Metabolism Explained by Grant

Metabolism is the sum of all the chemical processes carried out by living organisms. It includes anabolism, reactions that require energy to synthesize complex molecules from simpler ones, and catabolism, reactions that release energy by breaking complex molecules into simpler ones that can be reused as building blocks. Anabolism is needed for growth, reproduction, and repair of cellular structures. Catabolism provides an organism with energy for its life processes, including movement, transport, and the synthesis of complex molecules - that is, anabolism.

All catabolic reactions involve electron transfer, which allows energy to be captured in high-energy bonds in ATP and similiar molecules. Electron transfer is directly related to oxidation and reduction. Oxidation can be defined as the loss or removal of electrons. Although many substances combine with oxygen and transfer electrons to oxygen, oxygen need not be present if another electron acceptor is available. Reduction can be defined as the gain of electrons. When a substance loses electrons, or is oxidized, energy is released, but another substance must gain the electrons, or be reduced, at the same time. For example, during the oxidation of organic molecules, hydrogen atoms are removed and used to reduce oxygen to form water. In this reaction, hydrogen is an electron donor, or reducing agent, and oxygen is an electron acceptor, or oxidizing agent. Because oxidation and reduction must occur simultaneously, the reactions in which they occur are sometimes called redox reactions.

Among all living things, microorganisms are particularly versatile in the ways in which they obtain energy. The ways different microorganisms capture energy, and obtain carbon, can be classified as autotrophy - "self feeding" - or heterotrophy - "other-feeding". Autotrophs use carbon dioxide (an inorganic substance) to synthesize organic molecules. They include photoautotrophs, which obtain energy from light, and chemoautotrophs, which obtain energy from oxidizing simple inorganic substances such as sulfides and nitrites. Heterotrophs get their carbon from ready-made organic molecules, which they obtain from other organisms, living or dead. There are photoheterotrophs, which obtain chemical energy from light, and chemoheterotrophs, which obtain chemical energy from breaking down ready-made organic compounds.

Autotrophic metabolism (especially photosynthesis) is important as a means of energy capture in many free-living microorganisms. However, such microorganisms do not usually cause disease. We emphasize metabolic processes that occur in chemoheterotrophs because many microorganisms, including nearly all infectious ones, are chemoheterotrophs. These processes include glycolysis (oxidation of glucose to pyruvic acid), fermentation (conversion of pyruvic acid to ethyl alcohol, lactic acid, or other organic compounds), and aerobic respiration (oxidation of pyruvic acid to carbon dioxide and water). Glycolysis and fermentation (anaerobic processes) do not require oxygen, and only a small amount of the energy in a glucose molecule is captured as ATP. Aerobic respiration does require oxygen as an electron acceptor and captures a relatively large amount of the energy in a glucose molecule in ATP.

A large number of microorganisms obtain energy by photosynthesis, the use of light energy and hydrogen from water or other compounds to reduce carbon dioxide to an organic substance that contains more energy. Glucose is produced by photosynthesis in cyanobacteria, algae, and green plants. Photosynthetic organisms then use the glucose or other carbohydrates made in this way for energy.

Like nearly all other chemical processes in living organisms, glycolysis, fermentation, aerobic respiration, and photosynthesis each consist of a series of chemical reactions in which the product of one reaction serves as the substrate (reacting material) for the next: A -> B -> C -> D -> E, and so on. Such a chain of reactions is called a metabolic pathway. Each reaction in a pathway is controlled by a particular enzyme. In this pathway, A is the initial substrate, E is the final product, and B, C, and D are intermediates.

Metabolic pathways can be catabolic or anabolic (biosynthetic). Catabolic pathways capture energy in a form cells can use. Anabolic pathways make the complex molecules that form the structure of cells, enzymes, and other molecules that control cells. These pathways use building blocks such as sugars, glycerol, fatty acids, amino acids, nucleotides, and other molecules to make carbohydrates, lipids, proteins, nucleic acids, or combinations such as glycolipids (made from carbohydrates and lipids), glycoproteins (from carbohydrates and proteins), lipoproteins (from lipids and proteins), and nucleoproteins (from nucleic acids and proteins). ATP molecules are the links that couple catabolic and anabolic pathways. Energy released in catabolic reactions is captured and stored in the form of ATP molecules, which are later broken down to provide the energy needed to build up new molecules in biosynthetic pathways. Bacteria transfer approximately 40% of the energy in a glucose molecule to ATP during aerobic metabolism and 5% during anaerobic fermentation processes. Yields are higher in aerobic processes because their end products are highly oxidized, whereas end products of anaerobic processes are only partially oxidized.

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