Question

In an experiment in which photosynthesis is performed during the day you provide a plant with radioactive carbon dioxide $({}^{14}C{O_2})$ as a metabolic tracer. The ${}^{14}C$ is incorporated first into oxaloacetic acid. The plant is best characterised as a
a. ${C_4}$ plant
b. ${C_3}$ plant
c. CAM plant
d. Insectivorous plant

Answer 1

Hint: The amount of carbon dioxide will be good if photosynthesis takes place during the day. The process of photosynthesis stops at night and respiration will continue. The product of the photosynthesis experiment will be radioactive carbon dioxide and it will be considered as a stable compound which forms at the time of carbon dioxide fixation in green plants.

Complete answer:
There are certain tropical plants in which RuBisCO does not absorb $C{O_2}$ directly. So another pathway is followed because of the low concentration of $C{O_2}$ which is named the ${C_4}$ pathway. In this pathway, oxaloacetic acid (OAA) which is a 4-carbon compound is the first stable compound. The plants in which this pathway takes place is called ${C_4}$ plants.

OAA is formed by $C{O_2}$ initial fixation from the phosphoenolpyruvate carboxylation in the mesophyll cells. So with the introduction of radioactive ${}^{14}C{O_2}$ into the reaction, first oxaloacetic acid (OAA) will be incorporated.
$PEP+{}^{14}C{O_2}+{H_2}O\xrightarrow[]{PEPcarboxylase}OAA + {H_3}P{O_4}$

The plants in which the initial product is 3-phosphoglycerate formed from the assimilation of carbon dioxide through photosynthesis. 3-phosphoglycerate is a 3-carbon compound. The chloroplast is absent in the bundle sheath of ${C_3}$ plants. The fixation of carbon dioxide occurs in one place in ${C_3}$ plants.

The plants which have the capability to extract $C{O_2}$ from the environment are called CAM (Crassulacean acid metabolism) plants. For example – pineapple, jade plant, Spanish moss, orchids, agave, wax plant, etc.

The plants which obtain their nutrition from insects and other organisms by feeding on them are called insectivorous plants. These plants are mainly found in the humid area and are nutrient deficient. For example – Venus flytrap, pitcher plant, etc.

Hence, the correct answer is option (A).

Note: The radioactive carbon dioxide when used in an experiment then it will incorporate oxaloacetic acid which is the stable product in ${C_4}$ plant fixation. The function of carbon dioxide is different in different plant pathways but oxaloacetic acid (OAA) is the first and main product that is formed in almost every pathway for photosynthesis.