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 (O 2 ) anaerobic respiration: final electron acceptor is an inorganic molecule (not O 2 ) 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 CO 2-1 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 CO 2 -reduce 3 NAD + to 3 NADH -reduce 1 FAD (electron carrier) to FADH 2 -produce 1 ATP 15 5
16 Krebs Cycle After glycolysis, pyruvate oxidation, and the Krebs cycle, glucose has been oxidized to: - 6 CO 2-4 ATP - 10 NADH These electron carriers proceed - 2 FADH 2 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 FADH 2 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 + P i. 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 O 2 Respiration occurs without O 2 via either: 1. anaerobic respiration -use of inorganic molecules (other than O 2 ) as final electron acceptor 2. fermentation -use of organic molecules as final electron acceptor 26 Oxidation Without O 2 Anaerobic respiration by methanogens -methanogens use CO 2 -CO 2 is reduced to CH 4 (methane) Anaerobic respiration by sulfur bacteria -inorganic sulphate (SO 4 ) is reduced to hydrogen sulfide (H 2 S) 27 9
Oxidation Without O 2 Fermentation reduces organic molecules in order to regenerate NAD + 1. ethanol fermentation occurs in yeast -CO 2, 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 McGraw-Hill Companies, Inc. Permission required for reproduction or display. Alcohol Fermentation in Yeast Glucose G 2 ADP L Y C 2 ATP O L Y O S I S CH3 2 Pyruvate 2 NAD + 2 NADH CO2 H H H CH3 2 Ethanol H CH3 2 Acetaldehyde Lactic Acid Fermentation in Muscle Cells 2 ADP 2 ATP O Glucose G L Y C O L Y S I S CH3 2 Pyruvate 2 NAD + 2 NADH O H H CH3 2 Lactate 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