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Transcript
How Cells Harvest Chemical Energy Chapter 6 Cellular Respiration Energy Flow and Chemical Cycling in the Biosphere Fuel molecules in food represent solar energy traced back to the sun Animals depend on plants: to convert solar energy to chemical energy In form of sugars and other organic molecules Gas Exchange in the Body Cellular respiration and breathing are closely related Cellular respiration requires a cell to exchange gases with its surroundings Breathing exchanges these gases between blood and outside air Cellular respiration Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to ATP – – produces 38 ATP molecules from each glucose molecule Other foods (organic molecules) can be used as a source of energy as well Cellular Respiration Release of energy from molecules accompanied by the use of this energy to synthesize ATP molecules Metabolic pathway Main method that chemical energy is harvested from food and converted to ATP Aerobic Requires oxygen and gives off carbon dioxide Where Is the Energy in Food? The process of aerobic respiration requires oxygen and carbohydrates C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy The products are carbon dioxide, water, and energy (heat or ATP) How do we get the energy?? Energy contained in the arrangement of electrons in chemical bonds in organic molecules Cells tap energy from electrons “falling” from organic fuels to oxygen When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen – Oxygen has a strong tendency to attract electrons ATP Adenosine triphosphate (ATP) Nucleotide with the base adenine and the sugar ribose Main energy carrier in cells Formed during reactions that breakdown organic compounds to CO2 and water Requires ample oxygen Occurs within the mitochondrion Hydrolyzes phosphates to release energy form adenosine diphosphate (ADP) Redox Reaction (O-R) Chemical reaction that transfers electrons from one substance to another electrons retain their potential energy – Glucose loses its hydrogen atoms and is ultimately converted to CO2 – O2 gains hydrogen atoms and is converted to H2O Oxidation Reduction Redox Reaction Electrons pass from atoms or molecules to one another as part of many energy reactions Oxidation When an atom or molecule loses an electron Glucose is oxidized Reduction When an atom or molecule gains an elections Oxygen is reduced Other important players…… Enzymes are necessary to oxidize glucose and other foods – Dehydrogenase – – enzyme that removes hydrogen from an organic molecule requires a coenzyme called NAD+ – – – – (nicotinamide adenine dinucleotide) shuttle electrons NAD+ can become reduced when it accepts electrons and oxidized when it gives them up Reduced to NADH The Finale…. First step is transfer of electrons from organic molecule to NAD+ Other electron carrier molecules represent the electron transport chain Undergoes series of redox reactions Release energy to make ATP NADH NAD+ + ATP 2e– Controlled release of energy for synthesis of ATP H+ 2e– H+ H 2O 1 2 O2 Stages of Cellular Respiration: 1. 2. 3. Glycolysis Citric Acid Cycle Oxidative Phosphorylation NADH Mitochondrion High-energy electrons carried by NADH NADH FADH2 and OXIDATIVE GLYCOLYSIS Glucose PHOSPHORYLATION (Electron Transport and Chemiosmosis) CITRIC ACID CYCLE Pyruvate Cytoplasm Inner mitochondrial membrane CO2 CO2 ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation ATP Oxidative phosphorylation 1. Glycolysis Occurs in the cytoplasm Does not require oxygen to generate ATP Then, enters aerobic or anaerobic reactions Glucose Glycolysis 6 Carbon oxidizes glucose into 2 ADP + 2 molecules Pyruvate 3 Carbon breaking of the bond yields energy that is used to phosphorylate ADP to ATP in addition, electrons and hydrogen are donated to NAD+ to form NADH 2 NAD+ 2 P 2 NADH 2 ATP + 2 H+ 2 Pyruvate Enzyme Enzyme P ADP + P Substrate P Product ATP Anaerobic verse aerobic? Absence of oxygen Fermentation Make lactate or ethanol Presence of oxygen Oxidative respiration Pyruvate transported to mitochondria Oxidize pyruvate to form acetyl-coA When pyruvate is oxidized: A single carbon cleaved off by the enzyme pyruvate dehydrogenase This carbon leaves as part of a CO2 molecule hydrogen and electrons are removed from pyruvate donated to NAD+ to form NADH Remaining two-carbon fragment of pyruvate is joined to a cofactor called coenzyme A (CoA) Final compound called acetyl-CoA Acetyl-CoA The fate of acetyl-CoA depends on the availability of ATP in the cell Insufficient ATP The acetyl-CoA heads to the Krebs cycle Plentiful ATP The acetyl-CoA is diverted to fat synthesis for energy storage 2. Citric Acid Cycle “Krebs Cycle” occurs within the mitochondrion Breaks down pyruvate into carbon dioxide electrons passed to an electron transport chain in order to power the production of ATP Stages of Citric Acid Cycle acetyl (two-carbon) compound enters the citric acid cycle Acetyl-CoA enters the cycle and binds to a four-carbon molecule, forming a six-carbon molecule Two carbons are removed as CO2 and their electrons donated to NAD+ 1. 2. 3. In addition, an ATP is produced The four-carbon molecule is recycled and more electrons are extracted, forming NADH and FADH2 Acetyl CoA CoA CoA The Krebs cycle CITRIC ACID CYCLE Note: a single glucose molecule produces two turns of the cycle, one for each of the two pyruvate molecules generated by glycolysis 2 CO2 3 NAD+ FADH2 3 NADH FAD 3 H+ ATP ADP + P Cytoplasm Electron shuttle across membrane 2 NADH Mitochondrion 2 NADH (or 2 FADH2) 6 NADH 2 NADH GLYCOLYSIS Glucose 2 Pyruvate 2 Acetyl CoA CITRIC ACID CYCLE + 2 ATP by substrate-level phosphorylation + 2 ATP by substrate-level phosphorylation Maximum per glucose: About 38 ATP 2 FADH2 OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) + about 34 ATP by oxidative phosphorylation 3. Oxidative Phosphorylation Electron transport chain Shuttle molecules NADH and FADH take electrons to oxygen Final acceptor Forms H2O Carriers bind and release electrons in redox reactions Pass electrons down the “energy staircase” Use energy released from the transfers to transport H+ Intermembrane space Protein complex of electron carriers H+ H+ H+ H+ H+ H+ H+ Electron carrier H+ H+ ATP synthase Inner mitochondrial membrane FADH2 Electron flow NADH Mitochondrial matrix FAD NAD+ H+ 1 2 O2 + 2 H+ H+ H+ H2O Electron Transport Chain OXIDATIVE PHOSPHORYLATION ADP + P ATP H+ Chemiosmosis 3. Oxidative Phosphorylation Chemiosmosis Uses energy stored in a hydrogen ion gradient to drive ATP synthesis H+ concentration gradient stores potential energy ATP synthase drives hydrogen ions through Generates ATP Cytoplasm Electron shuttle across membrane 2 NADH Mitochondrion 2 NADH (or 2 FADH2) 6 NADH 2 NADH GLYCOLYSIS Glucose 2 Pyruvate 2 Acetyl CoA CITRIC ACID CYCLE + 2 ATP by substrate-level phosphorylation + 2 ATP by substrate-level phosphorylation Maximum per glucose: About 38 ATP 2 FADH2 OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) + about 34 ATP by oxidative phosphorylation Fermentation Occurs when O2 is not available Animal cells and bacteria convert pyruvate to lactate Other organisms convert pyruvate to alcohol and CO2 Glucose Is Not the Only Food Molecule Cells also get energy from foods other than sugars The other organic building blocks undergo chemical modifications that permit them to enter cellular respiration Food, such as peanuts Carbohydrates Fats Glycerol Sugars Proteins Fatty acids Amino acids Amino groups Glucose G3P GLYCOLYSIS Pyruvate Acetyl CoA ATP CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) DO PLANTS PERFORM CELLULAR RESPIRATION??