Aerobic (oxygen-requiring) organisms need oxygen because they use it for energy. When you breathe, oxygen is absorbed through the lungs and picked up by a protein in the blood (hemoglobin) that distributes it around the body. Within cells, tiny organelles (little organs) called mitochondria create energy by passing electrons through relay stations called cytochromes until they eventually join with oxygen to form water. The formation of oxygen releases energy that can be used by the body at a later point. Not all cells (or organisms) are aerobic. There are cells that use fermentation (an anaerobic, or without oxygen process) to produce energy. Examples of anaerobic organisms include yeast (which makes breads rise!) and certain bacteria; however, aerobic metabolism is more common because it provides more energy than metabolism without oxygen.