Brent Cornell

Cell respiration is the controlled release of energy from the breakdown of organic compounds (principally glucose) 

• Glucose can be partially broken down under anaerobic conditions (no oxygen) within the cytosol for a low energy yield
• The partially digested products can be completely broken down under aerobic conditions (oxygen) in mitochondria for a higher yield

ATP Production

When organic compounds are broken down, the energy released is transferred to one of two coenzyme molecules

• ATP is the primary energy carrier and can be produced directly from ADP (and Pi) via substrate-level phosphorylation
• Hydrogen carriers act as a transitional energy carrier and can transfer energy to form ATP via oxidative phosphorylation
  • Oxidative phosphorylation involves an electron transport chain located on the inner membrane (cristae) of the mitochondria
  • Oxidative phosphorylation requires oxygen to function, hence only aerobic respiration can produce ATP from hydrogen carriers

Anaerobic Respiration

Anaerobic respiration does not require oxygen and involves the partial breakdown of glucose via the process of glycolysis 

• It occurs in the cytosol and results in a low yield of ATP (net 2 molecules total) via substrate level phosphorylation

Glycolysis

• In glycolysis, glucose is broken down within the cytosol into two 3C-compounds called pyruvate (or pyruvic acid)
• This process uses two molecules of ATP but produces four molecules (for an overall net gain of 2 ATP molecules)
• Hydrogen atoms are also removed (via oxidation) and transferred to unloaded coenzymes (NAD)
• This results in a small yield of loaded hydrogen carriers (NADH) which can be used by the mitochondria

Aerobic Respiration

Aerobic respiration involves the complete breakdown of glucose in the mitochondria for a higher ATP yield (net 30 or 32 ATP)

• It is preceded by the anaerobic breakdown of glucose into two molecules of pyruvate (via glycolysis in the cytosol) 
• It occurs across two key stages: the Krebs cycle (in the matrix) and the electron transport chain (on the inner membrane)

Krebs Cycle

• The pyruvate produced via glycolysis is transferred to the mitochondrial matrix and oxidised to form acetyl coenzyme A
• Acetyl CoA is then introduced into the Krebs cycle, where it is combined with a 4C compound to make a 6C intermediate
• This 6C intermediate (citrate) is broken down into the original 4C compound over a series of sequential chemical reactions
• These reactions result in the formation of one ATP per pyruvate (2 in total) and a large number of loaded hydrogen carriers
• The breakdown of pyruvate also results in the formation of carbon dioxide (3 molecules per pyruvate, for 6 molecules in total)

Electron Transport Chain

• Hydrogen carriers (NADH and FADH₂) donate energised electrons (and protons) to an electron transport chain on the cristae
• The electron transport chain utilises the energy stored in the donated electrons to make ATP via oxidative phosphorylation
• A total of 26 or 28 molecules of ATP are produced as a result of the unloading of hydrogen carriers within the mitochondria
• For the electron transport chain to continue to function, the donated electrons (and protons) must be removed from the chain
• Oxygen acts as the final electron acceptor in the transport chain and is complexed with the protons to form water molecules 

Extension:  Chemiosmosis

• ATP is produced by the electron transport chain via the oxidation of hydrogen carriers (hence via oxidative phosphorylation)
• When hydrogen carriers (NADH or FADH₂) are oxidised, they release high energy electrons and protons (i.e. hydrogen)
• The electrons are transferred to a transport chain consisting of several transmembrane protein carriers
• As electrons pass through the chain, they lose energy – which is used by the chain to pump the protons from the matrix
• The accumulation of protons within the intermembrane space creates an electrochemical gradient (or a proton motive force)
• The proton motive force will cause the protons to move down their electrochemical gradient and diffuse back into matrix
• This diffusion of protons is called chemiosmosis and is facilitated by the transmembrane enzyme ATP synthase
• As the protons move through ATP synthase they trigger the molecular rotation of the enzyme, synthesising ATP
• The protons and de-energised electrons are coupled to oxygen to form water – if oxygen is not present to act as the final electron acceptor then the electron transport chain will be unable to continue functioning (hence, this process is aerobic)

Overview of Aerobic Respiration

Aerobic respiration requires oxygen and uses the mitochondria to produce a large yield of ATP (compared to anaerobic)

• Glycolysis produces net 2 ATP (via substrate level phosphorylation) and a small quantity of hydrogen carriers (2 NADH)
• Krebs cycle produces 2 ATP (via substrate level phosphorylation) and a large quantity of hydrogen carriers (8 NADH + 2 FADH₂)
• The electron transport chain uses the hydrogen carriers to produce 26 or 28 ATP via oxidative phosphorylation (requires oxygen)