Anabolism (from Greekana, "upward", and ballein, "to throw") is the set ofmetabolic pathways that construct molecules from smaller units. [1] These reactions require energy. One way ofcategorizing metabolic processes , whether at the cellular, organ or organism level is as 'anabo lic' or as ' catabolic' , which is the opposite. Anabolism is powered by catabolism , where large molecules are broken do wn into smaller parts and then used up in respiration. Many anabolic processes are powered by adenosine triphosphate (ATP). [2] Anabolic processes tend toward "building up" organs and tissues. These processes produce growth and differentiation of cells and increase in body size , a process that involves synthesis ofcomplex molecules . Examples of anabolic processes include the growth and mineralization ofbone and increases in muscle mass. Endocrinologists have traditionally classified hormones as anabolic or cat abolic , depending on which part of metabolism they stimulate. The c lassic anabolic hormones are the anabolic steroids, which stimulate protein synthesis and muscle growth. The balance betwee n anabolism and catabolism is also regulated by circadian rhythms , with processes such as glucose metabolism fluctuating to match an animal's normal periods o f activity throughout the day. [3] Catabolism (Greekkata = downward + ballein = to throw) is the set of pathways that breakdown molecules into smaller units and release energy. [1] In catabolism, large molecules such as polysaccharides ,lipids,nucleic acids andproteins are broken down into smaller uni ts such as monosaccharides ,fatty acids,nucleotides, and amino acids, respectively. As molecules such as polysaccharides , proteins , and nucleic acids are made from long chains of these small monomer units (mono = one + mer= part), the large molecules are ca lled polymers ( poly = many). Cells use the monomers released from breaking down polymers to either construct new polymermolecules , or degrade the monomers further to simple waste products , releasin g e nergy. Cellularwastes include lactic acid,acetic acid,carbon dioxide ,ammonia, and urea. The creation o f these wastes is usually an oxidation process involving a release of chemical free energy , some ofwhich is lost as heat, but the rest of which is used to drive the synt hesis ofadenosine triphosphate (ATP). This molecule acts as a wa y for the cell to transfer the energy released by catabolism to the energy-requiring reactions that make up anabolism . Catabolism therefore provides the chemical energy necessary for the maintenance and growth of cells. Examples ofcatabolic processes include glycolysis, the citric acid cycle, the breakdown of muscle prot ein in order to use amino acids as substrates forgluconeogenesis and breakdown offat in adipose tissueto fatty acids. There are many signals that control catabolism. Most of the known signals are hormones and the molecules involved in metabolism itself. Endocrinologists have traditionally classified many ofthe hormones as anabolic or catabolic, depending on which part o f metabolism they stimulate. The so-called classic catabolic hormones known since the early 20th century are cortisol ,glucagon, and adrenaline (and othercatecholamines ). In recent decades, many more hormones with at least some catabolic effects have been discovered , including cytokines,orexin (also known as hypocretin), and melatonin . [citation needed]
