Re: Can oxygen released via photosynthesis be used directly by the cell?

Yes, oxygen and carbon dioxide can be recycled within the plant without having 
to exit to the atmosphere first. Cellular respiration in the mitochondria occurs 
all the time so will be occurring at the same time as photosynthesis in the 
chloroplasts. Aquatic plants or plants that inhabit waterlogged soils often 
contain air spaces in their stems, so-called aerenchyma tissue, that allow an 
internal pathway for oxygen to reach the roots from above the water. A good 
example of carbon dioxide recycling is when leaves wilt during midday due to 
rapid transpiration. Despite plenty of light, photosynthesis grinds nearly to a 
standstill as the carbon dioxide supply is cut off when the stomata close. The 
carbon dioxide produced by cellular respiration in the leaf cannot escape and 
can be fixed via photosynthesis without escaping into the atmosphere first.

The stomata are the main pathway for both oxygen and carbon dioxide entry/exit 
in leaves. Stems have gas exchange structures called lenticels.

Oxygen entering the roots would be used in the roots for cellular respiration 
not transported to the leaves.