Photosynetheis and respiration are opposite processes...but, have to be balanced... i.e. the carbons can't be on the same side of the reaction:
Respiration: Glucose + O2 = CO2 + H2O
Photosynethesis: CO2 + H2O = Glucose + O2 (requires energy and a catalyst, chlorophyll)

Plants can, in fact, do both processes. Respiration, however is the one that releases energy, as it is the reverse of photosynthesis, which requires it. Photosynthetic reactions occur in two steps (note, this is overly simplified)...
1)H2O is converted to O2
2)CO2 is converted to a carbohydrate (glucose, etc.)
One of these reactions produce ATP, the other requires it. So, the ATP requirement cancels out in the net equation.