Cellular respiration takes place in the organelles called mitochondria. The mitochondria take in glucose, a type of sugar, and change it in the presence of oxygen into an energy-rich molecule called ATP, or adenosine triphosphate. For every molecule of glucose, 36 molecules of ATP are made.
Animal cells have an average of 1,000 to 2,000 mitochondria each. Cells that have a greater need for energy such as brain and muscle cells, have many more mitochondria. Mitochondria are made up of an inner and outer membrane. The outer membrane is more permeable and lets small proteins, ATP, ions and nutrients pass through. The inner membrane is less permeable and only allows carbon dioxide, oxygen and water to pass freely. The machinery and enzymes used to make ATP lie within the inner membrane.