If plants are grown in the light with CO2 containing heavy oxygen (18O), subsequent analysis will show that 18O incorporated in organic molecules first appears in If plants are grown in the light with CO2 containing heavy oxygen (18O), subsequent analysis will show that 18O incorporated in organic molecules first appears in photosystem II the thylakoid membranes. the stroma. the chloroplast intermembrane space. the thylakoid space.