TCA cycle and electron transport system (aerobic)

Aerobic respiration is the process by which organisms use oxygen to convert food into chemical energy (ATP). The two major stages of this process, following glycolysis, are the TCA Cycle and the Electron Transport System (ETS). These pathways are crucial for generating a large amount of ATP from the oxidation of glucose.

1.0The TCA Cycle (Krebs Cycle or Citric Acid Cycle) 

The TCA cycle is a central metabolic pathway that completes the oxidation of glucose derivatives. It occurs in the mitochondrial matrix of eukaryotes. The cycle begins with the end product of glycolysis, pyruvate, which is converted to acetyl-CoA by oxidative decarboxylation.

Steps of the TCA Cycle:

1. Condensation: Acetyl CoA (2-carbon molecule) combines with oxaloacetic acid (OAA) (4-carbon molecule) to form citric acid (6-carbon molecule). This is why it's also called the Citric Acid Cycle. The enzyme citrate synthase catalyses this step.
2. Isomerisation: Citric acid is rearranged to form isocitric acid.
3. Oxidative Decarboxylation: Isocitric acid is oxidised, and a molecule of CO_2​ is removed, producing α-ketoglutaric acid and a molecule of NADH.
4. Second Oxidative Decarboxylation: α-ketoglutaric acid is oxidised and decarboxylated to form succinyl CoA, producing another NADH and a molecule of CO₂​.
5. Substrate-level phosphorylation: Succinyl CoA is converted to succinic acid, generating one molecule of GTP(which is readily converted to ATP).
6. Oxidation: Succinic acid is oxidised to fumaric acid, producing a molecule of FADH2​.
7. Hydration: Water is added to fumaric acid to form malic acid.
8. Final Oxidation: Malic acid is oxidised back to oxaloacetic acid, generating the final molecule of NADH in the cycle. The OAA is now ready to accept another acetyl-CoA molecule and begin the cycle again.

Net Products from one Acetyl CoA molecule:

• 3 NADH molecules
• 1 FADH_2​ molecule
• 1 ATP (or GTP) molecule
• 2 CO₂​ molecules

2.0The Electron Transport System (ETS)=

The Electron Transport System (ETS) is a series of protein complexes and electron carriers located in the inner mitochondrial membrane. It is the final and most productive stage of aerobic respiration, where most of the ATP is generated. The ETS uses the high-energy electrons from NADH and FADH2​ produced during glycolysis and the TCA cycle.

Steps of the ETS:

1. Electron Transfer: NADH and FADH2​ donate their electrons to different complexes within the inner mitochondrial membrane.
2. Proton Pumping: As electrons move through the complexes, energy is released. This energy is used to pump protons (H⁺ ions) from the mitochondrial matrix into the intermembrane space.
3. Proton Gradient: This pumping action creates a high concentration of protons in the intermembrane space, forming an electrochemical gradient. This is also known as the proton-motive force.
4. ATP Synthesis: The protons flow back into the matrix through a specialised enzyme called ATP synthase. This flow of protons drives the rotation of ATP synthase, which catalyses the synthesis of a large number of ATP molecules from ADP and inorganic phosphate. This is called oxidative phosphorylation.
5. Final Electron Acceptor: At the end of the chain, electrons are accepted by oxygen (O₂​), which also combines with protons to form water (H₂​O). This is why oxygen is essential for aerobic respiration.

3.0Components of the Electron Transport System

1. Complex I (NADH Dehydrogenase):

• Accepts electrons from NADH and transfers them to ubiquinone (Coenzyme Q).
• Pumps protons into the intermembrane space.

2. Complex II (Succinate Dehydrogenase):

• Receives electrons from FADH₂ (from succinate oxidation).
• Passes electrons to Coenzyme Q but does not pump protons.

3. Coenzyme Q (Ubiquinone):

• Mobile electron carrier that transfers electrons from Complexes I and II to Complex III.

4. Complex III (Cytochrome bc₁ complex):

• Transfers electrons from CoQ to cytochrome c and pumps protons.

5. Cytochrome c:

• A small, mobile protein that carries electrons between Complex III and IV.

6. Complex IV (Cytochrome c Oxidase):

• Transfers electrons to oxygen, the final electron acceptor, forming water (H₂O).
• Pumps additional protons into the intermembrane space.