Question

During dark reaction, for the fixation of the carbon, the three carbon atoms of each molecule of 3-phosphoglyceric acid (PGA) are derived from
A. RuBP only
B. $C{O_2}$only
C. RuBP and $C{O_2}$
D. RuBP, $C{O_2}$and PEP

Answer 1

The dark reaction occurring in plants is also known as the Calvin cycle. This reaction is called so because this reaction occurs in the absence of the sunlight and in the presence of carbon dioxide. If carbon dioxide is absent, this reaction will not occur.

Complete answer:
To start the cycle, energy resources are required. These energy resources are met by ATP and NADPH present in the plant cells which are derived from the sun’s energy. Calvin cycle uses these energy molecules to convert the three molecules of carbon dioxide into one molecule of the three-carbon sugar. In the Calvin cycle, three molecules of carbon dioxide and water are converted into one molecule of glyceraldehyde. During this process, six atoms of oxygen are formed which are released into the atmosphere for the organism to use for respiration. In the first stage of the cycle, fixation of carbon dioxide occurs, as it is converted from an inorganic molecule to an organic molecule. This stage also witnesses the initiation of the light independent reactions. In the second stage of the cycle, which is a reduction reaction, ATP and NADPH are used to reduce 3-PGA into G-3-P, and the conversion of ATP and NADPH to ADP and $NAD{P^ + }$occurs. The last stage of the cycle is the regeneration stage, where RuBP is regenerated so that more carbon dioxide can be fixed in the plants. The regeneration of RuBP leads to carbon fixation.

Hence, the correct answer is option (C).

Note: The enzyme RuBisCO is used for the catalyzation of the RuBP molecule which is a five-carbon molecule. The carbon dioxide then splits itself in half, leading to the formation of the three-carbon molecule.