Cells trap free energy while metabolizing glucose

The familiar process of combustion (burning) is very similar to the chemical processes that release energy in cells. If glucose is burned in a flame, it reacts with O2, rapidly forming carbon dioxide and water and releasing a lot of energy. The balanced equation for this combustion reaction is

C6H12O6 + 6 O2 ^ 6 CO2 + 6 H2O + energy (heat and light)

The same equation applies to the metabolism of glucose in cells. The metabolism of glucose, however, is a multistep, controlled series of reactions. The multiple steps of the process permit about one-third of the energy released to be captured in ATP. That ATP can be used to do cellular work such as movement or active transport across a membrane, just as energy captured from combustion can be used to do work.

The change in free energy (AG) for the complete conversion of glucose and O2 to CO2 and water, whether by combustion or by metabolism, is -686 kcal/mol (-2,870 kJ/mol). Thus the overall reaction is highly exergonic and can drive the endergonic formation of a great deal of ATP from ADP and phosphate. It is the capture of this energy in ATP that requires the many steps characteristic of glucose metabolism.

Three metabolic processes play roles in the utilization of glucose for energy: glycolysis, cellular respiration, and fermentation (Figure 7.1). All three involve metabolic pathways made up of many distinct chemical reactions.

► Glycolysis begins glucose metabolism in all cells and produces two molecules of the three-carbon product pyruvate. A small amount of the energy stored in glucose is captured in usable forms. Glycolysis does not use O2.

► Cellular respiration uses O2 from the environment and completely converts each pyruvate molecule to three molecules of CO2 through a set of metabolic pathways. In the process, a great deal of the energy stored in the covalent bonds of pyruvate is released and transferred to ADP and phosphate to form ATP. ► Fermentation does not involve O2. Fermentation converts pyruvate into products such as lactic acid or ethyl alcohol (ethanol), which are still relatively energy-rich molecules. Because the breakdown of glucose is incomplete, much less energy is released by fermentation than by cellular respiration, and no ATP is produced.

Glycolysis and fermentation are anaerobic metabolic processes—that is, they do not involve O2. Cellular respiration is an aerobic metabolic process, requiring the direct participation of O2.

Redox reactions transfer electrons and energy

In Chapter 6, we described the addition of phosphate groups to ADP to make ATP as an endergonic reaction that can extract and store energy from exergonic reactions. Another way of transferring energy is to transfer electrons. A reaction in which one substance transfers one or more electrons to another substance is called an oxidation-reduction reaction, or redox reaction.

► Reduction is the gain of one or more electrons by an atom, ion, or molecule.

► Oxidation is the loss of one or more electrons.

Photosynthesis

Stored chemical energy

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