Question

if plants need oxygen for cellular respiration, then why do they release oxygen into the atmosphere?

Answer 1

The answer is, they do not. Plants actually hold on to a small amount of the oxygen they produced in photosynthesis and use that oxygen to break down carbohydrates to give them energy.

Answer 2

Since the plants use the sugar they make for more than just energy, they produce more oxygen than they use.

Answer 3

Found this:

Plants produce oxygen, because when they photosynthesize, they take carbon dioxide and water and combine them using light energy to produce sugars and oxygen. This stores the energy in chemical bonds (in the sugars) and releases O2.