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Why does photosynthesis produce O2 from CO2?

  1. Dec 8, 2018 #1
    My understanding is that the CO2 to O2 reaction is endothermic, so it takes more energy to be put in than you get back. So if plant life is attempting to get energy from photosynthesis, then why would it do an endothermic reaction which would take energy away from its cells?
  2. jcsd
  3. Dec 8, 2018 #2


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    They don't just need energy. To grow they need to make carbohydrates (sugars).

    In the daytime they use sunlight to convert CO2 and water into carbohydrate and O2. Some O2 is released and some is used to split some of the carbohydrate to make energy. At night they absorb O2 to split the carbs.

    Because not all of the carbohydrate produced is turned into energy there is a net excess of O2.
  4. Dec 8, 2018 #3

    jim mcnamara

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    If you look at the energy budgets of a plant, you find that plants have a net efficiency of ~3% - ~6%. The energy used goes into respiration (energy use) required for the photosynthetic process, growth, and reproduction. Plants store excess energy as lipids and starches, example: a carrot root. Farming is taking the excess plants have stored away and using it for human needs instead - heating, food, food preparation, and shelter.

    So, if photosynthesis were not so very efficient, then it is likely that life on earth would still be single celled anaerobic bacteria-like beasties.
  5. Dec 9, 2018 #4


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    The point of photosynthesis is for plants and other photosynthetic organisms to store the energy of sunlight as chemical fuels that can later be burned to produce energy. Therefore, the reactions involved in photosynthesis should be endothermic so that they can take energy from an external source (the sun) and produce molecules with higher chemical potential energy (sugars). Overall, water + CO2 --> sugar + O2 is an endothermic reaction power by sunlight. This process later allows the plant to burn the sugars to produce energy that can be used to power cellular functions through the exothermic reverse reaction sugar + O2 --> water + CO2.
  6. Dec 13, 2018 #5
    Good information so far, keep it coming folks.
  7. Dec 13, 2018 #6


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    The energy that runs the chemical reactions comes from sunlight.
    O2 is released when CO2 is used to synthesise organic molecules.
    Your question has been fully answered.
    It is time you did some reading on the subject rather than wasting the time of others.
  8. Dec 14, 2018 #7


    Staff: Mentor

    The OP question has been addressed. Thread closed.
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