The citric acid cycle
- Glycolysis, where the simple sugar glucose is broken down, occurs in the cytosol.
- Pyruvate, the product from glycolysis, is transformed into acetyl CoA in the mitochondria for the next step.
- The citric acid cycle, where acetyl CoA is modified in the mitochondria to produce energy precursors in preparation for the next step.
- Oxidative phosphorylation, the process where electron transport from the energy precursors from the citric acid cycle (step 3) leads to the phosphorylation of ADP, producing ATP. This also occurs in the mitochondria.
How does it happen?
- NADH: An energy shuttle which delivers high energy electrons to the electron transport chain where they will eventually power the production of 2 to 3 ATP molecules. When this electron shuttle is not carrying high energy electrons, meaning it has been oxidized (lost its electrons), it is left with a positive charge and is called NAD.
- FADH: Another energy shuttle that carries high energy electrons to the electron transport chain, where they will ultimately drive production of 1 to 2 ATP molecules. The oxidized form of FADH is FAD and happens just like in NADH.
- ATP: The basic energy currency of the cell. It’s a form of energy that cells can use right away.
- GTP: Similar to ATP, GTP can be easily converted to ATP in the cell.
A 6-carbon glucose molecule is split into two 3-carbon molecules called pyruvates. Pyruvate is needed in order to create acetyl CoA.
This is a very short step in between glycolysis and the citric acid cycle. The 3-carbon pyruvate molecule made in glycolysis loses a carbon to produce a new, 2-carbon molecule called acetyl CoA. The carbon that is removed takes two oxygens from pyruvate with it, and exits the body as carbon dioxide (CO). CO is the waste product that you release when you exhale.
The citric acid cycle is called a cycle because the starting molecule, oxaloacetate (which has 4 carbons), is regenerated at the end of the cycle. Throughout the citric acid cycle, oxaloacetate is progressively transformed into several different molecules (as carbon atoms are added to and removed from it), but at the end of the cycle it always turns back into oxaloacetate to be used again. Energy can be captured from this cycle because several of the steps are energetically favourable. When a step is favoured, it means that the products of the reaction have lower energy than the reactants. The difference in energy between the products and the reactants is the energy that is released when the reaction takes place (see enzyme kinetics). The released energy is captured as the electron shuttles (NAD and FAD) are reduced to NADH and FADH .
- An enzyme rearranges the atoms in the citric acid molecule (6 carbons) into a new 6-carbon arrangement.
- Energy is released when the 6-carbon arrangement is oxidized, causing one carbon to be removed. The removed carbon molecule combines with oxygen to produce CO. Some of the energy, in the form of electrons, is captured in formation of high-energy compound, NADH. (Recall that some of the energy released from the cycle is used to reduce NAD to create NADH.) The high energy electrons that are handed to NAD for reduction come from the oxidation (loss of electrons) from the carbon molecule here.
- Next, the same type of reaction happens again. Another carbon is cleaved off the 5-carbon molecule, leaving a 4-carbon molecule and CO, and some of the energy released is used to reduce NAD to NADH.
- Rearrangement occurs, allowing the 4-carbon molecule to find a more comfortable configuration (one that doesn’t use require a lot of energy or structural strain, and one that allows each bond to be satisfied). During this rearrangement, non-carbon groups are added to and removed from the molecule. GTP and FADH are made in these steps.
- The 4-carbon molecule rearranges its carbons one last time, producing oxaloacetate. Remember that oxaloacetate will be used again in the next cycle. Once again, some of the energy released is transferred to reduce NAD to NADH.
From one citric acid cycle, the following products are formed:
- 1 GTP
- 3 NADH
- 1 FADH
- 2 CO
- Regenerated oxaloacetate
During oxidative phosphorylation, NADH and FADH are transported to the electron transport chain, where their high energy electrons will ultimately drive synthesis of ATP.