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Q:Photosynthesis in C4 plants is relatively less limited by atmospheric CO2 levels because: 1) Four carbon acids are the primary initial CO2 fixation products 2) the primary fixation of CO2 is mediated via PEP carboxylase 3) Effective pumping of CO2 into bundle sheath cells 4) RuBisCO in C4 plants has higher affinity for CO2. Now the correct option is 2 but can you explain the reason for not choosing 4th option?

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    – tyersome
    Jan 7 '20 at 17:17
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If that is the case, why would C4 plants make PEPcarboxylase the primary acceptor instead of RuBisCO? Since it is not so, C4 plants were forced to take this alternative and make RuBisCO confined only to the bundle sheath cells. The problem in C3 was photorespiration, since oxygen used to competitively inhibit the uptake of carbon dioxide by RuBisCO. Hence it turns out that RuBisCO has more affinity for oxygen. If RuBisCO had more affinity for CO2, C4 plants would not have required to take the alternative pathway that they follow.

For the fact that why C4 remains unaffected by atmospheric CO2 levels is that because PEP carboxylase concentrates CO2 levels inside bundle sheath cells. So no matter what is the atmospheric CO2 level, the level inside bundle sheath cells remain always high and perfect for RuBisCO to act upon.

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