Question

Compare and contrast the energy consumption and pro/con of C3 carbon fixation (via Calvin cycle) vs C4 carbon fixation (via C4 pathway+Calvin cycle)

Answer 1

C₃ carbon fixation is the process where carbon dioxide and ribulose bisphosphate are converted to 3-phosphoglycerate by RuBisco enzyme by the following reaction-

CO₂ + H₂O + RuBP ? (2) 3-phosphoglycerate

C4 pathway is an elaboration of C3 pathway. In this CO2 is shuttled through malate from mesophyll cells to bundle sheath cells. These plants use PEP carboxylase to absorb more CO2 and then it is converted to oxaloacetic acid which is converted to malate. Malate releases CO2 in bundle sheath cells. All these additional steps require more energy (ATP). By using extra ATP, C4 plants are able to survive and fix carbon efficiently high temperature, drought, and CO2 or N2 limitations.

As every CO2 molecules is fixed twice, one by 4-carbon organic acid and 2nd by RuBisCO (Calvin cycle), the energy utilization in C₄ pathway is more than C₃ pathway. 18 molecules of ATP is used to synthesize one molecule of glucose by C₃ pathway. whereas 30 molecules of ATP are required by the C₄ pathway.

PROS and CONS

Pathway	PROS	CONS
C3 Pathway	low energy cost	high water loss in dry areas
		high photorespiration in hot regions
C4 Pathway	reduce photorespiration by using the efficient enzyme for CO2 fixation and shutting it by malate-aspartate shuttle	more energy consumption that C3 plants
	reduce water loss as PEP carboxylase brings CO2 fast preventing stomata open for a long time.
	ability to survive in drought and high-temperature conditions