| The \[{{C}_{4}}\] carbon cycle is a \[C{{O}_{2}}\] concentrating mechanism evolved to reduce photorespiration. The followings are stated as important features of the \[{{C}_{4}}\] pathway: |
| (i) The leaves of \[{{C}_{4}}\] plants have Kranz anatomy that distinguishes mesophyll and bundle sheath cells. |
| (ii) In the peripheral mesophyll cells, atmospheric \[{{\operatorname{CO}}_{2}}\] is fixed by phosphoenol pyruvate carboxylase yielding a four-carbon acid. |
| (iii) In the inner layer of mesophyll, NAD-malic enzyme decarboxylates four-carbon acid and releases\[{{\operatorname{CO}}_{2}}\]. |
| (iv) \[{{\operatorname{CO}}_{2}}\] is again re-fixed through Calvin cycle in the bundle sheath cells. |
| Which one of the following combinations is correct? |
A) (ii), (iii) and (iv)
B) (i), (ii) and (iii)
C) (i), (ii) and (iv)
D) (i), (iii) and (iv)
Correct Answer: C
Solution :
In the \[{{C}_{4}}\] pathway, initial carbon fixation takes place in mesophyll cells and the Calvin cycle takes place in bundle sheath cells. PEP carboxylase attaches an incoming carbon dioxide molecule to the three-carbon molecule PEP, producing oxaloacetate (a four-carbon molecule). The oxaloacetate so formed is then reduced to malate in the chloroplasts by NADP-malic dehydrogenase (NADP-MDH), or transformed to aspartate by transamination. These acids are then exported to the bundle sheath cells, where a decarboxylation occurs (via malic enzyme or PEP carboxykinase) to yield \[C{{O}_{2}}\]
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