Metabolism Oxidation and Reduction

All the chemical reactions in the body are collectively called metabolism. Metabolism has two divisions—catab-olism and anabolism. Catabolism10 (ca-TAB-oh-lizm) consists of energy-releasing decomposition reactions. Such reactions break covalent bonds, produce smaller molecules from larger ones, and release energy that can be used for other physiological work. Energy-releasing reactions are called exergonic11 reactions. If you hold a beaker of water in your hand and pour sulfuric acid into it, for example, the beaker will get so hot you may have to put it down. If you break down energy-storage molecules to run a race, you too will get hot. In both cases, the heat signifies that exergonic reactions are occurring.

Anabolism12 (ah-NAB-oh-lizm) consists of energy-storing synthesis reactions, such as the production of protein or fat. Reactions that require an energy input, such as these, are called endergonic13 reactions. Anabolism is driven by the energy that catabolism releases, so ender-gonic and exergonic processes, anabolism and catabolism, are inseparably linked.

Oxidation is any chemical reaction in which a molecule gives up electrons and releases energy. A molecule is cafa = down, to break down '' ex, exo = out + erg = work '2ana = up, to build up '3 end = in

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 2 The Chemistry of Life 71

oxidized by this process, and whatever molecule takes the electrons from it is an oxidizing agent (electron acceptor).

The term oxidation stems from the fact that oxygen is often involved as the electron acceptor. Thus, we can sometimes recognize an oxidation reaction from the fact that oxygen has been added to a molecule. The rusting of iron, for example, is a slow oxidation process in which oxygen is added to iron to form iron oxide (Fe2O3). Many oxidation reactions, however, do not involve oxygen at all. For example, when yeast ferments glucose to alcohol, no oxygen is required; indeed, the alcohol contains less oxygen than the sugar originally did, but it is more oxidized than the sugar:

Before You Go On

CgH^Og Glucose

2 CH3CH2OH Ethanol

2 CO2 Carbon dioxide

Reduction is a chemical reaction in which a molecule gains electrons and energy. When a molecule accepts electrons, it is said to be reduced; a molecule that donates electrons to another is therefore called a reducing agent (electron donor). The oxidation of one molecule is always accompanied by the reduction of another, so these electron transfers are known as oxidation-reduction (redox) reactions.

It is not necessary that only electrons be transferred in a redox reaction. Often, the electrons are transferred in the form of hydrogen atoms. The fact that a proton (the hydrogen nucleus) is also transferred is immaterial to whether we consider a reaction oxidation or reduction.

Table 2.5 summarizes these energy transfer reactions. We can symbolize oxidation and reduction as follows, letting A and B symbolize arbitrary molecules and e~ represent one or more electrons:

Ae~ + B ^ A + Be~ High-energy Low-energy Low-energy High-energy reduced oxidized oxidized reduced state state state state

Ae~ is a reducing agent because it reduces B, and B is an oxidizing agent because it oxidizes Ae~.

Answer the following questions to test your understanding of the preceding section:

12. Define energy. Distinguish potential energy from kinetic energy.

13. Define metabolism, catabolism, and anabolism.

14. What does oxidation mean? What does reduction mean? Which of them is endergonic and which is exergonic?

15. When sodium chloride forms, which element—sodium or chlorine—is oxidized? Which one is reduced?

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    When sodium chloride forms, which element—sodium or chlorine—is oxidized?
    7 years ago

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