Question

In photosynthesis $CO_2$ combines with
A) RUDP or RUBP
B) ATP
C) ADP
D) PGA

Answer 1

Photosynthesis is the process by which the plants prepare glucose using carbon dioxide and water. The fixation of carbon dioxide into glucose takes place during the dark reaction of photosynthesis. This reaction needs energy molecules produced in the light reaction. The dark reaction is also known as carbon reduction cycle.

Complete answer:
To answer this question, we must know about the dark reaction of photosynthesis. During the light reaction, the light energy is utilised to produce the energy molecules like NADPH and ATP. In dark reactions which take place in the absence of light, the energy molecules produced in the light reaction are consumed to synthesize carbohydrates.

The dark reaction or the Calvin cycle takes place in three steps-
- Carbon fixation- In this step, carbon dioxide combines with RUBP or RUDP in the presence of enzyme RUBISCO. This generates PGA or phosphor-glyceric acid.
- Reduction- In this step the PGA is reduced in the presence of ATP and NADPH to produce glyceraldehyde 3 phosphate.
- Regeneration- In this step the RUBP is regenerated by using ATP which is available for accepting the next carbon dioxide.
- ATP and ADP are Adenosine tri phosphate and biphosphate. It is produced during light reaction. It is consumed during the reduction and regeneration phase.
- PGA- Phospho-glyceraldehyde is a 3C compound which is produced as a result of carbon dioxide fixation.

Thus the correct answer is option ‘A’.

Note: The dark reaction is the biochemical phase of the photosynthesis which takes place in the stroma of chloroplast. The glucose is the end product. Each molecule of carbon dioxide requires 3ATP and 2NADPH.