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Return to Biology:  Cellular Respiration              Return to A&P I      Also see Cell Physiology

Cellular respiration is the process in which an organism breaks down fuel (usually glucose, but glycogen, protein, and/or lipids may be used) to capture energy in a usable form (ATP).

When a phosphate is passed from ATP to another molecule, that molecule gains energy; this is an endergonic (energy storage) reaction. Likewise, when that phosphate is removed, both energy and heat are given off (an exergonic reaction), and the molecule contains less energy than before. 

The overall reaction may be written as: C₆H₁₂O₆ + 6O₂ ---->  6CO₂ + 6H₂O, but, as you will see below, the process is much more complicated than the formula above.  Likewise, photosynthesis, which is covered in the next chapter, can be described as the opposite reaction.  Cellular respiration normally is aerobic (uses oxygen), but it is only used in the very last step in a very long process.  The process of respiration is divided into at least four distinct processes: 1) Glycolysis, which occurs in the cytoplasm of cells, and is anaerobic (does not use oxygen). In this process, glucose is broken down into two molecules of pyruvate in 10 steps, and a net gain of 2 ATPs occurs;  2) Entrance of pyruvate into the mitochondria and production of two molecules of Acetyl-Coenzyme A, a reaction in which 2 molecules of CO₂ are produced;  3) The Krebs Citric Acid Cycle (KCAC), 9 steps within the interior of the mitochondria, in which another 2 ATPs  and 4 more CO₂ are produced; and 4) the Electron Transport System (Cytochrome System), which involves enzymes located within the inner mitochondrial membrane, and in which up to 32 additional ATPs are produced, for a total of 36 ATPs.  

It is not necessary to memorize these steps, but an understanding of each process is necessary in order to understand normal physiology and abnormal conditions that may interfere with respiration and possibly lead to death.

Note the role of NAD as a hydrogen acceptor (carrier), and note how many molecules of NAD and FAD are used.  What eventually happens to the hydrogen that is passed to NAD and FAD?

The last half of glycolysis, where a net gain of two ATPs occurs.

When oxygen levels are not sufficient, pyruvate is converted into lactate in our muscles by the use of the enzyme LDH. Yeasts have different enzymes, and convert pyruvate into ethyl alcohol, a process utilized in brewing.  The last two figures in this group illustrates fermentation in more detail.  Note that no more energy (ATP) is obtained by these processes.

Glucose, proteins, and lipids may be processed into Acetyl Co-A.

Various amino acids may be converted into the molecules found within the KCAC, and may be utilized as fuel.

The beginning of the Krebs cycle, in which the first two CO₂s are produced.

Remember that for each molecule of glucose, two Acetyl Co-A molecules are produced; therefore the KCAC occurs twice for each glucose, so all products here are X 2..

The ETS uses Cytochoromes, iron containing pigments, each of which has a slightly higher electronegativity than the previous one used; this allows hydrogen electrons and ions to gradually be passed to the final step, which is the combination of hydrogen and oxygen, to produce water. Note that 6 molecules of water enter into the reactions at various steps, and that 12 molecules are produced in the end.  The original simplified formula is sometimes shown with 6 additional water molecules on each side of the equation.

The ETS occurs within the inner membrane of the mitochondria.

Chemiosmosis is the diffusion of H⁺ through the proteins within the inner membrane, which results in the production of ATP.  This process also occurs in photosynthesis, within the thylakoids.

Overview of ATP synthesis within the mitochondria.

Total theoretical yield of ATP in eukaryotes is 36.

Both deamination and beta-oxidation occur within the liver, thus allowing use of proteins and lipids as fuels.  

In the absence of oxygen in yeasts, two molecules of ethyl alcohol plus one molecule of CO₂ are produced from each molecule of glucose.

In most organisms, an absence of oxygen causes lactic acid to be produced following glycolysis.

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