The Respiratory Chain

• The respiratory chain (or electron transport chain, or oxidative phosphorylation) is an unusual metabolic pathway in that it takes place within the inner mitochondrial membrane, using integral membrane proteins.
• There are five of these proteins, called I, II, III, IV and ATP synthase.
• They each contain up to 40 individual polypeptide chains, which perform many different functions including enzymes and trans-membrane pumps.
• In the respiratory chain the hydrogen atoms from NADH gradually release all their energy to form ATP, and are finally combined with oxygen to form water.

I

• NADH molecules bind to protein I and release their hydrogen atoms as protons (H+) and electrons (e-).
• FADH binds to complex II rather than complex I to release its hydrogen.
• The NAD and FAD molecules then return to the Krebs Cycle to collect more hydrogen, so these coenzymes are constantly shuttling between their oxidised and reduced forms

II

• The electrons are passed along the chain of proteins.
• The energy of the electrons is used to pump protons across the inner mitochondrial membrane by  active transport through the proteins, forming a proton gradient across the membrane.

III

•  Finally, the electrons are combined with protons and molecular oxygen (O2) to form water, the final end-product of respiration.
• The oxygen diffused in  from the tissue fluid, crossing the cell and mitochondrial membranes by lipid diffusion.
• Oxygen is only involved at the very last stage of respiration as the final electron acceptor, but without it the whole respiratory chain stops.

IV

• The energy of the electrons is now stored in the form of the proton gradient across the inner mitochondrial membrane.
• It’s a bit like using energy to pump water uphill into a high reservoir, where it is stored as potential energy.
•  And just as the potential energy in the water can be used to generate electricity in a hydroelectric power station, so the potential energy in the proton gradient can be used
  to generate ATP in the ATP synthase enzyme.
• The ATP synthase enzyme has a proton channel through it, and as the protons “fall down” this channel their energy is used to make ATP, spinning the globular head as they go.
• It takes 4 protons to synthesise 1 ATP molecule.

This synthesis of ATP is called oxidative phosphorylation because it uses oxygen to phosphorylate ADP.
The method of storing energy by creating a proton gradient across a membrane is called chemiosmosis.

1. NADH releases its H and is oxidised to NAD, which returns to the Krebs cycle
(NADH → NAD + H+ + e-).

2. The electron is passed along the chain of proteins in the inner mitochondrial membrane, releasing its energy as it goes.

3. Oxygen combines with hydrogen to form water (O2+ H++ e- →H2O).

4. The energy of the electron is used to make ATP in the ATP synthase enzyme (ADP + Pi →ATP).

• If there is no oxygen (anaerobic conditions) then the final reaction to make water cannot take place,  no electrons can leave the respiratory chain, and so NADH cannot unload any hydrogens to the respiratory chain.
• This means that there is no NAD in the cell;it’s all in the form of NADH.
• Without NAD as a coenzyme, some of the enzymes of the Krebs cycle and glycolysis cannot work, so the whole of respiration stops.
• Some cells can get round this problem using anaerobic respiration.
•  This adds an extra step to the end  of glycolysis that regenerates NAD, so allowing glycolysis to continue and some ATP to be made.
•  Anaerobic respiration only makes 2 ATPs per glucose, but it’s better than nothing!

There are two different kinds of anaerobic respiration:

Lactic Acid Anaerobic Respiration

• In animals and bacteria the extra step converts pyruvate to lactate(or lactic acid).
• This is a reduction, so NADH is used and NAD is regenerated, to be used in glycolysis.

• The reaction is reversible, so the energy remaining in the lactate molecule can be retrieved when oxygen becomes available and the lactate is oxidised via the rest  of
  aerobic respiration.
• The bacteria used to make yogurt use this reaction,as do muscle cells and red blood cells in humans.
• Unfortunately the lactate is poisonous, causing acidosis in muscles cells, which stops enzymes working and causes muscle fatigue and cramp.
•  So anaerobic respiration in muscles cannot be continued for very long.

Ethanolic Anaerobic Respiration

• In plants and fungi the extra steps converts pyruvate to ethanol.
•  This is also a reduction, so NADH is used and NAD is regenerated, to be used in
  glycolysis.
•  Ethanol is a two-carbon compound and carbon dioxide is also formed.
•  This means the reaction is irreversible, so the energy in the ethanol cannot be retrieved by the cells.

• Ethanolic anaerobic respiration is also known as fermentation, and we make use of fermentation in yeast to make ethanol in beer and wine.

ATP is made in two different ways:

• Some ATP molecules are made directly by the enzymes in glycolysis or the Krebs cycle. This is called substrate level phosphorylation(since ADP is being phosphorylated to form ATP).
• Most of the ATP molecules are made by the ATP synthase enzyme in the respiratory chain. Since this requires oxygen it is called oxidative phosphorylation. Scientists don’t yet know exactly how many protons are pumped in the respiratory chain.

Two ATP molecules are used at the start of glycolysis to phosphorylate the glucose, and these must be subtracted from the total.

•  Other substances can also be used to make ATP.
•  Glycogen of course is the main source of glucose in humans.
• Triglycerides are broken down to fatty acids and glycerol, both of which enter the Krebs Cycle
• .A typical triglyceride molecule might make 50 acetyl  CoA molecules, yielding 500 ATP molecules.
• Fats arethus a very good energy store, yielding 2.5 times as much ATP per g dry mass as carbohydrates.
•  Proteins are not normally used to make ATP, but in starvation they can be broken down and used in respiration.
• They are first broken down to amino acids, which are converted into pyruvate and Krebs Cycle metabolites and then used to make ATP.