How Cells Harvest Energy Chapter 7 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 Cellular respiration is a series of reactions that: -are oxidations loss of electrons -are also dehydrogenations lost electrons are accompanied by hydrogen Therefore, what is actually lost is a hydrogen atom (1 electron, 1 proton). During redox reactions, electrons carry energy from one molecule to another. NAD + is an electron carrier. -NAD accepts 2 electrons and 1 proton to become NADH -the reaction is reversible 3 4 5 6 1
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 7 8 Aerobic respiration: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O G = -686kcal/mol of glucose G can be even higher than this in a cell This large amount of energy must be released in small steps rather than all at once. 9 The goal of respiration is to produce ATP. -energy is released from oxidation reaction in the form of electrons -electrons are shuttled by electron carriers (e.g. NAD + ) to an electron transport chain -electron energy is converted to ATP at the electron transport chain 10 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 11 12 2
Oxidation of Glucose The complete oxidation of glucose proceeds in stages: 1. glycolysis 2. pyruvate oxidation 3. Krebs cycle 4. electron transport chain & chemiosmosis 13 14 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 + 15 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 16 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 + 17 18 3
Pyruvate Oxidation In the presence of oxygen, pyruvate is oxidized. -occurs in the mitochondria in eukaryotes -occurs at the plasma membrane in prokaryotes -in mitochondria, a multienzyme complex called pyruvate dehydrogenase catalyzes the reaction 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. 19 20 Krebs Cycle The Krebs cycle oxidizes the acetyl group from pyruvate. -occurs in the matrix of the mitochondria -biochemical pathway of 9 steps -first step: acetyl group + oxaloacetate citrate (2 carbons) (4 carbons) (6 carbons) Krebs Cycle The remaining steps of the 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 -regenerate oxaloacetate 21 22 Krebs Cycle After glycolysis, pyruvate oxidation, and the Krebs cycle, glucose has been oxidized to: - 6 CO 2-4 ATP - 10 NADH - 2 FADH 2 These electron carriers proceed to the electron transport chain. 23 24 4
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 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. 25 26 Electron Transport Chain The higher negative charge in the matrix attracts the protons (H + ) back from the intermembrane space to the matrix. The accumulation of protons in the intermembrane space drives protons into the matrix via diffusion. 27 28 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. 29 30 5
Energy Yield of 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 31 32 Regulation of aerobic respiration is by feedback inhibition. Oxidation Without O 2 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 33 34 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) 35 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 36 6
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 -for example: alanine is converted to pyruvate aspartate is converted to oxaloacetate 37 38 Catabolism of Protein & Fat Catabolism of fats: -fats are broken down to fatty acids and glycerol -fatty acids are converted to acetyl groups by -oxidation The respiration of a 6-carbon fatty acid yields 20% more energy than glucose. 39 40 Evolution of Metabolism A hypothetical timeline for the evolution of metabolism: 1. ability to store chemical energy in ATP 2. evolution of glycolysis 3. anaerobic photosynthesis (using H 2 S) 4. use of H 2 O in photosynthesis (not H 2 S) 5. evolution of nitrogen fixation 6. aerobic respiration evolved most recently 41 42 7