MCAT Biology

Cellular Respiration

MCAT Biology > Cellular Respiration


All energy is ultimately derived from the sun. The only reason that life exists on planet Earth is that our planet resides within a certain radius of the sun that allows life. The temperature on Earth does not get to hot or cold and is suitable for life along with our nitrogen and oxygen based atmosphere.

Organisms on Earth are divided into two different groups, autotrophs and heterotrophs.

Autotrophs are organisms capable of using light energy to create organic molecules through anabolic reactions that can store energy in bonds

Heterotrophs derive energy from catabolic digestion of organic molecules from plants


See Photosynthesis


See Carbohydrates

Carbohydrates can either be mono, di, or polysaccharides depending on how many carbons are in the molecule.

A basic monosaccharide being glucose (C6H12O6). Carbohydrates are sugar polymers that can break down and stored as glycogen in the liver. Glycogen can then be converted to PGAL to be used in the citric acid cycle if needed.

Want to see how this topic related to organic chemistry? Click here to learn more about the organic chemistry behind carbohydrates.


See Proteins

Proteins are polypeptides and composed of amino acids. The removal of aminomoiety from amino acids by transaminoases results in a conversion to alpha-keto acids, which can then be converted to acetyl-CoA for use in the Krebs cycle

First, proteins are broken down through the enzyme peptidase and the nitrogen in the amino acids is converted into the waste product urea. In certain animals the urea is called uric acid. Next, the carbon in the amino acid is converted to pyruvate or acetyl-CoA depending on the variety of amino acid. The products from this catabolism can then enter the Krebs cycle and eventually the electron transport chain.

Want to see how this topic related to organic chemistry? Click here to learn more about the organic chemistry behind amino acids and proteins.

Fatty Acids

See Fatty Acids

Want to see how this topic related to organic chemistry? Click here to learn more about the organic chemistry behind lipids.


Cellular Respiration - Adenosine Triphosphate

Adenosine tri-phosphate (ATP) derives its powerful energy strength from the negative charges of the oxygen molecules that want to repel each other. However they are held in place by such strong bonds that this doesn’t happen. The breaking of these strong covalent bonds is what releases the energy stored in ATP.

Cellular Respiration - ATP

ATP breaks down to adenosine di-phosphate (ADP) and inorganic phosphorus (Pi) and or AMP.

ATP ⇒ ADP + Pi = 7 kcal/mol

ATP ⇒ AMP + PPi = 7 kcal/mol

Cellular Respiration - ADPCellular Respiration - Phosphate

Processes that require energy will be coupled with ATP release, for example muscular contraction, and active transport.

Glucose catabolism (substrate level phosphorylation) is the reaction that is required to reverse this reaction

ADP + Pi + 7 kcal/mol → ATP

NAD+ and FAD

Cellular Respiration - NAD+


NAD+ & FAD are coenzymes whose function is to act as a high-energy electron shuttle between the cytoplasm and the mitochondria. Both of these enzymes accept electrons during glucose oxidation (H:-) through Redox reactions.

When NAD+ and FAD accept hydride ions during glycolysis and the krebs cycle (citric acid cycle), they are reduced to NADH/FADH2 that means hydrogen was added.

Cellular Respiration - FAD


The hydride is then carried to the electron transport chain where they release the ion in conjunction with the oxygen molecule to make large amounts of ATP. The liberation of the hydride ion causes the reverse oxidation and NADH and FADH2 are converted back to NAD+ and FAD

Anaerobic Respiration

See Anaerobic Respiration

Aerobic Respiration

See Aerobic Respiration

Cellular Respiration Summary

See Cellular Respiration Summary

Cellular Respiration

Cellular Respiration Links

MCAT Biology

Biology Topics