Metabolism (chemistry)

I

Introduction

Metabolism (chemistry), inclusive term for the chemical reactions by which the cells of an organism transform energy, maintain their identity, and reproduce. All life forms—from single-celled algae to mammals—are dependent on many hundreds of simultaneous and precisely regulated metabolic reactions to support them from conception through growth and maturity to the final stages of death. Each of these reactions is triggered, controlled, and terminated by specific cell enzymes or catalysts, and each reaction is coordinated with the numerous other reactions throughout the organism.

II

Anabolism and Catabolism

Two metabolic processes are recognized: anabolism and catabolism. Anabolism, or constructive metabolism, is the process of synthesis required for the growth of new cells and the maintenance of all tissues. Catabolism, or destructive metabolism, is a continuous process concerned with the production of the energy required for all external and internal physical activity. Catabolism also involves the maintenance of body temperature and the degradation of complex chemical units into simpler substances that can be removed as waste products from the body through the kidneys, intestines, lungs, and skin.

Anabolic and catabolic reactions follow what are called pathways—that is, they are linked to produce specific, life-essential end products. Biochemists have been able to determine how some of these pathways weave together, but many of the finer intricacies are still only partly explored. Basically, anabolic pathways begin with relatively simple and diffuse chemical components, called intermediates. Taking their energy from enzyme-catalyzed reactions, the pathways then build toward specific end products, especially macromolecules in the forms of carbohydrates, proteins, and fats. Using different enzyme sequences and taking the opposite direction, catabolic pathways break down complex macromolecules into smaller chemical compounds for use as relatively simple building blocks.

When anabolism exceeds catabolism, growth or weight gain occurs. When catabolism exceeds anabolism, such as during periods of starvation or disease, weight loss occurs. When the two metabolic processes are balanced, the organism is said to be in a state of dynamic equilibrium.

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III

How Metabolism Derives Its Energy

In keeping with the first two laws of thermodynamics, organisms can neither create nor destroy energy but can only transform it from one form to another. Thus, the chlorophyll of plants, at the foundation of almost all food and energy-transfer webs (see Food Web), captures energy from sunlight and uses it to power the synthesis of living plant cells from inorganic substances such as carbon dioxide, water, and ammonia. This energy, in the form of high-energy products (carbohydrates, fats, and proteins), is then ingested by herbivores and secondarily by carnivores, providing these animals with their only source of energy and cell-building chemicals.

Virtually all living organisms, therefore, ultimately derive their energy from the sun. On reproducing, each species member—whether green plant, herbivore, or carnivore—passes on specific genetic instructions on how to intercept, transform, and finally release energy back into the environment during its life span. Metabolism, from a thermodynamic point of view, embraces the processes by which cells chemically intercept and distribute energy as it continuously passes through the organism.

IV

Food and Energy

All organisms depend on energy from food for life. Carbohydrates, fats, and proteins are synthesized in plants during periods of available sunlight and stored in tubers (potatoes) or roots (sugar maples), to be drawn on during periods when new growth calls for large energy expenditure.

Food energy is expressed in calories. (In energy metabolism this unit usually refers to the large calorie, or kilocalorie: the amount of heat energy required to raise the temperature of 1 kg of water by 1° C.) Carbohydrates have an average value of 4.1 calories per gram, proteins have 5.7 calories per gram, and fats have an average of 9.3 calories per gram. Organisms rely more heavily on one or another of these foods to suit particular needs. An arctic fox, for example, depends almost entirely on lightweight, high-energy-yielding fats. Seeds, which must be light in weight yet contain large amounts of energy, are likely to contain a high percentage of oils. A sugar maple, however, which leads a fixed existence and has ample storage space in its roots, relies almost entirely on carbohydrates in the form of sucrose.

When foods—especially in the form of carbohydrates and fats—are burned in the animal system, they yield the same calories per gram as when undergoing rapid combustion in a laboratory calorimeter. Mechanical engines, in fact, yield the same number of calories per weight of fuel as animal systems. Mechanical and animal systems also yield large amounts of heat energy and relatively small amounts of work energy. Animal muscle yields only about one calorie of work for every four given up as heat. In animal systems, however, heat does not go entirely wasted. It is needed (especially by warm-blooded animals) to maintain body temperature and to induce metabolic reactions, which at lower temperatures would take place too slowly to be able to maintain bodily functions.

Although living cells conform to the same laws of energy transformation as do machines, their modes of functioning are infinitely more versatile. One unique characteristic of living systems is their ability to consume their own tissues after they have exhausted all other food-energy stores. Another is that instead of radically releasing energy through rapidly combusting compounds, as an automobile engine does, living cells release energy in step-by-step chemical reactions. The energy yielded by one chemical reaction drives other reactions, enabling a gradual release of work energy with minimum fatigue to the cells.