12 - Respiration

• Respiration is the reaction that produces the ATP needed for metabolism. It is an enzyme controlled reaction and is therefore sensitive to temperature and pH changes.
• Glucose is broken down to release the ATP, but if there is no glucose, glycogen (in animals ) or starch (in plants) stores left then other respiratory substrates are used.

  
  

  Lipids can be hydrolysed to glycerol and fatty acids. The glycerol is phosphorylated into triose phosphate and enters aerobic respiration at the Krebs cycle. Proteins could also be modified and used.

  
  

  The amino group is removed (deamination) from the amino acids. The carbon compound that remains can enter the Krebs cycle.
• Where do the four stages of aerobic respiration occur?

  1. Glycolysis - cytoplasm
  2. Link Reaction - Mitochondrial matrix
  3. Krebs cycle - Mitochondrial matrix
  4. Oxidative Phosphorylation - Cristae/inner mitochondrial membrane
• What are the four stages of aerobic respiration?

  1. Glycolysis
  2. Link Reaction
  3. Krebs Cycle
  4. Oxidative Phosphorylation
• Use the diagram to describe the key stages in glycolysis.

  • Substrate level phosphorylation - The glucose has 2 phosphate groups added to it from 2 ATP molecules.
  • Phosphorylation makes the glucose 2 phosphate unstable and it splits into two x 3 carbon compounds, triose phosphate (TP).
  • The 2 TP molecules are oxidised to form 2 pyruvate molecules by removing an H from each. The H is picked up by 2 NAD molecules to become reduced NAD. This process also releases 4 ATP.

  
• What are the products of glycolysis?

  2x Pyruvate

  Net gain of 2 ATP

  2x reduced NAD
• Use the diagram to describe the key stages in the link reaction.

  • The pyruvate made in glycolysis is oxidised to acetate.
  • NAD picks up the hydrogen and becomes reduced NAD.
  • A carbon atom is lost as carbon dioxide.
  • Acetate combines with coenzyme A to produce acetyl coenzyme A.

  
• What are the products of the link reaction (from 1 glucose molecule originally)?

  The link reaction occurs twice for every glucose molecule creating:

  2 x Acetyl CoA

  2 x CO2 released

  2 x reduced NAD
• Use the diagram to describe the key stages in the Krebs cycle.

  • The acetyl CoA reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters the Krebs cycle.
  • In a series of oxidation-reduction reactions, the Krebs cycle generates 8 reduced coenzymes, 2 ATP by substrate-level phosphorylation, and 4 carbon dioxides are lost.

  
• What are the products of the Krebs cycle (from 1 glucose molecule originally)?

  • 6 x reduced NAD
  • 2 x reduced FAD
  • 4 x carbon dioxide
  • 2 x ATP
• What is the role of oxygen in oxidative phosphorylation?

  Oxygen is the final electron acceptor in the electron transport chain. The oxygen combines with the electrons and the protons to form water.
• What happens to the reduced co-enzymes in this stage?

  All of the accumulated reduced coenzymes release the hydrogens which are split into protons (H+) and electrons (e-).
• How is ATP is produced in oxidative phosphorylation?

  • Electrons are passed down a series of electron carrier proteins embedded in the inner mitochondrial membrane, losing energy as they move along (electron transport chain). The small amount of energy the electrons release pumps protons from the mitochondrial matrix into the intermembrane space by active transport.
  • This creates an electrochemical gradient across the membrane. Therefore, the protons move down the electrochemical gradient back into the matrix via ATP synthase making ATP.
• Where does anaerobic respiration occur?

  Cytoplasm
• The pyruvate produced in glycolysis is reduced to form ethanol and carbon dioxide (in plants and microbes) or lactate (in animals) by gaining the hydrogen from reduced NAD.

  
• Why is it important that NAD is re-oxidised in anaerobic respiration?

  So that it can be reused in glycolysis and ensure ATP continues to be produced.