an octahedron: basically two square based pyramids with their bases stuck together.
Most eukaryotic cells have mitochondria, which produce ATP from products of the citric acid cycle, fatty acid oxidation, and amino acid oxidation. At the mitochondrial inner membrane, electrons from NADH and succinate pass through the electron transport chain to oxygen, which is reduced to water.
The electron transport chain is the stepwise process of cellular respiration that is responsible for producing: Water (with the help of oxygen we breathe) up to 34 ATP (thanks to the proton gradient)