Question

Explain how the carbohydrates in the food are digested and in what form they are then absorbed into the bloodstream from the intestine?

Inside the cells, the carbohydrates are degraded in a reaction pathway called glycolysis. What is formed in glycolysis, ie what is the end product?

How many ATPs are formed, in glycolysis, when one glucose molecule is degraded? Are these ATPs formed by phosphorylation at the substrate level or by oxidative phosphorylation? Explain your answer.

If the cell has enough energy, glycolysis can be slowed down by its
speed regulating step. Which step is this, and which metabolite is formed from glucose up to this step?

What can the metabolite, which is formed in the glycolysis up to that
speed regulating step, used in the cell that has enough energy from before?

Answer 1

Carbohydrates that are eaten are first broken down into their monomeric forms. There exist various enzymes for this purpose. For example, maltase hydrolysis a maltose into two glucose monomers, alpha-smylase can break down glycogen, etc. Once the carbohydrates are broken down to monosaccharides, they enter the process of glycolysis at various steps. Galactose is converted to UDP-galactose and then UDP-glucose which then forms glucose-1-phosphate and then glucose-6-phosphate which can finally enter glycolysis. Fructose can either enter as fructose-6-phosphate or at the Glyceraldehyde-3-phosphate step. Glycolysis is the process by which the monosaccharides (glucose, fructose, galactose) are broken down into pyruvate.

Pyruvate is the end product of glycolysis.

Overall 2 ATP (net) are produced in glycolysis by degradation of one glucose molecule

These ATPs are formed by substrate level phosphorylation because the ADP is converted to ATP using the inorganic phosphate directly from the substrate.

The step in glycolysis involving the enzymes Phosphofructokinase-1 (PFK) is the speed regulating step. Upto this step, Fructose-6-phosphate is formed and then PFK converts it to Fructose-1,6-bisphosphate using ATP.

When the cell has enough energy, the fructose-6-phosphate can be converted to glucose-6-phosphate (reversibly) and then the glucose-6-phosphate becomes the starting material for glycogen synthesis. The glycogen acts as a storage of glucose for future use.

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