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Self-quiz

What are the three series of reactions involved in oxidative metabolism? Can you think of all three? Check here.

How is glucose converted to pyruvic acid?

This conversion, called glycolysis, involves a series of chemical reactions that convert glucose to pyruvic acid, In the process a small amount of energy is released and trapped for use by the cell. Glucose molecules are first broken in half, from six carbons to the three carbons that are present in pyruvic acid. No oxygen is used during glycolysis. Some bacteria and primitive organisms that depend highly on glycolysis can live in environments that do not have much oxygen. In higher animals, if oxygen is in short supply, the pyruvic acid may accumulate and convert into another three-carbon acid, called lactic acid. Lactic acid is what makes muscles sore when they are over-worked.

What happens to the pyruvic acid?

Pyruvic acid goes through a series of chemical  reactions, giving off carbon dioxide and water until pyruvic acid is regenerated. This occurs through a cyclic chain of reactions, called the Krebs' Cycle.  At several steps in the cycle, hydrogen and oxygen combine to form water, which explains what happens to the oxygen that is used in the cycle. In steps where a carbon atom is released, it reactions with oxygen to form carbon dioxide. At several points in the chain of reactions free hydrogen atoms are released and they are handed off to proteins that are anchored in the membranes inside the mitochondria.

Releasing food energy with the Krebs Cycle:

champgne.wmf (34806 bytes)The chemical  reactions that convert one acid to another compound take off carbon, hydrogen, and oxygen atoms to produce carbon dioxide, water and free hydrogen. It is the hydrogen atoms that are most interesting. Like the gas in a bottle of soda pop, once the lid is removed, the energy will all be lost unless there is some way to trap it. The electrons of hydrogen atoms have energy; they are flying around with a lot of energy until they get caught by some other molecule. Adenosine Triphosphate Image

So, how do mitochondria capture energy?

The proteins on the many membranes inside of mitochondria help to whip electrons from one to another. In the process the energy of these moving electrons can be captured. The energy is captured in a compound called adenosine diphosphate (ADP). The name indicates that the compound adenosine includes two molecules of phosphate. Phosphate is a molecule made up of one atom of phosphorus and four of oxygen. ADP can bind another phosphate molecule to become adenosine triphosphate (ATP) if their is enough free energy available to attach that third phosphate molecule. The phosphate bonds hold a lot of energy in a storage form, acting like a battery. The trick is to capture the energy released by food breakdown. In other words, the chemical reactions in mitochondria convert some of the energy of glucose into energy storage bonds in ATP.

Self-quiz

Can you think of reasons why so many chemical reactions are needed for glycolysis and Krebs cycle? Now (and only after you have done your best) check your answer here.

 

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