Question

The fixation of carbon dioxide takes place in _______ and results in the synthesis of _______which acts as a substrate for cellular respiration.
A. The stroma,ATP
B. The thylakoid space,ATP
C. The thylakoid space, carbohydrates
D. The stroma, carbohydrates

Answer 1

In the process of photosynthesis, carbon dioxide from the atmosphere is incorporated into the leaves through the stomata and is used to synthesis carbohydrates such as glucose and starch which are then utilized for cellular respiration. Photosynthesis occurs in the parts of the plants where chloroplast is present in large numbers. Photosynthesis has a photochemical phase and biochemical phase.

Complete answer:
The fixation of carbon dioxide and the formation of carbohydrates takes place in the chloroplast present in plant cells. The enzymatic reactions help incorporate carbon dioxide into the plants which lead to the synthesis of sugar molecules and it takes place in the stroma of chloroplasts. Stroma is limited by the inner membrane of the chloroplast. Both these processes take place in the biosynthetic phase of photosynthesis. The biosynthetic phase requires energy compounds such as ATP and NADPH which are formed in the photochemical phase of photosynthesis and includes the absorption of light, splitting of water, release of oxygen along with the formation of chemical intermediates such as ATP and NADPH. The CO2 combines with water to form carbohydrates.
The cyclic pathway of carbon dioxide fixation was discovered by Melvin Calvin. Calvin worked out the pathway called the Calvin cycle or C3 pathway. Another pathway called the C4 pathway was observed in plants which are adapted to dry regions. But the main biosynthetic pathway in C4 plants is the C3 cycle. The primary acceptor of CO2 is ribulose bisphosphate or RuBP, a five carbon sugar in Calvin cycle and phosphoenolpyruvate, a three carbon molecule in C4 cycle. The first product of CO2 fixation in a C3 cycle is a three carbon compound and a four carbon compound in C4 cycle. For the removal of one molecule of glucose from the C3 pathway, six molecules of carbon dioxide are fixed by six turns of the cycle. The glucose thus formed acts as the substrate for cellular respiration.
So, the correct answer is “Option D, The stroma, carbohydrates. ”

Note:
Cellular respiration is the breakdown of food materials within the cell to release energy molecules and the trapping of these energy molecules for the synthesis of ATP in cells. The synthesis of sugars takes place in the chloroplast of the plants whereas the breakdown of these complex molecules of photosynthesis takes place in the cytoplasm and in the mitochondria, which finally yields energy compounds like ATP.