Metabolism

Triglyceride synthesis.

When the glycogen storage areas are filled up, hepatocytes can transform the glucose to glycerol and fatty acids that can be used for lipogenesis (lip-ō-JEN-e-sis), the synthesis of triglycerides. Triglycerides then are deposited in adipose tissue, which has virtually unlimited storage capacity.

Metabolism

All of the chemical reactions that occur within an organism

Fate of glucose

ATP production, amino acid synthesis, glycogen synthesis, triglyceride synthesis

The 10 reactions of glycolysis

1.Glucose is phosphorylated, using a phosphate group from an ATP molecule to form glucose 6-phosphate. Glucose 6-phosphate is converted to fructose 6-phosphate. A second ATP is used to add a second phosphate group to fructose 6-phosphate to form fructose 1,6-bisphosphate. and Fructose splits into two 3-carbon molecules, glyceraldehyde 3-phosphate (G 3-P) and dihydroxyacetone phosphate, each having one phosphate group. Oxidation occurs as two molecules of NAD+ accept two pairs of electrons and hydrogen ions from two molecules of G 3-P to form two molecules of NADH. Body cells use the two NADH produced in this step to generate ATP in the electron transport chain. A second phosphate group attaches to G 3-P, forming 1,3-bisphosphoglyceric acid (BPG). through These reactions generate four molecules of ATP and produce two molecules of pyruvic acid (pyruvate*).

Amino acid synthesis

Cells throughout the body can use glucose to form several amino acids, which then can be incorporated into proteins.

anabolism

Chemical reactions that combine simple molecules and monomers to form the body's complex structural and functional components Anabolic reactions are the formation of peptide bonds between amino acids during protein synthesis, the building of fatty acids into phospholipids that form the plasma membrane bilayer, and the linkage of glucose monomers to form glycogen. Anabolic reactions are endergonic; they consume more energy than they produce. Metabolism is an energy-balancing act between catabolic (decomposition) reactions and anabolic (synthesis) reactions. The molecule that participates most often in energy exchanges in living cells is ATP (adenosine triphosphate), which couples energy-releasing catabolic reactions to energy-requiring anabolic reactions.

Glycogen synthesis.

Hepatocytes and muscle fibers can perform glycogenesis (glī′-kō-JEN-e-sis; glyco- = sugar or sweet; -genesis = to generate), in which hundreds of glucose monomers are combined to form the polysaccharide glycogen. Total storage capacity of glycogen is about 125 g in the liver and 375 g in skeletal muscles.

ATP production

In body cells that require immediate energy, glucose is oxidized to produce ATP. Glucose not needed for immediate ATP production can enter one of several other metabolic pathways.

catabolism

Those chemical reactions that break down complex organic molecules into simpler ones are collectively known as catabolism Overall, catabolic (decomposition) reactions are exergonic; they produce more energy than they consume, releasing the chemical energy stored in organic molecules.

Role of ATP

When complex molecules and polymers are split apart (catabolism, at left), some of the energy is transferred to form ATP and the rest is given off as heat. When simple molecules and monomers are combined to form complex molecules (anabolism, at right), ATP provides the energy for synthesis, and again some energy is given off as heat.

The fate of pyruvic acid

produced during glycolysis depends on the availability of oxygen (Figure 25.5). If oxygen is scarce (anaerobic conditions)—for example, in skeletal muscle fibers during strenuous exercise—then pyruvic acid is reduced via an anaerobic pathway by the addition of two hydrogen atoms to form lactic acid (lactate): When oxygen is plentiful, pyruvic acid enters mitochondria, is converted to acetyl coenzyme A, and enters the Krebs cycle (aerobic pathway). When oxygen is scarce, most pyruvic acid is converted to lactic acid via an anaerobic pathway. Each step in the oxidation of glucose requires a different enzyme, and often a coenzyme as well. The coenzyme used at this point in cellular respiration is coenzyme A (CoA), which is derived from pantothenic acid, a B vitamin. During the transitional step between glycolysis and the Krebs cycle, pyruvic acid is prepared for entrance into the cycle. The enzyme pyruvate dehydrogenase (pī-ROO-vāt dē-HĪ-drō-jen-ās), which is located exclusively in the mitochondrial matrix, converts pyruvic acid to a 2-carbon fragment called an acetyl group (AS-e-til) by removing a molecule of carbon dioxide (Figure 25.5). The loss of a molecule of CO2 by a substance is called decarboxylation (dē-kar-bok′-si-LĀ-shun). This is the first reaction in cellular respiration that releases CO2. During this reaction, pyruvic acid is also oxidized. Each pyruvic acid loses two hydrogen atoms in the form of one hydride ion (H−) plus one hydrogen ion (H+). The coenzyme NAD+ is reduced as it picks up the H− from pyruvic acid; the H+ is released into the mitochondrial matrix. The reduction of NAD+ to NADH + H+ is indicated in Figure 25.5 by the curved arrow entering and then leaving the reaction. Recall that the oxidation of one glucose molecule produces two molecules of pyruvic acid, so for each molecule of glucose, two molecules of carbon dioxide are lost and two NADH + H+ are produced. The acetyl group attaches to coenzyme A, producing a molecule called acetyl coenzyme A (acetyl CoA).

Glucose catabolism

• 1. Glycolysis. A set of reactions in which one glucose molecule is oxidized and two molecules of pyruvic acid are produced. The reactions also produce two molecules of ATP and two energy-containing NADH + H+. During glycolysis (glī-KOL-i-sis; -lysis = breakdown), chemical reactions split a 6-carbon molecule of glucose into two 3-carbon molecules of pyruvic acid (Figure 25.3). Even though glycolysis consumes two ATP molecules, it produces four ATP molecules, for a net gain of two ATP molecules for each glucose molecule that is oxidized. • Formation of acetyl coenzyme A. A transition step that prepares pyruvic acid for entrance into the Krebs cycle. This step also produces energy-containing NADH + H+ plus carbon dioxide (CO2). • Krebs cycle reactions. These reactions oxidize acetyl coenzyme A and produce CO2, ATP, NADH + H+, and FADH2. • Electron transport chain reactions. These reactions oxidize NADH + H+ and FADH2 and transfer their electrons through a series of electron carriers.