Krebs Cycle & Aerobic Exercise

Aerobic metabolism means ‘with oxygen’ and occurs when energy is produced in the body from chemical reactions that use oxygen. The aerobic system produces the largest amounts of energy, although at the lowest intensity, for example, in long-distance running.

This energy production can be sustained for long periods of time as long as breathing can supply the lungs with enough oxygen.

At the start of exercise, the body cannot deliver oxygen to the muscles fast enough to initiate the complex chemical reactions which occur during aerobic metabolism. Therefore the body relies on the anaerobic processes for the first couple of minutes.

The aerobic system

The aerobic system can be broken down into three sections:

  • Glycolysis
  • Kreb’s Cycle
  • Electron Transport Chain (ETC)

Aerobic Glycolysis

  • Glycolysis is the breakdown of Carbohydrates (in the form of Glucose or Glycogen) into Pyruvic acid and two ATP molecules.
  • A total of 10 chemical reactions are required to convert Carbohydrates into Pyruvic acid.
  • This takes place in the muscle Sarcoplasm, which is a gelatine type substance in the muscle fibres.
  • Gylcolysis can take place without the presence of oxygen in the cells, however, on finishing Glycosis the cell decides which process to carry out.
  • If Oxygen is present then the cell will perform Oxygen Respiration (aerobic respiration) and continue on to Kreb’s Cycle.

Kreb’s Cycle

Sometimes also known as the Citric acid cycle, or the Tricarboxylic acid cycle, this is the second phase in the process of aerobic metabolism.

  • The Pyruvic acid produced during Glycolysis enters the mitochondria and is immediately converted to Acetyl Coenzyme A.
  • This combines with Oxaloacetic acid to form a 6 carbon compound, known as Citric acid.
  • Further chemical reactions occur to wield enough energy to resynthesise 2 ATP molecules.
  • Bi-products of these reactions include Carbon Dioxide (CO2), which is exhaled by the lungs and Hydrogen (H) which is transported to the site of the Electron Transport Chain by carrier molecules NAD+ and FAD.

The process is termed a cycle due to the starting product of Oxaloacetic acid is also the end product, ready to start the process over again.

Electron Transport Chain

The hydrogen mentioned above is transported into the inner membranes of the Mitochondria where it is split into a proton (H+) and an electron (H-). The electrons are then subject to a series of redox reactions which release a large amount of energy in order to resynthesise ATP.

The protons also create energy by moving back through the inner membrane of the Mitochondria because of the redox reactions. This causes an imbalance of H+ and so they return through the membrane, producing energy.

A final exothermic reaction is the combination of hydrogen with oxygen, to form water. The total ATP production during all of the reactions of the electron transport chain is 34, meaning it is by far the highest producing phase of aerobic metabolism.

Aerobic Respiration Equation:

Glucose + Oxygen = Carbon Dioxide + Water + Energy

C6H12O6 + 6O2 = 6CO2 + 6H2O + Energy

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