H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 21
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For example, in neutral solutions (pH 7.0), aminoacids exist predominantly in the doubly ionized form inwhich the carboxyl group has lost a proton and the aminogroup has accepted one:A growing cell must maintain a constant pH in the cytoplasm of about 7.2–7.4 despite the metabolic production ofmany acids, such as lactic acid and carbon dioxide; the latter reacts with water to form carbonic acid (H2CO3). Cellshave a reservoir of weak bases and weak acids, calledbuffers, which ensure that the cell’s pH remains relativelyconstant despite small fluctuations in the amounts of H orOH being generated by metabolism or by the uptake or secretion of molecules and ions by the cell.
Buffers do this by“soaking up” excess H or OH when these ions are addedto the cell or are produced by metabolism.If additional acid (or base) is added to a solution thatcontains a buffer at its pKa value (a 1:1 mixture of HA andA), the pH of the solution changes, but it changes less thanit would if the buffer had not been present. This is becauseprotons released by the added acid are taken up by the ionized form of the buffer (A); likewise, hydroxyl ions generated by the addition of base are neutralized by protonsreleased by the undissociated buffer (HA). The capacity of asubstance to release hydrogen ions or take them up dependspartly on the extent to which the substance has already takenup or released protons, which in turn depends on the pH ofthe solution.
The ability of a buffer to minimize changes inpH, its buffering capacity, depends on the relationship between its pKa value and the pH, which is expressed by theHenderson-Hasselbalch equation.The titration curve for acetic acid shown in Figure 2-22illustrates the effect of pH on the fraction of molecules inthe un-ionized (HA) and ionized forms (A). At one pH unitbelow the pKa of an acid, 91 percent of the molecules are inthe HA form; at one pH unit above the pKa, 91 percent areNH3HCCOORwhere R represents the side chain.
Such a molecule, containing an equal number of positive and negative ions, is calleda zwitterion. Zwitterions, having no net charge, are neutral.At extreme pH values, only one of these two ionizablegroups of an amino acid will be charged.The dissociation reaction for an acid (or acid group in alarger molecule) HA can be written as HAH A .2.3 • Chemical Equilibrium8CH3COOHCH3COO − + H +6pHpK a = 4.754200.20.40.60.81.0Fraction of dissociated CH3COOHAdded OH−▲ FIGURE 2-22 The titration curve of acetic acid(CH3COOH).
The pKa for the dissociation of acetic acid tohydrogen and acetate ions is 4.75. At this pH, half the acidmolecules are dissociated. Because pH is measured on a logarithmicscale, the solution changes from 91 percent CH3COOH atpH 3.75 to 9 percent CH3COOH at pH 5.75. The acid has maximumbuffering capacity in this pH range.49in the A form. At pH values more than one unit above orbelow the pKa, the buffering capacity of weak acids andbases declines rapidly. In other words, the addition of thesame number of moles of acid to a solution containing a mixture of HA and A that is at a pH near the pKa will cause lessof a pH change than it would if the HA and A were notpresent or if the pH were far from the pKa value.All biological systems contain one or more buffers.
Phosphate ions, the ionized forms of phosphoric acid, are present in considerable quantities in cells and are an importantfactor in maintaining, or buffering, the pH of the cytoplasm.Phosphoric acid (H3PO4) has three protons that are capableof dissociating, but they do not dissociate simultaneously.Loss of each proton can be described by a discrete dissociation reaction and pKa as shown in Figure 2-23. The titrationcurve for phosphoric acid shows that the pKa for the dissociation of the second proton is 7.2. Thus at pH 7.2, about 50percent of cellular phosphate is H2PO4 and about 50 percent is HPO42 according to the Henderson-Hasselbalchequation.
For this reason, phosphate is an excellent bufferat pH values around 7.2, the approximate pH of the cytoplasm of cells, and at pH 7.4, the pH of human blood.KEY CONCEPTS OF SECTION 2.3Chemical EquilibriumA chemical reaction is at equilibrium when the rate ofthe forward reaction is equal to the rate of the reverse reaction (no net change in the concentration of the reactantsor products).■14pKa = 12.7HPO 42−12The equilibrium constant Keq of a reaction reflects theratio of products to reactants at equilibrium and thus is ameasure of the extent of the reaction and the relative stabilities of the reactants and products.■PO 43− + H+10pH8pKa = 7.2H2PO4−HPO 42− + H+6■ For any reaction, the equilibrium constant Keq equalsthe ratio of the forward rate constant to the reverse rateconstant (kf /kr).4pKa = 2.12■ The Keq depends on the temperature, pressure, andchemical properties of the reactants and products, but isindependent of the reaction rate and of the initial concentrations of reactants and products.H3PO4H2PO 4− + H+0Added OH−▲ FIGURE 2-23 The titration curve of phosphoric acid(H3PO4).
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.
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