Chapter 9. Cellular Respiration STAGE 1: Glycolysis

glucose      pyruvate Glycolysis Breaking down glucose “glyco – lysis” (splitting sugar) most ancient form of energy capture starting point for all cellular respiration inefficient generate only 2 ATP for every 1 glucose in cytosol why does that make evolutionary sense? glucose      pyruvate 6C 2x 3C Life on Earth first evolved without free oxygen (O2) in atmosphere energy had to be captured from organic molecules in absence of O2 Organisms that evolved glycolysis are ancestors of all modern life all organisms still utilize glycolysis

Glycolysis summary invest some ATP harvest a little more ATP & a little NADH Glucose is a stable molecule it needs an activation energy to break it apart. phosphorylate it = Pi comes from ATP. make NADH & put it in the bank for later. P is transferred from PEP to ADP kinase enzyme ADP  ATP

How is NADH recycled to NAD+? Another molecule must accept H from NADH aerobic respiration ethanol fermentation lactic acid fermentation NADH

Anaerobic ethanol fermentation Bacteria, yeast 1C 3C 2C pyruvate  ethanol + CO2 NADH NAD+ beer, wine, bread at ~12% ethanol, kills yeast Animals, some fungi Count the carbons!! Lactic acid is not a dead end like ethanol. Once you have O2 again, lactate is converted back to pyruvate by the liver and fed to the Kreb’s cycle. pyruvate  lactic acid 3C NADH NAD+ cheese, yogurt, anaerobic exercise (no O2) 2005-2006

Pyruvate is a branching point fermentation Kreb’s cycle mitochondria

Pyruvate oxidized to Acetyl CoA reduction Release CO2 because completely oxidized…already released all energy it can release … no longer valuable to cell…. Because what’s the point? The Point is to make ATP!!! oxidation Yield = 2C sugar + CO2 + NADH

reduction of electron carriers Count the carbons & electron carriers! pyruvate 3C 2C acetyl CoA citrate 4C 6C NADH x2 4C 6C This happens twice for each glucose molecule CO2 reduction of electron carriers NADH 5C 4C CO2 FADH2 4C 4C NADH ATP

What’s so important about NADH? NADH & FADH2 Krebs cycle produces: 8 NADH 2 FADH2 2 ATP Let’s go to ETC… What’s so important about NADH? 2005-2006

So why the Krebs cycle? If the yield is only 2 ATP, then why? value of NADH & FADH2 electron carriers reduced molecules store energy! to be used in the Electron Transport Chain

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! Why stop here… There’s got to be more to life than this.

Last stop and most important! Electron Transport Chain series of molecules built into inner mitochondrial membrane mostly transport (integral) proteins transport of electrons down ETC linked to ATP synthesis yields ~34 ATP from 1 glucose! only in presence of O2 (aerobic) That sounds more like it!

Don’t forget the Mito! Double membrane * form fits function! outer membrane inner membrane (ETC here!) highly folded cristae* fluid-filled space between membranes = intermembrane space Matrix (Kreb’s here!) central fluid-filled space * form fits function!

Electron Transport Chain

Remember the NADH? Kreb’s cycle Glycolysis 8 NADH 2 FADH2 2 NADH PGAL 2005-2006

Electron Transport Chain or Chemiosmosis NADH passes electrons to ETC H cleaved off NADH & FADH2 electrons stripped from H atoms  H+ (H ions) electrons passed from one electron carrier to next in mitochondrial membrane (ETC) transport proteins in membrane pump H+ across inner membrane to intermembrane space 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.

electrons flow downhill to O2 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 electrons flow downhill to O2

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 2005-2006

Why the build up H+? ATP synthase ADP + Pi  ATP enzyme in inner membrane of mitochondria ADP + Pi  ATP only channel permeable to H+ H+ flow down concentration gradient = provides energy for ATP synthesis molecular power generator! flow like water over water wheel flowing H+ cause change in shape of ATP synthase enzyme powers bonding of Pi to ADP “proton-motive” force 2005-2006

Cellular respiration 2005-2006

Metabolism Coordination of digestion & synthesis Digestion by regulating enzyme Digestion digestion of carbohydrates, fats & proteins all catabolized through same pathways enter at different points cell extracts energy from every source CO2 2005-2006

Summary of cellular respiration C6H12O6 6O2 6CO2 6H2O ~36 ATP  + Where did the glucose come from? Where did the O2 come from? Where did the CO2 come from? 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 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?