Respiration

• Life


  Hierarchical Organization
• Energy


  Our cells require energy from outside sources to work (for growth & repair; active transport; reproduction; molecule synthesis [e.g., proteins]; etc.)
• Energy flows into an ecosystem as sunlight and leaves as heat
• Importance of photosynthesis


  It generates O2 and organic molecules, which are used in cellular respiration
• Chemical energy


  Cells use chemical energy stored in organic molecules (e.g., Glucose) to generate ATP, which powers most cellular work
• ATP


  The cell's energy shuttle
• ATP


  • It is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups
  • The bonds between the phosphate groups of ATP's tail can be broken by hydrolysis (the addition of water that results in breaking bonds)
  • The result of broken bonds? Energy is released from ATP when the terminal phosphate bond is broken
• Metabolism


  An organism's metabolism (i.e., all its chemical reactions) transforms matter and energy
• Metabolism


  • Catabolic, where energy is released by breaking down complex molecules into simpler compounds (e.g., cellular respiration and the breakdown of glucose in the presence of oxygen)
  • Anabolic, where energy is consumed to build complex molecules from simpler ones (e.g., protein synthesis from amino acids)
• Cellular respiration


  It includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration
• Significant events in the transfer of energy between molecules

  1. Energy is derived from electrons in food/organic molecules (e.g., glucose/glucose metabolites)

  2. Glycolysis provides electrons via 2 NADH

  3. Citric acid cycle provides electrons via 6 NADH and 2 FADH2

  4. Electrons' energy powers proteins in the electron transport chain

  5. Water is the by-product. Note that the final electron acceptor – oxygen – is essential in the formation of water
• Summary of ATP production from complete oxidation of a molecule of glucose
• Flavin Adenine Dinucleotide (FAD)


  It exists in 2 forms - an oxidized (FAD) and a reduced (FADH2)
• Nicotinamide Adenine Dinucleotide (NAD)

  It also exists in 2 forms - an oxidized (NAD+) and reduced (NADH)
• Oxidation


  Loss of electrons
• Reduction


  Gain of electrons
• Oxidative phosphorylation


  1. Electron transport chain

  2. Chemiosmosis
• Harvesting of energy from glucose has three stages: Glycolysis, Citric acid cycle, and Oxidative phosphorylation
• Glycolysis


  It breaks down glucose into two molecules of pyruvate
• Glycolysis


  1. Energy investment phase (2 ATPs are used to start the process)

  2. Energy payoff phase (2 ATPs used are recovered)
• Glycolysis occurs whether or not O2 is present
• A smaller amount of ATP is formed in glycolysis by substrate-level phosphorylation
• Pyruvate Oxidation


  In the presence of O2, pyruvate enters the mitochondrion. Matrix is where the oxidation of glucose is completed. Oxidation of Pyruvate to Acetyl CoA: Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle
• Pyruvate Oxidation


  1. Pyruvate is stripped of CO2

  2. It gives up electrons & H+ to NAD+ and NADH is formed

  3. A coenzyme (CoA) joins its remaining 2-carbon fragment, forming acetyl-CoA
• Citric Acid Cycle


  It has eight steps, each catalyzed by a specific enzyme. First step - The acetyl group of acetyl CoA combines with oxaloacetate, forming citrate. The remaining steps (7 steps) decompose the citrate back to oxaloacetate, making the process a cycle. The 6 NADH and 2 FADH2 produced by the cycle relay electrons extracted from food (substrates) to the electron transport chain
• Oxidative phosphorylation accounts for almost 90% (87.5%) of the ATP generated by cellular respiration
• NADH and FADH2 account for most of the energy extracted from food (substrates)
• Citric Acid Cycle


  1. Decompose the citrate back to oxaloacetate

  2. Making the process a cycle
• NADH and FADH2


  Electron carriers produced by the Citric Acid Cycle
• The 6 NADH and 2 FADH2 produced by the cycle relay electrons extracted from food (substrates) to the electron transport chain
• Electron Transport Chain


  • Site: Cristae (inner membrane of the mitochondrion)
  • Source of electrons: NADH or FADH2
  • ATP generation: Not generated directly
  • Formation of water: Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O
• Chemiosmosis


  1. Electron transfer in the electron transport chain causes proteins to pump hydrogen ions from the mitochondrial matrix to the intermembrane space

  2. Hydrogen ions then move back across the membrane, passing through the protein complex - ATP synthase

  3. ATP synthase uses the flow of hydrogen ions to drive the phosphorylation of ATP (attaches a phosphate to ADP)
• NADH and FADH2 donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
• Electron carriers alternately pick up and release electrons and ultimately transfer them to their final acceptor, which is oxygen
• Oxygen is the final acceptor of electrons in the electron transport chain
• Most cellular respiration requires O2 to produce ATP
• Anaerobic Respiration


  1. Uses an electron transport chain with a final electron acceptor other than O2, for example, sulfate

  2. Mycobacterium (TB cause) - obligate anaerobe - carries out fermentation or anaerobic respiration and cannot survive in the presence of O2
• Fermentation


  1. Uses substrate-level phosphorylation instead of an electron transport chain

  2. Generates a small amount of ATP

  3. Regenerates NAD+
• Types of Fermentation


  • Alcohol fermentation (e.g., as in Yeast)
  • Lactic acid fermentation (e.g., in muscles)
• Lactic Acid Fermentation


  • Products: ATP, NADH & pyruvate
  • Pyruvate is reduced by NADH, forming lactate as an end product, with no release of CO2
  • Commercial use: Cheese and yoghurt production