Question

5. A scientist proposes using radioactive tracers to demonstrate that the carbon atoms from carbon dioxide end up incorporated into glucose molecules as the result of photosynthesis. In order to test this hypothesis, the scientist conducts the following experiment:

1. A group of plants are illuminated with light of a wavelength that is optimal for photosynthesis.

2. The light is turned off, and the plants are kept in darkness for all remaining steps in the investigation.

3. Radiolabeled CO2 is introduced to the plants.

4. After 30 minutes, plant leaves are homogenized and glucose extracted from the cell homogenate.

5. Radiolabels are found in the glucose samples.

Explain why the radiolabel was present in the homogenate. Describe the role of light in the synthesis of ATP and NADPH and how the absence of light may have affected this process.

Answer 1

Photosynthesis is the process by which plants make their own food by converting light energy of the sun to chemical energy.

There are two stages of photosynthesis-

a) light reaction- in this reaction the thylakoids of the leaves traps the light of the sun and splits water to produce oxygen. During this reaction, the electrons pass on and leads to the formation of NADPH and ATP.

b) dark reaction- this is the light independent reaction of photosynthesis which happens in stroma where energy from ATP and NADPH, which are produced during light reaction, are used to fix carbon dioxide (CO2) and ultimately leading to the formation of glucose.

If radiolabeled CO2 is introduced to the plants, the radiolabeled CO2 will get fixed to RUBP forming 6 carbon intermediate which splits up forming 3-phosphoglycerate. This gets reduced to glyceraldehyde 3 phosphate with the help of NADPH and ATP. This glyceraldehyde 3 phosphate results in the formation of sugar.

Thus the glucose which was extracted has radiolabeled carbon.

ATP provides the free energy to power the Calvin Cycle and NADPH is the key electron donor (reducing agent) for the formation of sugar. The process of photosynthesis will not take place, without light for a long time.