How Cells Release Chemical Energy

1. How Cells Release Chemical Energy Chapter 7

2. Producing the Universal Currency of Life All energy-releasing pathways • require characteristic starting materials • yield predictable products and by-products • produce ATP

3. ATP Is Universal Energy Source • Photosynthesizers get energy from the sun • Animals get energy second- or third-hand from plants or other organisms • Regardless, the energy is converted to the chemical bond energy of ATP

4. Making ATP • Plants make ATP during photosynthesis • Cells of all (including plants) organisms make ATP by breaking down carbohydrates, fats, and protein

5. Aerobic pathways Evolved later Require oxygen Start with glycolysis in cytoplasm Completed in mitochondria Anaerobic pathways Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm Main Types of Energy-Releasing Pathways

6. Energy-Releasing Pathways

7. Overview of Aerobic Respiration C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water dioxide

8. Main Pathways Start with Glycolysis • Glycolysis occurs in cytoplasm • Reactions are catalyzed by enzymes • ALL LIFEFORMS ON PLANET EARTH Glucose 2 Pyruvate (six carbons) (three carbons)

9. The Role of Coenzymes • NAD+ and FAD accept electrons and hydrogen from intermediates during the first two stages • When reduced, they are NADH and FADH2 • In the third stage, these coenzymes deliver the electrons and hydrogen to the transfer chain (3rd stage)

10. Overview of Aerobic Respiration

11. Glucose • A simple sugar (C6H12O6) • Atoms held together by covalent bonds

12. Glycolysis Occurs in Two Stages • Energy-requiring steps (supply energy) • ATP energy activates glucose and its six-carbon derivatives • Energy-releasing steps (gain energy) • The products of the first part are split into three-carbon pyruvate molecules • ATP and NADH form

13. Energy-Requiring Steps

14. Energy-Releasing Steps

15. Net Energy Yield from Glycolysis Energy requiring steps: 2 ATP invested (supply energy) Energy releasing steps: 2 NADH formed 4 ATP formed (gain energy) Net yield is 2 ATP and 2 NADH

16. Second-Stage Reactions(Stage 2) • Occur in the mitochondria • Pyruvate is broken down to carbon dioxide and water • More ATP is formed • More coenzymes are reduced - carry high energy

17. Two Parts of Second Stage • Preparatory reactions (Stage 2a) • Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide • NAD+ is reduced • Krebs cycle (Stage 2b) • The acetyl units are oxidized to carbon dioxide • NAD+ and FAD are reduced

18. Preparatory Reactions pyruvate + coenzyme A + NAD+ acetyl-CoA + NADH + CO2 • One of the carbons from pyruvate is released in CO2 • Two carbons are attached to coenzyme A and continue on to the Krebs cycle

19. Overall Products - resulting material Coenzyme A 2 CO2 3 NADH FADH2 ATP Overall Reactants - Starting material Acetyl-CoA 3 NAD+ FAD ADP and Pi The Krebs Cycle (Citric acid cycle)

20. Results of the Second Stage • All of the carbon molecules in pyruvate end up in carbon dioxide • Coenzymes are reduced (they pick up electrons and hydrogen) • One molecule of ATP is formed

21. Coenzyme Reductions during First Two Stages • Glycolysis (stage 1) 2 NADH • Preparatory reactions (stage 2a) 2 NADH • Krebs cycle 2 FADH2 + 6 NADH (stage 2b) • Total 2 FADH2 + 10 NADH

22. Electron Transfer Phosphorylation (chain) • Occurs in the mitochondria • Coenzymes deliver electrons to electron transfer chains • Electron transfer sets up H+ ion gradients • Flow of H+ down gradients powers ATP formation (from between membranes to inner matrix of mitochondria)

23. Second Stage of Aerobic Respiration

24. Electron Transfer Phosphorylation • Electron transfer chains are embedded in inner mitochondrial compartment • NADH and FADH2 give up electrons that they picked up in earlier stages to electron transfer chain • Electrons are transferred through the chain • The final electron acceptor is oxygen

25. Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT

26. ATP Formation-as the H+ rush back into the mitochondrial matrix they spin a protein that makes lots of ATP ATP INNER COMPARTMENT ADP+Pi

27. Summary of Transfers

28. Importance of Oxygen • Electron transfer phosphorylation (chain) requires the presence of oxygen • Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water

29. Summary of Energy Harvest(per molecule of glucose) • Glycolysis • 2 ATP formed by substrate-level phosphorylation • Krebs cycle and preparatory reactions • 2 ATP formed by substrate-level phosphorylation • Electron transfer phosphorylation • 32 ATP formed

30. Energy Harvest from Coenzyme Reductions • What are the sources of electrons used to generate the 32 ATP in the final stage? • 4 ATP - generated using electrons released during glycolysis and carried by NADH • 28 ATP - generated using electrons formed during second-stage reactions and carried by NADH and FADH2

31. Energy Harvest Varies • NADH formed in cytoplasm cannot enter mitochondrion • It delivers electrons to mitochondrial membrane • Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion • Electrons given to FAD yield less ATP than those given to NAD+

32. Energy Harvest Varies Liver, kidney, heart cells • Electrons from first-stage reactions are delivered to NAD+ in mitochondria • Total energy harvest is 38 ATP Skeletal muscle and brain cells • Electrons from first-stage reactions are delivered to FAD in mitochondria • Total energy harvest is 36 ATP

33. Efficiency of Aerobic Respiration • 686 kcal of energy are released per glucose molecule breakdown to CO2 and water. • 7.5 kcal are conserved in each ATP • When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP • Efficiency is 270 / 686 X 100 = 39 percent • Most energy (over 60%) is lost as heat

34. Anaerobic Pathways • Do not use oxygen • Produce less ATP than aerobic pathways • Two types of fermentation pathways • Alcoholic fermentation • Lactate fermentation

35. Fermentation Pathways • Begin with glycolysis • Do not break glucose down completely to carbon dioxide and water • Yield only the 2 ATP from glycolysis (stage 1) • Steps that follow glycolysis serve only to regenerate NAD+

36. Alcoholic Fermentation

37. Yeasts • Single-celled fungi • Carry out alcoholic fermentation • Saccharomyces cerevisiae • Baker’s yeast • Carbon dioxide makes bread dough rise • Saccharomyces ellipsoideus • Used to make beer and wine

38. Lactate Fermentation • Carried out by certain bacteria • Electron transfer chain is in bacterial plasma membrane • Final electron acceptor is compound from environment (such as nitrate), not oxygen • ATP yield is low

39. Lactate Fermentation

40. Carbohydrate Breakdown and Storage • Glucose is absorbed into blood • Pancreas releases insulin (a hormone) • Insulin stimulates glucose uptake by cells • Cells convert glucose to glucose-6-phosphate • This traps glucose in cytoplasm where it can be used for glycolysis

41. Making Glycogen • If glucose intake is high, ATP-making machinery goes into high gear • When ATP levels rise high enough, glucose-6-phosphate is diverted into glycogen synthesis (mainly in liver and muscle) • Glycogen is the main storage polysaccharide in animals

42. Using Glycogen • When blood levels of glucose decline, pancreas releases glucagon (a hormone) • Glucagon stimulates liver cells to convert glycogen back to glucose and to release it to the blood • (Muscle cells do not release their stored glycogen) -selfish ba**ards!

43. Energy Reserves • Glycogen makes up only about 1 percent of the body’s energy reserves • Proteins make up 21 percent of energy reserves • Fat makes up the bulk of reserves (78 percent)

44. Energy from Fats • Most stored fats are triglycerides (glycerol head linked to three fatty acid tails) - like a three-legged octupus!!! • Triglycerides are broken down to glycerol and fatty acids • Glycerol is converted to PGAL, an intermediate of glycolysis • Fatty acids are broken down and converted to acetyl-CoA, which enters Krebs cycle

45. Energy from Proteins • Proteins are broken down to amino acids • Amino acids are broken apart • Amino group is removed, ammonia forms, is converted to urea and excreted by cells • Carbon backbones can enter the Krebs cycle or its preparatory reactions

46. Reaction Sites

47. Evolution of Metabolic Pathways • When life originated, atmosphere had little oxygen • Earliest organisms used anaerobic pathways • Later, noncyclic pathway of photosynthesis increased atmospheric oxygen • Cells arose that used oxygen as final acceptor in electron transfer

48. Aerobic Respiration Reactants Sugar Oxygen Products Carbon dioxide Water Photosynthesis Reactants Carbon dioxide Water Products Sugar Oxygen Processes Are Linked

49. Life Is System of Prolonging Order • Powered by energy inputs from sun, life continues onward through reproduction • Following instructions in DNA, energy and materials can be organized, generation after generation • With death, molecules are released and may be cycled as raw material for next generation