Bill Nye explains that an atom is composed of protons, neutrons, and electrons. Protons and neutrons are found in the nucleus and have a positive and a neutral charge, respectively. Electrons, taking on a negative charge, move around the nucleus. Bill Nye demonstrates that an atom is mostly empty space by illustrating the distance between the nucleus and the electrons.
Brian Cox explains the origin of the mitochondrial symbiotic relationship with the cell. He starts with introducing the nucleus and gives context to the early cell types - prokaryotes and eukaryotes. Brian then explains what scientists believe was the beginning of a symbiotic relationship where a bacteria enters an archaea.