Electron Transport Chain & Chemiosmosis

1. Electron Transport Chain & Chemiosmosis The final stage of cellular respiration:

2. ATP accounting so far… • Glycolysis 2ATP • Kreb’s cycle 2ATP • Life takes a lot of energy to run, need to extract more energy than 4 ATP! There’s got to be a better way! What’s the point? A working muscle recycles over 10 million ATPs per second

3. O2 The Essence of ETC: • Electron Transport Chain • series of molecules built into inner mitochondrial membrane • along cristae • Consists of transport proteins& enzymes • transport of electrons down ETC linked to pumping of H+ to create H+ gradient* • yields ~36 ATP from 1 glucose! • only in presence of O2 (aerobic respiration)

4. The Electron Transport Chain • NADH and FADH2 molecules derived from glycolysis and the Kreb’s Cycle each contain electrons they gained from their formation • NADH (or FADH2) molecules carry these electrons to the innermembrane of the mitochondrion and electron transfer begins • *NAD+ and FAD = electron carriers – takes H’s (and their e-) to electron transport chain!

5. Remember the Electron Carriers? glucose Krebs cycle Glycolysis G3P 8 NADH 2 FADH2 4 NADH Time tobreak openthe bank!

6. Innermitochondrialmembrane Electron Transport Chain Intermembrane space C Q cytochromebc complex cytochrome coxidase complex NADH dehydrogenase Mitochondrial matrix

7. Electron Transport Chain (ETC) • A series of electron acceptors (proteins) are embedded in the cristae. • These proteins are arranged in order of increasing electronegativity. • The weakest attractor of electrons (NADH dehydrogenase) is at the start of the chain and the strongest (cytochromeoxidase) is at the end.

8. ETC & Redox • These electrons are passed through a series of electron carriers, one step at a time. As the electrons are passed along, one substance is oxidized, the other is reduced. (REDOX rxns) • What is the “driving force”?

9. ETC & Redox • As electrons move through the chain, a series of redox reactions occur. • This causes the electrons to have more stable positions relative to the nuclei of the atoms they associate with and so they release free energy. • This energy is used to pump protons (H+ ions) from the matrix into the intermembrane space.

10. ETC & Redox • This creates an electrochemical gradient, creating potential difference (voltage) similar to a battery. • At the end of the chain, the electrons are so stable that only oxygen is strong enough to oxidize the last protein complex. • Oxygen strips 2 electrons from the final protein complex and forms water with 2 protons from the matrix.

11. Electrons & Protons (H+) NADH passes electrons to ETC H cleaved off NADH & FADH2 electrons stripped from H atoms  H+ (protons) electrons passed from one electron carrier to next in mitochondrial membrane (ETC) transport proteins in membrane pump H+ (protons) across inner membrane to intermembrane space H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ C e– Q e– 1 2 e– FADH2 FAD NADH 2H+ + O2 H2O NAD+ cytochromebc complex NADH dehydrogenase cytochrome coxidase complex TA-DA!! Moving electronsdo the work! ADP+ Pi ATP

12. The ETC Structure • The ETC consists of: • 4 protein complexes • Associated cofactors (located in the protein complexes) • 2 mobile electron carriers

13. The ETC Structure Protein Complexes • Complex I: NADH dehydrogenase complex • Complex II:(really long name)(basically second entry point for electrons stripped from the molecule succinate & FADH2)

14. The ETC Structure • Complex III Cytochromebc complex • Complex IV Cytomechrome c oxidase complex

15. The ETC Structure Mobile Electron Carriers • Ubiquinone/Coenzyme Q - transfers electrons from Complex I and II to Complex III • Cytochrome C - Transfers electrons from Complex III to Complex IV

16. cytochromebc complex NADH dehydrogenase

18. ATP Synthase • Protons enter the matrix through proton channels associated with ATP synthase (ATPase). • For every H+ that passes through, enough free energy is released to create 1 ATP from the phosphorylation of ADP. • Conditions must be aerobic because oxygen acts as the final electron and H+ acceptor (water is formed as a byproduct).

20. Chemiosmosis & Oxidative Phosphorylation • Protons begin to accumulate in the intermembrane space which creates an electrochemical gradient that stores free energy known as a proton-motive force (PMF). This gradient has 2 components: • an electrical component caused by a higher positive charge • a chemical component caused by a higher concentration of protons

21. Chemiosmosis • The diffusion of ions across a membrane • build up of proton gradient just so H+ could flow through ATP synthase enzyme to build ATP Chemiosmosis links the Electron Transport Chain to ATP synthesis

22. Oxidative Phosphorylation • These protons are unable to diffuse through the phospholipidbilayer and are forced to pass through special proton channels called ATP synthase (ATPase). • When protons move through ATPase, the free energy drives the synthesis of ATP from ADP and Pi found in the matrix (oxidative phosphorylation). • ATP is then transported out into the cytoplasm by facilitated diffusion and can be used to drive endergonic processes such as movement and active transport.

23. Review!! 1. Glycolysis: occurs in the cytoplasm: anaerobic 2. Pyruvate enters the mitochondrion, converts to Acetyl-CoA 3. Kreb’s Cycle: Acetyl-CoA broken down into CO2… drives production of NADH, FADH2 and ATP 4. Electron Transport Chain: NADH powers the electron transport system that chemiosmotically produces ATP

24. Note on NADH & FADH • NADH and FADH2 enter the ETC at different locations thus releasing different amounts of free energy (NADH pumps 3 protons into intermembrane space, FADH2 only pumps 2 protons) • NADH produced in glycolysis cannot pass through the inner mitochondrial membrane into the matrix. 2 shuttle systems pass electrons from cytosolic NADH to the matrix. This results in a smaller production of ATP

25. ATP Balance Sheet

26. ATP Yield • The theoretic yield of ATP per glucose molecule is about 36. However, it turns out that the actual yield is only 30 realistically. (Due to the fact that NADH produces an average of 2.5, not 3 and FAD produces an average of 1.5, not 2). • Efficiency of energy conversion for aerobic respiration is much higher than the 2.2% of glycolysis. It is 32% efficient for aerobic. This allows multicellular organisms to exist. (compare to a car engine that is about 25% efficient!)

27.  C6H12O6 + 6CO2 + 6H2O + ~36 ATP 6O2 Summary of cellular respiration • Where did the glucose come from? • Where did the O2 come from? • Where did the CO2 come from? • Where did the CO2 go? • Where did the H2O come from? • Where did the ATP come from? • What else is produced that is not listed in this equation? • Why do we breathe?

28. Taking it beyond… • What is the final electron acceptor in Electron Transport Chain? O2 • So what happens if O2 unavailable? • ETC backs up • nothing to pull electrons down chain • NADH & FADH2 can’t unload H • ATP production ceases • cells run out of energy • and you die!