Carbohydrate Cycle

"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 that diagram.

Diagram showing life cycle of glucose (Example 28). (Mvc-006f.jpg]

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.

Photosynthesis portion of the life cycle of glucose diagram. [Mvc-001f.jpg]

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.

Glucose conversions portion of the life cycle of glucose diagram. [Mvc-002f.jpg]

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 CoA.

Aerobic conversion of pyruvate to acety CoA portion of the life cycle of glucose diagram. [Mvc-003f.jpg]

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.

Citric acid cycle portion of the life cycle of glucose diagram. [Mvc-004f.jpg]

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.

Electron transport chain portion of the life cycle of glucose diagram. [Mvc-005f.jpg]

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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