Ask A Scientist General Science Archive

name         Anne
status       educator
grade        K-3
location     MD

Question -   I understand plants need oxygen to produce sugar and
energy...but I'm still confused about whether plants produce more CO2 or
oxygen.
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Cellular respiration (process that uses oxygen to break sugar into energy 
for life functions) and photosynthesis (green plants converting sunlight 
energy into sugars using water and carbon dioxide) are distinct and 
separate processes.  Animals only use cellular respiration.  Plants do not 
have muscles and the other functions necessary in animals functioning that 
require a high levels of oxygen.  Plants also CAN NOT take the energy 
directly for their energy needs from photosynthesis.  Photosynthesis 
produces a far greater amount of oxygen and sugars, etc. then the small 
amount the plant requires to produce its energy needs.  OK - but only 
during the day when the sun light is available!  During the night, the 
plant actually uses oxygen it has left over from the daylight 
photosynthesis or takes the oxygen from the air surrounding the plant to 
meet its energy needs.  This is not nearly the amount of oxygen an animal 
needs over the same period of time.

To answer your question, during the day the plant produces far more oxygen 
from photosynthesis than it produces carbon dioxide from cellular 
respiration.  It is a waste gas and if not expelled from the plant, can 
cause serious harm.  Since plants do not utilize large amounts of oxygen 
over a twenty-four hour period (ideal conditions, winter is another issue), 
oxygen wins.

It is a good thing too, for ALL the oxygen in the atmosphere is the result 
of plants producing far more oxygen than they need!!!  Take a deep breath!

I hope this helps.

Steve Sample 
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