Final answer:
The oxygen we breathe is produced during photosynthesis, where water molecules are split in the reaction center of Photosystem II, releasing diatomic oxygen gas that is expelled into the atmosphere.
Step-by-step explanation:
The oxygen we breathe comes from the splitting of water molecules (H₂O) during the process of photosynthesis in plants. This process takes place in the reaction center of Photosystem II (PSII), specifically at a point called P680. Here, the high-energy electrons are transferred to a primary electron acceptor through the electron transport chain, and an electron from a water molecule is used to replenish the electron lost from P680. When a molecule of water splits, it releases a pair of electrons, two hydrogen ions, and one oxygen atom. To make one molecule of diatomic oxygen gas (O₂), which is the form we breathe, it is necessary to split two water molecules. The oxygen that is produced is then released from the plant into the atmosphere. About 10% of this oxygen is used by the plant's own mitochondria for cellular respiration, and the remainder is expelled, contributing to the oxygen in the air that is available for aerobic organisms, including humans, to inhale.