The purpose of cellular respiration is to convert glucose from food into energy.

Cells break down glucose in a series of complex chemical reactions and combine the reaction products with oxygen to store energy in adenosine triphosphate (ATP) molecules. The ATP molecules are used to power cell activities and act as the universal energy source for living organisms.

Cellular respiration in humans starts in the digestive and respiratory systems. Food is digested in the intestines and converted to glucose. Oxygen is absorbed in the lungs and stored in red blood cells. The glucose and the oxygen travel out into the body through the circulatory system to reach cells that need energy.

The cells use the glucose and oxygen from the circulatory system for energy production. They deliver the waste product, carbon dioxide, back to the red blood cells and the carbon dioxide is released to the atmosphere through the lungs.

While the digestive, respiratory and circulatory systems play a major role in human respiration, respiration on a cellular level takes place inside the cells and in the mitochondria of the cells. The process can be broken down into three distinct steps:

• Glycolysis: The cell splits the glucose molecule in the cell cytosol.
  
• Krebs cycle (or citric acid cycle): A series of cyclical reactions produces the electron donors used in the next step and takes place in the mitochondria.
• The electron transport chain: The final series of reactions that uses oxygen to produce ATP molecules takes place on the inner membrane of the mitochondria.

In the overall cellular respiration reaction, each glucose molecule produces 36 or 38 molecules of ATP, depending on the cell type. Cellular respiration in humans is a continuous process and requires a continuous supply of oxygen. In the absence of oxygen, the cellular respiration process stops at glycolysis.

The purpose of cell respiration is to produce ATP molecules through the oxidation of glucose.

For example, the cellular respiration formula for the production of 36 ATP molecules from a molecule of glucose is C₆H₁₂O₆ + 6O₂ = 6CO₂ + 6H₂O + energy (36ATP molecules). The ATP molecules store energy in their three phosphate group bonds.

The energy produced by the cell is stored in the bond of the third phosphate group, which is added to the ATP molecules during the cellular respiration process. When the energy is needed, the third phosphate bond is broken and used for cell chemical reactions. An adenosine diphosphate (ADP) molecule with two phosphate groups is left.

During cellular respiration, the energy from the oxidation process is used to change the ADP molecule back to ATP by adding a third phosphate group. The ATP molecule is then again ready to break this third bond to release energy for the cell to use.

In glycolysis, a six-carbon glucose molecule is split into two parts to form two pyruvate molecules in a series of reactions. After the glucose molecule enters the cell, its two three-carbon halves each receive two phosphate groups in two separate steps.

First, two ATP molecules phosphorylate the two halves of the glucose molecule by adding a phosphate group to each one. Then enzymes add one more phosphate group to each of the halves of the glucose molecule, resulting in two three-carbon molecule halves, each with two phosphate groups.

In two final and parallel series of reactions, the two phosphorylated three-carbon halves of the original glucose molecule lose their phosphate groups to form the two pyruvate molecules. The final splitting of the glucose molecule releases energy that is used to add the phosphate groups to ADP molecules and form ATP.

Each half of the glucose molecule loses its two phosphate groups and produces the pyruvate molecule and two ATP molecules.

Glycolysis takes place in the cell cytosol, but the rest of the cellular respiration process moves into the mitochondria. Glycolysis does not require oxygen, but once the pyruvate has moved into the mitochondria, oxygen is required for all further steps.

The mitochondria are the energy factories that let oxygen and pyruvate enter through their outer membrane and then let the reaction products carbon dioxide and ATP exit back into the cell and on into the circulatory system.

The citric acid cycle is a series of circular chemical reactions that generates NADH and FADH₂ molecules. These two compounds enter the subsequent step of cellular respiration, the electron transport chain, and donate the initial electrons used in the chain. The resulting NAD⁺ and FAD compounds are returned to the citric acid cycle to be changed back to their original NADH and FADH₂ forms and recycled.

When the three-carbon pyruvate molecules enter the mitochondria, they lose one of their carbon molecules to form carbon dioxide and a two-carbon compound. This reaction product is subsequently oxidized and joined to coenzyme A to form two acetyl CoA molecules. Over the course of the citric acid cycle, the carbon compounds are linked to a four-carbon compound to produce a six-carbon citrate.

In a series of reactions, the citrate releases two carbon atoms as carbon dioxide and produces 3 NADH, 1 ATP and 1 FADH₂ molecules. At the end of the process, the cycle re-constitutes the original four-carbon compound and starts again. The reactions take place in the mitochondria interior, and the NADH and FADH₂ molecules then take part in the electron transport chain on the inner membrane of the mitochondria.

The electron transport chain is made up of four protein complexes located on the inner membrane of the mitochondria. NADH donates electrons to the first protein complex while FADH₂ gives its electrons to the second protein complex. The protein complexes pass the electrons down the transport chain in a series of reduction-oxidation or redox reactions.

Energy is liberated during each redox stage, and each protein complex uses it to pump protons across the mitochondrial membrane into the inter-membrane space between the inner and outer membranes. The electrons pass through to the fourth and final protein complex where oxygen molecules act as the final electron acceptors. Two hydrogen atoms combine with an oxygen atom to form water molecules.

As the concentration of protons outside the inner membrane increases, an energy gradient is established, tending to attract the protons back across the membrane to the side that has the lower proton concentration. An inner membrane enzyme called ATP synthase offers the protons a passage back through the inner membrane.

As the protons pass through ATP synthase, the enzyme uses the proton energy to change ADP to ATP, storing the proton energy from the electron transport chain in the ATP molecules.

The complex biological and chemical processes that make up respiration on a cellular level involve enzymes, proton pumps and proteins interacting at a molecular level in very complicated ways. While the inputs of glucose and oxygen are simple substances, the enzymes and proteins are not.

An overview of glycolysis, the Krebs or citric acid cycle and the electron transfer chain helps demonstrate how cellular respiration works on a basic level, but the actual operation of these stages is much more complex.

To describe the process of cellular respiration is simpler on a conceptual level. The body takes in nutrients and oxygen and distributes the glucose in the food and the oxygen to individual cells as needed. The cells oxidize the glucose molecules to produce chemical energy, carbon dioxide and water.

The energy is used to add a third phosphate group to an ADP molecule to form ATP, and the carbon dioxide is eliminated through the lungs. ATP energy from the third phosphate bond is used to power other cell functions. That's how cellular respiration forms the basis for all other human activities.