During strenuous activity

(top right), large amounts of energy are required quickly, especially by the muscles. The immediate source of this energy is adenosine triphosphate, ATP (structure shown top left. The energy is released when ATP reacts with water to form ADP (adenosine diphosphate) and a phosphate ion (above). The reaction is re­versible. ADP and a phos­phate ion can be recon­verted to ATP by supplying energy.

The hydrogen and elec­tron carrier systemasso­ciated with the production of ATP is illustrated in the diagram below.

Biochemical energy

Energy is needed for nearly all the vital proc­esses that take place in animals and plants. All the energy used by animals is ultimately de­rived from plants eaten as food. Plants and many bacteria are able to trap light (or occa­sionally, chemical energy). They use it to con­vert inorganic materials, such as carbon diox­ide and water, into complex organic products. This energy is "stored" in carbohydrates and lipids. Carbohydrates and lipids are discussed in previous articles in this section on biochem­istry. The stored energy can then be used to do work in the organism. This may be the chemical work of the biochemical processes, electrical work in nerve cells, or mechanical work in the muscles. The amount of energy needed obviously depends on the size and complexity of the organism. But the levels of physiological and physical activity are also im­portant. For example, the energy requirements of a sleeping human being may increase ten­fold or more during times of strenuous exer­cise.

Anabolism and catabolism

Metabolism is the network of biochemical re­actions that underlies living processes. It con­sists of constructive (anabolic) and destructive (catabolic) pathways. The breaking down of large molecules into smaller units releases en­ergy that may be used to build other large units. Many thousands of these reactions are going on in the body all the time. The path­ways for particular types of molecules, such as proteins or carbohydrates, do not remain sep­arate. They converge so that energy can be re­leased from any available fuel. Similarly, in an­abolic processes, a particular compound in excess can be converted to a different material for use in growth or for storage. This flexibility ensures that anabolic and catabolic processes are in balance in a normal organism.

The role of ATP

Adenosine triphosphate (ATP) is a relatively simple compound derived from the purine base adenine, discussed in the previous arti­cle. It consists of adenosine (adenine and the sugar ribose) linked to three phosphate groups. ATP is the readily accessible energy

Biochemistry: Biochemical energy 117

used by all plants, animals, and bacteria. The ending phosphate group can be broken off. ATP thus becomes ADP, adenosine diphos­phate. ADP is energy available for immediate use. For example, this could be energy for electrical or mechanical work. Or it could be energy to form chemical bonds in new mole­cules, which may thus store some of the en­ergy released from the ATP. Alternatively, the energy released in other reactions can be used to convert ADP back to ATP. Hence, ATP provides "ready cash" for the organism to spend or save in its "bank account" of carbohy­drates and lipids. Energy is also stored in pro­teins. This is called upon to some extent dur­ing fasting. Protein energy can become the only energy source during starvation, when other resources are exhausted. Some other compounds also contain energy-transferring phosphate bonds, particularly creatine phos­phate. This compound provides an emergency reserve to regenerate ATP rapidly in contract­ing muscle. In terms of an organism's total en­ergy output, however, these compounds are of only minor importance.

ATP is produced from ADP by a highly complex sequence of reactions that is essen­tially the same in all organisms. The break­down of ATP releases energy. So the forming of ADP must involve the input of the same quantity of energy. The energy is produced by the gradual oxidation of molecules. These mol­ecules include hexose sugars and fatty acids, even though these may have been made origi­nally by breaking down other molecules. Even­tually, the molecules used to produce ATP are oxidized to carbon dioxide and water, just as they would be if burnt directly in air. For this reason, cellular respiration ("breathing") is often referred to as "burning" food. This de­scription is slightly misleading. Direct oxida­tion involves the production of heat energy, which is of only limited use to a cell. An excess of heat energy would be damaging. Instead, cell respiration involves the gradual stripping away of hydrogen atoms from the foodstuff and the controlled transfer of this hydrogen to oxygen. In this way, the production of water is coupled with the release of small "packets" of chemical energy. Thus, the conversion is more controlled than it would be in a single-step re­action. It is also a more efficient source of use­ful energy.

Date: 2015-12-11; view: 1113