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Metabolism
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Catabolic Pathways
Adenosine triphosphate (ATP) is a molecule in your muscles, which stores energy and allows it to be release for muscle contraction when it is broken down by myosin ATPase into adenosine diphosphate (ADP). This process lasts for 3 seconds at most. If your body continues to require energy after this initial 3 second period, then Creatine Phosphate (CP) is released from the muscle in order for the phosphate molecule on CP to bind to ADP, which creates ATP. The enzyme that allows this reaction to take place is called creatine kinase. This process lasts about one minute until CP stores run out (Source: Hatfield, PhD, Frederick C. Fitness: The Complete Guide. 8.1.5th ed. Santa Barbara: International Sports and Sciences, 2004. 19-22).
Picture taken from: Berg, Tymoczko, & Stryer. Metabolism: Basic Concepts and Design. Biochemistry, Fifth Edition (2002). W.H. Freeman & Co.
Glycolytic Pathway (Glycolysis)-Anaerobic Conditions
Once your CP stores are used up, your body must result to the breakdown of carbohydrates to produce ATP. Glycolysis, the breakdown of glucose molecules, can occur once glycogen (stored glucose) is broken down by a process known as glycogenolysis. During glycolysis, a glucose molecule gets broken down into two pyruvates. In humans, pyruvate can get converted into acetyl CoA only if oxygen is available (see oxidative pathway), or pyruvate can go through fermentation if there is not enough oxygen due to strenuous exercise. Fermentation produces lactic acid and NAD+. NAD+ is necessary for glycolysis to continue. Lactic acid buildup causes the burning sensation in your muscles; however, lactic acid can be used as energy or it can be used to produce more glucose through gluconeogenesis, which is the process of synthesizing glucose when energy needs are low. Glycolysis lasts a little under a minute and a half, because the accumulation of lactic acid known as your anaerobic threshold becomes too much. Glycolysis (glucose to pyruvate) produces a net gain of 2 ATP, and 2NADH molecules per glucose.
Picture taken from: Berg, Tymoczko, & Stryer. Glycolysis and Gluconeogenesis. Biochemistry, Fifth Edition (2002). W.H. Freeman & Co.
Oxidative Pathway: The Citric Acid Cycle & Oxidative Phosphorylation-Aerobic Conditions
Your body now must use the product of glycolysis, pyruvate, to produce ATP from ADP. This process involves the TCA cycle (A.K.A The Krebs cycle or the TCA cycle) and the electron transport chain (ETC). Under aerobic conditions pyruvate gets converted into acetyl CoA, which enters the TCA cycle and releases 2NADH molecules per glucose if energy needs are high. If the energy needs are low then acetyl CoA will be converted into fat (lipids). The TCA cycle includes a series of reactions that primarily involve oxidation-reduction (redox) reactions. The main point here is that each TCA cycle (2 cycles per glucose molecule) releases 3 NADH molecules, 1 FADH2 molecule, and 1 GTP. In other words, 8 electrons are captured by NADH and FADH2 per TCA cycle. The end result so far is that glycolysis, the conversion of pyruvate to acetyl CoA, and the TCA cycle produced a net gain of 10 NADH and 2 FADH2 molecules, which enter the electron transport chain. It is important to realize that NADH and FADH2 are both considered to be "high energy" electron carries, meaning that they are more than willing to transfer their protons (H+) to an electron acceptor. In other words NADH and FADH2 are readily oxidized while oxygen is readily reduced. Oxygen is therefore the "final electron acceptor." The transfer of electrons across the mitochondrial membrane results in a proton-motive-force, which drives ATP synthesis.
So, what on earth is the purpose of catabolism?...We just made approximately 30 ATP per glucose, which is used as energy and we regenerated NAD+ and FAD+, which keeps glycolysis and the TCA cycle running!
Picture taken from: Berg, Tymoczko, & Stryer. The Citric Acid Cycle. Biochemistry, Fifth Edition (2002). W.H. Freeman & Co.
Picture taken from: Berg, Tymoczko, & Stryer. Oxidative Phoshorylation. Biochemistry, Fifth Edition (2002). W.H. Freeman & Co.
Summary and Application to Exercise
Understanding metabolism can lead to a great deal of success in the gym. You now know that during weight lifting you mainly use ATP, CP, and glycogen stores. When you perform aerobic activities such as running on the treadmill for 2 minutes or more you are using mostly fat molecules. As your aerobic intensity increases more carbohydrates are used for energy than fat. It is also important to note that no matter what type of exercise you are performing energy will always be synthesized through the ATP/CP energy pathway first. Lastly, you should realize that although it is necessary for glucose to be created via gluconeogenesis in the glycolytic energy pathway you are using proteins as well as lactic acid to produce glucose (Source: Hatfield, PhD, Frederick C. Fitness: The Complete Guide. 8.1.5th ed. Santa Barbara: International Sports and Sciences, 2004. 19-22).
Picture taken from: Berg, Tymoczko, & Stryer. Metabolism: Basic Concepts and Design. Biochemistry, Fifth Edition (2002). W.H. Freeman & Co.
Click this link to expand your knowledge on how and when the macronutrients are used.
Last Updated:
April 14, 2008
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