Understanding Cellular Respiration

This article will help you to understand the process of cellular respiration -- including where it takes place, why it is important, and what it produces.

Cellular respiration is the process by which food is broken down by the body's cells to produce energy, in the form of ATP molecules. In plants, some of this ATP energy is used during photosynthesis to produce sugar. These sugars are in turn broken down during cellular respiration, continuing the cycle.

There are three main stages of cellular respiration: 1) glycolysis, 2) Krebs Cycle, and 3) the Electron Transport Chain (ETC).

Cellular Respiration Overview:

Cellular respiration is carried out by every cell in both plants and animals and is essential for daily living. It does not occur at any set time, and, at the same point in time, Neighboring cells may be involved in different stages of cellular respiration. Cellular respiration is an exergonic reaction, which means it produces energy. It is also a catabolic process - it breaks down polymers into smaller, more manageable pieces. The ultimate goal of cellular respiration is to take carbohydrates, disassemble them into glucose molecules, and then use this glucose to produce energy-rich ATP molecules. The general equation for cellular respiration is: one glucose molecule plus six oxygen molecules produces six carbon dioxide molecules, six water molecules, and approximately 36-38 molecules of ATP.

(Please Note: The three steps of cellular respiration have been summarized below. The description does not include all of the sub-steps involved. Unless you are planning to major in a biology field, no high school or college course will require you to memorize each individual step in these three processes. They are looking only for a general knowledge of the process as a whole and its major component steps)


Glycolysis involves the breaking down of glucose molecules from carbohydrates into molecules of pyruvate, which will continue on to the Krebs Cycle. This process occurs in the cytosol of the cell and can proceed regardless of the presence of oxygen. In the first stage of glycolysis, energy is actually used to phosphorylate the 6-carbon glucose molecule. This means that a phosphate is taken from ATP (which becomes ADP) and added to the glucose molecule. This addition of phosphate makes the molecule much more chemically reactive. The position of the glucose molecule is changed, so that it becomes its isomer, fructose. An enzyme then cuts the molecule apart, producing two 3-carbon molecules of pyruvate. Through several more steps, catalyzed by several different enzymes, the phosphate groups are removed and these pyruvate molecules are ready to enter the Krebs Cycle. The reactions of glycolysis produces a net gain of 2 ATP molecules, as well as a release of 2 water molecules and 2 NADH molecules (these are another type of energy-rich molecule)

The Krebs Cycle:

As pyruvate is being shuttled from the cytosol to the interior of the mitochondrion, a microenzyme removes one carbon and two oxygens from each molecule, producing Aceytl CoA. This two-carbon sugar that actually enters the Krebs Cycle. The Krebs Cycle is a series of steps, catalyzed by enzymes, which completely oxidize the Aceytl CoA molecule. The Krebs Cycle is an aerobic process, meaning it needs oxygen to function. Two complete turns of the Krebs Cycle must occur to produce: 4 carbon dioxide molecules, 6 NADH molecules, 2 ATP molecules and 2 FADH2 molecules (yet another energy-yielding molecule).

The Electron Transport Chain:

Very little energy has been produced during glycolysis and the Krebs Cycle. Most of the energy locked in the original glucose molecule will be released by the electron transport chain and oxidative phosphorylation. The electron transport chain is a network of electron-carrying proteins located in the inner membrane of the mitochondrion. These proteins transfer electrons from one to another, down the chain, much in the way a bucket brigade passes buckets of water. These electrons will eventually be added, along with protons, to oxygen, which is the final electron acceptor. This produces water, but does not produce any ATP. The ATP is actually produced by a proton motive force. This force is a store of potential energy created by the gradient formed when hydrogens (protons) are moved across a biological membrane. Therefore, the electron transport chain merely produces a gradient through which ATP can be made (this is known as chemiosmosis). The electron transport chain produces the remaining 32-34 ATP.

Fermentation - an Alternative to Cellular Respiration:

While some steps do not require oxygen, cellular respiration, as a whole, can only take place when oxygen is present. For organisms living in anaerobic conditions, complete cellular respiration is not possible. For these organisms, glycolysis is the first and last step of the cellular respiration process. Glycolysis proceeds normally, as in aerobic conditions, producing a net gain of 2 ATP. The two pyruvate molecules, however, are reduced and the NAD necessary for the initiation of glycolysis is recycled. In this way, the cells do not deplete their store of NAD, although they are only able to produce 2 ATP. As a by-product of fermentation, either ethanol or lactic acid is produced.

Cellular respiration is an almost universal process by which organisms utilize the sugars in their food to produce enough energy to perform all the necessary actions of living creatures.

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