What molecule is split to release oxygen?

Answer 1

Any molecule containing oxygen.

Any molecule, under the correct conditions, may be split into its component elements, e.g. water (H2O) may be split into hydrogen and oxygen:

2H20 <-> 2H2 + O2

O2 because the natural state of gaseous oxygen is diatomic (i.e. one molecule of oxygen is composed of two oxygen atoms). Exceptions exist, e.g. ozone (O3).

Answer 2

In photosynthesis, water molecules are split to release oxygen through a process called photolysis, which occurs in the thylakoid membranes of chloroplasts. The oxygen released is a byproduct of this process and is essential for aerobic respiration in plants and animals.