Mitochondria are the powerhouses for the cell. Cells with high energy demands, such as muscle cells, have large number of mitochondria. Mitochondria are double membrane bound organelles. The outer membrane is fairly smooth. The inner membrane is highly folded, as shown on the right. The folds are called cristae. The cristae greatly increase the inner membrane's surface area. It is on the membrane of the cristae that molecules derived from the breakdown of sugar are combined with oxygen to produce ATP, the primary energy source for the cell. This process is called cellular respiration.