Aerobic respiration keeps us alive on a moment-to-moment basis. It is often considered to be synonymous with breathing, but this is not quite accurate. Breathing is the process by which humans and most other land animals take air containing oxygen into our bodies, but aerobic respiration is the chemical reaction that allows cells to convert glucose in the presence of oxygen to useful energy, water and carbon dioxide at the microscopic level. The chemical equation for aerobic respiration is well known, and variations and relatives of this literally vital equation form one of the cornerstones of basic cell biology.
The cells of eukaryotes, or multicellular animals, rely on aerobic respiration for energy production. They take in oxygen gas and glucose molecules from their environment by breathing and eating respectively. In a straightforward chain of chemical reactions that occur inside parts of the cell called mitochondria, these molecules are converted to water, carbon dioxide, and a form of energy called ATP, or adenosine triphosphate.
The molecules involved in aerobic respiration cannot simply be thrown into a mixture to produce energy, any more than the parts of a car can become an automobile by rolling down an assembly line in the absence of instructions to the plant workers. Different foods containing the macromolecules we call proteins, carbohydrates and fats can all contribute glucose molecules, even though glucose itself is a sugar and therefore a carbohydrate. Once glucose is liberated from foods or body storage sources in the muscles, blood and liver, and it is taken into the cells of the body, it can attach to mitochondria within the cells, and special proteins called enzymes carry out the various reactions that sum to aerobic respiration.
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The Complete Reaction
Complete chemical reactions must be "balanced" – that is, the number of atoms of a given element (carbon, hydrogen and so on) on one side of the equation must be the same as that on the other side. This can mean adding multiplying factors, or coefficients, to the front of some of the molecules.
The complete, balanced reaction of aerobic respiration is:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + heat + 38 ATP
Heat is not an element, but the fact that it is given off during aerobic respiration results from energy in the chemical bonds of the glucose and oxygen molecules escaping into the environment.