Photosynthesis is the process of converting light to chemical energy and using that to produce glucose.

The general word formula for photosynthesis is -

$6{\mathrm{C}\mathrm{O}}_2+12{\mathrm{H}}_2\mathrm{O}\mathrm{ }\mathrm{ }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{H}}_2\mathrm{O}+6{\mathrm{O}}_2$ in presence of light and chlorophyll. The whole photosynthesis process is divided into two broad parts -

Light reaction where the light energy is used to generate ATP and ${\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}\mathrm{H}}_2$ which is later used in the Dark reaction to reduce carbon dioxide into glucose (carbohydrate) molecule. The free oxygen released as a byproduct in the photosynthesis occurs by splitting water. The oxygen of the carbon dioxide is used in making the glucose molecule through the Calvin cycle.

Splitting of water molecule into protons and oxygen in presence of sunlight is called as photolysis of water that occurs in the non-cyclic photophosphorylation. Thus, if ${\mathrm{H}}_2{\mathrm{O}}^{18}$ is used in the photosynthesis process, the oxygen released during photosynthesis will be ${\mathrm{O}}^{18}$ type.