The anaerobic energy system provides energy in the absense of Oxygen. This is used in the first few minutes of all exercise, before there is suffiecient oxygen available at the muscles for aerobic metabolism. It is also used for fast, powerful bursts of energy, for which the aerobic system is insufficient. There are two systems within Anaerobic metabolism, which are the ATP-PC system and the lactic acid system.
ATP as already discussed is a high energy molecule which is broken down to form ADP and release energy. PC or Phosphocreatine is another high energy molecule, found in the Sarcoplasm of muscle fibres. The breakdown of ATP and so increase in volume of ADP triggers an enzyme known as Creatine Kinase to initiate the breakdown of PC into Phosphate and Creatine. Being an exothermic reaction, this provides the energy required to resynthesise ATP at a fast rate.
We only have 120g of Creatine within our bodies and so this repeated breaking down of PC in order to produce energy to resynthesise ATP is temporary and can only last a maximum of 10 seconds. Therefore the ATP-PC system is used mainly for bursts of speed.
Lactic Acid System
Sometimes also known as Anaerobic Glycolysis due to the initial process being the same as aerobic glycolysis (as mentioned above), only without oxygen. So, as before 10 chemical reactions occur within the Sarcoplasm which turn Carbohydrate into Pyruvic acid and 2 molecules of ATP. The difference now being the lack of oxygen meaning the carrier molecule NAD+ cannot offload the Hydrogen (H+) by-product of glycolysis causing a build up in the cell.
To try to prevent an increase in acidity the pyruvic acid accepts the H+, forming Lactic acid. If oxygen was present the H+ would be transported to the Mitochondria for use in the Kreb's cycle. Lactic acid is thought to interefere with muscle contraction due to disrupting the binding of Calcium to Troponin. Acidity also stimulates free nerve endings within the muscle, causing pain. Due to lactic acid production, this energy system can only be predominant for up to 2 minutes.
Following anaerobic exercise, despite the metabolic process used not requiring oxygen, your body will be in Oxygen Debt and so your respiration rate will be very high.
Anaerobic Respiration Equation:
Glucose = Ethanol + Carbon Dioxide + Energy
C6H12O6 = 2C2H5OH + 2CO2 + Energy