Question

Photorespiration in ${C_3}$ plants start from
A. Photosynthesis
B. Glycerate
C. Glycine
D. Phosphoglycolate

Answer 1

Hint:- Using the primary photosynthetic enzyme Rubisco, photosynthesis is the mechanism used by plants to convert light, carbon dioxide and water into sugars that fuel the growth of plants. The oxygenation of RuBP by RUBISCO is photorespiration. Oxygen and carbon dioxide rivalry decreases the rate of carbon assimilation, the energy efficiency of photosynthesis, and can decrease the photosynthetic quotient.

Complete answer:
Most plant species on Earth use ${C_3}$ photosynthesis, where three carbon atoms are contained in the first carbon compound formed. In this process, through its stomata (microscopic pores on plant leaves), carbon dioxide enters a plant where, through the Calvin-Benson cycle, the enzyme Rubisco fixes carbon into sugar in the middle of a series of complex reactions.
In the chloroplast, photorespiration starts when Rubisco adds oxygen to RuBP in its oxygenase reaction. It creates two molecules: a three-carbon compound, 3-PGA, and phosphoglycolate, a two-carbon compound. 3-PGA is a regular Calvin cycle intermediary, but phosphoglycolate is unable to join the cycle, so its two carbons are excluded from the cycle, or "stolen." Plants bring phosphoglycolate via a series of reactions that require transport between different organelles to retrieve some of the lost carbon. Three-quarters of the carbon that reaches this pathway is recovered as phosphoglycolate, while one-quarter is lost as carbon dioxide.

So, the correct answer is option (D).

Note:- Specific leaf anatomy allows carbon dioxide to be concentrated in 'bundle sheath' cells around Rubisco in ${C_4}$ photosynthesis, where a four-carbon compound is formed. This framework delivers carbon dioxide directly to Rubisco, essentially eliminating its oxygen interaction and the need for photorespiration. Through the capacity to continue fixing carbon when stomata are closed, this adaptation enables plants to retain water. Using another enzyme called PEP, these plants resist photorespiration.