Anabolism
Anabolism is the building up aspect of metabolism. During anabolism, biochemical pathways assemble complex organic molecules from simpler ones. The breaking down aspect of metabolism, catabolism, involves processes by which living things extract energy from food. Reactions in metabolic pathways involve energy. All energy utilized by living things originates with sunlight. Green plants convert the sun's radiant energy into chemical energy during the process of photosynthesis, and store it initially in the bonds of glucose. When the cells of living things need energy, the process of cellular respiration transfers energy from the bonds of glucose to the bonds of adenosine triphosphate (ATP). The energy in ATP is then available for work by the cell. Photosynthesis is an anabolic process that builds up glucose from carbon dioxide and water and gives off oxygen. Cellular respiration is a catabolic process that breaks down glucose and gives off carbon dioxide and water. Metabolic pathways also use enzymes, organic catalysts, that lower the activation energy that cells need to start up individual reactions.
During anabolism, cells join small organic molecules together to form larger ones. They synthesize large molecules that belong to four classes of organic compounds: carbohydrates, lipids, proteins, and nucleic acids. Macromolecules are giant sized molecules that cells synthesize. Some proteins, for example, contain thousands of atoms and weigh more than 100,000 daltons. Polymers are large molecules that consist of many similar building blocks, monomers, linked together by chemical bonds.
In the presence of enzymes, molecular subunits attach to each other by condensation reactions also known as dehydration reactions. Two molecules bond covalently and give off a molecule of water. One provides a hydroxyl (-OH) and the other provides a hydrogen (-H). to form water. The chain grows as condensation reactions add more subunits.
Monosaccharides or simple sugars are the monomers of carbohydrates. Glucose, fructose, maltose, and ribose are examples of monosaccharides. Disaccharides are double sugars that contain two monosaccharides bonded by condensation. Maltose is a disaccharide synthesized from two glucose molecules. Polysaccharides are polymers that contain hundreds to thousands of bound monosaccharides. Starch is a polymer that stores food in plants. Lipids, consisting mostly of hydrogen, carbon, and oxygen, include fats, phospholipids, steroids, and waxes. Although lipids are macromolecules, they are not polymers because their subunits are different. For example, fats contain the subunits fatty acids and glycerol. Proteins, with their diverse functions in cells, differ greatly in structure. Nevertheless they are polymers constructed of peptide chains made from twenty different amino acids. Each peptide chain has a precise sequence of amino acids. A protein consists of one or more peptide chains that fold and twist into different shapes. The nucleic acids, DNA and RNA, function in heredity and protein synthesis. Each contains polymers know as nucleic acids. Nucleic acids are composed of a phosphate group, a five-carbon sugar, and a nitrogenous base. The nucleotides in DNA contain the sugar deoxyribose, while RNA nucleotides contain ribose. The sequence of the nitrogenous bases stores genetic information.
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