The organelle that uses molecular oxygen to convert and detoxify harmful substances is the peroxisome.

Here's why:

* Peroxisomes are small, membrane-bound organelles found in the cytoplasm of eukaryotic cells.

* They contain enzymes that break down fatty acids, amino acids, and other molecules through oxidation. This process uses molecular oxygen, producing hydrogen peroxide (H₂O₂) as a byproduct.

* Catalase, another enzyme present in peroxisomes, then breaks down the harmful hydrogen peroxide into water and oxygen, preventing cellular damage.

Other functions of peroxisomes:

* Synthesis of bile acids: Important for lipid digestion.

* Degradation of purines: Breakdown products of nucleic acids.

* Production of plasmalogens: A type of lipid found in myelin sheaths.

So, peroxisomes play a crucial role in detoxification and other essential metabolic processes.