Question

If 24G-3-P molecules are formed in \[{{C}_{3}}\] plants. Calculate a) the number of CO$_2$ used and b) the number of G3P used for regeneration RuBP molecules.
A. \[\text{a=12 }\]and \[\text{b=20}\]
B. \[\text{a=24}\]and \[\text{b=48}\]
C. \[\text{a=12 }\]and \[\text{b=24}\]
D. \[\text{a=20}\]and \[\text{b=12}\]

Answer 1

Hint: The word \[{{C}_{3}}\] plants refers to plants in which the first product formed is a 3-carbon compound, i.e., 3-phosphoglycerate, by assimilation of carbon dioxide through the process of photosynthesis. RuBP stands for Ribulose 1,5-bisphosphate that is involved in the photosynthesis process.

Complete answer: The process which involves the conversion of inorganic carbon into organic compounds by the living organism is called carbon fixation. The process of carbon fixation is involved in the photosynthesis process. Photosynthesis is the production of energy in the form of ATP molecules by the plants which involve the conversion of light energy into chemical energy. C$_3$ plants tend to survive by \[{{C}_{3}}\] carbon fixation. It is one of the metabolic pathways of photosynthesis. This process involves the conversion of carbon dioxide and ribulose phosphate into 2 molecules of 3-phosphoglycerate. The reaction which take place in \[{{C}_{3}}\] plants is \[\text{C}{{\text{O}}_{2}}\text{+}{{\text{H}}_{2}}\text{O + RuBP}\to \text{3-phosphoglycerate (2)}\]. This reaction occurs in almost all plants. \[{{C}_{3}}\] plants tend to grow in areas where the intensity of sunlight is moderate and carbon dioxide concentration is around 200 ppm or higher. 3 $C{{O}_{2}}$ particles must enter to give one G3P. 6 G3P atoms are made when three $C{{O}_{2}}$ particles enter. As one G3P leaves the cycle to make glucose, the other five is reused to recover three atoms of the RuBP acceptor. As one G3P leaves the cycle to make glucose, the other five is reused to recover three atoms of the RuBP acceptor. Therefore, 12 molecules of carbon dioxide are used to form 24G-3-P molecules and 20 molecules of G3P are used for the generation of RuBP molecules.
So, option A is the correct option.

Note: Carbon fixation which occurs in the absence of sunlight is called chemosynthesis. Autotrophs are organisms that grow by carbon fixation. Organisms that produce complex organic compounds using carbon are referred to as autotrophs. Carbon fixation is not possible by heterotrophs but they grow by consuming the carbon which is fixed by autotrophs.