Mitochondria give off water, carbon dioxide and energy, in the form of ATP molecules, during cellular respiration. Mitochondria produce these by combining glucose and oxygen molecules, which creates the molecules and releases large amounts of energy. This is the main process by which most organisms meet their energy needs.
There are two major steps to the creation of carbon dioxide, water and energy from glucose and oxygen. The first step is glycolysis, which does not take place in the mitochondria, but in the cytoplasm outside of them. The cell uses enzymes to break glucose, which has six carbon atoms, into two pyruvate molecules, which have three carbon atoms each. This process releases a small amount of energy, but does not require oxygen.
In the absence of oxygen, certain cells, such as muscle cells, can use glycolysis by itself to produce energy, but it is very inefficient relative to full cellular respiration. This process usually produces lactic acid or ethanol rather than water and carbon dioxide.
If oxygen is present, the pyruvate moves into the mitochondria to combine with oxygen in a process known as oxidative phosphorylation. This name refers to the fact that the mitochondria uses a reaction with oxygen to attach phosphorus ions to ADP, producing ATP.