4.1Enzyme Sets Determine the Organism’s Way of Life

Humans derive energy through respiration by oxidization of nutrients obtained from their food. Furthermore, these nutrients are utilized to synthesize the components of human bodies. Growing children need many nutrients, but adults must also have access to nutrients to maintain their own bodies, and they must all perform respiration. Why should this be so? Let us examine respiration at the cellular level because the same holds true for individual cells.

In aerobic organisms, energy for cellular activities is obtained through respiration of externally obtained nutrients. Furthermore, various biomolecules in a cell are degraded and reconstituted. The intracellular conversion of these biomolecules is called metabolism. Metabolism includes conversion of biomolecules and consumption of energy. In a cell, metabolism refers to the collection of biochemical reactions catalyzed by enzymes. Although digestive enzymes are secreted, most enzymes exist intracellularly, and each catalyzes different primary metabolic reactions. The type of biochemical reaction in a cell is determined by enzymes found in the cell. The enzymes also determine the metabolic pathways that occur in the cell. In this manner, the cellular “way of living” is considered to be evidenced by enzyme sets. In modern biology, the discovery of biological mechanisms mainly consists of investigation of enzymes produced in a cell for a specific biological phenomenon as well as biochemical reactions occurring within the cell. Our understanding of metabolism and enzymes includes content that is closer to chemistry even in the study of biology, and this chapter aims to broaden the understanding of biology using chemistry. Discussion in sections after Part II relates to how enzyme synthesis is regulated by genetic information, i.e., enzymes are synthesized only when required, and how reactions catalyzed by these enzymes form an organic network that comprises the biological pathways.

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