Respiration (3.5.2)

4 stages of respiration:

1) Glycolysis

2) Link reaction

3) Krebs cycle

4) Oxidative phosphorylation/Electron transport chain

Glycolysis doesn’t require oxygen to occur

In glycolysis:

1) Glucose is phosphorylated into glucose phosphate using the hydrolysis of 2 molecules of ATP

2) Glucose phosphate is unstable so breaks down into 2 molecules of triose phosphate

3) The triose phosphates are oxidised (loses a H) which is gained by the co-enzyme NAD to form 2 molecules of NADH (reduced NAD)

4) 4 molecules of ATP are formed as a result of substrate level phosphorylation (net output of 2 molecules of ATP)

5) This produces 2 molecules of pyruvate

In the cytoplasm

Overall yield of glycolysis:

2 x ATP (overall 4 produced, but 2 used up)

2 x NADH

2 x Pyruvate

In the link reaction:

1) Pyruvate is decarboxylated (loses C in the form of carbon dioxide) and is oxidised (loses H which reduces NAD into reduced NAD/NADH)

2) This forms acetate

3) Acetate combines with co-enzyme A to form acetyl co-A

4) No ATP is formed in this stage

The matrix

Products of the link reaction:

2 x Acetyl Co-A

2 x NADH

2 x Carbon dioxide

In the Krebs cycle:

1) Acetyl co-A (a 2C molecule) combines with oxaloacetate (a 4 carbon molecule) to form citrate (6 carbon molecule)

2) Citrate undergoes decarboxylation, releasing a molecule of carbon dioxide and is oxidised to form a 5 C compound

3) The 5 C molecule is decarboxylated again releasing another molecule of carbon dioxide and is oxidised further to form 2 extra reduced NAD (NADH) and 1 reduced FAD (FADH)

4) Oxaloacetate, the 4 C molecule, is regenerated as a result which allows the krebs cycle to continue

5) Substrate level phosphorylation produces ATP

Products of the Krebs cycle:

2 x ATP

6 x NADH

2 x FADH

4 x Carbon dioxide

In oxidative phosphorylation:

1) Reduced NAD (NADH) and reduced FAD (FADH) are transported to the cristae where they are oxidised (losing a H atom)

2) The hydrogen is split into H+ and electrons. The electrons enter the electron transport chain

3) The electrons are passed between electron carrier proteins by a series of redox reactions

4) The energy released from the redox reactions is directly used to supply energy to actively pump H+ ions out of the matrix and into the intermembrane space

5) This creates an electrochemical proton gradient, so the H+ ions diffuse back into the matrix down the electrochemical proton gradient through ATP synthase

6) The movement of H+ ions releases energy used to phosphorylate ADP into ATP (this process is called chemiosmosis)

7) Oxygen acts as the final electron acceptor, accepting H+ ions and electrons to form water

Products of oxidative phosphorylation:

NAD

FAD

ATP

Water

Lipids

Proteins

1) Anaerobic respiration occurs in the absence of oxygen

2) In an absence of oxygen, there is no final electron acceptor, so as a result, reduced NAD and reduced FAD are not oxidised to form NAD and FAD at the end of oxidative phosphorylation

3) So there would be no co-enzymes that are available to reduce triose phosphate into pyruvate

4) So the glycolysis and link reaction would stop and organism would no longer be able to respire properly

Lactate

Ethanol

Pyruvate is reduced into lactate (from the oxidation of NADH into NAD)

Therefore NAD is regenerated so glycolysis can occur

Pyruvate is decarboxylated into ethanal releasing carbon dioxide as a by product

Ethanal is reduced into ethanol (from the oxidation of NADH into NAD)

NAD is regenerated so glycolysis can occur