Theoretical yields

The yields in the table below are for one glucose and molecule being fully oxidised to carbon dioxide. It is assumed that all the reduced coenzymes are oxidised by the electron transport chain and used for oxidative phosphorylation.

Step 1: Glycolysis preparatory phase | coenzyme yield: None | ATP yield: -2 | Source of ATP: Phosphorylation of glucose and fructose 6-phosphate uses two ATP from the cytoplasm.

Step 2.1: Glycolysis pay-off phase | coenzyme yield: None | ATP yield: 4 | Source of ATP: Substrate-level phosphorylation

Step 2.2: Glycolysis pay-off phase | coenzyme yield: 2 NADH | ATP yield: 4 | Source of ATP: Oxidative phosphorylation. Only 2 ATP per NADH since the coenzyme must feed into the electron transport chain from the cytoplasm rather than the mitochondrial matrix.

Step 3: Oxidative carboxylation | coenzyme yield: 2 NADH | ATP yield: 6 | Source of ATP: Oxidative phosphorylation.

Step 4.1: Krebs cycle | coenzyme yield: None | ATP yield: 2 | Source of ATP: Substrate-level phosphorylation.

Step 4.2: Krebs cycle | coenzyme yield: 6 NADH | ATP yield: 18 | Source of ATP: Oxidative phosphorylation.

Step 4.3: Krebs cycle | coenzyme yield: 2 FADH2 | ATP yield: 4 | Source of ATP: Oxidative phosphorylation.

Total yield: 36 Source of ATP: From the complete oxidation of one glucose molecule to carbon dioxide and oxidation of all the reduced coenzymes.

Although there's a theoretical yield 36 ATP molecules per glucose during cellular respiration, such conditions are generally not realized due to losses such as the cost of moving pyruvate (from glycolysis), phosphate and ADP (substrates for ATP syhthesis) into the mitochondria. All are actively transported using carriers that utilise the stored energy in the proton electrochemical gradient.

- The pyruvate carrier is a symporter and the driving force for moving pyruvate into the mitochondria is the movement of protons from the intermembrane space to the matrix.
- The phosphate carrier is an antiporter and the driving force for moving phosphate ions into the mitochondria is the movement of hydroxyls ions from the matrix to the intermembrane space.
- The adenine nucleotide carrier is an antiporter and exchanges ADP and ATP across the inner membrane. The driving force is due to the ATP (-4) having a more negative charge than the ADP (-3) and thus it dissipates some of the electrical component of the proton electrochemical gradient.

The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H+ are needed to make 1 ATP. Obviously this reduces the theoretical efficiency of the whole process. Other factors may also dissipate the proton gradient creating an apparently leaky mitochondria. An uncoupling protein known as thermogenin is expressed in some cell types and is a channel that can transport protons. When this protein is active in the inner membrane it short circuits the coupling between the electron transport chain and ATP synthesis. The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in a babies brown fat, for thermogenesis, and hibernating animals.

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