Cellular respiration

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  • 1. Cellular Respiration Dr. Mark A. McGinleyHonors College and Department of Biological Sciences Texas Tech University
  • 2. Biological Work• Most of the energy used to do biological work comes from ATP• ATP breaks down and releases energy that is used to do biological work
  • 3. Energetics in a Nutshell• Photosynthesis converts light energy to potential energy stored in chemical bonds of glucose• Cellular Respiration converts potential energy in glucose to potential energy stored in ATP – ATP releases energy used to do work• Glucose links the two processes
  • 4. Breaking Down Glucose to Release Potential Energy• Starts with the process of glycolysis• Followed by either – Fermentation (in anaerobic environments) – Citric Acid Cycle (Krebs Cycle) + electron transport (in aerobic environments)
  • 5. Glycolysis• Glucose broken down into two molecules of pyruvate – Occurs in the cytosol• Breaking down ATP requires the input of energy from 2 molecules of ATP but releases energy in 4 molecules of ATP• Thus, net gain of energy of 2 ATPs in glycolysis
  • 6. Glycolysis• Glucose + 2ATP => 2 pyruvate + 4 ATP + H+
  • 7. Glycolysis• Glycolysis breaks down glucose to release energy in two ATPs – ATPs can release energy to do biological work
  • 8. Problem Facing the Cell• Glucose <= => 2 pyruvate + H+• This reaction will continue to break down glucose to release ATP until the reaction reaches an equilibrium• Once equilibrium is reached, glycolysis will stop, so no more ATP is released
  • 9. Solution• In order to allow glycolysis to continue cells must maintain the concentration gradient by removing pyruvate and H+ from the cell.• H+ picked up by NAD+ => NADH• Eventually NAD+ gets saturated
  • 10. Ultimate Solution• H+ must be removed from NADH in order to allow glycolysis to continue• Key Point- How this happens depends on whether or not there is oxygen in the environment
  • 11. Anaerobic Environment• When there is no oxygen in the environment then pyruvate and H+ are removed from the cell by fermentation• Several patterns of fermentation including – Alcohol fermentation – Lactic acid fermentation
  • 12. Alcohol Fermentation• Pyruvate and H+ => acetaldehyde => ethanol• Ethanol becomes the ultimate “hydrogen acceptor”
  • 13. Advantages and Disadvantages of Alcohol Fermentation• Benefit – End products of glycolysis are removed from the cell so glycolysis can continue• Disadvantage – Alcohol can be poisonous to cells – Pyruvate used to help remove H+ from the cell • Still lots of potential energy stored in pyruvate • Can’t break down pyruvate to release energy
  • 14. Lactic Acid Fermentation• Pyruvate + H+ => lactate• Lactate becomes the ultimate hydrogen acceptor
  • 15. Advantages and Disadvantages of Lactic Acid Fermentation• Benefit – End products of glycolysis are removed from the cell so glycolysis can continue• Disadvantage – lactate can be poisonous to cells – Pyruvate used to help remove H+ from the cell • Still lots of potential energy stored in pyruvate • Can’t break down pyruvate to release energy
  • 16. Review in Anaerobic Environments• Glucose broken down by glycolysis and fermentation• For each glucose molecule broken down there is a net gain of two ATPs
  • 17. Aerobic Environments• When oxygen is present – O2 + H+ => H20• Water becomes the ultimate hydrogen acceptor – Benefit • Water is non-toxic and in fact is beneficial • Pyruvate can be broken down to release more stored energy
  • 18. Energy From Pyruvate• Glycolysis occurs in the cytosol – NADH and pyruvate move into the mitochondria• In the mitochondria pyruvate is broken down to release ATP in two processes – Citric acid cycle (Krebs Cycle) – Electron transport
  • 19. Pyruvate Links Glycolysis and Citric Acid Cycle
  • 20. Citric Acid Cycle• The details of the Citric Acid Cycle are well know – Not super important for this course• Key Points – Inside of the mitochondrion pyruvate breaks down to produce CO2 + Acetyl CoA – Acetyl CoA enters Citric Acid Cycle • Acetyl CoA + oxaloacetate = > citrate – CO2 released – 1 ATP produced for each Acetyl CoA that enters the cycle • Thus, 2 ATPs per glucose
  • 21. Electron Transport• In a process very similar to what we talked about in cyclic electron flow in photosynthesis – An excited electron moves down an electron transport chain (located in inner membranes of mitochondria) • Energy released used to actively transport H+ • H+ concentration gradient powers Chemiosmosis – Releases lots of ATP – 26 or 28 ATP/glucose
  • 22. Electron Transport
  • 23. Review in Aerobic Environments• Glucose broken down by glycolysis, citric acid cycle, and electron transport• For each glucose molecule broken down there is a net gain of 30 - 32 ATPs – 2 per glucose from glycolysis – 2 per glucose from citric acid cycle – 26 – 28 per glucose from electron transport
  • 24. Advantages of Breaking Down Glucose in Aerobic Environments• Benefit – End products of glycolysis are removed from the cell so glycolysis can continue – Ultimate hydrogen acceptor (water) is beneficial to cells – Pyruvate can be broken down to release much more energy