Respiration

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  • Created by: Alicek_
  • Created on: 29-11-17 13:41

4 stages of aerobic respiration

1. Glycolysis is the first stage of respiration, and occurs in both aerobic and anaerobic respiration.

2. The link reaction is the next stage, which ends with the formation of acetylcoenzyme A.

3. The Krebs cycle is the penultimate stage, and yields reduced NAD and FAD.

4. The final stage is called oxidative phosphorylation, and concludes with ATP being synthesised. 

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Glycolysis

Glucose is made more reactive through its phosphorylation. The phosphate molecules come from the hydrolysis of ATP to ADP. This gives the energy to activate glucose, and it also lowers the activation energy for the enzyme-controlled reactions that follow. Each glucose molecule is split into two triose phosphate molecules, each with an inorganic phosphate molecule.

The 3-c triose phosphate is oxidised  and the hydrogen atoms are transferred to NAD, an electron carrier and a coenzyme, to create reduced NAD, or NADH. 

Each triose phosphate molecule is converted to a 3-c pyruvate molecule through a series of enzyme controlled reactions. In the process, two molecules of ATP are regenerated from ADP + Pi. 

The overall yield from glycolysis is therefore:

2 ATP

2 NADH

2 pyruvate

This reaction takes place in the cytoplasm of the cell.

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The link reaction

The pyruvate molecules from glycolysis are actively transported across the membrane of the mitochondria into the mitochondrial matrix. The 3-c pyruvate is then oxidised to a 2-c molecule of acetate.

The 3-c molecule has lost a CO2 molecule and 2H molecules. The CO2 molecule is directly transported into the bloodstream then to the lungs and breathed out. The 2H molecules are accepted by NAD to form NADH. 

The 2-c acetate combines with a molecule called coenzyme A (CoA) to produce a 2-c compound called acetylcoenzyme A. 

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The Krebs cycle

This involves a series of oxidation-reduction reactions taking place in the mitochondrial matrix. The acetylcoenzyme A combines with a 4-c molecule to form a 6-c molecule. This molecule then loses 2CO2 molecules, a single molecule of ATP, and several H molecules which reduce NAD and FAD to NADH and FADH. The 4-c molecule is reformed and combines with a new molecule of acetylcoenzyme A. 

The Krebs cycle is important for several reasons:

  • it breaks macromolecules into smaller ones e.g. pyruvate is broken down into CO2. 
  • it reduces coenzymes NAD and FAD which are carried to the electron transfer chain and provide energy for oxidative phosphorylation
  • it regenerates the 4-c molecule so it can recombine with another acetylcoenzyme A molecule, to prevent this molecule building up
  • it creates many intermediate compounds used by cells in the production of important substances such as fatty acids, amino acids and chlorophyll
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Oxidative phosphorylation

This takes place in the cristae of the mitochondria, and is the stage in which ATP is synthesised.

NADH and FADH donate H atoms and electrons to the first molecule of the electron transfer chain. The electrons pass along the electron transfer chain in a series of oxidation-reduction reactions which take place at successively lower energy levels, meaning energy is lost. They are passed along the electron transfer chain so their energy is released gradually rather than all at once, as less energy is therefore lost as heat energy. At the end of the chain the electrons combine with protons and oxygen to form water. Oxygen is therefore the final electron acceptor in the electron transfer chain.

1/2 O2 +2e- + 2H+ --> H2O

The released energy is used to actively transport protons across the inner mitochondrial membrane and into the inter-membranal space. The protons accumulate in the inter-membranal space before diffusing back into the matrix through ATP synathse channels embedded in the inner mitochondrial membrane. This synthesises a molecule of ATP, which is the final product of respiration. 

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Anaerobic respiration

In plants- 

The pyruvate molecule formed at the end of glycolysis loses a molecule of carbon dioxide and accepts hydrogen from reduced NAD to produce ethanol.

Pyruvate + NADH --> ethanol + carbon dioxide + oxidised NAD

In animals-

When oxygen is in short supply, NAD from each pyruvate molecule takes up the 2H atoms from NADH to form lactate.

Pyruvate + NADH --> lactate + oxidised NAD

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