Anatomy & Physiology

Anatomy & Physiology Study Guide either ribose (in RNA) or deoxyribose (in DNA). Five nitrogenous bases occur in nucleic acids: adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U). Both RNA and DNA contain adenine, guanine, and cytosine. Uracil occurs only in RNA and thymine only in DNA. The bases are joined in complementary base pairs: adenine with thymine (or uracil in RNA) and cytosine with guanine. A nucleotide will form when a phosphate group binds to a pentose already attached to a nitrogenous base. In the formation of a nucleic acid, dehydration synthesis attaches the phosphate group of one nucleotide to the sugar of another. The primary role of nucleic acids is the storage and transfer of information— specifically, information essential to the synthesis of proteins within our cells. In both DNA and RNA, it is the sequence of nitrogenous bases that carries the information. 5.9 ATP To perform their vital functions, cells must use energy obtained by breaking down organic substrates (catabolism). To be useful, that energy must be transferred from molecule to molecule or from one part of the cell to another. The usual method of energy transfer involves the creation of high-energy bonds by enzymes within cells. A high-energy bond is a covalent bond whose breakdown releases energy the cell can use directly. The resulting product is called a high-energy compound. Most high- energy compounds are derived from nucleotides, the building blocks of nucleic acids. The attachment of a phosphate group to another molecule is called phosphorylation . The creation of a high-energy compound requires (1) a phosphate group, (2) enzymes capable of catalyzing the reactions involved, and (3) suitable organic substrates to which the phosphate can be added. The most used substrate is the nucleotide adenosine monophosphate (AMP). Attaching a second phosphate group produces adenosine diphosphate (ADP). A significant energy input is required to convert AMP to ADP, and the second phosphate is attached by a high-energy bond. Even more energy is required to add a third phosphate and thereby create the high-energy compound adenosine triphosphate (ATP). The conversion of ADP to ATP is the most useful method of energy storage in our cells; the reversion of ATP to ADP is the greatest method of energy release.

©2018

Achieve Page 57

of 368