 Solution

The hydrogenosomes and mitosomes are specialized mitochondrion-related organelles found in various anaerobic or oxygen-limited eukaryotes. Typical hydrogenosomes support anaerobic ATP production, converting substrates such as pyruvate into products including acetate, carbon dioxide and molecular hydrogen. Substrate-level phosphorylation supplies ATP, while hydrogenase disposes of reducing equivalents as hydrogen; oxygen is not the terminal acceptor of a conventional respiratory chain in this pathway.^[ref]

The mitosomes retain selected essential metabolic functions but do not generate ATP by oxidative phosphorylation. In Giardia, their role includes iron-sulfur cluster assembly and protein maturation, demonstrated using organelle markers and biochemical reconstitution.^[ref] Their functions are lineage-dependent: Entamoeba histolytica mitosomes compartmentalize a sulfate activation pathway.^[ref] Thus neither organelle class should be described simply as a nonfunctional mitochondrion.