Why do plants take in oxygen and give off carbon dioxide at night?  

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Note that plants take in oxygen and give off carbon dioxide all the time, not only at night. The only reason we do not notice that during the day time is because plants produce more oxygen than they consume and they consume more carbon dioxide than they produce. In simpler...

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Note that plants take in oxygen and give off carbon dioxide all the time, not only at night. The only reason we do not notice that during the day time is because plants produce more oxygen than they consume and they consume more carbon dioxide than they produce. In simpler terms, during daytime, both photosynthesis and cellular respiration take place, however at night, only cellular respiration takes place.

Photosynthesis can be represented as:

`6CO_2 + 6H_2O + sunlight -> C_6H_12O_6 + 6O_2`

Cellular respiration can be represented as:

`C_6H_12O_6 + 6O_2 -> 6CO_2 + 6H_2O + ATP`

In the first process, plants produce glucose and oxygen, however it only takes place in the presence of sunlight. The second process, cellular respiration, is the one that generates energy (in the form of ATP molecules) and is needed throughout the life of a plant and hence takes place continuously. It is just that photosynthesis dominates respiration during daytime and hence we only notice the respiration process at night.

Hope this helps. 

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