How is ATP produced in the mitochondria?

The Krebs cycle produces ATP by substrate level phosphorylation. The cycle is completed twice per glucose molecule. It also produces NADH which goes on to donate an electron to the electron transport chain on the cristae. A series of redox reactions occur, releasing energy which is used to actively transport protons into the intermembrane space, thereby creating a proton gradient. The chemiosmosis of protons through the stalked particle on the cristae gives the ATP synthase the energy required to catalyse:ADP+Pi=ATP. This is oxidative phosphorylation.