All Carriers Can Change Direction

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In a normal animal cell, the primary source of energy is the Krebs cycle (page 283). This regenerates NADH from NAD+, making at the same time a small amount of ATP. Because the NADH currency is always being topped up, while the others are being used, the direction of operation of the energy conversion systems is usually that shown in Figure 12.3. However, all the carriers are reversible.

Yeast cells in a wine barrel, or muscle cells in the leg of a sprinter, are anaerobic; there is no oxygen available. In this situation cells can make NADH, but the electron transport chain cannot drive H+ out of the mitochondria because there is no molecular oxygen waiting to be reduced by NADH. Instead, the cell's energy needs are met by anaerobic glycolysis (page 286), which makes ATP. Figure 12.11 shows how the cell maintains the amounts of energy currencies. Any drain on the mitochondrial H+ gradient is counteracted by ATP synthase running in the opposite direction from that shown in Figure 12.3. ATP is hydrolyzed and H+ ions are pushed out of the mitochondrion. The ADP/ATP exchanger also reverses its direction. ATP is regenerated by anaerobic respiration in the cytosol and is used up by ATP synthase in the mitochondrial matrix.

Chloroplasts do not contain the enzymes of the Krebs cycle, so at night they have no internal source of energy and are dependent on their host cell. ATP from the cytosol (most of it created by mitochondria) enters the chloroplast by the ADP/ATP exchanger, which is carrying ADP and ATP in the opposite direction from that shown in Figure 12.3. ATP synthase hydrolyzes ATP and maintains the H+ gradient.

Anaerobic glycolysis high energy low energy electron transport chain

Anaerobic glycolysis high energy low energy electron transport chain

ATPase

ATPase does not function in absence of molecular oxygen

Figure 12.11. Energy flow between the currencies in an anaerobic cell.

Healthy cells maintain themselves in a steady state in which none of the energy currencies are allowed to run down. The direction in which energy moves between the four currencies depends on the primary source of energy for that cell.

IN DEPTH 12.3 Can It Happen? The Concept of Free Energy

We know from experience that some chemical reactions give out heat (the burning of organic matter for instance) while others absorb heat (so heat is used to make the reaction happen). The study of how energy affects matter and particularly how it affects chemical reactions is called thermodynamics. A vast amount of research in the late nineteenth and early twentieth centuries led to the development of this complex field. To make a very long story short, a process results in a change in two parameters: heat and entropy. Heat we are familiar with and understand as the random motion of molecules, but entropy may be unfamiliar. Entropy is the degree of disorder in a system: Entropy always tends to increase. The melting of ice is favorable because it results in an increase in entropy as the ordered, crystalline ice becomes more random water.

A physical process can absorb or emit heat and/or result in an increase or decrease in entropy. If we wish to know whether a process can occur or not, we need to consider both. The concept of free energy was formulated by the American physical chemist J. Willard Gibbs in 1878. It is now called the Gibbsfree energy and given the symbol G in his honour. The change in free energy in a process, AG (A is the Greek upper-case delta used to denote a change) is

Where AH is the heat change (called enthalpy) in the process, T is the absolute temperature in degrees Kelvin, and AS is the change in entropy. If AG for a process is negative, it will proceed, although the rate may be very slow in the absence of a catalyst. If it is not negative, the process cannot occur.

Unfortunately, we can only measure changes and not absolute values for G, H, and S in chemical processes. This difficulty is avoided by defining standard states so we can make comparisons. A free-energy change in a process under standard conditions is denoted AG0. Biochemists use a slightly different standard state, which is in water at pH 7, and this is shown as AG0'.

The free-energy change is related to the equilibrium constant for a reaction. Consider the reaction

where R is the gas constant and T the absolute temperature. At equilibrium AG = 0 so

AG0' = -RT ln Keq where K^q is the equilibrium constant under standard conditions.

Conditions in the cell are not usually the standard ones, but if we know AG0' for a reaction and the relevant concentrations, we can calculate AG for the reaction. For this book we have calculated AG values under human cellular conditions by using the following reasonable concentration values:

Glucose = 250 ^mol liter-1

Glucose-6-phosphate = 10 mmol liter-1

Temperature 370C

For example, we have used the hexokinase reaction (page 258) to illustrate coupling a reaction with a negative free-energy change with one which has a positive free energy change.

Under biochemical standard conditions this reaction has a AG0' of -36 kJmol-1. However, inside cells the concentration of glucose-6-phosphate is much higher than the concentration of unphosphorylated glucose; when this and other concentration effects are taken into account, the calculated overall free-energy change is -11 kJmol-1: The reaction will certainly still proceed but is not as strongly favored as the AG0' would suggest.

The AG values we have given for the four energy currencies are those that apply in an aerobic animal cell, as shown in Figure 12.3, where each of the conversion reactions has a negative AG, as it must if it is to proceed as shown. Under conditions where one or more of the conversion reactions is operating in the opposite directions, the concentrations, and therefore AG values, will of course be different.

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