Chemical Energy from Carbohydrates

The bacterial cell wall peptidomurein is hydrolyzed by hen egg-white lysozyme [53] to give di-, tetra-, and octasaccharides by the specific hydrolysis of the glycosidic bond of A -acetyl-D-muramic acid [53]. Lysozyme will also hydrolyze the P-1 4 GlcNAc bond of chitin, but at a much lower rate. There are specific chitinases that will also hydrolyze chitin to give chitobiose and chitodex-trins. 

For aerobic organisms, NADH is reoxidized in the electron transport system, see Fig. 11.4, and pyruvate enters the TCA cycle. 

For some anaerobic organisms, such as yeasts, pyruvate is decarboxylated and NAD+ is regenerated by NADH reduction of acetaldehyde to ethanol. 

Aerobic glycolysis, however, still had not oxidized the carbohydrate to CO2 and H2O. As the complexity of life-forms continued to increase, there was a need 

As with the photosynthetic process, we see that the respiratory process is also more complicated than the simple reaction (reaction 1.3) that we wrote in Chapter 1. The energy contained in the carbon-carbon bonds of carbohydrates that are formed in photosynthesis are built up in a series of small steps (reactions) that conserve the energy from the sun. This conserved chemical energy is also obtained in a series of small chemical degradation reactions that make available uti-lizable chemical energy in the form of ATP. 

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Nucleotides play important roles in all major aspects of metabolism. ATP, an adenine nucleotide, is the major substance used by all organisms for the transfer of chemical energy from energy-yielding reactions to energy-requiring reactions such as biosynthesis. Other nucleotides are activated intermediates in the synthesis of carbohydrates, lipids, proteins, and nucleic acids. Adenine nucleotides are components of many major coenzymes, such as NAD+, NADP+, FAD, and CoA. (See chapter 10 for structures of these coenzymes.)... 

Energy necessary for our daily activity and bodily function comes largely from the "combustion" of carbohydrates. Chemical energy from foods that is not used to maintain normal body temperature or in muscular activity is stored as fat. Thus "high-calorie" foods are implicated in obesity. 

Chloroplasts are the site of photosynthesis, the reactions by which light energy is converted to metabolically useful chemical energy in the form of ATP. These reactions occur on the thylakoid membranes. The formation of carbohydrate from CO9 takes place in the stroma. Oxygen is evolved during photosynthesis. Chloroplasts are the primary source of energy in the light. 

The substrates of catabolism—proteins, carbohydrates, and lipids—are good sources of chemical energy because the carbon atoms in these molecules are in a relatively reduced state (Figure 18.9). In the oxidative reactions of catabolism, reducing equivalents are released from these substrates, often in the form of hydride ions (a proton coupled with two electrons, H ). These hydride ions are transferred in enzymatic dehydrogenase reactions from the substrates... 

Electron-transfer reactions occur all around us. Objects made of iron become coated with mst when they are exposed to moist air. Animals obtain energy from the reaction of carbohydrates with oxygen to form carbon dioxide and water. Turning on a flashlight generates a current of electricity from a chemical reaction in the batteries. In an aluminum refinery, huge quantities of electricity drive the conversion of aluminum oxide into aluminum metal. These different chemical processes share one common feature Each is an oxidation-reduction reaction, commonly called a redox reaction, in which electrons are transferred from one chemical species to another. 

Photosynthesis occurs only in plants, algae, and some bacteria, but all forms of life are dependent on its products. In photosynthesis, electromagnetic energy from the sun is used as the driving force for a thermodynamically unfavorable chemical reaction, the synthesis of carbohydrates from C02 and H20 (Equation E9.1). 

In this process, the radiant energy of sunlight is stored as chemical energy in the molecules of carbohydrates and other compounds that are derived from them. 

Carbohydrates are formed in green plants by photosynthesis, which is the chemical combination, or fixation, of carbon dioxide and water by utilization of energy from the absorption of visible light. The overall result is the reduction of carbon dioxide to carbohydrate and the formation of oxygen ... 

Flow of energy in the biosphere. The sun s rays are the ultimate source of energy. These rays are absorbed and converted into chemical energy (ATP) in the chloroplasts. The chemical energy is used to make carbohydrates from carbon dioxide and water. The energy stored in the carbohydrates is then used, directly or indirectly, to drive all the energy-requiring processes in the biosphere. 

Photosynthesis. The biosynthesis that directly harnesses the chemical energy resulting from the absorption of light. Frequently used to refer to the formation of carbohydrates from CO2 that occurs in the chloroplasts of plants or the plastids of photosynthetic microorganisms. 

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