Question

Assertion: The dark reaction occurs only at night in the stroma of a chloroplast.
Reason: $CO_2$ fixation occurs only during the $C_3$ cycle.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C. Assertion is correct but Reason is incorrect
D. Both Assertion and Reason are incorrect

Answer 1

Photosynthesis is only the process by which green plants and certain other organisms transform light energy into chemical energy continuously in presence of sunlight. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds in a continuous and respective process.

Complete answer:
Dark reaction- generally any of the chemical reactions that take place during the second stage of photosynthesis or after light reaction and do not require light either day or night no specific is known as a dark reaction. During the dark reactions, the energy released from ATP which is only created by the light reactions, continues the fixation of carbon from carbon dioxide in organic molecules in a cyclic manner.

$CO_2$ fixation- Carbon fixation or carbon assimilation is the process by which inorganic carbon is converted to organic compounds by living organisms in a chemical process in presence of sunlight.

So the Dark reaction is additionally referred to as the light-independent part. Unlike photochemical reactions, it doesn't need light as a vital factor, thus, each will take place within the presence or absence of sunshine. The term dark reaction doesn't mean that it takes place solely in a dark period or at night-time. $CO_2$ fixation happens in each $C_3$ $C_4$ and cycle. In $C_3$ cycle, $CO_2$ is added by the catalyst, RuBisCo to a five-carbon compound RuBP that's reborn to two molecules of PGA. In$C_4$ cycle, the primary product of$CO_2$ fixation (takes place in mesophyll) could be a four-carbon compound, oxalic carboxylic acid. it's seen in some tropical plants.

Hence, the correct answer is option (D).

Note: So for the production of food i.e. Energy, the chemical process photosynthesis, there's a great need of sunlight but in light reaction only, but after this in dark reaction I.e. in Kelvin's cycle there is no need of sunlight. It may occur in day or night. And also due to this some plans are modified according to their physical changes known as $C_2$, $C_3$, $C_4$ etc. But$CO_2$ fixation occurs in every plant.