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Thus present-daynitrogen-fixing reactions can be said to perform a "topping-up" function for thetotal nitrogen supply.Vertebrates receive virtually all of their nitrogen from their dietary intake ofproteins and nucleic acids. In the body these macromolecules are broken downto amino acids and the components of nucleotides, and the nitrogen they contain is used to produce new proteins and nucleic acids-or utilized to makeother molecules. About half of the 20 amino acids found in proteins are essential amino acids for vertebrates (Figure 2-87), which means that they cannot besynthesizedfrom other ingredients of the diet. The others can be so synthesized,using a variety of raw materials, including intermediates of the citric acid cycleas described previously.The essentialamino acids are made by plants and otherorganisms, usually by long and energeticallyexpensivepathways that have beenlost in the course of vertebrate evolution.
Roshanl(eab 02l-66950639The nucleotides needed to make RNA and DNA can be synthesized usingspecializedbiosynthetic pathways. All of the nitrogens in the purine and pyrimidine bases (as well as some of the carbons) are derived from the plentifulamino acids glutamine, aspartic acid, and glycine, whereas the ribose anddeoxyribose sugars are derived from glucose. There are no "essentialnucleotides" that must be provided in the diet.Amino acids not used in biosynthesis can be oxidized to generatemetabolicenergy.Most of their carbon and hydrogen atoms eventually form COz or HzO,whereas their nitrogen atoms are shuttled through various forms and eventuallyappear as urea, which is excreted.Each amino acid is processeddifferently, anda whole constellation of enzymatic reactions exists for their catabolism.Sulfur is abundant on Earth in its most oxidized form, sulfate (SOaz-).To convert it to forms useful for life, sulfate must be reduced to sulfide (S2-),the oxidation state of sulfur required for the synthesis of essential biological molecules.These molecules include the amino acids methionine and cysteine,coenzymeA(seeFigure 2-62), and the iron-sulfur centers essentialfor electron transport (seeFigure 14-23).
The process begins in bacteria, fungi, and plants, where a specialgroup of enzymes use ATP and reducing power to create a sulfate assimilationpathway. Humans and other animals cannot reduce sulfate and must thereforeacquire the sulfur they need for their metabolism in the food that they eat.Metabolismls Organizedand RegulatedOne gets a sense of the intricacy of a cell as a chemical machine from the relation of glycolysis and the citric acid cycle to the other metabolic pathwayssketched out in Figure 2-88. This type of chart, which was used earlier in thischapter to introduce metabolism, represents only some of the enzymatic pathways in a cell. It is obvious that our discussion of cell metabolism has dealt withonly a tiny fraction of cellular chemistry.All these reactions occur in a cell that is less than 0.1 mm in diameter, andeach requires a different enzyme.
As is clear from Figure 2-88, the samemolecule can often be part of many different pathways. Pyruvate,for example, isa substrate for half a dozen or more different enzymes,each of which modifies itchemically in a different way. One enzyme converts pyruvate to acetyl CoA,another to oxaloacetate;a third enzyrne changespyruvate to the amino acid alanine, a fourth to lactate, and so on. All of these different pathways compete forthe same pyruvate molecule, and similar competitions for thousands of othersmall molecules go on at the same time.The situation is further complicated in a multicellular organism.
Differentcell tlpes will in general require somewhat different sets of enzymes. And different tissues make distinct contributions to the chemistry of the organism as awhole. In addition to differences in specialized products such as hormones orantibodies, there are significant differences in the "common" metabolic pathways among various types of cells in the same organism.Although virtually all cells contain the enzymes of glycolysis, the citric acidcycle, lipid synthesis and breakdown, and amino acid metabolism, the levels ofTHEESSENTIALAMINOACIDSFigure?-87The nine essentialaminobyacids.Thesecannotbe synthesizedhumancellsand so mustbe suppliedinthe diet.102Chapter2:CellChemistryand BiosynthesisFigure2-88 Glycolysisand the citricacid cycleare at the center ofmetabolism.Some500 metabolicreactionsof a typicalcellareshownwith the reactionsschematicallyofglycolysisand the citricacidcyclein red.Otherreactionseitherleadinto thesetwo centralpathways-deliveringsmallmoleculesto be catabolizedwithproductionof energy-or they leadoutwardand therebysupplycarboncompoundsfor the purposeofbiosynthesis.these processes required in different tissues are not the same.
For example,nerve cells, which are probably the most fastidious cells in the body, maintainalmost no reservesof glycogen or fatty acids and rely almost entirely on a constant supply of glucose from the bloodstream. In contrast, liver cells supply glucose to actively contracting muscle cells and recycle the lactic acid produced bymuscle cells back into glucose.All types of cells have their distinctive metabolictraits, and they cooperate extensivelyin the normal state, as well as in responseto stressand starvation. One might think that the whole system would need tobe so finely balanced that any minor upset, such as a temporary change indietary intake, would be disastrous.In fact, the metabolic balance of a cell is amazingly stable.\.A/heneverthe balance is perturbed, the cell reacts so as to restore the initial state.
The cell canadapt and continue to function during starvation or disease.Mutations of manykinds can damage or even eliminate particular reaction pathways, and yet-provided that certain minimum requirements are met-the cell survives.It does sobecause an elaborate network of control mechanismsregulates and coordinatesthe rates of all of its reactions.These controls rest, ultimately, on the remarkableabilities of proteins to change their shape and their chemistry in response tochangesin their immediate environment. The principles that underlie how largemolecules such as proteins are built and the chemistry behind their regulationwill be our next concern.103END-OF-CHAPTERPROBLEMSSu m m a r yGlucoseand otherfood moleculesare broken down by controlled stepwiseoxidation toprouide chemical energy in the form of ATP and NADH.
Thereare three main setsofreactions that act in series-the products of each being the starting material for thenext:glycolysis(which occursin the cytosol),the citric acid cycle(in the mitochondrialmatrix), and oxidatiue phosphorylation (on the inner mitochondrial membrane).Theintermediate products of glycolysk and the citric acid cycleare usedboth as sourcesofmetabolic energyand to produce many of the small moleculesusedas the raw materials for biosynthesis.Cellsstore sugar moleculesas glycogenin animals and starch inplants; both plants and animals also usefats extensiuelyas a food store.Thesestoragematerials in turn serueas a major sourceof food for humans, along with the proteinsthat comprisethe majority of the dry massof most of the cellsin thefoods we eet.PROBLEMSTableQ2-1 Radioactiveisotopesand someof theirproperties(Problem2-12).Whichstatementsare true?Explainwhy or why not.2-1Of the original radioactivityin a sample,only about1/ 1000will remain after 10 half-lives.2-2A 10-BM solution of HCI has a pH of B.2-3Most of the interactions between macromoleculescould be mediatedjust aswell by covalentbonds as by noncovalentbonds.14c3635s32PB particleB particleB particleB particle5730years12.3years87.4days14.3days0.06229149091202-4Animals and plants use oxidation to extract energyfrom food molecules.2-5If an oxidation occurs in a reaction, it must beaccompaniedby a reduction.2-6Linking the energetically unfavorable reaction A -+ Bto a second,favorablereaction B -+ C will shift the equilibrium constantfor the first reaction.2-7The criterion for whether a reaction proceedsspontaneouslyis AG not AGo,becauseAG takesinto accounttheconcentrationsof the substratesand products.2-8Becauseglycolysis is only a prelude to the oxidationof glucosein mitochondria, which yields l5-fold more AIBglycolysisis not really important for human cells.2-9The oxygen consumed during the oxidation of glucosein animal cellsis returned as COzto the atmosphere.Discussthe following problems.2- 10 The organicchemistryof living cellsis said to be special for two teasons:it occurs in an aqueous environmentand it accomplishessome very complex reactions.But doyou suppose it is really all that much different from theorganic chemistry carried out in the top laboratoriesin theworld? \A/tryor why not?2-11 The molecular weight of ethanol (CHgCHzOH)is 46and its density is 0.789g/cm3.A.
\A4ratis the molarity of ethanol in beer that is 5%ethanol by volume? [Alcohol content of beer varies fromabout 4Vo(lite beer) to B% (stout beer).1B. The legal limit for a driver's blood alcohol contentvaries,but 80 mg of ethanol per 100 mL of blood (usuallyreferredto as a blood alcohollevel of 0.08)is t)?ical. \ /hat isthe molarity of ethanol in a person at this legal limit?t.
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