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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:
 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.
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. |
