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glucose 1-phosphate and glucose 6-phosphate are dissolved in water, when equilibrium is attained, there will always be 95% glucose ATP: Transferring Energy in Cells

6-phosphate and 5% glucose 1-phosphate.

glucose 1-phosphate and glucose 6-phosphate are dissolved in water, when equilibrium is attained, there will always be 95% glucose ATP: Transferring Energy in Cells

6-phosphate and 5% glucose 1-phosphate.

start out with an aqueous solution of glucose 1-phosphate that has a concentration of 0.02 M. (M stands for molar concentration; see Chapter 2.) The solution is maintained under constant environmental conditions (25°C and pH 7). As the reaction proceeds slowly to equilibrium, the concentration of the product, glucose 6-phosphate, rises from 0 to 0.019 M, while the concentration of the reactant, glucose 1-phosphate, falls to 0.001 M. At this point, equilibrium is reached (Figure 6.4). From then on, the reverse reaction, from glucose 6-phos-phate to glucose 1-phosphate, progresses at the same rate as the forward reaction.

At equilibrium, then, this reaction has a product-to-reac-tant ratio of 19:1 (0.019/0.001), so the forward reaction has gone 95 percent of the way to completion ("to the right," as written). Therefore, the forward reaction is an exergonic reaction. This result is obtained every time the experiment is run under the same conditions. The reaction is described by the equation glucose 1-phosphate ^ glucose 6-phosphate

The change in free energy (AG) for any reaction is related directly to its point of equilibrium. The further toward completion the point of equilibrium lies, the more free energy is given off. In an exergonic reaction, such as the conversion of glucose 1-phosphate to glucose 6-phosphate, AG is a negative number (in this example, AG = -1.7 kcal/mol, or -7.1 kJ/mol).

A large, positive AG for a reaction means that it proceeds hardly at all to the right (A ^ B). But if the product is present, such a reaction runs backward, or "to the left" (A ^ B), (nearly all B is converted to A). A AG value near zero is characteristic of a readily reversible reaction: reactants and products have almost the same free energies.

The principles of thermodynamics we have been discussing apply to all energy exchanges in the universe, so they are very powerful and useful. Next, we'll apply them to reactions in cells that involve the biological energy currency, ATP.

All living cells rely on adenosine triphosphate, or ATP, for the capture and transfer of the free energy needed to do chemical work and maintain the cells. ATP operates as a kind of energy currency. That is, just as you may earn money from a job and then spend it on a meal, some of the free energy released by certain exergonic reactions is captured in ATP, which can then release free energy to drive endergonic reactions.

ATP is produced by cells in a number of ways (which we will describe in the next two chapters), and it is used in many ways. ATP is not an unusual molecule. In fact, it has another important use in the cell: it can be converted into a building block for DNA and RNA. But two things about ATP make it especially useful to cells: it releases a relatively large amount of energy when hydrolyzed, and it can phos-phorylate (donate a phosphate group to) many different molecules. We will examine these two properties in the discussion that follows.

### ATP hydrolysis releases energy

An ATP molecule consists of the nitrogenous base adenine bonded to ribose (a sugar), which is attached to a sequence of three phosphate groups (Figure 6.5). The hydrolysis of ATP yields ADP (adenosine diphosphate) and an inorganic phosphate ion (abbreviated P;, short for HPO2 ), as well as free energy:

The important property of this reaction is that it is exergonic, releasing free energy. The change in free energy (AG) is about -12 kcal/mol (-50 kJ/mol) at the temperature, pH, and substrate concentrations typical of living cells.*

What characteristics of ATP account for the free energy released by the loss of one of its phosphate groups? First and foremost, the free energy of the P—O bond between phos-

*The "standard" AG for ATP hydrolysis is -7.3 kcal/mol or -30 kJ/mol, but that value is valid only at pH 7 and with ATP, ADP, and phosphate present at concentrations of 1 M—concentrations that differ greatly from those found in cells.

ATP (space-filling model)

ATP (space-filling model)

Phosphate groups O O

ATP (structural formula)

Phosphate groups O O