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All organisms—not just plants and animals—use cellular respiration to supply the energy they need to live. When animals eat plants or other animals, they break down the complex molecules into simpler molecules for their own cells to use.
Digestion of complex carbohydrates by enzymes (special proteins that affect the rate of chemical reactions) produces simple sugars like glucose. These simple sugars are transported into cells and then into their mitochondria.
Next, the glucose is broken down further, a process that involves a series of chemical reactions controlled by other enzymes. In the final step, the chemical bonds of the glucose molecule are broken, and their potential energy is released. In many organisms, respiration requires the gas oxygen and is called aerobic. During aerobic respiration the mitochondria in each cell can capture the energy released from one glucose molecule and form new bonds in 36 high-energy molecules known as ATP (adenosine tri-phosphate). Carbon dioxide is produced as a waste product (the carbons were once a part of the glucose molecule) and must be excreted by the cells.
Almost every metabolic process that a cell performs to maintain homeostasis requires the use of energy stored and made available in these ATP molecules.
Some organisms do not have mitochondria (most are very simple and/or unicellular). They perform anaerobic respiration which does not require oxygen. Organisms that do have mitochondria can also perform anaerobic respiration when there is not a sufficient supply of oxygen available to the mitochondria. This process of releasing energy from organic molecules produces ATP, but in much smaller amounts. An additional byproduct is formed by anaerobic respiration. In many simple organisms, alcohol is produced in a related process called fermentation. In humans, our cells produce lactic acid when they perform anaerobic respiration.
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