Question

Oxidation of glucose to ${ CO }_{ 2 }$ and water occurs during
(a)Glycolysis
(b)Pentose phosphate pathway
(c)Krebs cycle
(d)All of the above

Answer 1

Hint: The pathway responsible for the oxidation of glucose to ${ CO }_{ 2 }$ and water is otherwise known as the phosphogluconate pathway or the hexose monophosphate shunt. This pathway is very important in red blood cells.

Complete answer:
The pentose phosphate pathway is the only pathway where glucose is completely oxidized to both ${ CO }_{ 2 }$ and ${ H }_{ 2 }{ O }$. It is also called HMP (hexose monophosphate shunt). In the presence of glucose- 6-phosphate dehydrogenase, the primary substrate glucose is first phosphorylated to glucose-6-phosphate by consuming 1 ATP. The pentose phosphate pathway is an alternative pathway for glycolysis.
The pathway has two distinct phases: the oxidative phase, and the non-oxidative phase. The first is the oxidative step which converts glucose-6-phosphate to ribulose-5-phosphate. Throughout this process, two ${ NADP }^{ + }$ molecules are reduced to NADPH. The complete overall reaction for this process is given below.
$Glucose-6-phosphate{ 2NADP }^{ + }+{ H }_{ 2 }O\rightarrow Ribulose-5-phosphate+2NADPH+2{ H }^{ + }+{ CO }_{ 2 }$
During the oxidative step, two ${ NADP }^{ + }$ molecules are reduced to NADPH, by using the energy from converting glucose-6-phosphate into ribulose-5-phosphate. Then these NADPH molecules can be used in other cells as an energy source. The non-oxidative step produces 5-carbon sugars, which can be used in nucleotide, nucleic acid, and amino acid synthesis. In addition, the cells can use NADPH to stop oxidative stress. NADPH reduces glutathione through glutathione reductase which converts ${ H }_{ 2 }{ O }_{ 2 }$ into ${ H }_{ 2 }{ O }$.
So, the correct answer is ‘Pentose phosphate pathway’.

Note: Pentose phosphate pathway (PPP) plays a very important role in regulating cancer cell growth by providing cells with both ribose-5-phosphate and NADPH for detoxification of intracellular reactive oxygen species, which is crucial for cancer progression. Thus, suppression of the PPP contributes to cell proliferation, survival, and cancer progression.