Question

The substrate for photorespiration is
(A) Glycolate
(B) Glucose
(C) Lipid
(D) Sucrose

Answer 1

Hint: Photorespiration is a metabolic pathway involving a network of enzymes in plants. It is undesirable as it leads to wastage of energy generated from photosynthesis. It occurs between mitochondria, peroxisome and chloroplasts.

Complete answer:
Now let us learn how photorespiration is undesirable for plants. The main enzyme involved is Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco). It is responsible for converting atmospheric carbon dioxide into compounds that produce energy like glucose, which is a process, called carbon fixation. To perform carbon fixation, rubisco catalyses the carboxylation of Ribulose 1,5-bisphosphate (RuBP), as follows:
$RuBP+C{{O}_{2}}\xrightarrow{Rubisco}2\ \ 3-phosphoglycerate$
This is a key step as well for the Calvin cycle, which at the end produces energy. However, enzyme rubisco is not only specific to carbon dioxide but also to oxygen. When rubisco binds with oxygen the cascade of reactions that are previously mentioned gets reversed. It catalyses oxygenation as follows,
$RuBP+{{O}_{2}}\xrightarrow{Rubisco}3-Phosphoglycerate+Phosphoglycolate+2{{H}^{+}}$
The 3-phosphoglycerate (PGA) is favourable and it passes onto Calvin cycle. But phosphoglycolate (PPG) inhibits some enzymes involved in carbon fixation. The plant cannot recycle PPG. It eventually enters the chloroplast through a transporter with glycerate. Once entered, the chloroplast at an expense of one ATP converts it into PGA and it enters into Calvin cycle.

Thus, the substrate for photorespiration is (A) glycolate.

Note:
The affinity of rubisco to carbon dioxide or oxygen depends on the temperature. In hot and arid conditions, the stomata of the leaves close. Hence, the plant cannot take in any carbon dioxide but photosynthesis continues. This increases the amount of oxygen and lessens carbon dioxide. So rubisco binds more to oxygen as it is abundant. Normally, rubisco adds oxygen to RuBP instead of carbon dioxide to around 25% of the reactions.