Chapter 9: Cellular Respiration – Harvesting Chemical Energy

1. Chapter 9: Cellular Respiration – Harvesting Chemical Energy

2. Overview: Life is Work • To perform their many tasks, living cells require energy from outside sources • Energy enters ecosystems as sunlight and leaves as heat • Photosynthesis generates oxygen and organic molecules • Mitochondria of eukaryotes use this as fuel for cellular respiration • Respiration has three key pathways: • Glycolysis • The citric acid cycle • Oxidative phosphorylation

3. Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels • Enzymes catalyze the degradation of organic molecules that are rich in energy to simpler waste products with less energy • Some the released energy is used to do work • The rest is dissipated as heat • Catabolic metabolic pathways release the energy stored in complex organic molecules • The breakdown of organic molecules is exergonic

4. Several processes are central to cellular respiration and related pathways: • Fermentation – partial degradation of sugars that occur WITHOUT oxygen • Aerobic respiration – consumes organic molecules and oxygen and yields ATP • Anaerobic respiration – similar to aerobic respiration, but consumes compounds other than oxygen

5. Cellular respiration – includes both aerobic and anaerobic respiration, but is often used to refer to aerobic respiration • Mitochondria are the site of most of the processes of cellular respiration • Cellular respiration is similar to the combustion of gas in a car engine after oxygen is mixed with hydrocarbon fuel • Food is the fuel for respiration • The exhaust is carbon dioxide and water

6. Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C6H12O6+ 6 O2 6 CO2 + 6 H2O + Energy (ATP + heat) • The catabolism of glucose is exergonic • Some of this energy is used to produce ATP, which can perform cellular work

7. Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation-reduction reactions • Aka redox reactions • The loss of electrons is called oxidation • The addition of electrons is called reduction

8. The formation of table salt from sodium and chloride is a redox reaction • Na + Cl Na+ + Cl- • Sodium is oxidized • Chlorine is reduced • Redox reactions require both a donor and acceptor

9. Redox reactions also occur when the transfer of electrons is not complete, but involves a change in the degree of electron sharing in covalent bonds • Energy must be added to pull an electron away from an atom

10. During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced

11. Organic molecules that have lots of H+ are good fuels because they have electrons that can be transferred to oxygen • This must happen in steps

12. In cellular respiration, glucose and other organic molecules are broken down in a series of steps • Each are catalyzed by a specific enzyme • At key steps, electrons are stripped from glucose • In many oxidation reactions, the electron is transferred with a proton, as a hydrogen atom

13. Hydrogen atoms are NOT transferred directly to oxygen • First passed to a coenzyme called NAD+ • ‘traps’ electrons from glucose • Enzymes strip 2 H+ from the fuel • The enzyme passes two electrons and 1 proton to NAD+ • Electron acceptor = functions as an oxidizing agent

14. Each NADH (the reduced form of NAD+) represents stored energy that is trapped to synthesize ATP • NADH passes the electrons to the electron transport chain • Another series of steps • Oxygen pulls electrons down the chain • The energy yielded is used to regenerate ATP • Food  NADH  Electron transport chain  oxygen

15. Respiration occurs in 3 metabolic steps: • Glycolysis • Occurs in cytoplasm • Breaks glucose down into two pyruvate molecules • Citric acid cycle • Completes breakdown of glucose • Electron transport chain/oxidative phosphorylation • Accounts for most of the ATP synthesis • Powered by redox reactions

16. OXPHOS accounts for almost 90% of the ATP generated by cellular respiration • A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation Substrate Product

17. Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate • Glycolysis – splitting of sugar • Breaks down glucose into two molecules of pyruvate • Occurs in the cytoplasm • Has two major phases: • Energy investment phase • Energy payoff phase • Glycolysis can happen without oxygen present

18. Energy investment phase: • Invests ATP to provide activation energy by phosphorylating glucose • Requires 2 ATP per glucose • Energy payoff phase: • ATP is produced • NAD+ is reduced to NADH • Caused by oxidation of glucose • Net yield for glycolysis: • 2 ATP • 2 NADH • No CO2

20. Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules • More than ¾ of the original energy in glucose is still present in the 2 pyruvate molecules • Exposed to oxygen = pyruvate enters the mitochondrion via active transport • When in the mitochondria, pyruvate is converted to a compound • Called acetyl coenzyme A or acetyl CoA

21. The citric acid cycle is also called the Krebs cycle • In honor of Hans Krebs • The dude that explained the pathways involved in it

22. The citric acid cycle oxidizes fuel derived from pyruvate • Generates: • 1 ATP • 3 NADH • 1 FADH2 • Has 8 steps • Each has its own specific enzyme

23. The acetyl group of acetyl CoA combines with oxaloacetate • Forms citrate • The next 7 steps decompose citrate back into oxaloacetate • This is why it is a cycle • 3 CO2 molecules are released during the process

24. Most of the chemical energy is transferred to NAD+ and FAD during redox reactions • Reduced coenzymes = NADH and FADH2 • These transfer high-energy electrons to the electron transport chain

25. Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • Only 4 of the 38 ATP ultimately produced by respiration of glucose are produced by substrate-level phosphorylation • 2 in glycolysis, 2 in Krebs • Following glycolysis and the Krebs cycle, NADH and FADH2 account for most of the energy extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

26. The electron transport chain is a collection of molecules embedded in the cristae • Folding  increase in surface area • = space for thousands of copies of the chain • Electrons drop free energy as the pass down the electron transport chain • During this, electron carriers go between reduced and oxidized states

27. Each component of the chain becomes reduced when it accepts electrons from its ‘uphill’ neighbor • Then returns to oxidized form when it passes electrons to its ‘downhill’ neighbor

28. Electrons carried by NADH are transferred to the first molecule in the electron transport chain (a flavoprotein) • The electrons continue along the chain that includes several cytochrome proteins and one lipid carrier • Each cytochrome has a heme group • Has an iron atom that accepts and donates electrons

29. The last cytochrome of the chain passes its electrons to oxygen • Each oxygen also picks up a pair of hydrogen ions • Forms water • For every two electron carriers (4 electrons), one O2 molecules is reduced to two molecules of water

30. The electrons carried by FADH2 have lower free energy than those carried by NADH • The electron transport chain provides about 1/3 less energy for ATP synthesis when the electron donor is FADH2

31. The electron transport chain generates no ATP directly • Function: break the large free energy drop from food to oxygen into a series of smaller steps • This releases energy in manageable amounts

32. How does the mitochondrion couple electron transport and energy release to ATP synthesis? • Chemiosmosis • Involves inner membrane of mitochondria • Basic process: • NADH and FADH2 release electrons to electron transport system • Electron energy used to pump H ions into intermembrane space, forming a concentration gradient • H ions return to intermembrane compartment and energy used to produce ATP • Results in 32 – 34 ATP and water • Overall result of breaking down one glucose molecule: • 6 CO2, 6 H2O, 36-38 ATP

33. The energy stored in the H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis • The H+ gradient is referred to as a proton-motive force • Emphasizes its capacity to do work

34. During cellular respiration, most energy flows in this sequence: • Glucose  NADH  electron transport chain  proton-motive force  ATP • About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP • The rest is lost as heat • Some of that heat is used to maintain our high body temperature • Car efficiency – only about 25%

35. Some generic conversion helps • 1 NADH molecule = 3 ATP • 1 FADH2 molecule = 2 ATP • 1 glucose molecule = 36-38 ATP • Why imprecise? • Conversions are averages • NADH from glycolysis crossing into mitochondria uses different shuttles • The proton-motive force can be used to drive other types of work in the cell

36. Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen • Most cellular respiration requires O2 to produce ATP • OXIDATIVE phosphorylation – need O2 as a final electron acceptor • Glycolysis can produce ATP with or without O2 • In aerobic or anaerobic conditions • In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP

37. Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2 • Ex) sulfate • Fermentation uses phosphorylation instead of an electron transport chain to generate ATP

38. Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis • Two common types are alcohol fermentation and lactic acid fermentation

39. In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 • Alcohol fermentation by yeast is used in brewing, winemaking, and baking • 2 ethanol, 2 CO2, 2 ATP, and 2 NAD+are the products from 1 glucose

40. In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 • Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt • Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce • 1 glucose makes 2 ATP, 2 lactate, and 2 NAD+

41. Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate • The processes have different final electron acceptors: • An organic molecule (such as pyruvate) in fermentation • O2 in cellular respiration • Cellular respiration produces 38 ATP per glucose molecule • Fermentation produces 2 ATP per glucose molecule

42. Obligate anaerobes – carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 • Ex) C. botulinum • Paralyzes muscles • Botox • Yeast and many bacteria are facultative anaerobes • They can survive using either fermentation or cellular respiration

43. Glucose CYTOSOL Pyruvate No O2 present Fermentation O2 present Cellular respiration MITOCHONDRION Ethanol or lactate Acetyl CoA Citric acid cycle Figure 9.18 • In a facultative anaerobes, pyruvate is a fork in the metabolic road • Leads to two alternative catabolic routes

44. Glycolysis has evolutionary significance • It occurs in nearly all organisms • Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere