Question

The \[{{\text{C}}_{\text{4}}}\] plants differ from \[{{\text{C}}_{\text{3}}}\] plants with respect to
A) Substance that accepts \[\text{C}{{\text{O}}_{\text{2}}}\] in carbon assimilation
B) Type of end product
C) Type of pigment involved in photosynthesis
D) No.of ATP consumed in synthesis of sugar

Answer 1

Hint:\[{{\text{C}}_{\text{3}}}\] and \[{{\text{C}}_{\text{4}}}\]are both carbon assimilation pathways in plants

Complete answer:
The \[{{\text{C}}_{\text{4}}}\] pathway of carbon fixation is also called Hatch and Slack pathway. It is mainly seen in monocots like maize, sugarcane. Since the final sugar us a 4-Carbon product, it is called the \[{{\text{C}}_{\text{4}}}\] pathway. These plants are called \[{{\text{C}}_{\text{4}}}\]plants. The chloroplast of \[{{\text{C}}_{\text{4}}}\]plants is dimorphic. The mesophyll cells chloroplasts are granal, whereas in bundle sheath cells they are agranal.
The granal chloroplasts contain thylakoids, stacked to form grana. In agranal chloroplasts of bundle sheath cells grana are absent. Thylakoids are present only as stroma lamellae.
The \[\text{C}{{\text{O}}_{\text{2}}}\]from the atmosphere enters the mesophyll cells through the stomata and are fixed by phosphoenol pyruvate or PEP. An enzyme called phosphoenolpyruvate carboxylase (PEPCase) catalyses the reaction. The product is a \[\text{4-C}\] compound oxalo acetic acid or OAA. This is called carboxylation or fixation of \[\text{C}{{\text{O}}_{\text{2}}}\].
The next step of reaction is transport of oxalo acetic acid (OAA) from cytosol of mesophyll cells to chloroplasts of bundle-sheath cells, where it is decarboxylation occurs of release fixed \[\text{C}{{\text{O}}_{\text{2}}}\].
The other product of decarboxylation reaction is pyruvic acid which is a 3-carbon compound. This is transported back to mesophyll cells, where it regenerates PEP for continuation of \[{{\text{C}}_{\text{4}}}\]pathway.

The \[{{\text{C}}_{\text{3}}}\] cycle is also known as the Calvin cycle. Mainly occurs in dicot plants and such plants are called \[{{\text{C}}_{\text{3}}}\] plants. It is a reaction occurring in the dark phase of photosynthesis. The reaction occurs in three distinct phases, carbonylative, reductive and regenerative phases.
The \[\text{C}{{\text{O}}_{\text{2}}}\]from the atmosphere is fixed by Ribulose bisphosphate, \[\text{RuBP}\] which is a \[\text{5-C}\] compound. An enzyme called \[\text{ribulose-1,5-bisphosphate carboxylase oxygenase}\] or simply \[\text{RuBisCO}\] is used as a catalyst. The end product of one turn of a \[{{\text{C}}_{\text{3}}}\]cycle are
\[\text{ }\!\!~\!\!\text{ 2 glyceraldehyde-3-phosphate }\left( \text{G3P} \right)\] molecules, \[\text{3 ADP}\], and \[\text{2 NADP+}\]. Each \[\text{G3P}\] molecule is composed of 3 carbons. Perhaps the cycle is called a \[{{\text{C}}_{\text{3}}}\] cycle.

Hence the correct answer is option (A.) Substance that accepts \[\text{C}{{\text{O}}_{\text{2}}}\]in carbon assimilation.

Note:Apart from the \[{{\text{C}}_{\text{3}}}\]and \[{{\text{C}}_{\text{4}}}\] cycles, plants, especially xerophytes have a special cycle called the Crassulacean acid mechanism or CAM pathway.