Anatomy & Physiology

Anatomy & Physiology Study Guide controlled by specific enzymes that each catalyze a series of steps. This reaction sequence is called a pathway. It takes activation energy to start a chemical reaction, but once it has begun, the reaction absorbs or releases energy, generally in the form of heat, as it proceeds to completion. There will be a net release of energy when the amount of energy released is greater than the activation energy needed to start the reaction. Reactions that release energy are exergonic or exothermic . If more energy is required to begin the reaction than is released, then the reaction will absorb energy. These reactions are called endergonic or endothermic . Exothermic reactions are common in the body; they generate the heat that maintains body temperature. Enzyme Function Almost everything that happens inside the human body occurs due to a specific enzyme. The reactants in enzymatic reactions are called substrates . For an enzyme to function as a catalyst, the substrates must bind to a special region of the enzyme called the active site. This site is a groove or pocket into which one or more substrates fit in. This action begins the first of three steps for enzymatic reactions. Step one is when substrates bind to the enzyme at its active site. Step two is when the substrate binding typically results in a temporary, reversible change in the shape of the protein; this change may further the reaction by placing physical stresses on the substrate molecules. Step three is when the enzyme promotes product formation. The completed product then detaches from the active site, and the enzyme is free to repeat the process. Enzymes work quickly, cycling rapidly between substrates and products. A cofactor is an ion or a molecule that must bind to the enzyme before substrates can also bind. Without a cofactor, the enzyme is intact but nonfunctional; with the cofactor, the enzyme can catalyze its specific reaction. 5.8 DNA and RNA Nucleic acids are large organic molecules composed of carbon, oxygen, hydrogen, nitrogen, and phosphorus. Nucleic acids store and process information at the molecular level. The two classes of nucleic acid molecules are deoxyribonucleic acid , or DNA, and ribonucleic acid , or RNA. These two classes of nucleic acids differ in composition, structure, and function. The DNA in cells determines the inherited characteristics, like eye color, hair color, and blood type. DNA affects all aspects of body structure and function. DNAmolecules encode the information needed to build proteins. By directing the synthesis of structural proteins, DNA controls the shape and physical characteristics of the body. DNA regulates protein synthesis and all aspects of cellular metabolism. These reactions include the creation and destruction of lipids, carbohydrates, and other vital molecules. Several forms of RNA cooperate to manufacture specific proteins by using the information provided by DNA. Structure of Nucleic Acids A nucleic acid consists of one or two long chains that are formed by dehydration synthesis. The individual subunits are called nucleotides . Each nucleotide has three components: (1) a pentose (five- carbon sugar) attached to both (2) a phosphate group and (3) a nitrogenous base. The pentose is Achieve Page 56 of 368 ©2018