Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Metabolism: An Overview
Cellular metabolism is an economical, strictly regulated process

Cellular METABOLISM relies on THE PRINCIPLE OF maximum efficiency. The overall rate of Catabolism, which supplies The Cell with energy, is not merely determined by the presence or concentration of cellular fuel; rather, it is dictated by the cell's energy demands in the form of ATP and NADPH. At any given moment, a cell consumes precisely The amount of nutrients required to meet its energetic needs. Similarly, the synthesis rate of the cell's building blocks and macromolecules is governed by its immediate requirements. In growing Cells, for instance, all 20 Amino Acids are synthesized at the exact rate and in the precise proportions necessary to facilitate the assembly of newly required Proteins. Consequently, none of the 20 amino acids are overproduced or left unused. Many animals and plants store reserve nutrients within their bodies that can serve as sources of energy and carbon—most notably, fats and CARBOHYDRATES. In contrast, proteins, Nucleic Acids, and simple Biomolecules acting as building blocks are generally not stored; instead, they are synthesized only when needed and strictly in the required quantities. There is, however, an exception to this rule: plant seeds and animal eggs frequently contain large amounts of storage proteins that serve as an amino acid source during embryonic development.

Catabolic processes exhibit high sensitivity, responding rapidly and delicately to any shifts in the cell's Energy Requirements. In the housefly, for example, oxygen consumption and cellular fuel utilization increase roughly 100-fold in less than a second during flight, driven by a sharp surge in the demand for ATP consumed by the flight Muscles. This example demonstrates that The regulatory mechanisms governing central metabolic pathways—particularly those supplying the cell with energy in the form of ATP—are highly responsive and capable of swiftly accommodating the cell's fluctuating metabolic needs.