Friday, January 18, 2008

Energy Production in a Cell

disclaimer: If you don't want to be confused don't read this post. It made me confused, and I wrote it!

Let's see if I can get this right.....
ATP (adenosine tri phosphate) is the "energy source" of the cell. Energy is released when the Hydrogen bonds between adenosine and one of it's phosphates is broken. To store energy in a form that a cell can use, ADP (adenosine di phosphate) must be converted to ATP by addition of a phosphate.
There are generally three steps to the production of ATP by a cell:

Gycolysis (literally "glucose -breaking")
Citric Acid Cycle or Kreb's Cycle
Electron Transport Chain

Let's start with Glycolysis.

Glycolysis is an anaerobic process occuring in the cytosol by which the six carbon molecule, glucose, is broken down into 2 three carbon molecules called pyruvic acid. When the carbon bond in 1 glucose molecule is broken to make the 2 pyruvic acids, enough energy is released to create 2 ATP molecules out of 2 ADP molecules and 2Phosphate molecules.
C-C-C-C-C-C-C --------> C-C-C + C-C-C + ATP + ATP
Now for the Citric Acid Cycle (this is the tough one):
This stage of energy production is aerobic because oxygen is required to move the pyruvic acids into the mitochondrion where the cycle takes place.
Let's consider one pyruvic acid at a time.

A pyruvic acid (3C) will be changed to acetic acid (2C) by removal of one carbon. The carbon that is removed will be exhaled as CO2 (carbon dioxide) [note: the oxygen atom originates from the glucose molecule] In addition, a hydrogen atom is also split off of the pyruvic acid as it becomes acetic acid. This hydrogen atom is picked up by the carrier molecules NADH and FADH.
C-C-C -------> C-C + CO2 [exhaled] + H (NAD/FAD) [carried away to be used later]
The two carbon, acetic acid is now picked up by coenzyme A and is dropped off at the Krebs cycle. Let's see if I can illustrate this

<ahem, Paint keeps blurring when I try to upload it. >

Inspite of my good intentions as an illustrator, I must refer you to this link. There you can take your pick of numerous Kreb's Cycle illustrations and learn alot more than you can here.'s+Cycle&hl=en&*:IE-SearchBox&rlz=1I7DMUS&um=1&ie=UTF-8&sa=X&oi=images&ct=title

Anyway, the two carbons of pyruvic acid added to the 4 carbons of oxaloacetic acid make six carbons and become citric acid. For some reason, I'm not quite sure what the step with the isocitric acid is; I'll have to ask my instructor. Isocitric acid loses a hydrogen ,which is carried off by a carrier molecule for later use, and a carbon in the form of carbon dioxide which is exhaled .[note: the oxygen in the CO2 originated from the glucose]

The resulting five carbon a-ketogluteric acid does the same thing, losing a hydrogen and carbon.

Now both of the carbons that originally were added to the cycle have been lost along with their oxygens. We are left with the four carbons that came from the oxaloacetic acid plus two hydrogens; this is succinyl CoA.

Succinyl CoA becomes succinic acid. As this change occurs, the energy that was released from the breaking off of the carbons changes 1 ADP and P into ATP.

Succinic acid sheds a hydrogen, which is again picked up by a carrier, to become fumaric acid.

Fumaric acid undergoes the same procedure to become malic acid.
I'm not quite sure how fumaric acid returns to oxaloacetic acid; again I'll have to ask my instructor and update this post.

Now the second pyruvic acid is changed into acetic acid and goes through the cycle.

So, the Kreb's Cycle now ended let's survey it's results:
We now have
-2ATP (one from each pyruvic acid)
-8 H (in the form of NADH or FADH, 4 from each pyruvic acid)
-4CO2 (exhaled as waste, 2 from each pyruvic acid)

So far including Glycolysis, from one molecule of glucose we have obtained
-8 H (in the form of NADH or FADH)

Now let's look at the Electron Transport Chain:

I wish I could illustrate this one, but after all the trouble with the last illustration, I'm not going to try.
Basically, as I understand it, the principle is this: those eight hydrogen atoms that were plucked off in the Kreb's Cycle have electrons in high energy orbits. The carrier molecules (NAD and FAD) drop off the 8 hydrogens at a chain of proteins on the cristae (folds of inner membrane) on the mitochondrion. Here, as the hydrogens are passed from protein to protein, their electrons drop from energy level to energy level emitting (what do you think?) ENERGY!

This huge amount of energy is enough to create 32 ATP molecules from 32 ADP plus 32P molecules.
Byproducts of the Electron Transport Chain are heat and H2O (water)

Let's conclude this mess. From one glucose molecule we get 2 ATP from Gycolysis, 2 ATP from the Kreb's Cycle, and 32 ATP fro the ETC.

In the end, our byproducts CO2 and H2O.

1 C6H12O6 ----------------------------> 32ATP

Not bad, huh?

Ok, so maybe my explanation is bad. I know that it's definitely flawed and didn't even begin to scratch the surface of energy production. At least I've now tried to write down the main points of my professor's lecture for somebody else. If anyone has any questions, go ask your own instructor. If anyone has any comments, don't tell me I did a good job, because I didn't. I anyone wants to point out mistakes, go right ahead!

No comments: