AP BIOLOGY

Detailed explanation-1: -The malic acid undergoes decarboxylation and releases carbon dioxide. It is the source of CO during Calvin cycle in C4 plant. Hence, option (a) is correct.

Detailed explanation-2: -C4-acids as the source of carbon dioxide for Calvin cycle photosynthesis by bundle sheath cells of the C4-pathway species Atriplex spongiosa.

Detailed explanation-3: -In C4 plants, the Hatch and Slack pathway involves two carboxylation reaction, one taking place in chloroplast of mesophyll cells and other in chloroplast of bundle – sheath cells. In mesophyll cells, phosphoenol pyruvate (PEP) is carboxylated to oxaloacetate (OAA) which is then reduced to malic acid.

Detailed explanation-4: -The four carbon containing organic acids (malic acid or oxalic acid) are pumped into bundle sheath cells, where they are decarboxylated to release carbon dioxide. The released carbon dioxide is fixed by the enzyme RuBisCO in bundle sheath cells.

Detailed explanation-5: -In C4 photosynthesis, where a four-carbon compound is produced, unique leaf anatomy allows carbon dioxide to concentrate in ‘bundle sheath’ cells around Rubisco. This structure delivers carbon dioxide straight to Rubisco, effectively removing its contact with oxygen and the need for photorespiration.