Question

The standard free energy change of phosphate hydrolysis is shown below for several molecules in the glycolytic pathway.

Molecule	?G? (kJ/mol)
Phosphoenolpyruvate	?61.9
1,3-Bisphosphoglycerate	?49.4
ATP ? ADP + Pi	?30.5
Fructose-6-phosphate	?15.9
Glucose-6-phosphate	?13.8

Which statement explains why glucose phosphorylation could not occur without ATP investment?
A.   Without ATP investment in stage I of glycolysis, the concentration of ATP would become too high and would inhibit phosphofructokinase, leading to inhibition of the entire glycolytic pathway.

B.   Without ATP investment, glucose would not become "trapped" in the cell and could easily be transported out as blood glucose levels decrease. This transport would reduce the amount of glucose available for some organs.

C.   Without ATP investment it would be impossible to regulate the entry of glucose into glycolysis.

D.   Without ATP investment, one or both of the substrates would need to exceed the solvent capacity of the cell for glucose phosphorylation to occur.

Answer 1

Answer

Glucose is the readily available source of energy for the cells. It is degraded to produce energy required by the cellular activities. Its degradation is done by a catabolic or degradative pathway called as glycolysis.

The process of glycolysis starts with the phosphorylation of glucose. Glucose is converted to glucose-6-phosphate using ATP by the enzyme glucokinase.

This glucose-6-phosphate is made available to the cell ceasing its permeability. Glucose can diffuse across the membrane but glucose-6-phosphate cannot. Glu-6P does not have carriers also for transporting across the membrane. So, it is made available to the cells.

Therefore, option B does not explain the reason for the statement.

If ATP is not consumed in stage 1 of glycolysis, it inhibits the rate limiting enzyme (phosphor-fructokinase) of glycolytic pathway.

Therefore, option A is also not the correct reason for the given statement.

Glucose enters the glycolytic pathway by its phosphorylation but not the reason for the given statement. Therefore, option C is also incorrect.

If either of the substrates, say glucose or ATP exceeds in the cell, then the solvent capacity of that substrate increases which favors the phosphorylation of glucose. Therefore, it explains that glucose phosphorylation occurs with ATP investment.

Hence, option D is the correct explanation for the given statement.