Question

The PAN blocks which of the following reactions?
(A) ATP synthesis
(B) Hill reaction
(C) Glycolysis
(D) $CO_2$ fixation

Answer 1

In photochemical smog, peroxyacyl nitrates (also known as Acyl peroxy nitrates, APN, or PANs) are potent respiratory and ocular irritants. They are nitrates formed by the gas-phase oxidation of a range of volatile organic compounds (VOCs) or by aldehydes and other oxygenated VOCs oxidising in the presence of $NO_2$ in the thermal equilibrium between organic peroxy radicals.

Complete answer:
PANs are secondary pollutants, meaning they are generated from other pollutants in the atmosphere by chemical processes rather than being directly discharged as exhaust from power plants or internal combustion engines. Unburned hydrocarbons are oxidised to aldehydes, ketones, and dicarbonyl compounds by free radical reactions accelerated by UV radiation from the sun. These secondary processes produce peroxyacyl radicals, which mix with nitrogen dioxide to generate peroxyacyl nitrates.
The process through which light energy is absorbed and transformed to chemical energy is known as photosynthesis. This chemical energy is finally employed by plants to convert carbon dioxide into sugar.
As part of photosynthesis, the Hill reaction involves the light-driven transfer of electrons from water to Hill reagents (non-physiological oxidants) in the opposite direction of the chemical potential gradient.
PAN induces Hill reaction during photosynthesis by damaging chloroplasts, inhibiting electron systems, and sabotaging enzyme systems that govern cellular metabolism.
As a result, option B is the proper answer.

Note:
Photochemical smog component peroxyacetyl nitrate (PAN) limits electron transport in both photosystems of isolated chloroplasts with no uncoupling (no increase in basal electron flow rate). The inhibition caused by PAN is stimulated by light, but only in old chloroplasts. PAN (peroxyacetyl nitrate) is a phytotoxic air pollutant formed when hydrocarbons and nitrogen oxides combine under the influence of light. In closed habitats as well as in the open, this pollutant can stifle plant development.