Cellular Respiration

Cellular Respiration The set of metabolic reactions used by cells to harvest energy from food. 234 kcal/mol is trapped as ATP (34%) Occurs in a series of small steps. 686 kcal/mol of Glucose

Under Aerobic Conditions: In the presence of O2 Glycolysis The 6-carbon monosaccharide glucose is converted into two 3-carbon molecules of pyruvate. Pyruvate Oxidation Two 3-carbon molecules of pyruvate are oxidized to two 2- carbon molecules of acetyl CoA and two molecules of CO2 Citric Acid Cycle  Two 2-carbon molecules of acetyl CoA are oxidized to four molecules of CO2

Glycolysis Takes place in the cytosol Involves ten enzyme-catalyzed reactions. Releases energy as C-H bonds are oxidized. Final product = 2 molecules of Pyruvate 2 molecules of ATP 2 molecules of NADH Two Stages The initial energy-investing reactions that consume chemical energy stored in ATP. Endergonic Reaction The energy-harvesting reaction that produce ATP and NADH. Exergonic Reaction

Two Types of reactions that occur repeatedly in glycolysis Oxidation-reduction: Exergonic Reaction The energy is trapped via the reductions of NAD+ to NADH. Substrate-level phosphorylation: Less energy is released. It is enough to transfer a phosphate from the substrate to ADP, forming ATP. End product of glycolysis is 2 pyruvate molecules.

Pyruvate Oxidation This is the link between glycolysis and the Krebs Cycle. The Oxidation of pyruvate to a 2-carbon acetate molecule and CO2 The acetate is then bound to coenzyme A (CoA).

Pyruvate Oxidation The overall reaction is exergonic One molecule of NAD+ is reduced. The main role of acetyl CoA is to donate its acetyl group to the 4-carbon compound oxaloacetate, forming the 6-carbon molecule citrate. This initiates the Kreb’s cycle.

Kreb’s Cycle Citric Acid Cycle Acetyl CoA is the starting point 8 reaction pathway The completely oxidation of 2-carbon acetyl group to two molecules of CO2 The free energy released from these reactions is captured by ADP and the electron carriers NAD+ and FAD. The citric acid cycle operates twice for each glucose molecule that enters glycolysis.

Kreb’s Cycle Starting material oxaloacetate is used then regenerated in the last step. It then accepts another acetate group from Acetyl CoA Operates twice for each glucose molecule. Once fore each pyruvate molecule made. Occurs in the Mitochondria. Exergonic Reaction Harvests A LOT OF chemical energy. NAD+  NADH FAD  FADH2

NADH & FADH2 Krebs cycle produces: 8 NADH 2 FADH2 2 ATP

So why the Krebs cycle? value of NADH & FADH2 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…

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)

Mitochondria Double membrane 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

Electron Transport Chain A series of carriers that pass electrons from one to the other. As electrons pass through the protein complexes of the respiratory chain, protons are pumped from the mitochondrial matrix into the intermembrane space. As the protons return to the matrix through ATP synthase, ATP is formed.

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

Oxidative Phosphorylation Occurs in the mitochondria NADH (and FADH2) oxidation is used to actively transport protons (H+ ions) across the inner mitochondrial membrane, resulting in a proton gradient across the membrane. This drives the synthesis of ATP In prokaryotes, this occurs at the cell membrane.

Oxidative Phosphorylation NADH + H+ + ½ O2  NAD+ + H2O Does not happen in a single step. Respiratory chain Series of redox electron carrier proteins Embedded in the inner membrane Electrons pass from one carrier to the next in a process called the electron transport. Key role of O2 in cells is to act as an electron acceptor and become reduced. 2H+ + 2e- + ½ O2  H2O

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.

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

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

Chemiosmosis The movement of ions across a semipermeable barrier from a region of higher concentration to a region of lower concentration. If the concentration of a substance is greater on one side of a membrane than the other, the substance will tend to diffuse across the membrane to an area of lower concentration. If a membrane blocks this diffusion, the substance at the higher concentration has a potential energy , which can be converted to other forms of energy. Because the interior of a membrane is nonpolar, protons cannot readily diffuse across the membrane, but can cross the membrane through the ATP synthase enzyme. ATP synthase converts potential energy of the proton gradient into the chemical energy ATP.

ATP Yield of electron Transport Chain About 34 molecules of ATP are produced per fully oxidized glucose molecule.

Evolutionary Advantage Those species that could exploit O2 for Oxidative Phosphorylation had a selective advantage as the atmosphere filled with from the ancient photosynthetic microorganisms.

Pyruvate is a branching point fermentation Kreb’s cycle mitochondria

Anaerobic Conditions Absence of Oxygen Respiratory chain cannot operate. Organisms use Fermentation to reoxidize NADH, thus allowing glycolysis to continue.

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

Fermentation Occurs in the cytoplasm. Operate to regenerate NAD+ Consequence of this is that the NADH made during glycolysis is not available for reoxidation by the respiratory chain to form ATP. Overall yield is restricted to the ATP made in glycolysis.

Lactic Acid Fermentation

Alcoholic Fermentation

Cellular Respiration Summary