Question

Read the assertion and reason carefully to mark the correct option in the question.

Assertion: Productivity and yield are better in $C_4$ plants.

Reason: $C_4$ plants lack photorespiration.

Answer 1

Assertion-Reason relation

When $RuBP$ binds $O_2$, instead of forming two molecules of 3-phosphoglycerate ($3-PGA$), it forms one molecule each of $3-PGA$ and 2-phosphoglycolate in a pathway called photorespiration.

There is loss of 2 carbon due to this process in $C_3$ plants. However, the presence of Kranz anatomy and the $C_4$ pathway in $C_4$ plants increases the concentration of $C{O}_2$ around the $RuB$is$CO$ enzyme and ensures that photorespiration does not occur.

Hence, the loss of carbon is prevented and yield is better.

Assertion-Reason relation

Photorespiration is a phenomenon which is observed in $C_3$ plants due to oxygenase activity of $RuB$is$CO$ enzyme.

Since photorespiration depends upon the activity of $RuB$is$CO$ enzyme, $C_4$ plants can avoid this. They have special mechanism that increases the concentration of $C{O}_2$ at the enzyme site, thereby preventing the binding of $O_2$ to $RuB$is$CO$.

Assertion-Reason relation

The synthesis of malic acid during carbon dioxide fixation occurs in the mesophyll cells.

Mesophyll cells lack the $RuB$is$CO$ enzyme. The enzyme responsible for fixation is $PEP$case. $PEP$case has higher efficiency to take up $C{O}_2$.

The malic acid formed is transported to bundle sheath cells for decarboxylation i.e., carbon dioxide is released. This carbon dioxide enters the Calvin cycle.

Final answer

(A) If both assertion and reason are true and the reason is the correct explanation of the assertion.