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Fuel for the muscle motors
Muscular contraction is one of the most wonderful things in the operation of our
body machinery. And though much about it remains a mystery, biochemists made many strides
towards the understanding of this operation. In whatever form sugars are eaten, whether
as starch, syrups, candy, or common table sugar, they are broken down by the juices of the
alimentary canal (the digestive tract) to simple sugars. All these sugars are then
changed to the simplest sugar, glucose, which is the only sugar found circulating
in the blood.
The amount of glucose in the blood stays remarkably the same. It increases in the
disease known as diabetes, otherwise the sugar content of the blood is about the same in
all healthy persons.
The body's heat comes mostly from the burning of glucose. Cold-blooded animals such as
the frog have less sugar in their blood than we do. Birds, which are warmer than humans,
have more.
If we could not store so vital a substance as glucose in our bodies, we would need to
eat constantly to maintain a steady supply of fuel. That would be a dangerous existence.
If we were to fall asleep, we would never wake. Without food, our body's machinery would
grind to a halt. On the other hand, large amounts of free glucose cannot be kept in the
body either. It is too readily used up. Our bodies can, however, store the glucose in a
more stable, less readily used form - glycogen. Glycogen is made up of many
glucose molecules that are hooked together.
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Every living thing contains
the chemical, adenosine triphosphate (ATP). ATP is used to provide energy for
heat, nerve electricity, light (as in fireflies), and muscle movement. An ATP molecule is
held together by strong electrical forces which are set free when the molecule is broken
apart in a chemical reaction. Somehow these forces are converted into the kind of
mechanical energy that will move our muscles. The part broken from the ATP molecule is a
smaller molecule called phosphate. After the phosphate is broken off, living
organisms can put it back and use the ATP molecule again. To do this, a rebuilding
chemical called phosphagen, is used for a while until the powerful glycogen
cycle takes over the production of fresh ATP energy. The glycogen cycle is a series of
chemical steps in which sugar and other substances are used to make fresh ATP in large
quantities. The whole process is so intricate that dozens of chemical reactions take
place, literally, in the wink of an eye!
