1. Tardigrade
  2. Question
  3. Biology
  4. Assertion: Plants possessing C4‐pathway of carbon fixation have a higher net primary productivity than the C3‐pathway possessing plants. Reason: For each unit weight of fixed carbon, C4‐pathway possessing plants require less water than the C3‐pathway possessing plants.

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Q. Assertion: Plants possessing C4‐pathway of carbon fixation have a higher net primary productivity than the C3‐pathway possessing plants.
Reason: For each unit weight of fixed carbon, C4‐pathway possessing plants require less water than the C3‐pathway possessing plants.

AIIMSAIIMS 1994

Solution:

C3 plants exclusively fix CO2 via the Calvin-Benson pathway. The leaf cells of C4 plants possess two types of chloroplast, bundle sheath cells and mesophyll cells. The mesophyll cells of C4 plants exhibit high activity of phosphoenolpyruvate (PEP) carboxylase, which catalyses the fixation of CO2 with PEP to form oxaloacetic acid. The bundle sheath cells exhibit high RuBP carboxylase and the other enzymes of the CalvinBenson cycle. C4 plants are highly productive photosynthetically in view of the low affinity of RuBP carboxylase for its substrate. But CO2 is concentrated as C4 acids due to the activity of the PEP carboxylase, which in effect maintains high pool levels of CO2 . C4 plants is related to very high light-harvesting saturation points. C4 plants exhibit low rates of photorespiration only in bundle sheath cells. C4 plants are adapted to high temperature. C4 plants show better utilisation of available water but it cannot be the cause of its higher productivity.