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How Cells Harvest Energy Chapter 7 Respiration Organisms can be classified based on how they obtain energy: autotrophs: are able to produce their own organic molecules through photosynthesis heterotrophs: live on organic compounds produced by other organisms All organisms use cellular respiration to extract energy from organic molecules. 2 Respiration During respiration, electrons are shuttled through electron carriers to a final electron acceptor. aerobic respiration: final electron receptor is oxygen (O2) anaerobic respiration: final electron acceptor is an inorganic molecule (not O2) fermentation: final electron acceptor is an organic molecule 3 1 4 Oxidation of Glucose Cells are able to make ATP via: 1. substrate-level phosphorylation – transferring a phosphate directly to ADP from another molecule 2. oxidative phosphorylation – use of ATP synthase and energy derived from a proton (H+) gradient to make ATP 5 Oxidation of Glucose The complete oxidation of glucose proceeds in stages: 1. glycolysis 2. pyruvate oxidation 3. Krebs cycle 4. electron transport chain & chemiosmosis 6 2 7 Glycolysis Glycolysis converts glucose to pyruvate. -a 10-step biochemical pathway -occurs in the cytoplasm -2 molecules of pyruvate are formed -net production of 2 ATP molecules by substrate-level phosphorylation -2 NADH produced by the reduction of NAD+ 8 Glycolysis For glycolysis to continue, NADH must be recycled to NAD+ by either: 1. aerobic respiration – occurs when oxygen is available as the final electron acceptor 2. fermentation – occurs when oxygen is not available; an organic molecule is the final electron acceptor 9 3 Glycolysis The fate of pyruvate depends on oxygen availability. When oxygen is present, pyruvate is oxidized to acetyl-CoA which enters the Krebs cycle Without oxygen, pyruvate is reduced in order to oxidize NADH back to NAD+ 10 11 Pyruvate Oxidation In the presence of oxygen, pyruvate is oxidized. -occurs in the mitochondria in eukaryotes -occurs at the plasma membrane in prokaryotes 12 4 Pyruvate Oxidation The products of pyruvate oxidation include: -1 CO2 -1 NADH -1 acetyl-CoA which consists of 2 carbons from pyruvate attached to coenzyme A Acetyl-CoA proceeds to the Krebs cycle. 13 Krebs Cycle The Krebs cycle oxidizes the acetyl group from pyruvate. -occurs in the matrix of the mitochondria -biochemical pathway of 9 steps 14 Krebs Cycle -release 2 molecules of CO2 -reduce 3 NAD+ to 3 NADH -reduce 1 FAD (electron carrier) to FADH2 -produce 1 ATP 15 5 16 Krebs Cycle After glycolysis, pyruvate oxidation, and the Krebs cycle, glucose has been oxidized to: - 6 CO2 - 4 ATP - 10 NADH - 2 FADH2 These electron carriers proceed to the electron transport chain. 17 Electron Transport Chain The electron transport chain (ETC) is a series of membrane-bound electron carriers. -embedded in the mitochondrial inner membrane -electrons from NADH and FADH2 are transferred to complexes of the ETC -each complex transfers the electrons to the next complex in the chain 18 6 Electron Transport Chain As the electrons are transferred, some electron energy is lost with each transfer. This energy is used to pump protons (H+) across the membrane from the matrix to the inner membrane space. A proton gradient is established. 19 20 Electron Transport Chain Most protons move back to the matrix through ATP synthase. ATP synthase is a membrane-bound enzyme that uses the energy of the proton gradient to synthesize ATP from ADP + Pi. 21 7 22 23 Energy Yield of Respiration theoretical energy yields - 38 ATP per glucose for bacteria - 36 ATP per glucose for eukaryotes actual energy yield - 30 ATP per glucose for eukaryotes - reduced yield is due to “leaky” inner membrane and use of the proton gradient for purposes other than ATP synthesis 24 8 25 Oxidation Without O2 Respiration occurs without O2 via either: 1. anaerobic respiration -use of inorganic molecules (other than O2) as final electron acceptor 2. fermentation -use of organic molecules as final electron acceptor 26 Oxidation Without O2 Anaerobic respiration by methanogens -methanogens use CO2 -CO2 is reduced to CH4 (methane) Anaerobic respiration by sulfur bacteria -inorganic sulphate (SO4) is reduced to hydrogen sulfide (H2S) 27 9 Oxidation Without O2 Fermentation reduces organic molecules in order to regenerate NAD+ 1. ethanol fermentation occurs in yeast -CO2, ethanol, and NAD+ are produced 2. lactic acid fermentation -occurs in animal cells (especially muscles) -electrons are transferred from NADH to pyruvate to produce lactic acid 28 Copyright © The M cG raw -Hill Com panies, Inc. Perm ission required for reproduction or display. Alcohol Ferm entation in Yeast 2 ADP 2 ATP O– C C Glucose G L Y C O L Y S I O S O CH3 H H C OH 2 NAD+ CH3 2 Ethanol 2 NADH H CO2 2 Pyruvate C O CH3 2 Acetaldehyde Lactic Acid Ferm entation in Muscle Cells Glucose 2 ADP 2 ATP O– C O C O CH3 G L Y C O L Y S I S O– C O H C OH 2 NAD+ CH3 2 Lactate 2 NADH 2 Pyruvate 29 Catabolism of Protein & Fat Catabolism of proteins: -amino acids undergo deamination to remove the amino group -remainder of the amino acid is converted to a molecule that enters glycolysis or the Krebs cycle 30 10 Catabolism of Protein & Fat Catabolism of fats: -fats are broken down to fatty acids and glycerol -fatty acids are converted to acetyl groups by b-oxidation 31 32 11