Question

Assume you are researching the process of photosynthesis using O18, a radioactive isotope of oxygen that allows you to trace its movements. If a plant was grown in the presence of CO2 that contained O18 (this means the oxygen atoms in the carbon dioxide molecules are the radioactive isotopes), would you expect to find the radioactive O18 in G3P or in the O2 produced by the plant. EXPLAIN! *
Note: the radioactive isotopes of oxygen are not distinguishable to the plant.

Answer 1

Photosynthesis takes place in two phases.

a. Light dependent phase

b. Light independent phase

During the light dependent phase plants split the water molecules using the light energy and generate NADPH and ATP molecules. Photolysis of the water generates oxygen as a by product.

During the light independent phase plants fix the carbon dioxide using the enzyme RubisCO. CO2 is received by the molecule ribulose1,5bisphosphate and 3 phosphoglyceric acid is produced. 3 phosphoglyceric acid is subsequently converted into glyceraldehyde-3-phosphate (G3P).

Plants grown in O18 containing carbon dioxide would assimilate this radioactive CO2. O18 would appear in G3P instead of O2 produced because the oxygen released by the plants come from the water not from the CO2.  O18 would appear in the products of the Calvin cycle like 3-PGA, G3P etc.