Cellular Respiration: Harvesting Chemical Energy

1. Cellular Respiration: Harvesting Chemical Energy

2. The Metabolic Pathway of Cellular Respiration • Cellular respiration is an example of a metabolic pathway • A series of chemical reactions in cells • All of the reactions involved in cellular respiration can be grouped into three main stages • Glycolysis • The Krebs cycle • Electron transport

3. A Road Map for Cellular Respiration Cytosol Mitochondrion High-energy electrons carried mainly by NADH High-energy electrons carried by NADH Glycolysis Krebs Cycle 2 Pyruvic acid Electron Transport Glucose Figure 6.7

4. Stage 1: Glycolysis • A molecule of glucose is split into two molecules of pyruvic acid • These molecules then donate high energy electrons to NAD+, forming NADH • Glycolysis makes some ATP directly when enzymes transfer phosphate groups from fuel molecules to ADP • Glycolysis occurs in the cytosol of the cell.

5. 2 Pyruvic acid Glucose PGAL Figure 6.8

6. If oxygen is available, pyruvic acid enters the pathways of aerobic respiration (respiration with oxygen)

7. 2 Acetic acid Pyruvic acid Acetyl-CoA (acetyl-coenzyme A) Coenzyme A CO2 Stage 2: The Krebs Cycle • The Krebs cycle completes the breakdown of sugar • Occurs inside of the mitochondria (mitochondrial matrix) • In the Krebs cycle, pyruvic acid from glycolysis is first “prepped” into a usable form, Acetyl-CoA

8. Mitochondria

9. The Krebs cycle extracts the energy of sugar by breaking the acetic acid molecules all the way down to CO2 • The cycle uses some of this energy to make ATP • The cycle also forms NADH and FADH2 Hans Krebs

10. 2 3 4 6

11. One glucose molecule causes two turns of the Krebs cycleThe two turns produce 6 NADH, 2 FADH2, 2 ATP, and 4 CO2.So now there have been 4 molecules of ATP created up to this point (remember the 2 created during glycolysis) Things to Remember

12. Stage 3: Electron Transport • Electron transport releases the energy your cells need to make the most of their ATP • The molecules of electron transport chains are built into the inner membranes of mitochondria • The chain functions as a chemical machine that uses energy released by the “fall” of electrons to pump hydrogen ions across the inner mitochondrial membrane • These ions store potential energy

13. When the hydrogen ions flow back through the membrane, they release energy • The ions flow through ATP synthase • ATP synthase takes the energy from this flow and synthesizes ATP (Chemiosmosis)

14. Protein complex Electron carrier Inner mitochondrial membrane Electron flow ATP synthase Electron transport chain Figure 6.12

15. The Versatility of Cellular Respiration • Cellular respiration can “burn” other kinds of molecules besides glucose • Diverse types of carbohydrates • Fats • Proteins

16. Food Polysaccharides Fats Proteins Sugars Glycerol Fatty acids Amino acids Amino groups Acetyl- CoA Krebs Cycle Glycolysis Electron Transport Figure 6.13

17. Adding Up the ATP from Cellular Respiration Cytosol Mitochondrion Glycolysis 2 Acetyl- CoA Krebs Cycle 2 Pyruvic acid Electron Transport Glucose Maximum per glucose: by ATP synthase by direct synthesis by direct synthesis Figure 6.14

18. FERMENTATION: ANAEROBIC HARVEST OF FOOD ENERGY • Some of your cells can actually work for short periods without oxygen • For example, muscle cells can produce ATP under anaerobic conditions • Fermentation • The anaerobic harvest of food energy

19. Fermentation in Human Muscle Cells • Human muscle cells can make ATP with and without oxygen • They have enough ATP to support activities such as quick sprinting for about 5 seconds • A secondary supply of energy (creatine phosphate) can keep muscle cells going for another 10 seconds • To keep running, your muscles must generate ATP by the anaerobic process of fermentation

20. Glycolysis is the metabolic pathway that provides ATP during fermentation • Pyruvic acid is reduced by NADH, producing NAD+, which keeps glycolysis going • In human muscle cells, lactic acid is a by-product

21. 2 ADP+ 2 Glycolysis 2 NAD 2 NAD Glucose 2 Pyruvic acid + 2 H 2 Lactic acid (a) Lactic acid fermentation Figure 6.15a

22. Fermentation in Microorganisms • Various types of microorganisms perform fermentation • Yeast cells carry out a slightly different type of fermentation pathway • This pathway produces CO2 and ethyl alcohol

23. The food industry uses yeast to produce various food products Figure 6.16

24. Two Types of Fermentation