Energy Generation in Mitochondria: The Process of Oxidative Phosphorylation

1. Chapter 13 &14 Energy Generation in Mitochondria

2. Generation of Energy • Millions of years ago there was no O2 available for oxidative phosphorylation to occur • Organisms produced energy from fermentation, still see this today • As O2 became available, a more efficient method of energy production developed • Based on the transfer of e- along the membrane

3. Glycolysis – sugar breaking rxn • Each step of rxns mediated by specific enzymes • ATP input necessary to get process started • Glycolysis occurs in the cytosol – anaerobic (no oxygen necessary)

4. Organisms Energy Source • Small amount of ATP from glycolysis in the cytosol of cells • Majority made by a membrane based process in 2 stages • Stage 1 – e- transport chain • e-transferred along e- carriers in the membrane • Stage 2 – flow of H+ down an electrochemical gradient to produce ATP • Use a complex called ATP synthase

5. Stage 1 • NADH (from the Kreb’s cycle) brings in the e- and transfers them to the carrier molecules • The e- moves down the chain and looses energy at each step – as this happens, H+ are pumped across the membrane • This creates an electro-chemical gradient across the membrane

6. Stage 2 • The electrochemical gradient is a form of stored energy – it has the potential to do work • The H+ can now move down the gradient and return to the other side of the membrane thru ATP synthase – in this process, generates ATP from ADP and Pi

7. Chemiosmotic Coupling • Once called the chemiosmotic hypothesis • Chemi from making ATP, osmotic because of crossing the membrane • Now known as chemiosmotic coupling

8. Proton Pumping • Many molecules can supply the e- - carbohydrates and fatty acids • O2 ultimate e- acceptor producing H2O as waste

9. Movement of Electrons

10. Mitochondria • Produce most of a cells ATP – acetyl groups in the Kreb’s cycle producing CO2 and NADH • NADH donates the e- to the electron transport chain and becomes oxidized to NAD+ • e- transfer promotes proton pump and ATP synthesis in process called oxidative phosphorylation • Cells that require large amounts of energy such as the heart have large numbers of mitochondria

11. Oxidative Phosphorylation

12. Mitochondria • Contain their own copies of DNA and RNA along with transcription and translation system (ribosomes) • Are able to regenerate themselves without the whole cell undergoing division • Shape and size dependent on what the cell’s function is

13. Mitochondria • Double membrane creates 2 spaces • Matrix – large internal space • Intermembrane space – between the membranes • Outer membrane • Inner membrane

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15. Inner Membrane • Inner membrane is the site of the e- transport chain, across which the proton pump occurs and contains ATP synthase • Inner membrane is highly folded – called cristae – increasing the surface area on which the above reactions can take place

16. High Energy e- • Mitochondria use pyruvate and fatty acids and convert it to acetyl CoA in the matrix • Citric acid cycle generates NADH and FADH2 which carry the e- to the electron transport chain

17. Summary – MUST KNOW

18. Location of H+

19. 4 Complexes in Membrane

20. Electron Transport Chain • Resides in the inner mitochondrial membrane – also called respiratory chain • 15 proteins involved in the chain – grouped in 3 large respiratory enzyme complexes • NADH dehydrogenase complex • Cytochrome b-c1 complex • Cytochrome oxidase complex • Pumps protons across the membrane as e- are transferred thru them

21. Respiratory Enzyme Complexes

22. Proton Gradient • e- transfer is an oxidation/reduction reaction • NADH has high-energy e- has a low electron affinity so the e- is readily passed to NADH dehydrogenase and so on down the chain • Each transfer couples the energy released with the uptake of a H+ from the matrix to the intermembrane space setting up the electrochemical gradient

23. Proton Gradient • Gradient of proton (H+) concentration across the inner mitochondrial membrane – a pH gradient with the pH in the matrix higher than in the intermembrane space • Proton pumping also generates a membrane potential – matrix side is negative and intermembrane space is positive

24. Electrochemical Gradient

25. Oxidative Phosphorylation • ATP synthase is the protein complex responsible for making ATP by creating a path for H+ thru the membrane • ATP synthase is an enzyme

26. ATP Synthase • Multisubunit protein responsible for making ATP

27. Summary

28. Bidirectional Pump

29. Coupled Transport Can Move Other Molecules

30. Oxidation of Sugar and Fats

31. Protons In H2O • H+ can move along the H-bonds in H2O • Dissociating from one molecule to associate with the next one

32. Transfer of H+ and e- • The transfer of an e- sets up a negative charge which is rapidly neutralized by adding a H+, the molecule is reduced • Reverse is true when things are oxidized

33. e-Transport Chain

34. Redox Potential • Redox = oxidation-reduction reactions • Depends on the affinity for electrons of the molecules involved in each reaction • Redox pairs – two molecules such as NADH and NAD+ - NADH is a strong electron donor (oxidizing agent) while NAD+ is a weak electron acceptor (reducing agent) • Redox Potential – a measure of the tendency of a given system to donate or accept electrons

35. Versatile Electron Carriers • The respiratory complexes are made up of a metal ion bound to a protein molecule • The metal ion is responsible for the movement of the e-, skipping from one ion to another • Ubiquinone, a hydrophobic molecule, that can move electrons without being bound to a protein

36. Quinone Electron Carriers • Can carry either 1 or 2 e- and picks up 1 H+ for each e-

37. Other e- Carriers • Dehydrogenation complex • Flavin group • Iron-sulfur centers – carry 1 e- at a time • Cytochrome b-c1 and cytochrome oxidase complexes • Proteins that contain a heme group that can accept an e- • Cytochromes are colored due to the Fe