The scientific way of saying that a process will proceed (although a catalyst may be necessary to achieve a reasonable reaction speed) is to say that the change in Gibbs free energy, expressed as AG, is negative. One reaction that we will meet again is the hydrolysis of glucose-6-phosphate to yield glucose and a phosphate ion:
Glucose-6-phosphate + H2O ^ Glucose + HPO42- AG = -19 kJmol-1
Since this reaction has a negative AG, it can proceed, releasing 19 kJ of energy for every mole of glucose-6-phosphate hydrolyzed. Another reaction we will meet again is the hydrolysis of nucleotides, the building blocks of DNA and RNA (Chapter 4). Simply losing the terminal phosphate from the nucleotide ATP releases 30 kJmol-1:
Adenosine triphosphate + H2O ^ Adenosine diphosphate + HPO42- + H+ AG = -30 kJmol-1
The reverse of these reactions will of course not proceed. For instance, cells need to phos-phorylate glucose to make glucose-6-phosphate but cannot use the reaction
Glucose + HPO42- ^ Glucose-6-phosphate + H2O AG = +19 kJmol-1
The reaction will not proceed because it has a positive AG. Crucially, though, an unfavorable (positive AG) reaction can occur if it is tightly coupled to a second reaction that has a negative free-energy change (negative AG) so the overall change for the reactions put together is negative. Thus cells phosphorylate glucose by carrying out the following reaction:
Glucose + Adenosine triphosphate ^ Glucose-6-phosphate + Adenosine diphosphate + H+ AG = -11 kJmol-1
Adenosine triphosphate, or ATP, has given up the energy of its hydrolysis to drive an otherwise energetically unfavorable reaction forward. We call ATP a cellular currency to draw an analogy with money in human society. Just as we can spend money to cause someone to do something they would not otherwise do, such as give us food or build us a house, the cell can spend its energy currency to cause processes that would otherwise not occur. However, the analogy is not exact because energy currencies are not hoarded. There is a continuous turnover of ATP to ADP and back again. ATP is therefore not an energy store but simply a way of linking reactions. It can be thought of as a truck that carries metabolic energy to where it is needed and that returns empty to be refilled. The number of trucks is small but the amount moved can be large. An average person hydrolyzes about 50 kg of ATP per day but makes exactly the same amount from ADP and inorganic phosphate. We will see how this happens in this chapter. The cell has a number of energy currencies of which four—NADH, ATP, the hydrogen ion gradient across the mitochondrial membrane, and the sodium gradient across the plasma membrane—are the most important. We will now discuss each of these in turn.
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