Metabolism refers to biochemical processes that occur with any living organism - including humans - to maintain life. These biochemical processes allow us to grow, reproduce, repair damage, and respond to our environment.
Most people use the term "metabolism" incorrectly for either anabolism or catabolism.
The modern English word "metabolism" comes from the Greek noun metabole, meaning "change," and the Greek verb metaballein, meaning "to change."
Contents of this article:
Anabolism and catabolism
Anabolism is the building up of things - a succession of chemical reactions that constructs or synthesizes molecules from smaller components, usually requiring energy in the process.
Catabolism is the breaking down of things - a series of degradative chemical reactions that break down complex molecules into smaller units, and in most cases releasing energy in the process.
During anabolism, simple molecules such as amino acids are built up into larger and more complex molecules.
Anabolism builds things and consumes energy - making bigger things out of smaller things and using up energy in the process. Anabolism, or biosynthesis, allows the body to grow new cells and maintain all the tissues.1
The body uses simple molecules to create complex ones. In the same way a builder will use simple building blocks, such as bricks, to create a house. Anabolic reactions in our bodies utilize a few simple chemicals and molecules to manufacture (synthesize) a vast array of finished products. The growth and mineralization of bone and increases in muscle mass are examples of anabolism.
The anabolic process uses monomers to build polymers. A polymer is a large complex molecule made of many small molecules that are similar to each other. Those small molecules are called monomers. For example:
- Amino acids, which are simple molecules (monomers)
- ...through a series of anabolic chemical reactions build...
- Proteins, which are large and complex molecules (polymers).
Classic anabolic hormones include:2
- Growth hormone - a hormone made in the pituitary gland. Growth hormone stimulates the release of the hormone somatomedin by the liver, causing growth.
- IGF1 and other insulin-like growth factors - hormones that simulate the production of protein and sulfate. IGF I and II are involved in the growth of the uterus and placenta, as well as the early growth of the fetus during pregnancy.
- Insulin - a hormone made by the beta cells in the pancreas. It regulates the level of sugar glucose in the blood. Cells cannot utilize glucose without insulin.
- Testosterone - a male hormone produced mainly in the testes. Testosterone causes the development of male sex characteristics, such as a deep voice and a beard. It also strengthens muscles and bone mass.
- Estrogen - a female hormone produced mainly in the ovaries. It is also involved in strengthening bone mass, as well as developing female characteristics such as breasts. It is also involved in the thickening of the inner membrane of the uterus (endometrium) and other aspects of regulating the menstrual cycle.
Adrenaline, a hormone produced by the adrenal gland, plays a part in the process of catabolism.
Catabolism breaks things down and gives out energy. Using bigger things to make smaller things and releasing energy in the process. Catabolism provides the energy our bodies need for physical activity, from a cellular level right up to whole body movements.3
The catabolic chemical reactions in the living cell break down polymers into their constituent monomers. For example:
- Polysaccharides are broken down into monosaccharides
Complex carbohydrates, such as starch, glycogen, and cellulose are polysaccharides.
Simple carbohydrates, such as glucose, ribose, and fructose are monosaccharides.
- Nucleic acids are broken down into nucleotides
Nucleic acids are the chemical basis of life and heredity - they encode our genetic information. They serve as transmitters of genetic information. Examples are RNA (Ribonucleic acid) and DNA (Deoxyribonucleic acid).
Nucleic acids are broken down to purines, pyrimidines and pentoses, which among other things, are involved in our body's energy supply.
- Proteins are broken down into amino acids
The amino acids produced by catabolism may be directly recycled, used to make new amino acids, or be converted to other compounds. Sometimes protein is broken down into amino acids to make glucose, which appears in the blood.
When we eat our body breaks down the organic nutrients - this breaking down process releases energy, which is stored inside molecules of adenosine triphosphate (ATP) in the body.
Classic catabolic hormones include:2
- Cortisol - known as the "stress hormone," as it is involved in response to stress and anxiety. It is produced by the adrenal cortex, which is part of the adrenal gland. It increases blood pressure and blood sugar, and reduces immune responses.
- Glucagon - a hormone made by the alpha cells in the pancreas. It stimulates the breakdown of glycogen by the liver which causes blood sugar levels to rise. Glycogen is carbohydrate that is stored in the liver and used as a fuel during physical activity. When the hormone glucagon is released it stimulates the liver to the break down glycogen which enters the blood as fuel (sugar).
- Adrenalin - a hormone produced by the medulla of the adrenal gland, adrenalin is also known as epinephrine. Adrenalin causes the heart beat to accelerate, strengthens the force of the heart's contraction, and opens up the bronchioles in the lung. This hormone is part of the "fight-or-flight" reaction animals and humans have in response to fear.
- Cytokines - this hormone is a small protein that has a specific effect on how interactions between cells occur, how they communicate, and how they behave. Examples are interleukins and lymphokines which are released in the generation of an immune response.
The energy stored in ATP is the fuel for anabolic reactions. Catabolism creates the energy that anabolism consumes for synthesizing hormones, enzymes, sugars and other substances for cell growth, reproduction, and tissue repair.
If catabolism is producing more energy than anabolism requires there will be excess energy. The human body stores this excess energy in fat or glycogen.
Fat tissue is relatively inactive compared to muscle or organ tissue and other systems in our body. Fat cells, because of their relative inactivity, do not use much energy to maintain themselves, compared to or types of cells.
On the next page, we look at tests and diagnosis of and the available treatment options for the condition.