Aerobic Metabolism

Aerobic metabolism occurs in the cell's mitochondria and involves the citric acid cycle and oxidative phosphoryla-tion. It is here that hydrogen and carbon molecules from the fats, proteins, and carbohydrates in our diet are broken down and combined with molecular oxygen to form car­bon dioxide and water as energy is released. Unlike lactic acid, which is an end product of anaerobic metabolism, carbon dioxide and water are generally harmless and eas­ily eliminated from the body. In a 24-hour period, oxida­tive metabolism produces 300 to 500 mL of water.

The citric acid cycle, sometimes called the tricarboxylic acid or Krebs cycle, provides the final common pathway for the metabolism of nutrients (see Fig. 4-15). In the citric acid cycle, an activated two-carbon molecule of acetyl coenzyme A (acetyl-CoA) condenses with a four-carbon molecule of oxaloacetic acid and moves through a series of enzyme-mediated steps. This process produces hydrogen and carbon dioxide. As hydrogen is generated, it combines with one of two special carriers, NAD⁺ or flavin adenine dinucleotide (FADH⁺), for transfer to the electron transport system. The carbon dioxide is converted to bicarbonate or carried to the lungs and exhaled. In the citric acid cycle,

each of the two pyruvate molecules formed in the cyto­plasm from one molecule of glucose yields another mole­cule of ATP along with two molecules of carbon dioxide and eight hydrogen atoms. These hydrogen atoms are trans­ferred to the electron transport system on the inner mito­chondrial membrane for oxidation. Besides pyruvate from the glycolysis of glucose, products of amino acid and fatty acid degradation enter the citric acid cycle and contribute to the generation of ATP.

Oxidative metabolism, which supplies 90% of the body's energy needs, is the process by which hydrogen generated during the citric acid cycle combines with oxy­gen to form ATP and water. It is accomplished by a series of enzymatically catalyzed reactions that split each hy­drogen atom into a H⁺ ion and an electron. During the process of ionization, the electrons removed from the hy­drogen atoms enter an electron transport system found on the inner membrane of the mitochondrion (Fig. 4-15). This electron transport chain consists of electron acceptors that can be reversibly reduced or oxidized by accepting or giving up electrons. Among the members of the electron transport system are several proteins, including a set of iron-containing molecules called cytochromes. Each elec­tron is shuttled from one acceptor to another until it reaches the end of the chain, where its final two electrons are used to reduce elemental oxygen, which combines with the hydrogen ions to form water. As the electrons move along the electron transport chain, large amounts of en­ergy are released. This energy is used to convert ADP to ATP. Because the formation of ATP involves the addition of a high-energy phosphate bond to ADP, the process is sometimes called oxidative phosphorylation. Cyanide poi­soning kills by binding to the enzymes needed for a final step in the oxidative phosphorylation sequence.

In summary, cells communicate with each other by means of chemical messenger systems. In some tissues, chemical mes­sengers move from cell to cell through gap junctions without entering the extracellular fluid. Other types of chemical mes­sengers bind to receptors on or near the cell surface. Three classes of cell surface receptor proteins are: G-protein linked, ion-channel linked, and enzyme linked. G-protein-linked receptors rely on a class of molecules called G proteins that function as an on-off switch to convert external signals (first messengers) into internal signals (second messengers). Ion-linked signaling is mediated by neurotransmitters that tran­siently open or close ion channels formed by integral proteins in the cell membrane. Enzyme-linked receptors interact with certain peptide hormones, such as insulin and growth factors, and directly initiate the activity of the intracellular protein-tyrosine kinase enzyme.

The life of a cell is called the cell cycle. It is usually divided into five phases: G₀, or the resting phase; G1, during which the cell begins to prepare for division through DNA and protein synthesis; the S or synthetic phase, during which DNA replica­tion occurs; G₂, which is the premitotic phase and is similar to G1 regarding RNA and protein synthesis; and the M phase, dur­ing which cell division occurs. Cell division, or mitosis, is the process during which a parent cell divides into two daughter

cells, with each receiving an identical pair of chromosomes. The process of mitosis is dynamic and continuous and is di­vided into four stages: prophase, metaphase, anaphase, and telophase.

Metabolism is the process by which the carbohydrates, fats, and proteins we eat are broken down and subsequently con­verted into the energy needed for cell function. Energy is con­verted to ATP, the energy currency of the cell. Two sites of energy conversion are present in cells: the mitochondria and the cytoplasmic matrix. The most efficient of these pathways is the aerobic citric acid pathway in the mitochondria. This pathway requires oxygen and produces carbon dioxide and water as end products. The glycolytic pathway within the cy­toplasm involves the breakdown of glucose to form ATP. This pathway can function without oxygen by producing lactic acid.