Catabolism is the part of the metabolism responsible for breaking complex molecules down into smaller molecules. The other part of the metabolism, anabolism, builds simple molecules into more complex ones. During the catabolism energy is released from the bonds of the large molecules being broken down.
Typically, that energy is then stored in the bonds of adenosine triphosphate (ATP). The catabolism increases the concentration of ATP in the cell as it breaks down nutrients and food. The ATP, in such high concentrations, becomes much more likely to give up its energy in the release of a phosphate. The anabolism then uses this energy to combine simple precursors into complex molecules that add to the cell and store energy for cell division.
Many pathways in the catabolism have similar versions in the anabolism. For example, large fat molecules in an organism’s food must be broken down into the small fatty acids that it is comprised of. Then, for the organism to store energy for winter, large fat molecules must be created and stored.
Catabolic reactions break the fats down, and anabolic pathways rebuild them. These metabolic pathways often use the same enzymes. To decrease the chance that the pathways will undo each other’s progress, the pathways often inhibit each other and are separated into different organelles in eukaryotes.
Examples of Catabolism
Carbohydrate and Lipid Catabolism
Almost all organisms use the sugar glucose as a source of energy and carbon chains. Glucose is stored by organisms in larger molecules called polysaccharides. These polysaccharides can be starches, glycogen, or other simple sugars like sucrose. When an animal’s cells need energy, it sends signals to the parts of the body that store glucose, or it consumes food.
Glucose is released from the carbohydrates by special enzymes, in the first part of the catabolism. The glucose is then distributed into the body, for other cells to use as energy. The catabolic pathway glycolysis then breaks glucose down even further, releasing energy that is stored in ATP. From glucose, pyruvate molecules are made. Further catabolic pathways create acetate, which is a key metabolic intermediate molecule. Acetate can become a wide variety of molecules, from phospholipids, to pigment molecules, to hormones and vitamins.
Fats, which are large lipid molecules, are also degraded by the metabolism to produce energy and to create other molecules. Similar to carbohydrates, lipids are stored in large molecules, but can be broken down into individual fatty acids. These fatty acids are then converted through beta-oxidation into acetate.
Again, acetate can be used by the anabolism, to produce larger molecules, or as part of the citric acid cycle which drives respiration and ATP production. Animals use fats to store large amount of energy for future use. Unlike starches and carbohydrates, lipids are hydrophobic, and exclude water. In this way, a lot of energy can be stored without the heavy weight of water slowing the organism down.
Most catabolic pathway are convergent in that they end in the same molecule. This enables organisms to consume and store energy in a variety of different forms, while still being able produce all the molecules it needs in the anabolic pathways. Other catabolic pathways, such as protein catabolism discussed below, create different intermediate molecules are precursors, known as amino acids, to build new proteins.
All proteins in the known world are formed of the same 20 amino acids. That means that the proteins in plants, animals, and bacteria are all just different combinations of the 20 amino acids. When an organism consumes a smaller organism, all of the protein in that organism must be digested in the catabolism.
Enzymes known as proteinases break the bonds between the amino acids in each protein, until the acids are completely separated. Once separated, the amino acids can be distributed to the cells of the body. According to the organism’s DNA, the amino acids will be recombined into new proteins.
If no source of glucose is present, or there are too many amino acids, the molecules will enter further catabolic pathways to be broken down into carbon skeletons. These small molecules can be combined in gluconeogenesis to create new glucose, which the cells can use as energy or store in large molecules.
During starvation, cellular proteins can go through the catabolism to allow an organism to survive on its own tissues until more food is found. In this way, organisms can live with only small amounts of water for extremely long times. This makes them much more resilient to changing environmental conditions.