In order to maintain optimum body temperature, mammals need thermal energy acquired from internal chemical reactions to make up for heat loss to the environment. Additionally, the free energy of the body's chemistry sustains functions ranging from muscle contraction to the thought processes and instinctual responses of the brain. Thermal energy and free energy, represented by AH and AG respectively, are supplied mainly by reaction of oxygen with food-
borne nutrients such as carbohydrates, proteins and fats. The end products of these chemical conversions are carbon dioxide and water.
Glucose is produced in plants by photosynthesis, in which sunlight causes carbon dioxide and water to combine. The reaction products are glucose and oxygen:
Here hv stands for photons from the sun that provide the free energy that overcomes the highly unfavorable energy (enthalpy) difference between the products and the reactants. C6H12O6 is the chemical formula for the glucose molecule that is shown in a structural representation in Fig. 11.2. The oxygen product of photosynthesis is returned to the atmosphere. The radiant energy that drives this reaction is stored in the glucose molecule, to be released after vegetable matter has been consumed by humans (and other mammals) and its charge of glucose released to the body.
Two forms of the glucose molecule are shown in Fig. 11.2: the ring and the chain. Attached to the ring of 5 carbon atoms plus an oxygen atom are four OH groups and an aldehyde group (CH2OH). This form of glucose is the monomer for the polymer produced by formation of H2O by removal of the H atom at the dashed line at the top of the ring form in Fig. 11.2 from one glucose molecule and an OH group at the same location from a second glucose molecule. The two glucose molecules are then joined by a common oxygen atom. The polymeric form is called starch.
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