NEET Practice Questions (MCQs) with Answers & Solutions

The NCERT text mentions: 'RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants: photorespiration.' This implied that under certain conditions, particularly when $\text{O}_2$ levels are high and $\text{CO}_2$ levels are low, RuBisCO binds with $\text{O}_2$ instead of $\text{CO}_2$, initiating photorespiration.

The NCERT text refers to photorespiration as a 'wasteful oxygenation reaction'. This 'wasteful' nature is due to the consumption of $\text{O}_2$ and the release of $\text{CO}_2$ without the net synthesis of sugars or $\text{ATP}$, effectively undoing some of the work of photosynthesis.

While not explicitly stated as 'high temperature' in the provided excerpt relating to photorespiration, it is a known biological fact that high temperatures contribute to stomatal closure, reducing internal $\text{CO}_2$, and increasing the solubility of $\text{O}_2$ relative to $\text{CO}_2$, thus favoring the oxygenase activity of RuBisCO. The text indirectly mentions factors affecting photosynthesis, and photorespiration is a consequence of environmental conditions impacting RuBisCO's affinity.

The NCERT text states: 'RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants: photorespiration.' This explicitly limits the occurrence of this 'wasteful oxygenation reaction' to C3 plants in this context, distinguishing them from C4 plants mentioned later as having a 'special type of photosynthesis called C4 pathway'.

Photorespiration is a 'wasteful oxygenation reaction' which consumes $\text{ATP}$ and produces $\text{CO}_2$ and other intermediates like phosphoglycolate and glycerate, but it does not produce $\text{ATP}$ as a net gain; rather, it consumes it. The context implies photorespiration as a process that does not lead to efficient energy capture or sugar synthesis.

The NCERT text states: 'Some tropical plants show a special type of photosynthesis called C4 pathway.' It also asks, 'Why do you think RuBisCO carries out more carboxylation in C4 plants?' This implies that C4 plants have adaptations that favor RuBisCO's carboxylation activity, which is indeed the $\text{CO}_2$ concentrating mechanism, minimizing photorespiration.

The text highlights that 'RuBisCO also catalyses a wasteful oxygenation reaction' in addition to its role in $\text{CO}_2$ fixation. This 'oxygenation reaction' arises from the competition between $\text{CO}_2$ and $\text{O}_2$ for RuBisCO's active site.

According to the NCERT text, 'The carbonyl carbon atom is sp2-hybridised and forms three sigma (s) bonds... The bond angles are approximately 120° as expected of a trigonal coplanar structure.' This directly indicates sp2 hybridization and trigonal planar geometry.

The context clearly states, 'The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen relative to carbon.'

The NCERT text explains, 'Hence, the carbonyl carbon is an electrophilic (Lewis acid), and carbonyl oxygen, a nucleophilic (Lewis base) centre.'