Question

The first step in dark reaction of photosynthesis is

• A. formation of ATP
• B. ionization of water
• C. attachment of $CO_2$ to a pentose sugar
• D. excitement of electron of chlorophyll by a photon of light

Answer 1

The Correct Option is C

The dark phase of photosynthesis involves fixation of $CO_2$ by the enzyme ribulose bisphosphate carboxylase [RUBISCO]. The two substrates are $CO_2$ and 1,5- bisphosphate (RuBP) and the product is the 3 - carbon compound, 3 - PGA (3 - phosphoglyceric acid). RuBP is a 5 - C sugar.

Answer 2

The dark reactions can occur in both the presence and absence of sunlight. These reactions are totally independent to the sun. It also involves the fixation of carbon dioxide, resulting in the formation of carbohydrates. Therefore this process is 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. The dark reaction is also known as the Blackman reaction, which is 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 3-carbon compound (phosphoglyceric acid). This pathway is also known as the C3 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), which forms two molecules of 3-phosphoglycerate (3PGA). Rubio catalyzes this reaction. Hence, from the above information, we can say that the first step in the dark reaction of photosynthesis is the attachment of carbon dioxide to a pentose sugar. 

Therefore, option C is the correct answer.