A photosynthesis experiment uses Chlorella to track the route taken by 14C in photosynthesis. The 14C is provided from CO2 and no new sources of CO2 are available. After the experiment the scientist extracts all of the starch produced and analyses the Carbon present. She finds that only half of the carbon is 14C. She repeats the experiment with more time and twice as much CO2 and finds that three quarters of the Carbon in the starch is radioactive.

Answer the following questions about the experiment:

Why is there radioactive Carbon in the starch? 2 marks

Why is only some of the Carbon in starch radioactive after the experiment? 2 marks

Using ideas from the Calvin cycle explain why doubling the amount of radioactive Carbon does not lead to all of the Carbon in starch being radioactive. 5 marks

How did Calvin himself use radioactive carbon? 5 marks

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Why is there radioactive Carbon in the starch?

There is radioactive Carbon in the starch because Chlorella, used in the experiment, undergoes photosynthesis. During photosynthesis, Chlorella takes in Carbon dioxide (CO2) from its environment, which contains the radioactive Carbon-14 isotope (14C). This radioactive Carbon becomes incorporated into the organic molecules synthesized by the Chlorella, including starch.

Why is only some of the Carbon in starch radioactive after the experiment?

After the experiment, only some of the Carbon in starch is radioactive because the Chlorella is not able to incorporate all the radioactive Carbon-14 into starch molecules. Some Carbon gets used for other biological processes within the Chlorella, such as cellular respiration or the synthesis of other organic compounds. Therefore, not all the Carbon in the Chlorella is converted into radioactive starch.

Using ideas from the Calvin cycle, explain why doubling the amount of radioactive Carbon does not lead to all of the Carbon in starch being radioactive.

In the Calvin cycle, Carbon dioxide (CO2) is fixed into organic molecules, such as sugars, through a complex series of reactions. Even with an increased concentration of radioactive CO2 (Carbon-14), doubling the amount of radioactive Carbon will not lead to all of the Carbon in starch being radioactive due to the regulation of the Calvin cycle.

There are several steps in the Calvin cycle where Carbon dioxide is converted into different molecules before ultimately being used to generate glucose or starch. If there is an excess of non-radioactive Carbon dioxide (Carbon-12) available, it will compete with the radioactive Carbon-14 for incorporation into organic molecules. This competition can cause a dilution effect, resulting in a lower percentage of radioactive Carbon being present in the final starch product.

Additionally, the Calvin cycle contains multiple enzymatic reactions, some of which may not have high specificity for Carbon-14. This can further contribute to the lower percentage of radioactive Carbon in the starch, as some steps of the cycle may preferentially use Carbon-12 over Carbon-14.

How did Calvin himself use radioactive carbon?

Melvin Calvin, an American biochemist, utilized radioactive Carbon-14 to trace the pathway of Carbon fixation during photosynthesis. In the 1950s, he performed experiments using the radioactive isotope to track how Carbon dioxide is converted into glucose through the Calvin cycle in plants.

Calvin and his colleagues exposed plants to CO2 containing Carbon-14 and then traced the movement of this radioactive Carbon throughout different reactions within the plant, including those in the chloroplasts where photosynthesis occurs. By analyzing the distribution of Carbon-14 in various organic compounds produced by the plants, Calvin was able to deduce the steps and intermediates involved in the Calvin cycle, ultimately leading to a better understanding of the process of photosynthesis.