B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 26
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The control is exerted throughspecialized biological catalysts. These are almost always proteins called enzymes,although RNA catalysts also exist, called ribozymes. Each enzyme accelerates, orcatalyzes, just one of the many possible kinds of reactions that a particular molecule might undergo. Enzyme-catalyzed reactions are connected in series, sothat the product of one reaction becomes the starting material, or substrate, forthe next (Figure 2–13). Long linear reaction pathways are in turn linked to oneanother, 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 catabolicpathways break down foodstuffs into smaller molecules, thereby generating botha useful form of energy for the cell and some of the small molecules that the cellneeds as building blocks, and (2) the anabolic, or biosynthetic, pathways use the(A)20 nm(B)50 nm(C)10 µm(D)0.5 mm(E)20 mmFigure 2–12 Biological structures are highly ordered.
Well-defined, ornate, and beautiful spatial patterns can be found at every level oforganization 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 spiralarrangement 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.)MBoC6 e3.03/2.12Chapter 2: Cell Chemistry and Bioenergetics52moleculemoleculemoleculemoleculemoleculemoleculeABCDEFcatalysis byenzyme 1catalysis byenzyme 2catalysis byenzyme 3catalysis byenzyme 4catalysis byenzyme 5ABBREVIATED ASFigure 2–13 How a set of enzyme-catalyzed reactions generates a metabolic pathway.
Each enzyme catalyzes a particularchemical reaction, leaving the enzyme unchanged. In this example, a set of enzymes acting in series converts molecule A tomolecule F, forming a metabolic pathway. (For a diagram of many of the reactions in a human cell, abbreviated as shown, seeFigure 2–63.)MBoC6 m2.34/2.13small molecules and the energy harnessed by catabolism to drive the synthesis ofthe many other molecules that form the cell. Together these two sets of reactionsconstitute the metabolism of the cell (Figure 2–14).The details of cell metabolism form the traditional subject of biochemistry andmost of them need not concern us here.
But the general principles by which cellsobtain energy from their environment and use it to create order are central to cellbiology. We begin with a discussion of why a constant input of energy is neededto sustain all living things.Biological Order Is Made Possible by the Release of Heat Energyfrom CellsThe universal tendency of things to become disordered is a fundamental law ofphysics—the second law of thermodynamics—which states that in the universe, orin any isolated system (a collection of matter that is completely isolated from therest of the universe), the degree of disorder always increases.
This law has suchprofound implications for life that we will restate it in several ways.For example, we can present the second law in terms of probability by statingthat systems will change spontaneously toward those arrangements that have thegreatest probability. If we consider a box of 100 coins all lying heads up, a seriesof accidents that disturbs the box will tend to move the arrangement toward amixture of 50 heads and 50 tails. The reason is simple: there is a huge numberof possible arrangements of the individual coins in the mixture that can achievethe 50–50 result, but only one possible arrangement that keeps all of the coinsoriented heads up. Because the 50–50 mixture is therefore the most probable, wesay that it is more “disordered.” For the same reason, it is a common experiencethat one’s living space will become increasingly disordered without intentionaleffort: the movement toward disorder is a spontaneous process, requiring a periodic effort to reverse it (Figure 2–15).The amount of disorder in a system can be quantified and expressed as theentropy of the system: the greater the disorder, the greater the entropy.
Thus,another way to express the second law of thermodynamics is to say that systemswill change spontaneously toward arrangements with greater entropy.Living cells—by surviving, growing, and forming complex organisms—aregenerating order and thus might appear to defy the second law of thermodynamics.
How is this possible? The answer is that a cell is not an isolated system: it takesin energy from its environment in the form of food, or as photons from the sun (oreven, as in some chemosynthetic bacteria, from inorganic molecules alone). Itthen uses this energy to generate order within itself. In the course of the chemicalreactions that generate order, the cell converts part of the energy it uses into heat.The heat is discharged into the cell’s environment and disorders the surroundings.As a result, the total entropy—that of the cell plus its surroundings—increases, asdemanded by the second law of thermodynamics.To understand the principles governing these energy conversions, thinkof a cell surrounded by a sea of matter representing the rest of the universe.
Asthe cell lives and grows, it creates internal order. But it constantly releases heatenergy as it synthesizes molecules and assembles them into cell structures. Heatis energy in its most disordered form—the random jostling of molecules. WhenfoodmoleculesCATABOLICPATHWAYSthe many moleculesthat form the cellusefulforms ofenergy+ANABOLICPATHWAYSlostheatthe many building blocksfor biosynthesisFigure 2–14 Schematic representation ofthe relationship between catabolic andanabolic pathways in metabolism. Assuggested in this diagram, a major portionof the energy stored in the chemical bondsof food molecules is dissipated as heat. Inaddition, the mass of food required by anyMBoC6 e3.02/2.14organism that derives all of its energy fromcatabolism is much greater than the massof the molecules that it can produce byanabolism.CATALYSIS AND THE USE OF ENERGY BY CELLS53Figure 2–15 An everyday illustration ofthe spontaneous drive toward disorder.Reversing this tendency toward disorderrequires an intentional effort and an input ofenergy: it is not spontaneous.
In fact, fromthe second law of thermodynamics, wecan be certain that the human interventionrequired will release enough heat to theenvironment to more than compensate forthe reordering of the items in this room.“SPONTANEOUS“ REACTIONas time elapsesORGANIZED EFFORT REQUIRING ENERGY INPUTthe cell releases heat to the sea, it increases the intensity of molecular motionsthere (thermal motion)—thereby increasing the randomness, or disorder, of thesea.
The second law of thermodynamics is satisfied because the increase in theamount of order inside the cell is always more than compensated for by an evenMBoC6 m2.37/2.15greater decrease in order (increasein entropy) in the surrounding sea of matter(Figure 2–16).Where does the heat that the cell releases come from? Here we encounter another important law of thermodynamics. The first law of thermodynamicsstates that energy can be converted from one form to another, but that it cannotbe created or destroyed. Figure 2–17 illustrates some interconversions betweendifferent forms of energy.
The amount of energy in different forms will changeas a result of the chemical reactions inside the cell, but the first law tells us thatthe total amount of energy must always be the same. For example, an animal celltakes in foodstuffs and converts some of the energy present in the chemical bondsbetween the atoms of these food molecules (chemical-bond energy) into the random thermal motion of molecules (heat energy).The cell cannot derive any benefit from the heat energy it releases unless theheat-generating reactions inside the cell are directly linked to the processes thatgenerate molecular order.
It is the tight coupling of heat production to an increasesea of mattercellHEATincreased disorderincreased orderFigure 2–16 A simple thermodynamicanalysis of a living cell. In the diagram onthe left, the molecules of both the cell andthe rest of the universe (the sea of matter)are depicted in a relatively disorderedstate. In the diagram on the right, the cellhas taken in energy from food moleculesand released heat through reactions thatorder the molecules the cell contains.
Theheat released increases the disorder in theenvironment around the cell (depicted byjagged arrows and distorted molecules,indicating increased molecular motionscaused by heat). As a result, the secondlaw of thermodynamics—which statesthat the amount of disorder in the universemust always increase—is satisfied asthe cell grows and divides.
For a detaileddiscussion, see Panel 2–7 (pp. 102–103).54Chapter 2: Cell Chemistry and Bioenergeticsfalling brick haskinetic energyraised brickhas potentialenergy dueto pull ofgravity1heat is releasedwhen brick hitsthe floorpotential energy due to positionkinetic energyheat energy+two hydrogengas molecules2oxygen gasmoleculerapid vibrations androtations of two newlyformed water moleculesrapid molecularmotions in H2Ochemical-bond energy in H2 and O2battery–heat dispersed tosurroundingsheat energyfanmotor–++wiresfan3chemical-bond energysunlight4electromagnetic (light) energyelectrical energychlorophyllmoleculechlorophyll moleculein excited statehigh-energy electronskinetic energyphotosynthesischemical-bond energyin order that distinguishes the metabolism of a cell from the wasteful burning offuel in a fire. Later, we illustrate how this coupling occurs.
For now, it is sufficientto recognize that a direct linkage of the “controlled burning” of food molecules tothe generation of biological order is required for cells to create and maintain anisland of order in a universe tending toward chaos.Cells Obtain Energy by the Oxidation of Organic MoleculesMBoC6 m2.39/2.17All animal and plant cells are powered by energy stored in the chemical bondsof organic molecules, whether they are sugars that a plant has photosynthesizedas food for itself or the mixture of large and small molecules that an animal haseaten.
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