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CHAPTER 2 - Molecules, Cells, and Theories

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Cells are the smallest units of the body that can survive on their own under favorable conditions (i.e., homeostasis) and have all the characteristics of life. These characteristics include organization, constant chemical activity, external or internal movement, an active response to stimulation, and reproduction. By possessing and carrying out the characteristics of life, cells and the substances they produce constitute all the larger components in the body and perform all body functions. Though cells in the body have many features in common, they are also specialized in a variety of ways and therefore can perform different functions. For example, muscle cells can contract to provide gross movements, and bone cells can secrete materials that make hard and rigid bones, which provide support and protection.

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Fig. 2.11

(a) Two amino acids, each with a short segment of -N-C-C- and side groups of hydrogen (H), oxygen (O), and R-groups. (b) A chain of amino acids (i.e., a polypeptide).

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Common reactions with *FRs involving fatty acids are shown next, where PUFA represents a polyunsaturated fatty acid. These reactions occur in three processes called initiation, propagation, and termination. The reactions are chain reactions because propagation or reinitiation may occur repeatedly before termination occurs. Each time propagation or reinitiation occurs, another damaged fatty acid is produced and a new *FR is formed, leaving a wake of oxidized damaged molecules. Oxidation damage from free radicals spreads though the cell like oxidation damage from a fire spreads from one house to the next along a street in a neighborhood. Since free radicals form in many areas in a cell, many molecular “fires” can be spreading at once through a molecular “neighborhood” such as a cell membrane (Fig. 2.18). 

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Glucose joins chemically with certain amino acids in proteins. No enzymes are needed for these reactions, and they usually occur with the side groups of arginine and lysine (Fig. 2.11, see R-). The products are altered amino acids attached to glucose (i.e., Amadori products). The amino acid/glucose portion may break down to form a distorted protein plus an ROS (e.g., H₂O₂), and the ROS may form *FRs. Distorting proteins in this way is like bending and twisting a wire clothes hanger until it is useless.
 

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mitochondria produce more ^*FRs and ROS. Though mitochondria contain antioxidants and enzymes to eliminate them, some ^*FRs and ROS escape from the mitochondria and cause damage in other parts of the cell or the body. The ^*FRs and ROS also damage the mitochondria, especially their inner membrane and their DNA. Mitochondrial DNA (mtDNA) damage is greatest in active non-dividing cells (e.g., heart, muscle, brain). Damaged mitochondria also produce less ATP. All these changes increase with age and may be a main cause of aging. (see Mitochondrial Theory and Mitochondrial DNA Theory below).

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Genes play several significant roles in aging. Many genes seem to influence the very different life spans among different species (e.g., flies, mice, humans). Of the estimated 30,000 genes in humans, scientists estimate that 70 to 7,000 of them many influence aging itself.

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Physiological Theories
Information about new theories of aging will be added to this section.