B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 35
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Examples of different types of phosphate bonds withtheir sites of hydrolysis are shown in the molecules depicted on the left. Those starting with a gray carbon atomshow only part of a molecule. Examples of molecules containing such bonds are given on the right, with thestandard free-energy change for hydrolysis in kilojoules. The transfer of a phosphate group from one moleculeto another is energetically favorable if the free-energy change (ΔG) for hydrolysis of the phosphate bond ofthe first molecule is more negative than that for hydrolysis of the phosphate bond in the second.
Thus, understandard conditions, a phosphate group is readily transferred from 1,3-bisphosphoglycerate to ADP to formATP. (Standard conditionsMBoC6often donot pertain to living cells, where the relative concentrations of reactants andm2.74/2.50products will influence the actual change in free energy.) The hydrolysis reaction can be viewed as the transferof the phosphate group to water.subunits in the large branched polysaccharide glycogen, which is present assmall granules in the cytoplasm of many cells, including liver and muscle.
Thesynthesis and degradation of glycogen are rapidly regulated according to need.When cells need more ATP than they can generate from the food molecules takenin from the bloodstream, they break down glycogen in a reaction that producesglucose 1-phosphate, which is rapidly converted to glucose 6-phosphate for glycolysis (Figure 2–52).Quantitatively, fat is far more important than glycogen as an energy store foranimals, presumably because it provides for more efficient storage. The oxidationof a gram of fat releases about twice as much energy as the oxidation of a gramof glycogen. Moreover, glycogen differs from fat in binding a great deal of water,producing a sixfold difference in the actual mass of glycogen required to store thesame amount of energy as fat.
An average adult human stores enough glycogenChapter 2: Cell Chemistry and Bioenergetics80large glycogengranules inthe cytoplasmof a liver cellbranch pointglucose units(A)(B)1 µmchloroplast envelopevacuolegranathylakoidstarchfat dropletcell wallgrana1 µm(C)(D)50 µmFigure 2–51 The storage of sugars and fats in animal and plant cells. (A) The structures of starch and glycogen, thestorage form of sugars in plants and animals, respectively.
Both are storage polymers of the sugar glucose and differ only in thefrequency of branch points. There are many more branches in glycogen than in starch. (B) An electron micrograph of glycogengranules in the cytoplasm of a liver cell. (C) A thin section of a chloroplast from a plant cell, showing the starch granules and lipid(fat droplets) that have accumulated as a result of the biosyntheses occurring there. (D) Fat droplets (stained red) beginning toaccumulate in developing fat cells of an animal. (B, courtesy of Robert Fletterick and Daniel S. Friend; C, courtesy of K. Plaskitt;D, courtesy of Ronald M. Evans and Peter Totonoz.)for only about a day of normal activities, but enough fat to last for nearly a month.If our main fuel reservoir had to be carried as glycogen instead of fat, body weightwould increase by an average of about 60 pounds.The sugar and ATP needed by plant cells are largely produced in separateorganelles: sugars in chloroplasts (the organelles specialized for photosynthesis),HOCH2HOCH2OHOOHOOHOOHOglycogen polymerOHP OCH2HOCH2PiOglycogenphosphorylaseHOOHO POHHOCH2Oglucose 1-phosphateHOOHOOHOHglucose 6-phosphateOHOGLYCOLYSISMBoC6 m2.75,e13.21/2.51OHOHglycogen polymerFigure 2–52 How sugars are producedfrom glycogen.
Glucose subunits arereleased from glycogen by the enzymeglycogen phosphorylase. This producesglucose 1-phosphate, which is rapidlyconverted to glucose 6-phosphate forglycolysis.HOW CELLS OBTAIN ENERGY FROM FOOD81Figure 2–53 Some plant seeds thatserve as important foods for humans.Corn, nuts, and peas all contain rich storesof starch and fat that provide the youngplant embryo in the seed with energy andbuilding blocks for biosynthesis.
(Courtesyof the John Innes Foundation.)and ATP in mitochondria. Although plants produce abundant amounts of bothATP and NADPH in their chloroplasts, this organelle is isolated from the rest of itsplant cell by a membrane that is impermeable to both types of activated carriermolecules. Moreover, the plant contains many cells—such as those in the roots—that lack chloroplasts and therefore cannot produce their own sugars. Thus, sugars are exported from chloroplasts tothe mitochondriapresent in all cells of theMBoC6m2.77/2.53plant. Most of the ATP needed for general plant cell metabolism is synthesized inthese mitochondria, using exactly the same pathways for the oxidative breakdownof sugars as in nonphotosynthetic organisms; this ATP is then passed to the rest ofthe cell (see Figure 14–42).During periods of excess photosynthetic capacity during the day, chloroplastsconvert some of the sugars that they make into fats and into starch, a polymer ofglucose analogous to the glycogen of animals.
The fats in plants are triacyl-glycerols (triglycerides), just like the fats in animals, and differ only in the types offatty acids that predominate. Fat and starch are both stored inside the chloroplastuntil needed for energy-yielding oxidation during periods of darkness (see Figure2–51C).The embryos inside plant seeds must live on stored sources of energy for aprolonged period, until they germinate and produce leaves that can harvest theenergy in sunlight.
For this reason plant seeds often contain especially largeamounts of fats and starch—which makes them a major food source for animals,including ourselves (Figure 2–53).Most Animal Cells Derive Their Energy from Fatty Acids BetweenMealsAfter a meal, most of the energy that an animal needs is derived from sugarsobtained from food. Excess sugars, if any, are used to replenish depleted glycogenstores, or to synthesize fats as a food store. But soon the fat stored in adipose tissueis called into play, and by the morning after an overnight fast, fatty acid oxidationgenerates most of the ATP we need.Low glucose levels in the blood trigger the breakdown of fats for energy production. As illustrated in Figure 2–54, the triacylglycerols stored in fat dropletsin adipocytes are hydrolyzed to produce fatty acids and glycerol, and the fattyacids released are transferred to cells in the body through the bloodstream.
Whileanimals readily convert sugars to fats, they cannot convert fatty acids to sugars.Instead, the fatty acids are oxidized directly.Sugars and Fats Are Both Degraded to Acetyl CoA inMitochondriaIn aerobic metabolism, the pyruvate that was produced by glycolysis from sugarsin the cytosol is transported into the mitochondria of eukaryotic cells. There, it isChapter 2: Cell Chemistry and Bioenergetics82hydrolysisstored fatfatty acidsbloodstreamglycerolFAT CELLMUSCLE CELLfatty acidsoxidation inmitochondriaFigure 2–54 How stored fats aremobilized for energy production inanimals. Low glucose levels in the bloodtrigger the hydrolysis of the triacylglycerolmolecules in fat droplets to free fattyacids and glycerol. These fatty acids enterthe bloodstream, where they bind to theabundant blood protein, serum albumin.Special fatty acid transporters in theplasma membrane of cells that oxidize fattyacids, such as muscle cells, then passthese fatty acids into the cytosol, fromwhich they are moved into mitochondria forenergy production.CO2ATPrapidly decarboxylated by a giant complex of three enzymes, called the pyruvatedehydrogenase complex.
The products of pyruvate decarboxylation are a moleculeof CO2 (a waste product), a molecule of NADH, and acetyl CoA (see Panel 2–9).The fatty acids imported from the bloodstream are moved into mitochondria,where all of their oxidation takes place (Figure 2–55). Each molecule of fatty acid(as the activated molecule fatty acyl CoA) is broken down completely by a cycle ofMBoC6 m2.78/2.54reactions that trims two carbonsat a time from its carboxyl end, generating onemolecule of acetyl CoA for each turn of the cycle. A molecule of NADH and a molecule of FADH2 are also produced in this process (Figure 2–56).Sugars and fats are the major energy sources for most nonphotosyntheticorganisms, including humans. However, most of the useful energy that can beextracted from the oxidation of both types of foodstuffs remains stored in the acetyl CoA molecules that are produced by the two types of reactions just described.The citric acid cycle of reactions, in which the acetyl group (–COCH3) in acetylCoA is oxidized to CO2 and H2O, is therefore central to the energy metabolismof aerobic organisms.
In eukaryotes, these reactions all take place in mitochondria. We should therefore not be surprised to discover that the mitochondrion isthe place where most of the ATP is produced in animal cells. In contrast, aerobicbacteria carry out all of their reactions, including the citric acid cycle, in a singlecompartment, the cytosol.The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groupsto CO2In the nineteenth century, biologists noticed that in the absence of air cells produce lactic acid (for example, in muscle) or ethanol (for example, in yeast), whilein its presence they consume O2 and produce CO2 and H2O.
Efforts to define thepathways of aerobic metabolism eventually focused on the oxidation of pyruvate and led in 1937 to the discovery of the citric acid cycle, also known as theplasma membraneSugars andpolysaccharidessugarsglucosepyruvatepyruvateacetyl CoAFatsfatty acidsfatty acidsfatty acidsMITOCHONDRIONCYTOSOLFigure 2–55 Pathways forthe production of acetyl CoAfrom sugars and fats. Themitochondrion in eukaryotic cellsis where acetyl CoA is producedfrom both types of major foodmolecules. It is therefore theplace where most of the cell’soxidation reactions occur andwhere most of its ATP is made.Amino acids (not shown) canalso enter the mitochondria, tobe converted there into acetylCoA or another intermediate ofthe citric acid cycle.
The structureand function of mitochondria arediscussed in detail in Chapter 14.HOW CELLS OBTAIN ENERGY FROM FOOD83tricarboxylic acid cycle or the Krebs cycle. The citric acid cycle accounts for abouttwo-thirds of the total oxidation of carbon compounds in most cells, and its majorend products are CO2 and high-energy electrons in the form of NADH.
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