Metabolic processes are biochemical reactions that maintain life. Catabolic processes break down food into smaller units for energy, while anabolic processes use energy to build new chemicals. Enzymes are organic catalysts that make these reactions possible. Cellular respiration breaks down glucose into CO2 and H2O, producing ATP. Anaerobic respiration occurs without oxygen. Lipids and proteins are also broken down for energy. Anabolic processes use energy from catabolic processes to build new materials. Excess energy is stored as fat, leading to weight gain.
Metabolic processes are sequences of biochemical reactions that occur 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 series of organic catalysts known as enzymes.
Together, the two types of metabolic processes enable the transformation of raw materials, or nutrients, which are absorbed by an organism into tissue. A compound, common to all cellular life, is essential for these transformations. Adenosine triphosphate (ATP) is used to store energy obtained from nutrients, such as carbohydrates, and to release energy when it is needed for the construction of new molecules.
Catabolic processes
Some organisms, such as green plants, produce their food from inorganic materials, while others, such as animals, consume organic materials to obtain their nourishment. Food consumed by animals can be divided into three main types: carbohydrates, lipids (fats and oils), and proteins. Digestion involves catabolic processes that break them 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 can be used by anabolic processes to build new materials, or they can be broken down further 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 which is stored in ATP. The procedure involves oxidation and, where atmospheric oxygen is available, it is used in so-called aerobic respiration. This is the process that occurs in animals, plants and some microorganisms. The overall reaction can be summarized as
C6H12O6 + 6O2 -> 6CO2 + 6H2O
but the process takes place in stages and the details are quite complex.
Under conditions where there is no free oxygen, anaerobic respiration occurs. This is only found 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 Earth’s history – before there was 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 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 cleaners can help clean up oil spills. This is done by soap-like substances contained in the bile released from the gallbladder 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 break them down into their amino acids, which can be absorbed and then used within cells. Generally, protein is not used to provide energy, but instead amino acids are used to make new proteins to build tissue and muscle. In cases where carbohydrates or fats are not available in the diet and the body has depleted its fat stores, proteins can be used to generate energy, by oxidizing their amino acids. In these cases, the body may begin to break down muscle protein.
Anabolic processes
Also known as biosynthesis, these are reactions that use up the energy stored in ATP from catabolic processes. They include building proteins from amino acids and building DNA from nucleotides. In animals, muscle contractions that fuel movement may also be included, as they 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 can be used immediately, through 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. Likewise, the amount used by various forms of exercise can also be measured this way. If more calories are consistently consumed than used, fat will accumulate, resulting in weight gain. Conversely, if more calories are consumed than consumed, the body will obtain energy by using its fat reserves, thus losing weight.
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