HMP also known as the Pentose phosphate pathway. Consists of two irreversible oxidative rxs followed by a series of reversible Pentose Phosphate Pathway Reactions in Photosynthesizing Cells Explain how Inactivity or absence of Glucose 6 phosphate dehydrogenase ; a rate limiting enzyme in the Pentose phosphate pathway could lead to oxidative stress.* Pentose Phosphate Pathway made simple. This pathway has two major jobs: Generate NADPH Produce ribose 5 phosphate NADPH protects cells from oxidative stress and supports fatty acid, cholesterol, steroid, nitric oxide, and respiratory burst