"Life Cycle" of Glucose
In your workbook, there is a diagram called the "life cycle" of glucose
(Example 28) that attempts to summarize the steps in the synthesis, conversion,
polymerization, hydrolysis and oxidation of glucose. Let's take some time to work through
|We could start anywhere on this diagram, but I like to start with water
and carbon dioxide going through the process of photosynthesis to make
glucose and oxygen.
|Once the plants have done that and created glucose by that means, glucose
can undergo quite a number of different kinds of conversions. Plant cells can convert
glucose into cellulose, which can be used for structural materials. A
plant can also convert glucose into starch by polymerization, cellulose
is also polymerization, but convert glucose into starch to store it for future energy
needs. Also, glucose can be converted into a wide variety of different kinds of
chemical compounds that the plants need.
|From our focus as animals rather than plants, let's take a
look at the digestion that can take place. The starch created by plants
can be digested by animals. In that process, starch is hydrolyzed to form glucose. Cellulose
can be digested by some animals and converted into glucose. For humans, cellulose that's
eaten pretty much stays cellulose and still serves the valuable function known as
"fiber" in our diet.
Once in an animal, the glucose can
be oxidized right away
or converted to other useful biochemicals. Excess glucose that's not immediately needed
for energy is converted into glycogen for later use. Glucose in excess of
the amount of glycogen that the animal can make and store can, through a variety of
methods, be converted into fat.
|In its role as an energy source, glucose can go through a process in which
it's changed into pyruvic acid, indicated here in the ionic form, pyruvate.
The pyruvate can react with coenzyme A (CoA) and give off carbon dioxide and become acetyl
|Acetyl CoA then enters the citric acid cycle, which is
shown here as the TCA cycle, and is also known as the Krebs cycle.
As the remnants of glucose loop through the citric acid cycle, they give off hydrogen
atoms and also carbon dioxide.
|The hydrogen atoms then pass through the electron transport system
to ultimately combine with the oxygen to form water. In that process, energy is released
that is used for generating ATP for energy use in the cells of the body.
|That brings the cycle to the point where water and carbon
dioxide are now available to be used by plants to photosynthesize glucose anew.
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