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.
This is called the electron transport chain. As the molecules pass the electron they pump hydrogen ions from one side of the membrane to the other. The high concentration of hydrogen ions pumped into the inner thylakoid space (the thylakoid lumen) can then be used to make ATP by a process called chemiosmosis. The electron finally gets dumped on the molecule NADP (Nicotinamide adenine dinucleotide phosphate) changing that molecule into NADPH.