The nutritionists generally advise the sprinters to have a carbohydrate rich diet prior to sprinting or any such physical exercise. The rationale behind this is that during physical exercise, our muscles do respiration at a faster rate to obtain more energy. The respiration process involves the breakdown of glucose in the presence of oxygen to yield carbon dioxide, water and a large amount of energy. This energy is used by the person doing physical exercise.
If there would be large amount of carbohydrate available then that could be a source of glucose for a long time and the sprinter could run for a long without getting fatigued easily. Otherwise, in the absence of excessive glucose, the sprinter would get fatigued easily in a short while.
The enzyme transaldolase is essential in the pentose phosphate shunt but it is not used in Calvin Cycle. This is because the plants have aldolase enzyme, which is more efficient than transaldolase whereas the animals use transaldolase mechanism. The plants cannot use this enzyme while animals can use it efficiently to get the required carbohydrates.
The Calvin cycle in plants is almost similar to the pentose phosphate shunt except at one step. In plants, only GAP is the input, while in animals there are two inputs X5P and R5P. When the GAP (glyceraldehydes-3-phosphate) reacts with S7P (sedoheptulose-7-phosphate) then transaldolase acts in animals and form F6P (fructose-6-phosphate) and E4P (eryhtrose-4-phosphate). On the other hand, in plants the same step occurs via DHAP (dihydroxyacetone phosphate). Thus, this is the reason that transaldolase is more active in animals while not in plants (Calvin cycle).
The muscle contraction is the process that requires energy when two proteins roll over each other. The glucose is taken up by the muscle cells, which are broken down into carbon dioxide, water and energy. The muscle cells require excessive energy which is compensated from glycogen reserves. Thus glycogenolysis occurs during the muscle contraction.
During starvation, all the body cells urge for energy, which is available in the form of glucose. So, the body obtains glucose from the stored glycogen reserves of liver.