Chapter 7 Cellular Pathways that Harvest Chemical Energy

1. Chapter 7 Cellular Pathways that Harvest Chemical Energy Biology 101 Tri-County Technical College Pendleton, SC

2. Principles of Metabolic Pathways • Complex chemical transformations in cell occur in number of small steps connected in a pathway • Each reaction catalyzed by specific enzyme • Pathways similar in all organisms • Some pathways compartmentalized with certain steps occurring inside organelle • Pathways regulated by activities of a few enzymes

3. Pass the glucose…please? • Glucose MOST common fuel for living cells • Many other compounds serve as food but almost all converted to glucose or intermediate compounds • Cells obtain energy from glucose by process called oxidation • If glucose burned in flame, it forms carbon dioxide and water and lots of energy

4. Glucose, cont. • Happens ONLY if oxygen gas is present • C6H12O6 + 6O2 6CO2 + 6H2O + energy • Energy in the form of heat and light • Same equations applies to metabolism of glucose in cells except is multi-step controlled series of reactions • Captures about 40% of energy in form of ATP • Complete oxidation of glucose = -686 kcal/mol • Some cells, unable to obtain or use oxygen, metabolize glucose incompletely and obtain LESS ATP per glucose

5. Metabolic Pathways • Glycolysis = splitting of sugar and begins metabolism of glucose in ALL cells • Universal pathway • Produces 2 three-carbon compounds called pyruvate (pyruvic acid) • Small amount of ATP and NADH produced • NADH is electron carrier (NAD+) • Cellular respiration occurs when environment is aerobic (contains oxygen gas)

6. Pathways, cont. • Converts pyruvate into carbon dioxide • Includes “preparatory Krebs,” Krebs, and ETC • Great deal of energy stored in pyruvate is released and trapped in ATP • Fermentation occurs when environment is anaerboic (lacking oxygen gas) • Instead of energy poor CO2 relatively energy rich molecules (lactic acid/ethanol) are produced • Energy harvest MUCH less

7. Pathways III • True fermenter MUST exist on only 2 ATP per glucose • Even see a 160 lb. yeast cell? • Estes will make a distinction here between aerobic and anaerobic respiration • We will leave the fermenters alone…or will we?

8. Redox Reactions • Reaction in which one substance transfers one or more electrons to another substance is called oxidation-reduction reaction • Redox for short • Gain or 1 or more electrons by atom, ion, or molecule is called reduction • Loss of 1 or more electrons by atom, ion, or molecule is called oxidation

9. Redox, cont. • Defined in terms of electrons, but can be thought of as gain/loss of hydrogen atoms • Reducing agents and oxidizing agents • You will not get this by osmosis…give it some thought • In redox reactions, energy is transferred • Some key reactions in glycolysis and cellular respiration are highly exergonic redox reactions

10. Redox Visual

11. Ones to Remember • Main pair of oxidizing and reducing agents in cells is based on NAD (nicotinamide adenine dinucleotide) • NAD+ is oxidized form and NADH is reduced form • FAD is another important electron carrier • Redox reactions important in glycolysis, PreKrebs, Krebs, and ETC

12. Coenzymes • Defined as nonprotein molecule that plays role in catalysis by an enzyme • May be part of enzyme molecule or free in solution • Some coenzymes are oxidizing and reducing agents • NAD+/NADH; FAD/FADH2; NADP+/NADPH

13. Metabolic Pathways • When O2 available as FEA, four pathways operate • Glycolysis, Pyruvate oxidation (preKrebs), Krebs (citric acid cycle), and respiratory chain (ETC) • When O2 unavailable, pyruvate oxidation, citric acid cycle, and respiratory chain do NOT function, and fermentation is added to the pathway • NOT QUITE TRUE…ONE MORE TIME WITH FEEING….

14. Pathways Visual

15. Pathways Visual II

16. Glycolysis • glucose2 pyruvate + 2 ATP (net) + 2 NADH • Known as Universal Pathway • Occurs in cytoplasm of cell and does NOT require the presence of O2 • Has energy investment stage (2 ATP) and energy payback stage • Chalk talk time on glycolysis; we will begin with concept of 3 kinds of phosphorylation

17. Oxidation of Pyruvate • 2 pyruvic acid produced by glycolysis are in cytoplasm of cell • In eukaryotes, they must be moved into the mitochondrion • Enzyme complex for pyruvate oxidation attached to inner mitochondrion membrane (cristae) • Oxidized to acetyl group and CO2 released • Part of energy from this step saved by reduction of NAD+ to NADH + H+

18. Pyruvate Oxidation, cont. • Some of remaining energy stored temporarily by combining acetyl group with CoA • Oxidation of pyruvate to acetate is link between glycolysis and cellular respiration • Pyruvate cannot enter Krebs cycle • Chalk talk time on preparatory Krebs (oxidation of pyruvate)

19. Krebs Cycle • Krebs occurs in mitochondrial matrix • Starts with acetyl-CoA • Chalk talk time on Krebs (citric acid cycle) • Inputs are acetate, water, and oxidized electron carriers (NAD+ and FAD) • Outputs are carbon dixoide, reduced electrons carriers, and ATP (yield of 2 per substrate level phosphorylation)

20. Krebs Visual

21. Respiratory Chain (ETC) • ETC located in inner mitochondrial membrane (cristae) • Series of electron carriers located there • Series of redox reactions provide energy to actively pump H+s across inner membrane • Critical thinking time on the three “things” that result from ETC • Eventually, H+s diffuse back across membrane through ATP synthase (ATPase)

22. Respiratory Chain, cont. • Diffusion of H+s back across membrane through ATPase coupled with synthesis of ATP (oxidative phosphorylation) • Several Key Questions: • What does redox reactions of ETC accomplish? • What actually synthesizes the ATP? • Process actually called “chemiosmosis”

23. Chemiosmosis and ATP • Each NADH (on average) traded for 3 ATP • Each FADH2 traded for 2 ATP • Chalk talk time on chemiosmosis and critical thinking on why the “trades” • Mitochondrion has 2 membranes (outer and inner) • This creates intramembranous space essential for establishment of both [ ] for H+s and for increasing membane potential across inner • Called Proton Motive Force

24. Chemiosmosis, cont. • Inner membrane highly folded to > surface area and provide more room for ETC • Each contains three large protein complexes with carrier molecules and associated enzymes • Have small protein called cytochrome c and nonprotein component called ubiquinone (Q) • Why are preparatory Krebs and Krebs located within the matrix

25. Jethroe and Ciphering • Glycolysis: 2 ATP (net) and 2 NADH • Preparatory Krebs: 2 NADH • Krebs Cycle: 2 ATP, 6 NADH, and 2 FADH2 • Totals (net): 4 ATP, 10 NADH, and 2 FADH2 • Swapping ECs yields 32-34 ATP so total is 36-38….why?

26. To ferment…or not!! • If O2 supply to aerobic respiring cell is cut off and cell has no other metabolic pathway available to itauf Wiedersehen • Many cells can switch to fermentation • With fermentation, ATP comes solely from glycolysis (2 ATP net per glucose) • Review: glycolysis produces 2 pyruvate, 2 ATP (net) and 2 NADH

27. Fermentation, cont. • NAD+ is required for glycolysis and is reduced to NADH • Fermenting cell has to regenerate NAD+ or glycolysis will SHUT down • Catch 22 to be sure—what to do with all that pyruvate that is essentially “worthless” • What a plan—use the pyruvate to regenerate NAD+

28. Fermentation III • Using pyruvate as oxidizing agent for NADH results in changing it into something else • Some organisms choose to ferment even in presence of oxygen • Some do anaerobic respiration by using inorganic molecule other than oxygen as FEA • nitrate to nitrite; sulfate to hydrogen sulfide; carbonate to methane

29. Why Fermentation? • Allows glycolysis to produce small but sustained amount of ATP • Glycolysis rate >s 10X to produce needed ATP • Allows some cells lacking oxygen to perform needed functions • “Aerobic” cells cannot ferment indefinitely

30. Products of Fermentation • Lactic acid fermentation reduces pyruvate to lactate (lactic acid) • Human muscle cells can ferment but NOT neurons or cardiac muscle cells • Is what makes one’s muscles sore • Eventually, lactate converted back to pyruvate and “burned” in process of paying “oxygen debt”

31. Products, cont. • Certain yeasts and plant cells perform alcoholic fermentation • Requires two enzymesone removes a carbon dioxide from pyruvate leaving acetaldehyde • Acetaldehyde reduced by NADH producing NAD+ and ethyl alcohol • Where would be be without beer and bread? • Note that some microorganism produce wide variety of acids and end products other than two above

32. In for a penny…in for a pound • Aerobic respiration can produce 36-38 ATP per glucose • Anaerobic respiration can produce an equivalent amount • Fermentation produces only 2 ATP (net) per glucose • How does one “burn” lipids, proteins, and nucleic acids?