Cellular respiration occurs in three distinct, yet interconnected, series of reactions: glycolysis (gli-kol' i-sis), the citric acid cycle, and the electron transport chain
(oxidative phosphorylation) (fig. 4.6). The products of these reactions include CO2, water, and energy. Although most of the energy is lost as heat, almost half is captured in a form that the cell can use through the synthesis of ATP (adenosine triphosphate), an energy-rich molecule.
Cellular respiration includes aerobic reactions (require oxygen) and anaerobic reactions (do not require oxygen). For each glucose molecule that is decomposed completely by cellular respiration, up to thirty-eight molecules of ATP can be produced. All but two ATP molecules are formed by the aerobic reactions.
Each ATP molecule consists of three main parts—an adenine, a ribose, and three phosphates in a chain (fig. 4.7). The third phosphate of ATP is attached by a high-energy bond, and the chemical energy stored in that bond may be quickly transferred to another molecule in a metabolic process. When such an energy transfer occurs, the terminal, high-energy bond of the ATP molecule breaks, releasing its energy. Energy from the breakdown of ATP powers cellular work such as skeletal muscle contraction, active transport across cell membranes, or secretion.
An ATP molecule that loses its terminal phosphate becomes an ADP (adenosine diphosphate) molecule, which has only two phosphates. However, ATP can be resynthesized from an ADP by using energy released from cellular respiration to reattach a phosphate, a process known as phosphorylation. Thus, as shown in figure 4.8, ATP and ADP molecules shuttle back and forth between the energy-releasing reactions of cellular respiration and the energy-utilizing reactions of the cell.
ATP is not the only kind of energy-carrying molecule within a cell, but it is the primary one. Without enough ATP, cells quickly die.
99 What is anaerobic respiration? Aerobic respiration?
Q What happens to the energy that cellular respiration releases?
H What are the final products of cellular respiration? Q What is the function of ATP molecules?
Both aerobic and anaerobic pathways begin with glycoly-sis. Literally "the breaking of glucose," glycolysis is a series of ten enzyme-catalyzed reactions that break down the 6-carbon glucose molecule into two 3-carbon pyruvic acid molecules. Glycolysis occurs in the cytosol (see fig. 4.6), and because it does not itself require oxygen, it lire 4.6
Anaerobic respiration occurs in the cytosol and does not require oxygen, whereas aerobic respiration occurs in the mitochondria only in the presence of oxygen.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.