Kreb Cycle Help

[Gentoo]

I've been avoiding and faking it for too long... I'm taking microbiology this semester, and right now we're learning about glycolysis, the Kreb Cycle, and Electron Transport Chain. However, I've never learned the Kreb Cycle and it's confusing me now. Can someone explain it to me as if I were four years old?

 

[Sunstone]

I wish I could help. I'm fascinated by the Krebs Cycle and the Electron Transport Chain, but I've never attempted to explain them to anyone, and I doubt I recall the details well enough to explain them. However, I have seen videos of the Cycle and Chain on YouTube -- do you think if you could find those videos they might help?

 

[Gentoo]

I wish I could help. I'm fascinated by the Krebs Cycle and the Electron Transport Chain, but I've never attempted to explain them to anyone, and I doubt I recall the details well enough to explain them. However, I have seen videos of the Cycle and Chain on YouTube -- do you think if you could find those videos they might help?

I'm willing to try anything! (I have a test on Tuesday :cover

 

[Sunstone]

I'm willing to try anything! (I have a test on Tuesday :cover

Good! Do you want me to hunt them down for you, or would you prefer doing that yourself?

 

[Gentoo]

Good! Do you want me to hunt them down for you, or would you prefer doing that yourself?

I'll take a look, thanks!

 

[lunamoth]

Do you have a specific question Gentoo?

I will try to help.

 

[Gentoo]

Do you have a specific question Gentoo?

I will try to help.

Unfortunately, I'm like a deer in headlights with this subject... I know I'm not much help :sorry1:

 

[lunamoth]

Unfortunately, I'm like a deer in headlights with this subject... I know I'm not much help :sorry1:

Do you have your textbook with you?

 

[lunamoth]

You can probably find a diagram that looks like this at the beginning of the section on cellular respiration:

 

[lunamoth]

It looks scary, but you can start by thinking of it as a machine that breaks down sugar and makes ATP, the 'energy molecule.'

The machine is described to have three phases of production, glycolysis (which takes place in the cytosol), the Krebs Cycle and Electron transport (which both take place in the mitochondria).

 

[lunamoth]

Most of the ATP is formed during the last phase, electron transport.

You get 2 ATP during glycolysis, 2 more during the Krebs Cycle, and a whopping 32 more from Electron Transport for each molecule of glucose 'processed.'

 

[lunamoth]

You've asked about the Krebs Cycle specifically, so you know that Glycolysis is a series of ten enzymatic steps that break glucose (six carbons) down to two pyruvic acid moleucles (3 carbons each) plus some energy carrying molecules called NADH, in addition to the 2 ATP energy molecules.

C6H12O6 (glucose) + 2 NAD + 2 ADP + 2P ---> 2 C3H4O3 (pyruvic acid) + 2 NADH + 2H + 2 ATP + 2 H2O

The pyruvic acid and NADH are used in the Krebs Cycle as substrates.

 

[lunamoth]

The link between Glycolysis and the Krebs Cycle is the formation of Acetyl CoA (2 carbons each) from the pyruvic acid (3 carbons each) molecules. This happens as the pyruvic acid molecules enter the mitochondria.

You see that this also a step where carbon dioxide (CO2) is formed. More CO2 will be formed during the Krebs Cycle.

Further, you get another molecule of the energy carrier NADH for each purvic acid molecule.

All those NADHs will be used in the last phase of respirtation, electron transport, to make a lot of ATP.

 

[Gentoo]

Okay, that gives me a start-off point

I don't understand what NAD+, NADH, FADH2 are... I mean, I know they're organic molecules but the light bulb just isn't going off yet..

 

[lunamoth]

Okay, that gives me a start-off point

I don't understand what NAD+, NADH, FADH2 are... I mean, I know they're organic molecules but the light bulb just isn't going off yet..

For now you can think of them as 'energy carriers.' What is really important about them are the Hydrogens they carry. Those will be used during the last phase, electron transport.

When they have the extra 'H', either as NADH or FADH2, they are useful for making ATP.

 

[lunamoth]

The Krebs Cycle is a series of enzymatic steps that are a powerhouse for turning out NADH and FADH2 energy carriers. This happens as the two-carbon (2-C) Aetyl CoA is broken down into two more carbon dioxide molecules (1-C each).

 

[lunamoth]

If you need to memorize all the enzymatic steps, I would first start by getting familiar with following just what is happening to the carbons and noting where the CO2 molecules are released. It's a cycle, rather than a linear chain of steps, so you start and end with the same substrate, oxaloacetic acid.

2 Acetyl CoAs (2 carbons each) -----> 4 CO2 (1 carbon each).



Then note at which steps the energy carriers NADH and FADH2 are released.

It really helps to draw the cycle out for yourself using balls for the carbons, until you get a feel for the overall process.

Finally, remember that you also get an ATP from each turn of the cycle, so that is two more since two Acetyl CoAs go in for each glucose you start with.

 

[lunamoth]

The last phase is the electron transport chain and oxidative phosphorylation. Don't be scared by the long names!

I found two diagrams for this. The first one here I like because it shows the main point of what is happening during the electron transport chain:

As electrons are passed down the chain by the enzymatic steps (the enzymes are proteins embedded in the inner mitochondrial membrane), protons (H+) are moved from one side of the membrane to the other. Yes, this is the most important thing!

It's like pumping water up into an upper reservoir so you can capture the energy from it as it flows downhill.

The way the energy is captured as the H+ 'flow downhill' is called oxidative phosphorylation. That's done by the ATP Synthase enzyme depicted at the end of the chain.

 

[lunamoth]

I think this figure is a bit better at showing how the NADH (and FADH2, not shown but enters the chain at a later 'lower energy' step than NADH) is used:

 

[Valjean]

Kreb's cycle, AKA citric acid cycle, is a really basic and well understood energy producing cascade. It's been in biology textbooks practically forever (I think the Neanderthals first described it)

I know your textbook diagrams the cascade. Every textbook does.
Does your professor want you to memorize the sequence? If so, just learn it. If s/he wants a discussion of karyotic vs eukaryotic respiration, the evolution of the cycle, or some such thing you can find all of this pretty much everywhere. Like I said. This is really fundamental and extensively covered stuff.