Physical Foundations-: Energy Coupling Links Reactions in Biology

The central issue in bioenergetics (the study of energy transformations in living systems) is the means by which energy from fuel metabolism or light capture is coupled to a cell’s energy-requiring reactions. In thinking about energy coupling, it is useful to consider a simple me chanical example, as shown in Figure 1–26a. An object at the top of an inclined plane has a certain amount of potential energy as a result of its elevation. It tends to slide down the plane, losing its potential energy of position as it approaches the ground. When an appropriate string-and-pulley device couples the falling object to another, smaller object, the spontaneous downward motion of the larger can lift the smaller, accomplishing a

certain amount of work. The amount of energy available to do work is the free-energy change, ΔG; this is always somewhat less than the theoretical amount of energy released, because some energy is dissipated as the heat of friction. The greater the elevation of the larger object, the greater the energy released (ΔG) as the object slides downward and the greater the amount of work that can be accomplished. How does this apply in chemical reactions? In closed systems, chemical reactions proceed spontaneously until equilibrium is reached. When a system is at equilibrium, the rate of product formation exactly equals the rate at which product is converted to reactant. Thus there is no net change in the concentration of reactants and products; a steady state is achieved. The energy change as the system moves from its initial state to equilibrium, with no changes in temperature or pressure, is given by the free-energy change, G. The magnitude of G depends on the particular chemical reaction and on how far from equilibrium the system is initially. Each compound involved in a chemical reaction contains a certain amount of potential energy, related to the kind and number of its bonds. In reactions that occur spontaneously, the products have less free energy than the reactants, thus the reaction releases free energy, which is then available to do work. Such reactions are exergonic; the decline in free energy from reactants to products is expressed as a negative value. Endergonic reactions re quire an input of energy, and their Gvalues are posi tive. As in mechanical processes, only part of the energy released in exergonic chemical reactions can be used to accomplish work. In living systems some energy is dissipated as heat or lost to increasing entropy. In living organisms, as in the mechanical example in Figure 1–26a, an exergonic reaction can be coupled to an endergonic reaction to drive otherwise unfavorable

FIGURE 1–26 Energy coupling in mechanical and chemical processes. (a)The downward motion of an object releases potential energy that can do mechanical work. The potential energy made available by spontaneous downward motion, an exergonic process (pink), can be coupled to the endergonic upward movement of another object (blue). (b) In reaction 1, the formation of glucose 6-phosphate from glucose and inorganic phosphate (Pi) yields a product of higher energy than the two reactants. For this endergonic reaction, Gis positive. In reaction 2, the exergonic breakdown of adenosine triphosphate (ATP) can drive an endergonic reaction when the two reactions are coupled. The exergonic reaction has a large, negative free-energy change (ΔG2), and the endergonic reaction has a smaller, positive free energy change (ΔG1). The third reaction accomplishes the sum of re actions 1 and 2, and the free-energy change, ΔG3, is the arithmetic sum of ΔG1and ΔG2. Because ΔG3 is negative, the overall reaction is exergonic and proceeds spontaneously.

reactions. Figure 1–26b (a type of graph called a reaction coordinate diagram) illustrates this principle for the conversion of glucose to glucose 6-phosphate, the first step in the pathway for oxidation of glucose. The simplest way to produce glucose 6-phosphate would be: Reaction 1: Glucose+Pi→glucose 6-phosphate (endergonic; ΔG1is positive) (Pi is an abbreviation for inorganic phosphate, HPO₄^-2. Don’t be concerned about the structure of these com pounds now; we describe them in detail later in the book.) This reaction does not occur spontaneously; ΔG is positive. A second, very exergonic reaction can occur in all cells:

Reaction 2: ATP → ADP+Pi

(exergonic; ΔG2 is negative)

These two chemical reactions share a common inter mediate, Pi, which is consumed in reaction 1 and produced in reaction 2. The two reactions can therefore be coupled in the form of a third reaction, which we can write as the sum of reactions 1 and 2, with the common intermediate, Pi, omitted from both sides of the equation:

Reaction 3: Glucose+ATP →glucose 6-phosphate +ADP

Because more energy is released in reaction 2 than is consumed in reaction 1, the free-energy change for re action 3, ΔG3, is negative, and the synthesis of glucose 6-phosphate can therefore occur by reaction 3. The coupling of exergonic and endergonic reactions through a shared intermediate is absolutely central to the energy exchanges in living systems. As we shall see, the breakdown of ATP (reaction 2 in Fig. 1–26b) is the exergonic reaction that drives many endergonic processes in cells. In fact, ATP is the major carrier of chemical energy in all cells.

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مواضيع ذات صلة

Physical Foundations:- Creating and Maintaining Order Requires Work and Energy

Physical Foundations:- The Flow of Electrons Provides Energy for Organisms

Physical Foundations:- Organisms Transform Energy and Matter from Their Surroundings

Chemical Foundations

Cellular Foundations

Nucleic acids exist in a double helix

Anions from sulfones

Organocopper reagents undergo conjugate addition

Hydroboration

تثبيت النتروجين Nitrogen Fixation

دورة اليوريا urea cycle

المصير الايضي للأحماض الامينية