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Assignment 3.1
The overall (summary) reaction for Respiration is below. Put into words,
it says that Sugar and Oxygen are used to make ATP with Carbon Dioxide
and water as by-products. It also shows us that the Carbons of the sugar
are oxidized (H's and electrons removed) to form the waste product CO2.
To better understand the overall reaction, we will divide Respiration into three components, and look at each in turn.
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Glycolysis - involves the initial oxidation (and partial breakdown) of Glucose Krebs cycle - is the further removal of the electrons from the remnants of Glucose (Pyruvate) electron transport chain - is where the energy of the electrons from the above oxidation reactions is used to make ATP. |
Glycolysis is the oxidative
catabolism of glucose (or other sugars) to yield two molecules of pyruvate
(a 3-carbon molecule) plus two ATP and two NADH. These reactions occur
in the cellular cytoplasm. The term Glycolysis literally means "Sugar-splitting".
| Glycolysis (a very general list of the reactions) |
| 1. Phosphorylation of Glucose |
| 2. Isomerization to Fructose-P |
| 3. Phosphorylation of Fructose-P |
| 4. Splits into 2, 3-Carbon sugars |
| 5. Isomerization |
| 6. Oxidation (forms NADH) and substrate-level phosphorylation |
| 7. De-phosphorylation, make ATP |
| 8. Isomerization |
| 9. De-phosphorylation, make ATP |
| 10. Pyruvate is formed |
The Bookkeeping of Glycolysis: As glucose is broken down, some energy is captured by substrate-level phosphorylation, some energy is captured by the reduction of NAD+, but most of the energy is still in the end product, Pyruvate. Since the purpose of respiration is to liberate useful Energy (aka make ATP), we should take a quick look at the ATP produced by Glycolysis.
- In steps 1 and 3, we invested ATP. We used ATP to phosphorylate our Glucose/Fructose molecules. This was necessary to get the remaining reactions to occur. We used 2 ATPs per 6-Carbon sugar, so we are "down" 2 ATPs so far. (- 2 ATP/6 carbon sugar).
- In step 4, each 6-Carbon sugar split into 2, 3-Carbon sugars. Keeping this in mind is paramount for for our bookkeeping.
- In step 7, each 3-Carbon sugar is used to Phosphorylate an ADP, forming ATP. Since we have 2 3-Carbon sugars from each 6-Carbon sugar, lets call this a gain of 2 ATP (+ 2 ATP/6 carbon sugar).
- In step 9, Just like step 7. Each 3-Carbon sugar is used to Phosphorylate an ADP, forming ATP. (+ 2 ATP/6 carbon sugar).
- In total: - 2ATP + 2ATP +2ATP = +2ATP for each 6 Carbon sugar that went thru Glycolysis.
We should also remember that NADH was formed during Glycolysis (2 NADH/6 carbon sugar). The electrons received by NAD+ are high-energy electrons. As a matter of fact, there is more Energy in the electrons of the H of NADH than there is in the P of ATP (about 3 times more). However, NADH is rarely used as a direct Energy source by cells. ATP is the molecule they need do most stuff.
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