Metabolic processes are sequences of biochemical reactions that take place within living cells to maintain life. They can be divided into two main types. Catabolic processes, or pathways, involve the breakdown of complex molecules from food into smaller units that can be used as building blocks for new molecules or to provide energy. Anabolic pathways involve the use of energy to build new chemicals that become components of cells. These reactions are made possible by a number of organic catalysts known as enzymes.
Together, the two types of metabolic processes allow the transformation of the raw materials, or nutrients, that are taken in by an organism into tissue. One compound, common to all cellular life, is essential to these transformations. Adenosine triphosphate (ATP) is used to store energy obtained from nutrients, such as carbohydrates, and to release energy when it is required for the building of new molecules.
Some organisms, such as green plants, make their own food from inorganic materials, while others, such as animals, consume organic materials to obtain their nutrition. The food consumed by animals can be broken down into three main types — carbohydrates, lipids (fats and oils), and proteins. Digestion involves catabolic processes that break these down into simpler components. For example, relatively complex carbohydrates, such as polysaccharides and disaccharides are broken down into glucose, and proteins are broken down into amino acids. These simpler compounds may be used by anabolic processes to build new materials, or they may be further broken down to provide energy.
Cellular respiration is the process by which the carbohydrate glucose (C6H12O6) is broken down into carbon dioxide (CO2) and water (H2O), producing energy that is stored in ATP. The procedure involves oxidation, and where there is atmospheric oxygen available it is used in what is known as aerobic respiration. This is the process that takes place in animals, plants and some microorganisms. The overall reaction can be summarized as
but the process takes place in a number of stages, and the details are quite complex.
In conditions where no free oxygen is present, anaerobic respiration takes place. This is found only in certain microorganisms that live in soil, decaying organic matter, under the sea, deep underground, and in the intestines of animals. These organisms use alternatives such as nitrates, sulfates, fumarate, and even sulfur in place of free oxygen. Anaerobic respiration is much less efficient than the aerobic process, and produces much less ATP, but early in the Earth’s history — before there was any free oxygen in the atmosphere — this was the only form of respiration possible.
In animals, lipids are also oxidized to carbon dioxide and water, but the first few steps are different. The chemistry of organisms takes place in a water-based environment, but fats and oils do not mix with water. The first step is to emulsify these substances, which means converting them into a form that will mix with water, in the same way that detergents can help clean up oil spills. This is done by soap-like substances contained in bile released by the gall bladder into the small intestine. The lipids are then broken down into fatty acids and glycerol, which can be absorbed through the intestines, and which can then undergo oxidation reactions similar to those performed on carbohydrates.
Proteins are very large, complex molecules, made up of building blocks known as amino acids. They are metabolized by various reactions that split them up into their amino acids, which can be absorbed, and then used within cells. Generally, proteins are not used to provide energy, but instead the amino acids are utilized to manufacture new proteins to build tissue and muscle. In cases where no carbohydrate or fat is available in the diet, and the body has used up its fat reserves, proteins may be used to generate energy, by oxidation of their amino acids. In these cases, the body may start breaking down muscle proteins.
Also known as biosynthesis, these are reactions that use up the energy stored in ATP by catabolic processes. They include the building of proteins from amino acids and the construction of DNA from nucleotides. In animals, the muscle contractions that power movement can also be included, as these require the use of stored energy. In plants, the synthesis of glucose from carbon dioxide and water through photosynthesis is another anabolic pathway.
Metabolic Processes and Body Weight
The energy obtained from food by catabolism may be used straight away, by anabolic processes, but if it is not used, much of it is stored as fat. The amount of energy that can be extracted from a food can be measured in calories. Similarly, the amount used up by various forms of exercise can also be measured this way. If consistently more calories are consumed than are used, fat will build up, resulting in weight gain. Conversely, if more calories are used up than are consumed, the body will obtain energy by using its fat reserves, losing weight.