Mitochondria are like tiny power plants in our cells that make energy in the form of ATP. They have two membranes, with the inner one folded to make lots of surface area. The fluid inside the mitochondria is where acetyl CoA is made from pyruvate. The citric acid cycle happens in this same fluid and makes molecules that help create energy. The electron transport chain is on the inner membrane and is a series of steps that make a proton gradient. This gradient is used to make ATP, which is energy for our cells.