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    The leaves of plant growing in bright light were exposed to radioactive carbon dioxide for an hour. This contains radioactive carbon atoms. Several hours later the roots of the plant had become radioactive.

    a ) Describe fully ,in sequence, the different processes that were responsible for the radioactivity getting from the air around the plant to its roots.

    b ) The same experiment was tried with an identical plant , but in dimmer light. No radioactivity reached this plant’s roots. Suggest why this was the case.

    Hello, I would like some help with answering this question. I have read the appropriate chapters from my textbook but I am still a little unsure and would appreciate help in answering or correcting my initial answers.

    For the first question (which is worth 9 Marks) I have answered as follows:
    The overall process at work in this experiment is translocation, the movement of sucrose and amino acids in the phloem.
    The first process to take place will be photosynthesis. The air surrounding the plant is known to contain radioactive carbon dioxide, as such when photosynthesis occurres the radioactive carbon dioxide, in addition to water and energy are taken into the plant to produce glucose and oxygen.

    However, since the plant has now been exposed to radioactive carbon dioxide it will cause the glucose produced to be radioactive (contain radioactive carbon atoms).The second process is therefore the production of radioactive glucose.

    These are the third processes:
    -The manufactured glucose is then either turned into starch and stored in the chloroplasts until it is needed.
    - Used in combination with nitrate ions in solution in the xylem to produce amino acids.
    - Or converted into less reactive sucrose to allow for greater room for glucose and starch production.

    Organs within the plant require amino acids to make proteins and glucose for energy release during respiration, even more so within growing plants where new leaves, roots, etc. are being produced.
    Consequently, the amino acids and sucrose are translocated in the phloem in sieve tubes. The sieve tubes contain cytoplasm and are assisted by by companion cells which supply energy for the transportation of sucrose and amino acids. The sucrose and amino acids are transloacted from the leaves to travel throughout the plant, often to the growing regions of the root or shoot. Since the glucose was radioactive the converted sucrose in addition to the amino acids and stored starch are all radioactive. The radioactive sucrose and amino acids travel to the growing regions, or roots, where amino acids are used to produce proteins to create cytoplasm, and the sucrose is used for energy release during growth and respiration. When the radioactive sucrose and amino acids reach the roots of the plant, and these processes take place, the roots of the plant henceforth become radioactive.
    This is the fourth process.


    For the second question I am not sure how to answer. It is worth two marks. Would it be to do with the light intensity providing less energy for respiration in dimmer light? This would reduce the amount carbohydrates and amino acids being produced because the plant does not photosynthesis as quickly in dimmer light. As such they do not reach the roots via translocation as quickly, or within the same time frame as the other plant. Additionally would the carbohydrates and amino acids that were produced be used up by the cells producing them?


    Any help would be appreciated 😊
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    Dimmer light would result in a decrease is photosynthesis as less light is being provided. If there is a decrease in photosynthesis then there will be less organic substances (Such as glucose) being made and as a direct result less CO2 will be absorbed.
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    (Original post by Lord Nutter)
    Dimmer light would result in a decrease is photosynthesis as less light is being provided. If there is a decrease in photosynthesis then there will be less organic substances (Such as glucose) being made and as a direct result less CO2 will be absorbed.
    Thank you for your reply. It helped. I did not mean to write respiration, I thought I had written photosynthesis, sorry I was very tired when i posted this.
 
 
 
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