BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.2. ATP

9.2.2. The Role of ATP

The breakdown of ATP into ADP and inorganic phosphate (P_i) releases energy:

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The reaction involves the consumption of Water, i.e., it is a Hydrolysis reaction (we encountered this very common type of biochemical reaction numerous times in Chapter 3). The third phosphate group cleaved from ATP remains in The Cell as inorganic phosphate (P_i). The Free energy yield of this reaction is 30.6 kJ per 1 mol of ATP.

ATP can be resynthesized from ADP and phosphate, but this requires an input of 30.6 kJ of energy per 1 mol of newly formed ATP.

In this reaction, known as a Condensation reaction, water is released. The addition of phosphate to ADP is called phosphorylation. The two equations above can be combined:

This reversible reaction is catalyzed by an enzyme called ATPase.

As already mentioned, all Cells require energy to perform their work, and for all cells in any Organism, ATP serves as the source of this energy. Therefore, ATP is referred to as the "universal energy carrier" or the "energy currency" of the cell. An apt analogy is an electric battery. Think of all the different ways we use them: we can use them to obtain light in one case, sound in another, sometimes mechanical motion, or sometimes we simply need electrical energy from them. The convenience of batteries lies in the fact that a single energy source—a battery—can be used for A wide variety of purposes depending on where we place it. ATP plays this exact role in cells. It supplies energy for such diverse processes as Muscle contraction, Nerve Impulse transmission, active Transport of substances, or Protein Synthesis, as well as all Other forms of cellular activity. To do this, it simply needs to be "plugged in" to the appropriate part of the cellular machinery.

The analogy can be extended further. Batteries must first be manufactured, and some of them (rechargeable ones), much like ATP, can be recharged. When batteries are manufactured in a factory, a certain amount of energy must be put into them (and thus expended by the factory). The synthesis of ATP also requires energy; its source is The oxidation of organic substances during Respiration. Because energy is released during oxidation to phosphorylate ADP, this type of phosphorylation is called Oxidative Phosphorylation. During Photosynthesis, ATP is produced using light energy, a process known as Photophosphorylation (see Section 7.6.2). Cells also contain "factories" that produce the bulk of ATP: these are the Mitochondria, which house the chemical "assembly lines" where ATP is generated during aerobic respiration. Finally, cells also recharge depleted "batteries": once ATP has released its stored energy and turned into ADP and P_i, it can be rapidly resynthesized from ADP and P_i using energy derived from the oxidation of a fresh batch of organic nutrients during respiration.

The amount of ATP in a cell at any given moment is very small. Therefore, ATP should be viewed merely as a carrier of energy, not as a storage depot. Long-term energy storage is served by substances such as fats or Glycogen. Cells are extremely sensitive to ATP levels. As soon as The rate of its utilization increases, the rate of respiration—which maintains this level—increases correspondingly.

The Role of ATP as a linking bridge between cellular respiration and energy-consuming processes is illustrated in Fig. 9.3. While the diagram appears simple, it illustrates a very important principle.

Thus, it can be said that the overall function of respiration is to produce ATP.

Fig. 9.3. Energy extracted from glucose is directed—via ATP—to perform useful work. Before the energy stored in ATP is lost (dissipated as heat), it can be used within the cell for various purposes. ATP is continuously produced through respiration and utilized in various cellular reactions. The resynthesis of ATP also occurs continuously within the cell.

9.2. Complete the left-hand side of the diagram in Fig. 9.3 to show how solar energy is ultimately stored in the form of glucose.

To briefly summarize the above:

1. The synthesis of ATP from ADP and inorganic phosphate requires 30.6 kJ of energy per 1 mol of ATP.

2. ATP is present in all living cells and is therefore a universal energy carrier. No other energy carriers are used. This simplifies matters—the required cellular machinery can be simpler and operate more efficiently and economically.

3. ATP readily delivers energy to any part of the cell for any energy-requiring process.

4. ATP releases energy rapidly, requiring only a single reaction: hydrolysis.

5. The rate of ATP regeneration from ADP and inorganic phosphate (the rate of respiration) is easily regulated to meet physiological demands.

6. ATP is synthesized during respiration using the chemical energy released from the oxidation of organic molecules such as glucose, and during photosynthesis using solar energy. The formation of ATP from ADP and inorganic phosphate is referred to as phosphorylation. When the energy for phosphorylation is supplied by oxidation, the process is known as oxidative phosphorylation (which occurs during respiration); when light energy is utilized, it is called photophosphorylation (which takes place during photosynthesis).