Cellular Respiration Stage 4: Electron Transport Chain Modified from Kim Foglia 2006-2007

Cellular respiration

What’s the point? ATP The Point is to Make ATP! 2006-2007

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

There is a better way! O2 Electron Transport Chain series of molecules built into inner mitochondrial membrane mostly transport proteins transport of electrons down ETC linked to ATP synthesis yields ~32-34 ATP from 1 glucose! only in presence of O2 (aerobic) O2

Mitochondria Matrix Double membrane central fluid-filled space outer membrane inner membrane highly folded cristae fluid-filled space between membranes = intermembrane space

Electron Transport Chain Inner mitochondrial membrane Intermembrane space C Q NADH dehydrogenase Cytochrome bc complex Cytochrome oxidase complex Mitochondrial matrix

Time to break open the bank! Remember the NADH? Krebs cycle Glycolysis G3P 8 NADH 2 FADH2 4 NADH Time to break open the bank!

Electron Transport Chain What powers the proton pumps?? Inner mitochondrial membrane Intermembrane space H+ H+ H+ H  e- + H+ C Q e– e– e– FADH2 H FAD 1 2 NADH 2H+ + O2 NAD+ H2O NADH dehydrogenase Cytochrome bc complex Cytochrome oxidase complex Mitochondrial matrix

Electron Transport Chain 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+ across inner membrane to intermembrane space NAD+ Q C NADH H2O H+ e– 2H+ + O2 FADH2 1 2 NADH dehydrogenase bc1 complex Cytochrome oxidase complex FAD H+ Oxidation refers to the loss of electrons to any electron acceptor, not just to oxygen. Uses exergonic flow of electrons through ETC to pump H+ across membrane. ADP + Pi ATP

O2 is 2 oxygen atoms both looking for electrons But what “pulls” the electrons down the ETC? Pumping H+ across membrane … what is energy to fuel that? Can’t be ATP! that would cost you what you want to make! Its like cutting off your leg to buy a new pair of shoes. :-( Flow of electrons powers pumping of H+ O2 is 2 oxygen atoms both looking for electrons oxidative phosphorylation

Electrons flow downhill Electrons move in steps from carrier to carrier downhill to O2 each carrier more electronegative controlled oxidation controlled release of energy Electrons move from molecule to molecule until they combine with O & H ions to form H2O It’s like pumping water behind a dam -- if released, it can do work

“proton-motive” force We did it! H+ ADP + Pi Set up a H+ gradient Allow the protons to flow through ATP synthase Synthesizes ATP ADP + Pi  ATP ATP

Chemiosmosis links the Electron Transport Chain to ATP synthesis 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 Chemiosmosis is the diffusion of ions across a membrane. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane. Hydrogen ions (protons) will diffuse from an area of high proton concentration to an area of lower proton concentration. Peter Mitchell proposed that an electrochemical concentration gradient of protons across a membrane could be harnessed to make ATP. He likened this process to osmosis, the diffusion of water across a membrane, which is why it is called chemiosmosis.

Peter Mitchell 1961 | 1978 Proposed chemiosmotic hypothesis revolutionary idea at the time proton motive force 1920-1992

Pyruvate from cytoplasm synthase Krebs cycle Mitochondrial matrix + Q C 32 ATP 2 Pyruvate from cytoplasm Electron transport system synthase H2O CO2 Krebs cycle Intermembrane space Inner mitochondrial membrane 1. Electrons are harvested and carried to the transport system. 2. Electrons provide energy to pump protons across the membrane. 3. Oxygen joins with protons to form water. 2H+ NADH Acetyl-CoA FADH2 4. Protons diffuse back in down their concentration gradient, driving the synthesis of ATP. Mitochondrial matrix 1 e-

38 ATP Cellular respiration + + + 2 ATP ~2 ATP 2 ATP ~32-34 ATP

Summary of cellular respiration C6H12O6 6O2 6CO2 6H2O ~38 ATP  + 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? Where did the glucose come from? from food eaten Where did the O2 come from? breathed in Where did the CO2 come from? oxidized carbons cleaved off of the sugars Where did the CO2 go? exhaled Where did the H2O come from? from O2 after it accepts electrons in ETC Where did the ATP come from? mostly from ETC What else is produced that is not listed in this equation? NAD, FAD, heat!

Taking it beyond… What is the final electron acceptor in electron transport chain? O2 So what happens if O2 unavailable? ETC backs up ATP production ceases cells run out of energy and you die! What if you have a chemical that punches holes in the inner mitochondrial membrane?

What’s the point? ATP The Point is to Make ATP! 2006-2007

Summary 2006-2007