Commenting on two classmates discussion on Photosynthesis and Cellular Respiration

This what the discussion is about Consider the two following statements: “Photosynthesis and cellular respiration are opposite reactions” and “Photosynthesis and cellular respiration are complementary reactions.”


In response to your peers, explain how plants can help reduce excess carbon dioxide in the atmosphere. Predict the possible consequences for terrestrial and oceanic life if carbon dioxide emissions from fossil fuels continue to rise.

Classmate # 1 Robert

I would agree with both. All living things need energy to survive. To say that both of these statements are false, or one is true while the other is not would be contradicting. Photosynthesis is the process of the cellular conversion of sunlight and carbon dioxide into useable energy, sugars. The sugar is then broken down using cellular respiration, giving off oxygen and water as a byproduct. In plants, without cellular respiration, photosynthesis is useless, and without photosynthesis, cellular respiration could not occur. So, I would say both statements are correct.

Autotrophs are the foundation of every ecosystem. Every ecosystem has a flow of energy. The biggest carnivore would not be alive had there been no autotrophic level feeding the prey, the predators feed on. There would be no life at all if there was an absence of autotrophs.

Classmate # 2 Darren

To claim that the two mechanisms are opposite one would have to ignore the finer details of each and limit the comparison exclusively to the context of carbon dioxide and oxygen. In that limited sense one could say that photosynthesis uses O2 to produce CO2 and cellular respiration uses O2 to produce CO2. That in my opinion is reducing away too much detail for me to find it an acceptable usage of the word opposite.

Beginning with the definition of complementary given by Google which uses the Oxford Pocket English Dictionary “combining in such a way as to enhance or emphasize the qualities of each other or another,” (Thompson 1996) or, in a biochemical context related by the rules of base pairing regarding nucleotides. I would consider than in an organism that uses both mechanisms to produce ATP they could technically ‘combine’ to enhance the qualities of the total energy production for the organism but I do not see how they would ’emphasize the qualities of each other’ without actively recycling the O2 and CO2 back and forth.

That said I would consider this a loaded question and take the pedantic standpoint that neither statement is correct as to do so would be too much of a stretch for each word. Instead if one must compare the two mechanisms it would be best to include a bit more detail and avoid misleading words. I would say that photosynthesis and cellular respiration are ATP producing mechanisms one which uses light and CO2 producing O2 and the other uses O2 with an output of CO2.

I do agree that autotrophs are the foundation of all ecosystems. In order for life to function it requires a constant input of energy. Without that energy the organism can no longer function once it uses up its reserves. There are chemotrophs which do not require light to produce energy but for the most part the added energy for life of earth comes from the sun and phototrophs.

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