Question

Using Fick's equation for photosynthesis, explain why plants that have evolved to live in very dry areas use PEP carboxylase for the initial CO2 fixation enzyme rather than RUBISCO.

Your answer should include which term Fick's equation is affected by using PEP carboxylase rather than RUBISCO

Answer 1

C4 plants those plant in which first stable product is Oxaloacetate (OAA) and occur most at high temp a,low soli water, high saline soil. Eg sugarcane, maize ,sorghum

It's efficiency increases as compared to C3 plant due to absence of photorespiration which caused loss of carbon in form of CO2.

C4 avoid photorespiration by doing several physilogical changes which includes -

1. Formation of thick and closed associated cells called bundle sheath around vascular bundle in leaves. This cells are thick and thightly associated with each other so there is no excahnge or diffusion of gas from these cells

2.Absence of Rubisco enzyme in mesophyll cells so in place of Rubisco, enzyme PEP carboxylase present which has only carboxylase property opposite of Rubisco enzyme which is having both carboxylase and oxygenase property (oxygenase property is responsible for photorespiration)

3. Mesophyll to guard cells are connected through plasmodespmata for exchange of OAA and PEP carboxylase enzyme.

4.RUBP substrate and RUBISCO enzyme both are absent in mesophyll but present in. Bundle sheath in C4 plant.

Now O2 and CO2 both diffused to mesophyll through stomata but only CO2 combine to PEP in prsence of PEP carboxylase to form OAA (4C) which transported to bundle sheath through plasmodesmata.

Now in bundle sheath, OAA is dissociated into PEP and CO2 and hence cause increase of local concentration of CO2 i. Bundle sheath away from mesophyll cells. The CO2 cannot diffuse back as the cell wall of bundle sheath is very thick so follows Ficks law more thicker boundry less diffusion.

Now CO2 combine with RUBP in presence of Rubisco enzyme to 3PGA ( 3C) which start C3 cycle to form glucose. In this way Rubisco binds only with CO2 not with O2 and O2 cannot diffuse to bundle sheath due thick boundry of bundle sheath.

Hope it's clear..thanks