Expert: John Locke - 2/22/2008

Question
Hello.
I'm looking for a detailed answer for : What happens to glucose when it's brokendown, after the digestive system?
Any points would help me piece together the puzzle.

Thank you!

Answer
Thanks for using AllExperts, Dara. The breakdown of glucose is fundamental to the survival of life; all organisms that use glucose as a source of energy (which is virtually all organisms) have evolved complicated enzyme pathways that use the chemical energy of glucose bonds to manufacture other essential compounds. This can occur by a variety of pathways, each of which involves multiple steps. Broadly, let me describe the process to you.

In the first stage, known as glycolosis, the glucose molecule is broken down into two three-carbon molecules known as pyruvates. This stage also produces high-energy electrons held by two molecules of the compound NADH, and two energy-carrying molecules of ATP. As you can see from the links below, this process itself includes multiple steps; the ultimate product is pyruvate, NADH, and ATP. NADH and ATP are both in an oxidized state at the start of this process; consequently, they're written as NAD+ and ADP, respectively. Glycolosis leads to the reduction of both.

If you were to write an equation for this step, it would look like this:

1 glucose+ 2 NAD+ + 2 ADP -->2 pyruvate + 2 NADH + 2 ATP

The pyruvate is sent on to a process known as the Krebs cycle, in which the energy of the pyruvate is used to produce more high-energy electrons, which are carried by the two compounds NADH and FADH2. All the carbons originally present in the glucose molecule are discarded as carbon dioxide; it's only the NADH and FADH2 that proceed to the final step, the electron transport chain. There, high-energy electrons are used to produce more of the energy-carrying molecule ATP; this step requires oxygen to complete. Organisms use ATP directly to fuel their cellular processes; glucose is one source for ATP, and the primary source for most of the ATP produced by humans.

An overall reaction for aerobic respiration looks like this:

1 glucose + 6 O2 + 38 ADP --> 6 CO2 + 6 H2O + 38 ATP

As you'll note, it is aerobic respiration that forms the carbon dioxide we exhale as a waste gas.

To better clarify this, I'm including links below that describe the process with pictures. Feel free to ask me any other questions if they arise. Good luck.

This link describes the entire process of the breakdown of food through cellular respiration.
http://library.thinkquest.org/C004535/cellular_respiration.html

http://www.cbu.edu/~seisen/AerobicRespiration.htm