Muscles

• Lactic acid metabolism 

  1. During exercise (muscles) do not generate enough oxygen for aerobic respiration
  2. Pyruvate is converted to lactic acid 
  3. This involves the transfer of hydrogen from NADH (produced during glycolysis) 
  4. This regenerates the NAD needed to maintain ATP production by glycolysis 
  5. Lactic acid builds up (in muscles) causing fatigue/ an oxygen debt 
  6. (When exercise stops) lactic acid is converted back into pyruvate/glucose (in the liver) 
• Slow twitch muscle fibres 

  1. Contract slowly but maintain contractions for a long time 
  2. They rely on aerobic respiration (to generate ATP) 
  3. They have many mitochondria / a large blood supply / a high concentration of myoglobin 
  4. Their (main) storage fuel/ energy source is fats. 
• Fast twitch muscle fibres 

  1. Contract quickly but cannot maintain contractions for a long time 
  2. They generate ATP through glycolysis 
  3. They have few mitochondria and a low blood supply 
  4. Their (main) storage fuels / energy sources are glycogen and creatine phosphate.