CELLULAR RESPIRATION

• Cellular respiration

  1. Glycolysis

  2. Krebs Cycle

  3. Electron Transport Chain
• Cellular respiration


  • Vital to all living organisms
  • Purpose is to transfer chemical energy stored in glucose (C6H12O6) into ATP which is used to power cellular reactions

  
• Why use ATP to provide energy for living organisms

  • Energy can be easily released in a single step: ATP → ADP + Pi + Energy
  • ATP has a fast turn over rate (it only takes a few seconds to covert ATP to ADP and then back from ADP to ATP)
• Why not just use glucose

  • Release of energy from glucose occurs via a complex multistep pathway which is much slower
  • Energy in glucose is high (3000kJ) compared to ATP (30kJ)
• Types of cellular respiration

  • Aerobic Cellular Respiration
  • Anaerobic Fermentation
• Aerobic Cellular Respiration

  • Requires oxygen
  • Produces 30-32 ATP molecules per one glucose molecule

  
• Anaerobic Fermentation

  • Does not require oxygen
  • Produces 2 ATP molecules per one glucose molecule
  • Produces harmful by-products (lactic acid or ethanol)
• Aerobic cellular respiration

  1. Glycolysis

  2. Krebs Cycle

  3. Electron Transport Chain
• Mitochondria


  • Crucial to aerobic cellular respiration as they are the site at which Krebs Cycle and Electron Transport Chain occur
  • Complex organelles made up of many different structures
• Mitochondrial structures

  • Inner and outer membrane
  • Mitochondrial matrix
  • Cristae
  • Intermembrane Space
• Glycolysis


  1. Occurs in Cytosol of cells

  2. Involves the breakdown of 6-carbon glucose into two 3-carbon pyruvate molecules via a sequence of 10 enzyme-regulated reactions

  3. Results in net production of 2 ATP molecules
• As glucose is broken down into pyruvate


  Energy is released
• The Link Reaction

  Pyruvate moves from cytosol to matrix of mitochondria and combines with coenzyme A (CoA) to form acetyl coenzyme A (acetyl-CoA)
• The Krebs Cycle (also known as the Citric Acid Cycle)

  1. Takes place in mitochondrial matrix

  2. Consists of 8 reactions that break down acetyl CoA

  3. Breaking down of acetyl CoA results in the release of protons and high-energy electrons which are loaded onto NAD+ and FAD to generate high-energy coenzymes NADH and FADH2

  4. Produces 2 ATP
• During formation of acetyl-CoA from pyruvate – 1 CO2 molecule is produced
• During one full turn of the Krebs Cycle – 2 CO2 molecules are produced (totaling 3 CO2 molecules)
• In ONE turn of the Krebs Cycle, acetyl-CoA is metabolised into: 2 CO2 molecules, 1 ATP molecule, 3 NADH molecules, 1 FADH2 molecule
• High Energy

  NADH, FADH2 and ATP
• Low Energy

  NAD+, FAD and ADP
• The Electron Transport Chain (ETC)

  1. Takes place on inner membrane of mitochondria (Cristae)

  2. Place where majority of ATP is produced

  3. Converts high-energy coenzymes NADH and FADH2 back to NAD+ and FAD forms which are then recycled for continued use in glycolysis and the Krebs cycle
• 26 or 28 ATP produced in the Electron Transport Chain
• Steps involved in making ATP at Electron Transport Chain

  1. NADH and FADH unload protons (H+) and electrons at first and second protein of ETC

  2. Excited electrons power active transport of protons (H+) from mitochondrial matrix into narrow intermembrane space

  3. Concentration of protons (H+) increases

  4. To move down concentration gradient, protons (H+) need to move through protein channel (ATP synthase) – ADP + Pi → ATP

  5. Large amounts of ATP are made

  6. Unbound protons (H+) and electrons bind with oxygen to produce water
• In total, for every glucose molecule metabolised, 30 – 32 ATP molecules are formed: 2 ATP (from glycolysis), 2 ATP (from Krebs Cycle), 26-28 ATP (from ETC)
• Enzymes and coenzymes in cellular respiration

  • Enzymes, with the help of their coenzymes, catalyse the reactions of cellular respiration to allow them to proceed at a fast rate
  • Cells can breakdown and extract energy from glucose at a rate fast enough to sustain energy-dependent processes
  • Each enzyme is only capable of catalysing one specific reaction, so there is a wide range of enzymes involved in cellular respiration

  
• Anaerobic fermentation

  1. Glycolysis to generate ATP

  2. Lactic Acid Fermentation (occurs in bacteria and animals)

  3. Alcoholic Fermentation (occurs in yeast and plants)
• Lactic Acid Fermentation

  Occurs in skeletal muscle cells when supply of oxygen to cells cannot keep up with their energy demands
• During strenuous exercise, lactic acid levels rise
• Lactic acid dissociates – forming lactate and H+ ions which lowers the pH of muscle tissue causing pain and fatigue (can be toxic in high amounts)
• Lactic acid is metabolised back into pyruvate when oxygen is present
• Anaerobic fermentation in yeasts

  1. Glycolysis

  2. Pyruvate is converted to ethanol and carbon dioxide
• Yeasts are used in bread making and in the production of alcoholic beverages (predominantly wine and beer)
• Yeasts are unable to metabolise ethanol into any useful products
• Ethanol diffuses out of cells
• If concentration of ethanol rises within a confined area – it can become toxic
• Temperature and pH


  They have a large effect on the rate at which cellular respiration occurs due to their effect on enzymes

  
• Respiration rate and ATP production are greatest when temperature aligns to enzymes optimal temperature

  
• Below optimal temperature, enzymes and substrates have less kinetic energy so there are fewer reaction-inducing collisions = lower rate of cellular respiration
• When temperature rises above optimum, enzymes denature and respiration rate drops significantly due to loss of enzyme function
• Above or below optimal pH, enzymes begin to denature and the rate of cellular respiration slows

  
• Glucose availability


  Increasing glucose availability increases the rate of cellular respiration until enzymes reach saturation point