METABOLISM

Metabolism (from Greek: μεταβολή "metabolē", "change" or Greek: μεταβολισμός metabolismos, "outthrow") is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter, for example to harvest energy in cellular respiration. Anabolism uses energy to construct components of cells such as proteins and nucleic acids. The chemical reactions of metabolism are organized into metabolic pathways , in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes . Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. As enzymes act as catalysts they allow these reactions to proceed quickly and efficiently. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell's environment or signals from other cells. The metabolism of an organism determines which substances it will find nutritious and which it will find poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals.[1] The speed of metabolism, the metabolic rate, influences how much food an organism will require, and also affects how it is able to obtain that food. A striking feature of metabolism is the similarity of the basic metabolic pathways and components between even vastly different species. [2] For example, the set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, being found in species as diverse as the unicellular bacteria Escherichia coli and huge multicellular organisms like elephants.[3] These striking similarities in metabolic pathways are likely due to their early appearance in evolutionary history , and being retained because of their efficacy. [4][5] Key biochemicals Further information: Biomolecule, cell (biology) and biochemistry Structure of a triacylglycerol lipid Most of the structures that make up animals, plants and microbes are made from three basic classes of molecule: amino acids, carbohydrates and lipids (often called fats). As these molecules are vital for life, metabolic reactions either focus on making these molecules during the construction of cells and tissues, or breaking them down and using them as a source of energy, in the digestion and use of food. Many important biochemicals can be joined together to make polymers such as DNA and proteins. These macromolecules are essential. Type of molecule Name of monomer forms Name of polymer forms Examples of polymer forms Amino acids Amino acids Proteins (also called polypeptides) Fibrous proteins and globular proteins Carbohydrates Monosaccharides Polysaccharides Starch, glycogen and cellulose Nucleic acids Nucleotides Polynucleotides DNA and RNA Amino acids and proteins Proteins are made of amino acids arranged in a linear chain and joined together by peptide bonds. Many proteins are the enzymes that catalyze the chemical reactions in metabolism. Other proteins have structural or mechanical functions, such as the proteins that form the cytoskeleton , a system of scaffolding that maintains the cell shape.[6] Proteins are also important in cell signaling, immune responses, cell adhesion, active transport across membranes, and the cell cycle .[7] Lipids Lipids are the most diverse group of biochemicals. Their main structural uses are as part of biological membranes such as the cell membrane, or as a source of energy. [7] Lipids are usually defined as hydrophobic or amphipathic biological molecules that will dissolve in organic solvents such as benzene or chloroform.[8] The fats are a large group of compounds that contain fatty acids and glycerol ; a glycerol molecule attached to three fatty acid esters is a triacylglyceride .[9] Several variations on this basic structure exist, including alternate backbones such as sphingosine in the sphingolipids, and hydrophilic groups such as phosphate in phospholipids. Steroids such as cholesterol are another major class of lipids that are made in cells.[10] Carbohydrates Glucose can exist in both a straight-chain and ring form. Carbohydrates are aldehydes or ketones with many hydroxyl groups that can exist as straight chains or rings. Carbohydrates are the most abundant biological molecules, and fill numerous roles, such as the storage and transport of energy (starch, glycogen ) and structural components (cellulose in plants, chitin in animals).[7] The basic carbohydrate units are called monosaccharides and include galactose, fructose, and most importantly glucose. Monosaccharides can be linked together to form polysaccharides in almost limitless ways.