Question

The first step in the dark reaction of photosynthesis is
A. Formation of ATP
B. Attachment of carbon dioxide to a pentose sugar
C. Excitement of an electron of chlorophyll by photon of light
D. Ionization of water

Answer 1

Dark reaction of photosynthesis is the reaction which is independent to light. Fixation and reduction of carbon dioxide can occur even in darkness. Photosynthesis takes place in the stroma of the chloroplasts which are the cell organelles and also known as the kitchen of the cells. Calvin cycle is the dark reaction. Carbon dioxide fixation occurs when it is accepted by 5-carbon sugar.

Complete answer:
Dark reactions can occur in both presence and absence of sunlight. These reactions are independent to the sunlight. It involves fixation of carbon dioxide, resulting in the formation of carbohydrates. This process does not directly depend on the presence of light but it utilizes ATP and NADPH (assimilatory power) obtained as a result of light reactions for the reduction and fixation of carbon dioxide.
Dark reaction is also known as Blackman reaction, named after the scientist who reported it for the first time. It is the biochemical phase.
In the Calvin cycle, the first product in the series of reactions is a three carbon compound (phosphoglyceric acid). This pathway is also called the ${C_3}$ cycle. This cycle is also called the Calvin-Benson-Bassham cycle. The primary acceptor of carbon dioxide in this cycle is Ribulose 1, 5-bisphosphate (RuBP) and forms two molecules of 3- phosphoglycerate (3PGA). RUBISCO catalyzes this reaction.
From the above information we know that the first step in the dark reaction of photosynthesis is attachment of carbon dioxide to a pentose sugar.

Therefore, option B is the correct answer.

Note: Calvin cycle includes three distinct phases: carboxylation, glycolytic reversal, and regeneration of RuBP. These three phases take place in a stroma. Carboxylation is the process of fixation of $C{O_2}$. In glycolytic reversal or reduction leads to the formation of glucose. Regeneration of $C{O_2}$ acceptor molecules is crucial if the cycle is to continue uninterrupted. Regular supply of ATP and NADPH is required for continuous functioning of the cycle.