Aerobic Respiration (Leaving Cert Biology): Revision Notes

Aerobic Respiration

• Aerobic respiration involves the complete breakdown of glucose.
• It produces more energy than anaerobic respiration.
• There are two stages of aerobic respiration:
  1. Glycolysis
  2. Krebs cycle and the electron transport chain

Mitochondrion:

Stage 1: Glycolysis

• Glycolysis takes place in the cytosol of the cell.
• The cytosol is the cytoplasm without any organelles.
• This stage is anaerobic, meaning it does not require oxygen to occur.
• As this stage is anaerobic, it releases very little energy - only 2 ATP molecules are produced for each glucose molecule broken down.
• Glucose is broken down into two 3-carbon molecules called pyruvic acid (or pyruvate).
• 2 ADP molecules are converted into 2 ATP molecules.
• When glucose is broken into 2 pyruvic acids, 4 high-energy electrons and 2 protons are released.
• These are captured by 2 NAD⁺ molecules, forming 2 NADH.
• Most of the energy originally stored in the bonds of glucose remains in the bonds of the two 3-carbon molecules (pyruvate).

Stage 2: Krebs Cycle and the Electron Transport Chain

• Stage 2 takes place in the mitochondria.

• This stage is aerobic, meaning it requires oxygen.

• A large amount of energy is released in this stage as many ATP molecules are produced.

• NADH molecules are also produced and sent to the electron transport chain. When oxygen is present, pyruvic acid enters the mitochondrion:

• Pyruvic acid is converted into a small 2-carbon molecule called acetyl coenzyme A (acetyl CoA).

• Acetyl CoA then enters a series of reactions known as the Krebs cycle.

Krebs Cycle

• In the Krebs cycle, acetyl CoA is broken down into carbon dioxide (CO₂) and water (H₂O).
• High-energy electrons are released during this process.
• These electrons are picked up by NAD⁺, forming NADH, which is sent to the electron transport chain.
• Additionally, ADP is converted into ATP during the cycle.
• As part of the process, two CO₂ molecules are produced.

Electron Transport System/Chain

• The NADH molecules produced during the Krebs cycle move to the inner membrane (cristae) of the mitochondrion.
• Here, NADH breaks down to release high-energy electrons and protons (H⁺).
• The electrons pass along a series of chemicals in the electron transport chain.
• As the electrons move through the chain, they release energy, which is used to convert ADP into ATP.
• At the end of the chain, the electrons combine with oxygen and hydrogen ions to form water (H₂O).