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Captain's Cards
Let me out! It's so hard to breathe in here while surrounded by the other carbons, hydrogens, and oxygens in glucose. We seem to be in a very tight hallway and every few seconds a huge crowd pushes us all into the wall beside us. Oh, no! Here they come... give me some oxygen... on second thought, skip the oxygen we don't need any CO2 in this artery. I think I see the cell up ahead, but it's so dark and claustrophobic in here, I can't be sure. My glucose friends tell me we're going to go into the cell as glucose and the lucky atoms will become part of pyruvic acid! I wonder what will happen to the others... well, there's no time to worry about that, the cell is approaching even faster. I'm starting to get a touch of the dreaded Brownian motion sickness, so I'm going to close my eyes while we enter the cell.... and we made it! I've been told that everything else that happens is so quick that I won't be able to even write about it while its happening. All I know is that once this nightmare is over, I should be a part of a pyruvic acid molecule. Let's see what happens!
Although Captain Carbon is scared of continuing his journey, glycolysis is a vital part of human life. If glycolysis and the Krebs Cycle were not utilized by the body, it would be impossible to contract muscles for exercise, transmit nerve impulses for thought, or even digest food after a meal. The important factor in all of these actions is ATP, which functions as a fuel for the body. This "gasoline" of the body is not the only product of respiration, energized electrons carried by NADHs are manufactured as well. However, since oxygen is not present to break apart glucose efficiently, very little fuel is produced in glycolysis. Instead, most of the energy remains in pyruvic acid until it can be cultivated more effectively by oxidation and the Krebs Cycle.
glucose -----> 2 pyruvic acid + energy
We begin glycolysis with the glucose molecule, characterized by its six-sided ring. A phosphate group from an ATP molecule attaches itself to the sixth position of the glucose molecule, forming glucose 6-phosphate. This addition destabilizes the glucose molecule which disables it from resisting the future changes.
Glucose 6-phosphate is reorganized to form fructose 6-phosphate, a five-sided ring.
Another phosphate group attaches itself to the molecule; however, this phosphate is in the first position forming fructose 1,6-diphosphate. The second investment of energy is for the same reason as the first, to destabilize the glucose molecule.
Fructose 1,6-diphosphate is split into dihydroxyacetone phosphate and glyceraldehyde phosphate.
Since the glyceraldehyde phosphate will be consumed in the following reactions, step 5 is a continuous reaction where dihydroxyacetone phosphate is converted to glyceraldehyde phosphate as needed by the cell.
Hydrogen atoms accompanied by their electrons are removed from the glyceraldehyde phosphate molecules - a process known as oxidation. The two hydrogen atoms and electrons reduce NAD+ to NADH which then carries the electrons in an electron transport chain. In addition, a phosphate group is attached to the 1 position of the molecule forming two molecules of 1,3-diphosphoglycerate.
The phosphate just added to the first position of the molecule is removed and added to an ADP molecule forming ATP. Remember that there are two molecules involved now which means two molecules of ATP are produced which "cancel out" the two inputed in steps one and three. The remaining molecules at the end of this step are 3-phosphoglyceric acid molecules.
The phosphate group that was previously in the third position is placed in the second position with the aid of an enzyme.
A molecule of water is removed from the structure leaving phosphoenolpyruvic acid. As can be seen in the picture, the removal of the water causes the energy of the molecule to now be situated in the center instead of the outskirts.
The remaining phosphate group is attached to an ADP molecule forming ATP as in step seven. Therefore, the net gain of glycolysis is two ATP molecules per molecule of glucose. Two molecules of pyruvic acid are the remaining products of the complex process of glycolysis.
Therefore, the overall chemical equation of glycolysis is:
glucose + 2ATP + 4ADP + 2Pi +2NAD+ ---> 2pyruvic acid + 2ADP + 4ATP + 2NADH + 2H+ +2H2O
The net gain of this process is two ATP molecules and two NADH molecules. NADH acts as an electron carrier, keeping the electrons until they are needed later for ATP production. However, the net gain of energy by glycolysis is not too impressive when considering the numbers. The free energy of the total oxidation of glucose into CO2 and H2O is -686 Kilocalories per mole. ATP's bonds have an energy content of 7.3 Kilocalories per mole. So, for two ATP's:
7.3 x 2 = 14.6 and 14.6/686 = 2%
Glycolysis only harvests 2% of the energy present in glucose. Of course, this is why the cell eventually evolved (and Captain Carbon must continue) to aerobic respiration, which produces 34 more ATP's.