Asked by: Anonymous
The reason we breathe in oxygen is because it’s a highly reactive molecule that can combine with the compounds in our food to build new molecules and release energy. Other metabolic pathways can achieve a similar result, albeit less efficiently, using nitrate or sulphate instead of oxygen, but they all share the chemical property of being oxidizing agents. Carbon dioxide (CO2) is already fully oxidized, so it can’t be used to power respiration in the same way.
Of course, plants take in CO2 for photosynthesis, but this is a completely separate metabolic process from breathing. Plants make their food using photosynthesis, but they still need to breathe in oxygen to ‘burn’ this food. So the CO2 they absorb doesn’t count as breathing, but even if it did, it’s doubtful that a photosynthetic organism would ever evolve intelligence. Brains are metabolically very costly organs and photosynthesis doesn’t generate energy fast enough to run on
Also, creatures that can make their own food just by pointing at the Sun don’t need to hunt, plan or even move much. So there’s little selective pressure for them to evolve intelligence in the first place.
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