Energy Systems - Aerobic System

Aerobic System

• Glycolysis
• The Krebs Cycle
• Oxidative Phosphorylation

Glycolysis - The first stage is known as glycolysis, which produces 2 ATP molecules, 2 reduced molecules of NAD (NADH), and 2 pyruvate molecules which move on to the next stage - the Krebs cycle. Glycolysis takes place in the cytoplasm of normal body cells, or the sarcoplasm of muscle cells.

The Krebs Cycle - This is the second stage, and the products of this stage of the aerobic system are a net production of 1 ATP, 1 carbon dioxide Molecule, three reduced NAD molecules, 1 reduced FAD molecule (The molecules of NAD and FAD mentioned here are electron carriers, and if they are said to be reduced, this means that they have had a H+ ion added to them). The things produced here are for each turn of the Krebs Cycle. The Krebs cycle turns twice for each molecule of glucose that passes through the aerobic system - as 2 pyruvate molecules enter the Krebs Cycle. In order for the Pyruvate molecules to enter the Krebs cycle they must be converted to Acetyl Coenzyme A. During this link reaction, for each molecule of pyruvate that gets converted to Acetyl Coenzyme A, an NAD is also reduced. This stage of the aerobic system takes place in the matrix of the cells' mitochondria.

Oxidative Phosphorylation - This is the last stage of the aerobic system and produces the largest yield of ATP out of all the stages - a total of 34 ATP molecules. It is called 'Oxidative Phosphorylation' because oxygen is the final acceptor of the electrons and hydrogen ions that leave this stage of aerobic respiration (hence oxidative) and ADP gets phosphorylated (an extra phosphate gets added) to form ATP (hence phosphorylation).

This stage of the aerobic system occurs on the cristae (infoldings on the membrane of the mitochondria). The NADH+ from glycolysis and the Krebs cycle, and the FADH+ from the Krebs cycle pass down electron carriers which are at decreasing energy levels, in which energy is released to reform ATP. Each NADH+ that passes down this electron transport chain provides enough energy for 3 molecules of ATP, and each molecule of FADH+ provides enough energy for 2 molecules of ATP. If you do your math this means that 10 total NADH+ molecules allow the rejuvenation of 30 ATP, and 2 FADH+ molecules allow for 4 ATP molecules to be rejuvenated (The total being 34 from oxidative phosphorylation, plus the 4 from the previous 2 stages meaning a total of 38 ATP being produced during the aerobic system). The NADH+ and FADH+ get oxidized to allow the NAD and FAD to return to be used in the aerobic system again, and electrons and hydrogen ions are accepted by oxygen to produce water, a harmless by-product.