As we follow the pathway through the Krebs cycle we should recall that there are two molecules of pyruvate generate for every molecule of glucose that enters glycolysis.

Pyruvate easily enters the matrix of the mitochondria where it enters a circular pathway (Krebs cycle) which occurs within the matrix of the mitochondria.

The first step is to convert pyruvate into a two-carbon fragment, then attach it to another coenzyme known as coenzyme A or CO-A.

The enzyme that accomplishes thus is a large enzyme known as the pryruvate dehydrogeneses complex. Inhibited by high ATP, Acetyl-CoA, and NADH. In the process NAD⁺ is converted to NADH + H⁺ and CO₂ (a carboxyl group) is lost. Therefore over all simplified Krebs cycle is as follows…

Imbedded in the inner mitochondria membrane are a series of electron carriers. These electron carriers pass electrons from NADH and FADH to one another down a red-ox stairway. The net result of this series of step-wise electron exchanges is to pump H⁺ (protons) out of the matrix into the outer compartment between the outer and inner membrane of the mitochondria.

This process establishes a steep proton gradient across the inner mitochondrial membrane. This creates a great deal of potential energy across the inner membrane.

This potential energy can be harnessed by an enzyme system to form ATP from ADP + Pi. This is known as chemiosmotic phosphorylation.

The Enzyme called ATP synthetase has two major components Fo and F1 factors.

The Fi component resembles a doorknob protruding into the matrix from the inner membrane. It is attached by a stalk to the Fo part, which is embedded in the inner membrane and extends across it.

The chemiosmotic principle postulates that the High H⁺ gradient creates the protomotive force needed to link the electron transport chain to the ATP synthetase molecule.

As 2H⁺ (two protons) pas back into the mitochondria matrix through the ATP synthetase molecule enough free energy is released to create one ATP molecule from ADP + Pi.

For each 2H⁺ 1 ATP is created. Therefore since each NADH causes 6 H⁺s to be pumped out 3 ATP can be made for each NADH entering the respiratory chain.

Also each FADH₂ can pump 4 H⁺s. Therefore each FADH₂ can cause 2ATP to be made.

What about the 2 NADH's produce in the Cytoplasm during glycolysis?

The two NADH's are know not to be able to enter the mitochondria in one of two ways.

These two shuttles are called the:

Glycerol phosphate shuttle NADH ----> FADH₂ ----> etc.

And the:

Malate shuttle NADH ----> NADH ----> etc.

Therefore if the glycerol phosphate shuttle is used then FADH₂ picks up the two electrons and the proton (plus one other [proton) and therefore only 4 more ATPs result.

But if the malate shuttle is used then NADH picks up the electrons and 6 ATPs result.

Overall glucose tally sheet

Glycolysis

Output Total

2 ATP in 4 ATP ------> 2 ATP

2 NADH ------> 6 ATP

or ------> 4 ATP

8 NADH

2 FADH₂

2 ATP ------> 2 ATP

3 X 8 NADH ------> 24 ATP

2 X 2 FADH₂ -----> 4 ATP

TOTAL

36 or 38 ATPs per Glucose

Glucose is not the only material that can be metabolized to generate energy. Many carbohydrates can be broken down in glycolysis and enter the Krebs Cycle. Proteins can be broken down into amino acids and those can be deaminated and the carbon chains feed into the Krebs Cycle. The very long carbon chains of fatty acids can be chopped into two carbon pieces by a process known as Beta Oxidation. Since the fatty acid chains can be up to 20 carbons long there is a very great deal of energy stored in fats.