Biology 3.3 - The Stages of Respiration

There are three types:

1. OXIDATIVE PHOSPHORYLATION - occurs on the inner membrane of mitochonria in aerobic respiration. It is where energy for ATP comes from oxidation-reduction reactions, and is released by the transfer of electrons along a chain of electron carriers. 

2. PHOTOPHOSPHORYLATION - occurs on the thylakoid membranes of chloroplasts in photosynthesis. It is where energy for making ATP comes from light and is released by the transfer of electrons along a chain of electron carriers.

3. SUBSTRATE LEVEL PHOSPHORYLATION - when a phosphate group is transferred from a donor molecule.

• First stage of both aerobic and anaerobic respiration.
• Occurs in the cytosol, as glucose cannot diffuse through the mitochondrial membrane due to its size.

Mechanism:

• A glucose molecule is phosphorylated by the addition of two phosphate groups from two ATP molecules. Glucose diphosphate is produced.
• The glucose diphosphate splits into two molecules of triose phosphate.
• Each triose phosphate is dehydrogenated to form pyruvate (3C). The hydrogens lost combine with the hydrogen carrier NAD to produce NADH. 
• These steps release enough energy to synthesise 4 molecule of ATP by substrate level phosphorylation.

NET GAIN:

• 2 x NADH
• 2 x ATP from substrate level phosphorylation
• 6 x ATP from oxidative phosphorylation.

• Occurs in the mitochondrial matrix.

Steps:

• The pyruvate molecules diffuse from the cytosol into the mitochondrial matrix.
• Each pyruvate is dehydrogenated and decarboxylated to form a 2C acetate group. NADH and carbon dioxide is also produced.
• The acetate group combines with coenzyme A to form acetyl coenzyme A, which enters the Krebs cycle. 

NET GAIN:

• 2 x NADH
• 6 x ATP from oxidative phosphorylation.

• Occurs in the mitochondrial matrix.

Steps:

• AcCoA combines with a 4C acid to produce a 6C compound. CoA is regenerated.
• This 6C compound is then dehydrogenated and decarboxylated to produce a 5C compound, NADH and carbon dioxide. 
• This 5C compound is then dehydrogenated, forming NADH and FADH, and decarboxylated, evolving carbon dioxide and forming a 4C compound. An ATP molecule is also synthesised. 
• This 4C compound can now recombine with AcCoA and begin the cycle again.

Summary:

• Each glucose molecule gives two turns of the Krebs cycle.
• Decarboxylation happens twice (one turn).
• Dehydrogenation happens four times, producing 3 x NADH and 1 x FADH (one turn).

NET GAIN:

• 6 x NADH (3 per turn).
• 2 x FADH (1 per turn).
• 22 x ATP from oxidative phosphorylation.
• 2 x ATP from substrate level phosphorylation.

• Occurs on the cristae of the inner mitochondrial membrane.
• Oxygen is the final electron acceptor.

Steps:

• NADH donates electrons to the first of a series of electron carriers in the ETC.
• NADH provides energy for three proton pumps: as the electrons pass along the ETC, they provide energy to each of the three proton pumps in turn.
• FADH only provides enough energy for two proton pumps.
• Both FADH and NADH also donate their protons, whcih are pumped into the inter-membrane space by the energised proton pumps.
• The inner membrane is impermeable to protons so they accumulate in the inter-membrane space. 
• The concentration of protons in the inter-membrane space eventually exceeds that in the matrix, so an electrochemical gradient is established.
• The protons diffuse down the gradient, through ATP synthetase and into the matrix. This generates energy to phosphorylate ADP into ATP..
• At the end of the chain the protons combine with oxygen and electrons to form water. 

For each molecule of glucose entering the Krebs cycle, the ETC receives:

• 10 NADH = 10 x 3 ATP = 30 ATP.
• 2 FADH = 2 x 2 ATP = 4 ATP.
• 34 ATP from oxidative phsophorylation.

4 x ATP is also rpoduced via substrate level phsophorylation = 38 ATP.