Respiration

• Why do organisms need to respire?

  • Produces ATP as energy currency for:
  • active transport against concentration gradients
  • metabolic reactions
  • muscle contraction
  • Releases heat energy for thermoregulation
• Mitochondrion


  • Surrounded by double membrane
  • Folded inner membrane forms cristae: site of electron transport chain
  • Fluid matrix: contains mitochondrial DNA, respiratory enzymes, lipids, proteins
• Aerobic respiration


  1. Glycolysis: cytoplasm

  2. Link reaction: mitochondrial matrix

  3. Krebs cycle: mitochondrial matrix

  4. Oxidative phosphorylation via electron transfer chain: membrane of cristae

  
• Glycolysis


  1. Glucose is phosphorylated to hexose bisphosphate by 2x ATP

  2. Hexose bisphosphate splits into 2x triose phosphate (TP)

  3. 2x TP is oxidised to 2x pyruvate

  4. Net gain of 2x reduced NAD & 2x ATP per glucose

  
• Pyruvate from glycolysis enters the mitochondria via active transport
• Link reaction


  1. Oxidation of pyruvate to acetate

  2. Acetate combines with coenzyme A (CoA) to form Acetylcoenzyme A

  
• Link reaction summary equation

  pyruvate + NAD + CoA → Acetyl CoA + reduced NAD + CO2

  
• Krebs cycle


  1. Series of redox reactions produces: ATP by substrate-level phosphorylation, reduced coenzymes, CO2 from decarboxylation

  2. Begins when acetyl group from Acetyl CoA (2C) reacts with oxaloacetate (4C)

  3. Cycle regenerates oxaloacetate

  
• Electron transfer chain (ETC)


  • Series of carrier proteins embedded in membrane of the cristae of mitochondria
  • Produces ATP through oxidative phosphorylation via chemiosmosis during aerobic respiration

  
• Electron transfer chain (ETC)

  1. Electrons released from reduced NAD & FAD undergo successive redox reactions

  2. The energy released is coupled to maintaining proton gradient or released as heat

  3. Oxygen acts as final electron acceptor

  
• Chemiosmosis


  1. Some energy released from the ETC is coupled to active transport of H+ ions (protons) from mitochondrial matrix into intermembrane space

  2. H+ ions move down concentration gradient into mitochondrial matrix via channel protein ATP synthase

  3. ATP synthase catalyses ADP + Pi → ATP

  
• Role of oxygen in aerobic respiration


  Final electron acceptor in electron transfer chain (produces water as a byproduct)
• Stages in respiration that produce ATP by substrate-level phosphorylation

  • Glycolysis (anaerobic)
  • Krebs cycle (aerobic)
• Anaerobic respiration in animals

  1. Only glycolysis continues

  2. Reduced NAD + pyruvate → oxidised NAD (for further glycolysis) + lactate

  
• Anaerobic respiration in some microorganisms e.g. yeast and some plant cells


  1. Only glycolysis continues, so much less ATP is produced compared to aerobic respiration

  2. Pyruvate is decarboxylated to form ethanal

  3. Ethanal is reduced to ethanol using reduced NAD to produce oxidised NAD for further glycolysis
• Benefits of being able to respire anaerobically
• Investigating effect of variable on rate of respiration of single-celled organism

  1. Use respirometer (pressure changes in boiling tube cause a drop of coloured liquid to move)

  2. Use a dye as the terminal electron acceptor for the ETC
• Purpose of sodium hydroxide solution in respirometer

  Absorbs CO2 so that there is a net decrease in pressure as O2 is consumed

  
• Calculating rate of respiration using respirometer

  1. Volume of O2 produced or CO2 consumed/ time x mass of sample

  2. Volume = distance moved by coloured drop x (0.5 x capillary tube diameter)2 x π
• Alternative respiratory substrates


  • (amino acids from) proteins
  • (glycerol and fatty acids from) lipids
• Respiratory quotient (RQ)


  • RQ = carbon dioxide produced / oxygen consumed
  • Can be used to determine: respiratory substrate being used, if organism is undergoing anaerobic respiration
• Why different respiratory substrates have different relative energy values