Krebs Cycle (Citric Acid Cycle)

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Model and understand aerobic respiration demonstrating the flow of matter and energy out of a cell and explain energy transfer systems. Also, compare aerobic respiration to alternative processes of glucose metabolism.

Article includes ideas, images, and content from Troy Smigielski (2021-10)

Krebs Cycle (Citric Acid Cycle)

(This sounds like something off of Sponge Bob!)

Source, 2021-11

So, this process has three names, actually!

Krebs Cycle = Citric Acid Cycle = tricarboxylic acid cycle

(Must be important!)

Also, I head this guy on YouTube call it the Hans cycle, because that was Krebs' first name… Hans Krebs. So, yeah… That…

The Krebs cycle is a part of cellular respiration by which adenosine triphosphate (ATP) is produced. It's… complicated! 

It "is a central driver of cellular respiration. It takes acetyl CoA—produced by the oxidation of pyruvate and originally derived from glucose—as its starting material and, in a series of redox reactions, harvests much of its bond energy in the form of NADH, FADH2, and ATP molecules. The reduced electron carriers—NADH and FADH2—generated in the TCA cycle will pass their electrons into the electron transport chain and, through oxidative phosphorylation, will generate most of the ATP produced in cellular respiration (Source 2021-11).

Source, 2021-11

The whole goal of cellular respiration is to produce ATP that can be used to power cellular functioning. 

The Krebs cycle is one of the three steps, the middle step, in cellular respiration. 

Although it is complicated at a bio-chemical process, let's see if we can break it down into less complex steps!

If, in cellular respiration, we take a step backward from the Krebs cycle, we are in glycolysis.

Glycolysis is an anaerobic reaction taking place in the cytosol. If there is no oxygen present, then fermentation occurs. But!!! If oxygen is present, then it passes two 3-carbon pyruvate (pyruvic acid) on to the Krebs cycle.

aerobic - with oxygen anerobic - without oxygen

Recall that glycolysis begins with glucose. And… recall that glucose (along with oxygen) is produced by photosynthesis.

As discussed above, glucose is a six carbon molecule that is broken down in glycolysis. The process produces a net gain of 2 ATP, then passes 2 pyruvate to the Krebs cycle. 

Whereas glycolysis is anaerobic (and… you know… takes place in the cytosol), the Krebs cycle takes place in the mitochondria and is aerobic.

In between glycolysis and the Krebs cycle, the pyruvate (the pyruvic acid molecules) breaks down to produce one NADH and one CO2, leaving behind a 2-carbon molecule.

This 2-carbon molecule is called Acetyl CoA, and this is what is taken into the Krebs cycle. 

At this point, we are ready to start looking at the Krebs cycle.

Source 2021-11

The Krebs cycle occurs in the mitochondrion. Specifically, the Krebs Cycle takes place in the matrix inside the mitochondrion.

The matrix is the space inside the organelle… kinda like a curvy track.

Recall that between glycolysis and the Krebs cycle, the pyruvate molecules are turned into acetyl CoA, a 2-carbon molecule.

Before the Citric Acid Cycle begins, one pyruvate (3-C) from glycolysis is converted into Acetyl CoA (2-C) so the cycle can begin. This creates one CO2 that will go into the atmosphere. This also creates one NADH.

So, just getting ready to start the Krebs cycle, on of the electron carriers, NADH, and on CO2 have been created.

Source, 2021-11

The most important function of the Krebs cycle is to energize the electron carriers and send them to the electron transport chain. The electron transport chain will use the electron carriers to produce… a lot of ATP!

There are two electron transport molecules in cellular respiration. They are usually called electron carriers. Why? Because the… carry electrons! 

One of the two electron carriers in cellular respiration might remind you of the electron carrier in photosynthesis. It is NADH. (In photosynthesis, the electron carrier is NADPH.) The other electron carrier in cellular respiration is called FADH2. So, for cellular respiration, the electron carriers are NADH and FADH2.

The Krebs cycle is the source for most of the NADH and FADH2 that will be used in the electron transport chain.

Next, entering the Krebs cycle, Acetyl CoA (2-C) combines with oxaloacetate (4-C) to form citric acid (6-C).

After rearrangement, citric acid releases two of its Carbons as CO2 into the atmosphere.

As each CO2 leaves, one NADH is also produced. So, if 2 CO2 are released, then 2 NADH are also produced.

These CO2  molecules are exhaled and released into the atmosphere. They are byproducts not needed by the cell.

The remaining four carbons go through reactions to regenerate oxaloacetate so that the cycle can repeat. In the regeneration process, one ATP, one NADH, and one FADH2 are made.

One round of the Krebs cycle produces:

• 2 CO2
• 3 NADH
• 1 FADH2
• 1 ATP

These numbers are for one round. One round started with only one pyruvate. Remember, one glucose at the beginning makes 2 pyruvate. Therefore, we need to multiply these numbers by 2 to see the total number of each product that are made by one glucose molecule.

One molecule of glucose produces:

• 4 CO2
• 6 NADH
• 2 FADH2
• 2 ATP

The energy in these electrons that are carried by NADH and FADH2 will go the the ETC (Step 3 of the cellular respiration process) to help produce many more molecules of ATP.