Glucose combustion

Modem calorimetric data indicate that combustion of carbon (graphite) sufficient to produce exactly 1 ounce of carbon dioxide would melt 26.86 ounces of ice. If sufficient glucose (CgHj Og) were burned to collect the same 1 ounce of CO, one might naively have expected in 1783 that 26.86 ounces of ice would be melted. However, we know that formation of 0.41 ounces of H O would accompany the 1 ounce of CO formed in glucose combustion. The extra heat from formation of water added to the heat from formation of carbon dioxide would melt 31.89 ounces of ice. 

Reconsider Example 10.9 with active transport of Na ions from the intracellular to the extracellular space. Assume that 25% of the energy of hydrolysis of ATP can be used for the transport and that 38 mol of ATP are formed per mole of glucose (G) combusted. In the steady state, the extracellular concentration of Na" " is observed to be 145 mM and the intracellular concentration is observed to be 12 mM and the electrical potential of the intracellular fluid is determined to be -90.5 mV in relation to the extracellular space. The transcellular flux of Na+ ions is 1 mmol/min and the temperature of the tissue is 37 °C. Estimate the oxygen consumption, heat output and energy expenditure of the process when A//r,AXP = -20 kJ/mol for hydrolysis of ATP and A//r,G= —2867 kJ/mol for glucose combustion. Assume 44/ 5 = 4/5. 

We have seen that a constant-pressure calorimeter and a constant-volume bomb calorimeter measure changes in different state functions at constant volume, the heat transfer is interpreted as A U at constant pressure, it is interpreted as AH. However, it is sometimes necessary to convert the measured value of AU into AH. For example, it is easy to measure the heat released by the combustion of glucose in a bomb calorimeter, but to use that information in assessing energy changes in metabolism, which take place at constant pressure, we need the enthalpy of reaction. 

We could not live without combustion reactions the oxidation of glucose powers our bodies, and the burning of fossil fuels (coal, petroleum, and natural gas) powers our homes and vehicles. Because fossil fuels reserves are limited, alternatives are being sought (Box 6.2), but even these new fuels will be burned. Consequently, the study of combustion is critically important for our survival. 

Self-Test 6.15A Calculate the standard enthalpy of combustion of glucose from the standard enthalpies of formation in Table 6.5 and Appendix 2A. 

When 1 mol of glucose is combusted in a calorimeter to CO2 and water, approximately 2870 kj are hberated as heat. When oxidation occurs in the tissues, approximately 38 mol of ATP are generated per molecule of glucose oxidized to CO2 and water. In vivo, AG for the... 

ATP synthase reaction has been calculated as approximately 51.6 kJ. It follows that the total energy captured in ATP per mole of glucose oxidized is 1961 kJ, or approximately 68% of the energy of combustion. Most of the ATP is formed by oxidative phosphorylation resulting from the reoxidation of reduced coenzymes by the respiratory chain. The remainder is formed by substrate-level phosphorylation (Table 17—1). 

In 1886, Brown11 discovered an organism which formed extremely tough membranes when cultivated m suitable nutrient solutions containing carbohydrates such as D-fructose, D-mannitol or D-glucose ethanol, sucrose or starch did not support membrane formation by this organism which Brown called Bacterium xylinum ) (Acetobacter xylinum). The membranes were readily soluble in cuprammonium hydroxide solution and yielded a dextrorotatory sugar upon acid hydrolysis. These properties and the results of combustion analysis led him to believe that the membrane was cellulose. 

If all of the energy of combustion of 1 mole of glucose is employed in the conversion of ADP to ATP, then the maximum number of moles ATP produced is... 

Using the concentrations and pressures provided we can calculate the Gibbs energy for the combustion of glucose under biological conditions. First, we need to replace the activities by the appropriate effective concentrations. That is,... 

The value of At/(Combustion) for glucose is huge, but most values of AU are smaller, and are expressed in kilo joules per mole. 

SAQ 3.9 Calculate the standard enthalpy of combustion A He for burning S-D-glucose, C6Hi206. The required values of AHf may be found in Table 3.1. 

Carbon black is derived from vegetable material, usually peat, by complete combustion to residual carbon. The particle size is very small, usually less than 5 /rm, and consequently is very difficult to handle. It is usually sold to the food industry in the form of a viscous paste in a glucose syrup. Carbon black is very stable and technologically a very effective colorant. It is widely used in Europe and other countries in confectionery. 

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