Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 22

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This biologically ubiquitous molecule has three hydrogenatoms that dissociate at different pH values; thus, phosphoricacid has three pKa values, as noted on the graph. The shadedareas denote the pH ranges—within one pH unit of the three pKavalues—where the buffering capacity of phosphoric acid is high.In these regions the addition of acid (or base) will cause relativelysmall changes in the pH.Within cells, the linked reactions in metabolic pathwaysgenerally are at steady state, not equilibrium, at which rateof formation of the intermediates equals their rate of consumption (see Figure 2-21).■The dissociation constant Kd for a reaction involving thenoncovalent binding of two molecules is a measure of thestability of the complex formed between the molecules(e.g., ligand-receptor or protein-DNA complexes).■The pH is the negative logarithm of the concentrationof hydrogen ions (–log [H]). The pH of the cytoplasm isnormally about 7.2–7.4, whereas the interior of lysosomeshas a pH of about 4.5.■50CHAPTER 2 • Chemical FoundationsAcids release protons (H) and bases bind them.

In biological molecules, the carboxyl and phosphate groups arethe most common acidic groups; the amino group is themost common basic group.■Buffers are mixtures of a weak acid (HA) and its corresponding base form (A), which minimize the change inpH of a solution when acid or alkali is added. Biologicalsystems use various buffers to maintain their pH within avery narrow range.■2.4Biochemical EnergeticsThe production of energy, its storage, and its use are centralto the economy of the cell. Energy may be defined as the ability to do work, a concept applicable to automobile enginesand electric power plants in our physical world and to cellular engines in the biological world.

The energy associatedwith chemical bonds can be harnessed to support chemicalwork and the physical movements of cells.Several Forms of Energy Are Importantin Biological SystemsThere are two principal forms of energy: kinetic and potential. Kinetic energy is the energy of movement—the motionof molecules, for example. The second form of energy, potential energy, or stored energy, is particularly important inthe study of biological or chemical systems.Kinetic Energy Heat, or thermal energy, is a form of kineticenergy—the energy of the motion of molecules.

For heat todo work, it must flow from a region of higher temperature—where the average speed of molecular motion is greater—toone of lower temperature. Although differences in temperature can exist between the internal and external environmentsof cells, these thermal gradients do not usually serve as thesource of energy for cellular activities. The thermal energy inwarm-blooded animals, which have evolved a mechanism forthermoregulation, is used chiefly to maintain constant organismic temperatures. This is an important function, since therates of many cellular activities are temperature-dependent.For example, cooling mammalian cells from their normalbody temperature of 37 ºC to 4 ºC can virtually “freeze” orstop many cellular processes (e.g., intracellular membranemovements).Radiant energy is the kinetic energy of photons, or wavesof light, and is critical to biology.

Radiant energy can be converted to thermal energy, for instance when light is absorbedby molecules and the energy is converted to molecularmotion. During photosynthesis, light energy absorbed byspecialized molecules (e.g., chlorophyll) is subsequently converted into the energy of chemical bonds (Chapter 8).Mechanical energy, a major form of kinetic energy in biology, usually results from the conversion of stored chemicalenergy. For example, changes in the lengths of cytoskeletalfilaments generates forces that push or pull on membranesand organelles (Chapter 19).Electric energy—the energy of moving electrons or othercharged particles—is yet another major form of kineticenergy.Potential Energy Several forms of potential energy are biologically significant.

Central to biology is chemical potentialenergy, the energy stored in the bonds connecting atoms inmolecules. Indeed, most of the biochemical reactions described in this book involve the making or breaking of atleast one covalent chemical bond. We recognize this energywhen chemicals undergo energy-releasing reactions. For example, the high potential energy in the covalent bonds of glucose can be released by controlled enzymatic combustion incells (see later discussion). This energy is harnessed by thecell to do many kinds of work.A second biologically important form of potential energyis the energy in a concentration gradient. When the concentration of a substance on one side of a barrier, such as amembrane, is different from that on the other side, a concentration gradient exists. All cells form concentration gradients between their interior and the external fluids byselectively exchanging nutrients, waste products, and ionswith their surroundings.

Also, organelles within cells (e.g.,mitochondria, lysosomes) frequently contain different concentrations of ions and other molecules; the concentration ofprotons within a lysosome, as we saw in the last section, isabout 500 times that of the cytoplasm.A third form of potential energy in cells is an electricpotential—the energy of charge separation. For instance,there is a gradient of electric charge of ≈200,000 volts per cmacross the plasma membrane of virtually all cells.

We discusshow concentration gradients and the potential differenceacross cell membranes are generated and maintained inChapter 7.Cells Can Transform One Typeof Energy into AnotherAccording to the first law of thermodynamics, energy is neither created nor destroyed, but can be converted from oneform to another. (In nuclear reactions mass is converted toenergy, but this is irrelevant to biological systems.) In photosynthesis, for example, the radiant energy of light is transformed into the chemical potential energy of the covalentbonds between the atoms in a sucrose or starch molecule. Inmuscles and nerves, chemical potential energy stored in covalent bonds is transformed, respectively, into the kineticenergy of muscle contraction and the electric energy of nervetransmission. In all cells, potential energy, released by breaking certain chemical bonds, is used to generate potential energy in the form of concentration and electric potentialgradients.

Similarly, energy stored in chemical concentrationgradients or electric potential gradients is used to synthesize2.4 • Biochemical Energeticschemical bonds or to transport molecules from one side of amembrane to another to generate a concentration gradient.This latter process occurs during the transport of nutrientssuch as glucose into certain cells and transport of manywaste products out of cells.Because all forms of energy are interconvertible, they canbe expressed in the same units of measurement.

Although thestandard unit of energy is the joule, biochemists have traditionally used an alternative unit, the calorie (1 joule 0.239calories). Throughout this book, we use the kilocalorie tomeasure energy changes (1 kcal = 1000 cal).The Change in Free Energy Determinesthe Direction of a Chemical ReactionBecause biological systems are generally held at constanttemperature and pressure, it is possible to predict the direction of a chemical reaction from the change in the free energyG, named after J. W. Gibbs, who showed that “all systemschange in such a way that free energy [G] is minimized.” Inthe case of a chemical reaction, reactantsproducts, thechange in free energy G is given byG Gproducts GreactantsThe relation of G to the direction of any chemical reactioncan be summarized in three statements:If G is negative, the forward reaction (from left toright as written) will tend to occur spontaneously.■51more bond energy than the reactants, heat is absorbed, andH is positive.

The combined effects of the changes in the enthalpy and entropy determine if the G for a reaction is positive or negative. An exothermic reaction (H 0) in whichentropy increases (S 0) occurs spontaneously (G 0).An endothermic reaction (H 0) will occur spontaneouslyif S increases enough so that the T S term can overcomethe positive H.Many biological reactions lead to an increase in order,and thus a decrease in entropy (S 0). An obvious example is the reaction that links amino acids together to form aprotein. A solution of protein molecules has a lower entropythan does a solution of the same amino acids unlinked, because the free movement of any amino acid in a protein isrestricted when it is bound into a long chain.

Often cellscompensate for decreases in entropy by “coupling” such synthetic reactions with independent reactions that have a veryhighly negative G (see below). In this fashion cells can convert sources of energy in their environment into the buildingof highly organized structures and metabolic pathways thatare essential for life.The actual change in free energy G during a reactionis influenced by temperature, pressure, and the initial concentrations of reactants and products and usually differsfrom Gº. Most biological reactions—like others that takeplace in aqueous solutions—also are affected by the pH ofthe solution.

We can estimate free-energy changes for different temperatures and initial concentrations, using theequation[products][reactants]■If G is positive, the reverse reaction (from right to leftas written) will tend to occur.G Gº RT ln Q Gº' RT lnIf G is zero, both forward and reverse reactions occurat equal rates; the reaction is at equilibrium.where R is the gas constant of 1.987 cal/(degree·mol), T isthe temperature (in degrees Kelvin), and Q is the initial ratioof products to reactants. For a reaction A BC, inwhich two molecules combine to form a third, Q in Equation2-7 equals [C]/[A][B]. In this case, an increase in the initialconcentration of either [A] or [B] will result in a large negative value for G and thus drive the reaction toward moreformation of C.Regardless of the Gº for a particular biochemicalreaction, it will proceed spontaneously within cells only ifG is negative, given the usual intracellular concentrationsof reactants and products.

For example, the conversion ofglyceraldehyde 3-phosphate (G3P) to dihydroxyacetonephosphate (DHAP), two intermediates in the breakdown ofglucose,■The standard free-energy change of a reaction Gº is thevalue of the change in free energy under the conditions of298 K (25 ºC), 1 atm pressure, pH 7.0 (as in pure water), andinitial concentrations of 1 M for all reactants and productsexcept protons, which are kept at 107 M (pH 7.0). Most biological reactions differ from standard conditions, particularly in the concentrations of reactants, which are normallyless than 1 M.The free energy of a chemical system can be defined asG H TS, where H is the bond energy, or enthalpy, ofthe system; T is its temperature in degrees Kelvin (K); and Sis the entropy, a measure of its randomness or disorder.

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