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# Identifying the Type of Photosynthetic Biochemical Pathway

BIOCHM-8WJZUR

The C3 and C4 pathways of photosynthesis were worked out in large part by radiolabeling plants with $_{ }^{ 14 }{ C }{ O }_{ 2 }$ and determining which molecules were initially labeled and then following the label over time into molecules further down the metabolic pathway.

A researcher found a new plant in her field plot and wanted to determine what type of photosynthesis it carried out. She labeled the plant in the light with $_{ }^{ 14 }{ C }{ O }_{ 2 }$ for 5 sec and 2 min and analyzed the products in each case.

Considering the schematics of the C3 or Calvin cycle, the photorespiratory cycle, and the C4 pathways shown below and your understanding of the biochemistry of each of these pathways, which of the following sets of data is MOST likely to be observed AND is also correctly interpreted?

A

After 5 sec, almost all of the label was found in RuBP (ribulose 1,5-bisphosphate), strongly supporting that the plant was carrying out C3 photosynthesis.

B

After 2 min, almost all of the label was found in 3-PGA (3-phosphoglyerate), strongly supporting that the plant was carrying out C3 photosynthesis.

C

After 5 sec, almost all of the label was found in PEP (phosphoenolpyruvate), strongly supporting that the plant was carrying out C4 photosynthesis.

D

After 5 sec, almost all of the label was found in 3-PGA, and after 2 min, the label in 3-PGA decreased, whereas the label in sugar-phosphates and phosphoglycolate increased. These data strongly support that the plant was carrying out C3 photosynthesis.

E

After 5 sec, almost all of the label was found in aspartic acid, and after 2 min, the label in aspartic acid decreased, whereas the label in phosphoglycolate and glycine increased, strongly supporting that the plant was carrying out C4 photosynthesis.