Cellular Respiration

Let's first take a look at the movement of energy through the biosphere.

Be certain to keep in mind the First and Second Laws of Thermodynamics. The First Law states that energy can be neither created nor destroyed; the Second Law deals with the fact that while the conversion process is 100% efficient, some energy is always 'lost' to entropy and shows up as heat. Be certain that you understand these energy relationships. They are useful concepts in your everyday life.

The process of the stripping of the energy associated with a glucose molecule and its association with a molecule of ATP is called respiration. It can be defined more precisely as the overall process by which the energy stored in carbohydrates is gradually released and transferred to ATP.

Respiration burns or oxidizes the glucose produced by photosynthesis and provides usable energy for both plants and animals. Keep in mind that both plants and animals respire, but only plants (and a few bacteria) carry on photosynthesis.

The cyclical nature of this energy transformation between photosynthesis and respiration is also diagrammed for you.

Respiration is initiated in the cytoplasm, and completed in the mitochondria. The energy is released from simple sugar molecules that are broken down during a series of steps controlled by enzymes. No oxygen is needed to initiate the process, but in aerobic respiration, the process cannot be completed without oxygen. Carbon dioxide and water are the products of respiration.

Anaerobic respiration and fermentation are two forms of respiration carried out by certain bacteria and other organisms in the absence of oxygen. These forms of respiration release much less energy than aerobic respiration.

Respiration uses the products of photosynthesis, and is essentially the reversal of photosynthesis.

The first series of reactions in respiration are called glycolysis. They occur in the cytoplasm and are diagrammed for you.

Glycolysis can take place under both aerobic or anaerobic conditions. Glycolysis begins with glucose, and after several chemical reactions, eventually forms pyruvic acid or pyruvate, which acts as an intermediate between glycolysis and the Krebs cycle. The Krebs cycle is also known as the tricarboxylic acid cycle.

Pyruvate then moves from the cytoplasm through the outer mitochondrial membrane and enters the Krebs cycle chemistry.

The number of mitochondria present varies from tissue to tissue. Mitochondria are far more numerous than chloroplasts and may number a thousand or more per cell. The internal membrane of the mitochondrion is thrown into a series of folds and provides a large surface area. These folds are called cristae. You may want to review the three dimensional nature of the mitochondrion. The matrix between the cristae houses the enzymes responsible for the Krebs cycle; the surface of the cristae is the site of the enzymes responsible for electron transport.

In the Krebs cycle, pyruvate is first transformed and then combined with coenzyme A, to form acetyl CoA, which, after many different reactions, produces six CO₂ molecules, two ATPs, and an acetyl CoA fragment. To fully use up one glucose molecule, one must "spin" the Krebs cycle twice.

The third and final phase of aerobic respiration is electron transport (oxidative phosphorylation). This is where energy is captured in an orderly manner, as electrons are passed from the oxidation of Krebs cycle organic acids to the electron acceptor NAD and subsequently through a series of reactions. In this chain reaction, energy is transferred as both NADH, FADH₂ (electron carriers), and these carriers eventually produce ATP. Look again at the diagram.

The overall consequences of electron transport are to synthesize ATP and deliver electrons and protons to oxygen, forming water.

Some microorganisms are able to carry on respiration without oxygen. Such anaerobic organisms have a special group of enzymes which permit a partial oxidation of sugars to produce ethanol. This process is called fermentation, and is similar to anaerobic respiration in animals, which produces lactic acid.

In plants, fermentation takes place in anaerobic conditions, and alcohol is produced from many carbohydrate sources (such as in making beer or wine).

During fermentation, the chemistry of glycolysis is exactly the same, but once pyruvate has been produced it moves along a different chemical patheway and produces ATP and ethanol.

The entire process of fermentation goes on in the cytoplasm; mitochondria and the Krebs cycle are not involved.

Some microorganisms are able to switch metabolic pathways depending on the presence or absence of oxygen. Such organisms, called facultative anaerobes, have a great adaptive advantage.

Which mode of respiration is most efficient? In aerobic respiration, a net total of 36 ATPs can be made from one glucose molecule. By contrast, only two ATPs are netted during fermentation.

What does this mean evolutionarily? Probably that primitive anaerobic organisms were not very efficient in preserving the energy trapped by organic molecules.

You may take a quiz on the respiration material in this module. No record of the quiz is made. You decide after the quiz if you really know this material.

Go on to the Photosynthesis module.

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