SAS' Cell Biology Page

Aerobic Respiration (Glycolysis)

In the last steps of glycolysis, 2ATP and 2NADPH were produced, as summarized below.

But with a little oxygen, so much more energy can be made. That is what occurs in the pathways of aerobic respiration. Remember that in aerobic respiration, 36ATP are produced as shown below:

So aerobic respiration produces 34 more ATP molecules than glycolysis, anaerobic respiration, does. How does this happen?

The key to aerobic respiration lies within the mitochondria. The mitochondria is an organelle with a complex series of chemical reactions similar to the chloroplast The mitochondria, like the chloroplast, contains its own DNA, and it is has a double-membrane system. The outer membrane is similar to any other organelle, but the inner membrane is folded, creating a lot of surface area. The chemical reactions for aerobic respiration occurs on the surface of the inner mitochondrial membrane.

Pyruvate, the end-product of glycolysis, passes through the outer mitochondrial membrane, where CO2 is split off of the pyruvate, making a Acetyl group. At the same time, Vitamin C is turned into Coenzyme A. The Acetyl group combines with Coenzyme A, making Acetyl-CoA.

Acetyl-CoA enters the first major cycle for aerobic respiration, the Krebs Cycle. The Krebs cycle is similar to the Calvin cycle, in that organic molecules are being recycled, in a loop of chemical reactions. An organic molecule, in this case Acetyl-CoA, enters the cycle, charging NADH and ATP. In the case of the Krebs cycle, another battery molecule, FAD, is charged, making FADH2.

The Krebs cycle is a series of ten enzymatic reactions, where a 4-carbon organic molecule, oxaloacetate, is recycled back into the cycle. As each molecule of Acetyl-CoA runs through the Krebs cycle, it combines with the 4-carbon molecule to make Citric acid. Citric acid has six carbons.

In a series of five (5) enzymatic reactions, Citric acid has two more CO2 split off, and charges NAD+ into NADH.

We now have a 4-carbon molecule called succinyl-CoA. Succinyl-CoA now runs through two more reactions to produce ATP, FADH2, and an energy reduced 4-carbon compound. At the end of the Krebs cycle, one last bit of energy is squeezed out of the 4-carbon compound, producing NADH and oxaloacetate.

Finally, oxaloacetate is recycled back into the Krebs cycle, being energized by another Acetyl-CoA. The Krebs cycle is summarized below.

Ok, ok, told you that ATP drives the cellular processes, so what do cells do with the NADH and FADH2? Simple, they turn it into ATP in the second step of aerobic respiration, the electron transport chain.

Like a series of power converters, the electron transport chain takes the energy placed in NADH and FADH2, and uses it to charge ADP into ATP. This happens in a series of nine steps in the inner membrane of the mitochondria. The first charged molecule to enter the electron transport chain is NADH. NADH charges a flavoprotein at the beginning of the electron transport chain. This flavoprotein then sends its energy down a set of enzymatic steps, releasing a little energy during each step until cytochrome a3 takes the last bit of energy to produce O2. I bet you did not know that you made O2. Unfortunately this O2 is quickly hydrolyze, picking up H2 to form H2O.

Q, just like the Q in photosynthesis, is involved in the electron transport chain also. Q takes the energy from FADH2, reducing it to FAD. Q then adds FADH2's energy to the chain, sending it down to cytochrome a3, so that more O2, then H2O, is produced. And that is the electron transport chain.

But where are those 34ATPs that is supposed to be charged? Remember that the electron transport chain occurs in the inner membrane of the mitochondria. Well this membrane also houses a protein called ATP synthase, which does nothing but produces ATP. ATP synthase is an ion pump, such as the ones used in active transport. Except this ion pump runs in reverse, and it gets its energy from the electron transport chain.

ATP synthase is plugged into the electron transport chain, producing 32 ATP from the NADH and FADH2 that the electron transport chain got from the Krebs cycle and glycolysis. The Krebs cycle made 2ATP, NADH, and FADH2 from the pyruvate that came from glycolysis. Glycolysis made 2 ATP and NADH from glucose that came from that danish and candy bar. And that is Aerobic Respiration.