CHAPTER 6 How Cells Harvest Chemical Energy

1. CHAPTER 6How Cells Harvest Chemical Energy Modules 6.8 – 6.18

2. STAGES OF CELLULAR RESPIRATION AND FERMENTATION 6.8 Overview: Respiration occurs in three main stages • Cellular respiration oxidizes sugar and produces ATP in three main stages • Glycolysis occurs in the cytoplasm • The Krebs cycle and the electron transport chain occur in the mitochondria

3. High-energy electrons carried by NADH • An overview of cellular respiration GLYCOLYSIS ELECTRONTRANSPORT CHAINAND CHEMIOSMOSIS KREBSCYCLE Glucose Pyruvicacid Cytoplasmicfluid Mitochondrion Figure 6.8

4. 6.9 Glycolysis harvests chemical energy by oxidizing glucose to pyruvic acid Glucose Pyruvicacid Figure 6.9A

5. PREPARATORYPHASE(energy investment) Steps – A fuelmolecule is energized,using ATP. Glucose 1 3 Step 1 Glucose-6-phosphate 2 Fructose-6-phosphate 3 Fructose-1,6-diphosphate • Details of glycolysis Step A six-carbonintermediate splits into two three-carbon intermediates. 4 4 Glyceraldehyde-3-phosphate (G3P) ENERGY PAYOFF PHASE 5 Step A redoxreaction generatesNADH. 5 1,3-Diphosphoglyceric acid(2 molecules) 6 Steps – ATPand pyruvic acidare produced. 3-Phosphoglyceric acid(2 molecules) 6 9 7 2-Phosphoglyceric acid(2 molecules) 8 2-Phosphoglyceric acid(2 molecules) 9 Pyruvic acid (2 moleculesper glucose molecule) Figure 6.9B

6. 6.10 Pyruvic acid is chemically groomed for the Krebs cycle • Each pyruvic acid molecule is broken down to form CO2 and a two-carbon acetyl group, which enters the Krebs cycle Pyruvicacid Acetyl CoA(acetyl coenzyme A) CO2 Figure 6.10

7. 6.11 The Krebs cycle completes the oxidation of organic fuel, generating many NADH and FADH2 molecules Acetyl CoA • The Krebs cycle is a series of reactions in which enzymes strip away electrons and H+ from each acetyl group 2 KREBSCYCLE CO2 Figure 6.11A

8. 2 carbons enter cycle Oxaloaceticacid 1 Citric acid CO2 leaves cycle 5 KREBSCYCLE 2 Malicacid 4 Alpha-ketoglutaric acid 3 CO2 leaves cycle Succinicacid Step Acetyl CoA stokesthe furnace Steps and NADH, ATP, and CO2 are generatedduring redox reactions. Steps and Redox reactions generate FADH2and NADH. 1 2 3 4 5 Figure 6.11B

9. 6.12 Chemiosmosis powers most ATP production • The electrons from NADH and FADH2 travel down the electron transport chain to oxygen • Energy released by the electrons is used to pump H+ into the space between the mitochondrial membranes • In chemiosmosis, the H+ ions diffuse back through the inner membrane through ATP synthase complexes, which capture the energy to make ATP

10. Proteincomplex • Chemiosmosis in the mitochondrion Intermembranespace Electroncarrier Innermitochondrialmembrane Electronflow Mitochondrialmatrix ELECTRON TRANSPORT CHAIN ATP SYNTHASE Figure 6.12

11. 6.13 Connection: Certain poisons interrupt critical events in cellular respiration Rotenone Oligomycin Cyanide,carbon monoxide ELECTRON TRANSPORT CHAIN ATP SYNTHASE Figure 6.13

12. 6.14 Review: Each molecule of glucose yields many molecules of ATP • For each glucose molecule that enters cellular respiration, chemiosmosis produces up to 38 ATP molecules Cytoplasmic fluid Mitochondrion Electron shuttleacrossmembranes KREBSCYCLE GLYCOLYSIS 2AcetylCoA KREBSCYCLE ELECTRONTRANSPORT CHAINAND CHEMIOSMOSIS 2Pyruvicacid Glucose by substrate-levelphosphorylation used for shuttling electronsfrom NADH made in glycolysis by substrate-levelphosphorylation by chemiosmoticphosphorylation Maximum per glucose: Figure 6.14

13. 6.15 Fermentation is an anaerobic alternative to aerobic respiration • Under anaerobic conditions, many kinds of cells can use glycolysis alone to produce small amounts of ATP • But a cell must have a way of replenishing NAD+

14. This recycles NAD+ to keep glycolysis working • In alcoholic fermentation, pyruvic acid is converted to CO2 and ethanol released GLYCOLYSIS 2 Pyruvicacid 2 Ethanol Glucose Figure 6.15A Figure 6.15C

15. As in alcoholic fermentation, NAD+ is recycled • Lactic acid fermentation is used to make cheese and yogurt • In lactic acid fermentation, pyruvic acid is converted to lactic acid GLYCOLYSIS 2 Pyruvicacid 2 Lactic acid Glucose Figure 6.15B

16. INTERCONNECTIONS BETWEEN MOLECULAR BREAKDOWN AND SYNTHESIS 6.16 Cells use many kinds of organic molecules as fuel for cellular respiration • Polysaccharides can be hydrolyzed to monosaccharides and then converted to glucose for glycolysis • Proteins can be digested to amino acids, which are chemically altered and then used in the Krebs cycle • Fats are broken up and fed into glycolysis and the Krebs cycle

17. Food, such as peanuts • Pathways of molecular breakdown Polysaccharides Fats Proteins Sugars Glycerol Fatty acids Amino acids Amino groups Pyruvicacid ELECTRONTRANSPORT CHAINAND CHEMIOSMOSIS Glucose G3P AcetylCoA KREBSCYCLE GLYCOLYSIS Figure 6.16

18. 6.17 Food molecules provide raw materials for biosynthesis • In addition to energy, cells need raw materials for growth and repair • Some are obtained directly from food • Others are made from intermediates in glycolysis and the Krebs cycle • Biosynthesis consumes ATP

19. ATP needed todrive biosynthesis • Biosynthesis of macromolecules from intermediates in cellular respiration GLUCOSE SYNTHESIS KREBSCYCLE AcetylCoA Pyruvicacid G3P Glucose Aminogroups Amino acids Fatty acids Glycerol Sugars Proteins Fats Polyscaccharides Cells, tissues, organisms Figure 6.17

20. 6.18 The fuel for respiration ultimately comes from photosynthesis • All organisms have the ability to harvest energy from organic molecules • Plants, but not animals, can also make these molecules from inorganic sources by the process of photosynthesis Figure 6.18