Aerobic Respiration

Respiration

Glucose break down can be divided into four stages: 

• Glycolysis
• Link reaction
• Krebs cycle
• Oxidative phosphorylation

Glycolysis

It involves the phosphorylation of glucose and the subsequent splitting of fructose 1, 6-biphosphate (6C) into two triose phosphate molecules, which are further oxidised to pyruvate with a small yield of ATP and reduced NAD.

• It takes place in the cytoplasm of a cell
• Phosphorylation
  • this is the first stage
  • glucose is phosphorylated using ATP (because although glucose is energy rich it does not react easily, so to tap its bond energy, energy must first be used to make the reaction easier)
  • 2 ATP molecules are used for each molecule of glucose
  • this makes first glucose phosphate, then fructose phosphate and then fructose biphosphate
  • fructose biphosphate (6C)  breaks down to produce two molecules of triose phosphate (3C)
• Removal of hydrogen
  • hydrogen is then removed from triose phosphate and transferred to carrier molecule NAD
  • 2 molecules of reduced NAD are produced for each molecule of glucose entering glycolysis
  • hydrogens carried by reduced NAD can easily be transferred to other molecules to be used in oxidative phosphorylation to generate ATP
  • the end product of glycolysis is 2 molecules of pyruvate (3C)

Pyruvate still contains a great deal of chemical potential energy. When free oxygen is available, some of this energy can be released via the Krebs cycle and oxidatie phosphorylation. However, pyruvate first enters the link reaction.

The link reaction

It involves the conversion of pyruvate into acetyl (2C) coenzyme A. Occurs only if oxygen is available (aerobic respiration).

• It takes place in the mitochondrial matrix
  • pyruvate passed by active transport from the cytoplasm, through the outer and inner membranes of a mitochondria and into the mitochondrial matrix
• Pyruvate (3C) undergoes a series of reactions;
  • it is decarboxylated (removal of carbon diroxide)
  • it is dehydrogenated (removal of hydrogen)
  • It is combined with co enzyme A (CoA) to give acetyl (2C) coenzyme A 
    
• This is known as the link reaction
• The hydrogen removed from pyruvate is transferred to NAD

Fatty acids from fat metabolism may also be used to produce acetyl CoA. Fatty acids are broken down in the mitochondrion in a cylce of reactions in which each turn of the cycle shortens the fatty acid chain by a two-carbon acetyl unit. Each of these can react with CoA to produce acetyl CoA, which like that produced from pyruvate,enters the Krebs cycle

Coenzyme A is a complex molecule and acts as a carrier…