolic traits, and • Did chemiosmosis precede they cooperate extensively in the normal state, as well as in response to stress and fermentation as the source of starvation. One might think that the whole system would need to be so finely bal-biological energy, or did some form of anced that any minor upset, such as a temporary change in dietary intake, would fermentation come first, as had been be disastrous. assumed for many years? In fact, the metabolic balance of a cell is amazingly stable. Whenever the balance is perturbed, the cell reacts so as to restore the initial state. The cell can adapt • What is the minimum number of and continue to function during starvation or disease. Mutations of many kinds components required to make a living can damage or even eliminate particular reaction pathways, and yet—provided cell from scratch? how might we find out?
that certain minimum requirements are met—the cell survives. It does so because an elaborate network of control mechanisms regulates and coordinates the rates of all of its reactions. These controls rest, ultimately, on the remarkable abilities • are other life chemistries possible of proteins to change their shape and their chemistry in response to changes in besides the single one known on earth (and described in this chapter)? When their immediate environment. The principles that underlie how large molecules screening for life on other planets, such as proteins are built and the chemistry behind their regulation will be our what type of chemical signatures next concern.
should we search for?
• is the shared chemistry inside all Glucose and other food molecules are broken down by controlled stepwise oxidation living cells a clue for deciphering the to provide chemical energy in the form of ATP and NADH. There are three main sets environment on earth where the first of reactions that act in series, the products of each being the starting material for the cells originated? for example, what next: glycolysis (which occurs in the cytosol), the citric acid cycle (in the mitochon-might we conclude from the universally shared high K+/na+ ratio, neutral ph, drial matrix), and oxidative phosphorylation (on the inner mitochondrial mem and central role of phosphates?
brane). The intermediate products of glycolysis and the citric acid cycle are used both as sources of metabolic energy and to produce many of the small molecules used as the raw materials for biosynthesis. Cells store sugar molecules as glycogen in animals and starch in plants; both plants and animals also use fats extensively as a food store. These storage materials in turn serve as a major source of food for humans, along with the proteins that comprise the majority of the dry mass of most of the cells in the foods we eat.
Which statements are true? explain why or why not. Discuss the following problems.
2–1 A 10–8 M solution of HCl has a pH of 8. 2–8 The organic chemistry of living cells is said to be special for two reasons: it occurs in an aqueous environ2–2 Most of the interactions between macromolecules ment and it accomplishes some very complex reactions. could be mediated just as well by covalent bonds as by
But do you suppose it is really all that much different from noncovalent bonds.
the organic chemistry carried out in the top laboratories in the world? Why or why not?
2–3 Animals and plants use oxidation to extract energy from food molecules.