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CELLULAR RESPIRATION
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A.) Life is Work To perform their many tasks, living cells require energy from outside sources. Energy enters most ecosystems as sunlight and leaves as heat Photosynthesis generates oxygen and glucose that the mitochondria of eukaryotes use as fuel for cellular respiration
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Light Energy Heat Energy Photosynthesis In Chloroplasts CO2 + H2O
Glucose + O2 Cellular Respiration In Mitochondria ATP Powers most cellular work Heat Energy
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Cells harvest the chemical energy stored in
organic molecules (glucose) to regenerate ATP, the molecule that drives most cellular work. ***Remember*** Cells cannot use the energy from glucose directly. Why? The breakdown of glucose is exergonic, having a free-energy Release of -686 kcal/mole. Much too powerful for cellular work. 4. Cellular Respiration has 3 key pathways: GLYCOLYSIS - (splitting of glucose) CITRIC ACID CYCLE - (Kreb’s Cycle) OXIDATIVE PHOSPHORYLATION - (Electron Transport Chain) & (Chemiosmosis)
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Glycolysis Glycolysis Glucose Pyruvate Cytosol Mitochondrion ATP
Substrate-level phosphorylation
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Citric Acid Cycle Glycolysis Citric acid cycle Glucose Pyruvate
Cytosol Mitochondrion ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation
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Oxidative Phosphorylation
Electrons Carried via NADH Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Glucose Pyruvate Cytosol Mitochondrion ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation
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B.) It’s All About REDOX!! The loss of electrons is oxidation The gain of electrons is reduction 2. The reducing agent donates electrons (is oxidized) The oxidizing agent accepts electrons (is reduced) Oxygen has a high electronegativity. Oxygen is a STRONG OXIDIZING AGENT When electrons are relocated closer to oxygen, chemical energy is released that can do work In Cellular Respiration, glucose is oxidized in a series of steps, each catalyzed by a specific enzyme In many oxidation reactions, the electron is transferred with a proton as a hydrogen atom
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Reactants Products Becomes oxidized CO2 + Energy + 2H2O CH4 + 2O2
Becomes reduced Methane Reducing Agent Oxygen Oxidizing Agent Carbon Dioxide Water
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DO NOW: What type of energy is used in cells and what is the ultimate source of this energy? 2. Explain how the process of cellular respiration and photosynthesis are linked. 3. Write the overall reaction for photosynthesis: Write the overall reaction for cellular respiration:
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C.) Glycolysis - Pathway #1
During glycolysis, glucose, a six carbon sugar, is split into two three-carbon sugars. These smaller sugars are oxidized and rearranged to form 2 molecules of pyruvate, the ionized form of pyruvic acid Each of the 10 steps of glycolysis is catalyzed by a specific enzyme.
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Energy Investment Phase The cell invests 2 ATP
to provide activation energy by phosphorylating glucose
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Energy Payoff Phase 4 ATP are produced by substate-level
phosphorylation (net gain of 2 ATP!) 2 NAD+ are reduced to NADH by electrons released by the oxidation of glucose
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4. What is “NAD+” you ask?? NAD+ is a coenzyme (nicotinamide adenine dinucleotide) a derivitive from the vitamin niacin. NAD+ is an electron receptor (oxidizing agent) during respiration. When NAD+ is reduced to NADH, it acts as a carrier molecule. (carries electrons to Electron Transport Chain) NAD C4H6O5 NADH + H+ + C4H4O5
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5. What is “substate-level phosphorylation” you
ask?? A process in which an enzyme converts ADP to ATP with a phosphate group from an organic molecule instead of an inorganic phosphate (Pi) **The substrate is an organic molecule generated from the catabolism of glucose**
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Glycolysis occurs in the cytoplasm
6. Summary of Glycolysis: 2 ATP + Glucose 2 Pyruvate + 4 ATP Net Gain 2 ATP** Also, 2 NADH produced from oxidizing glucose Glycolysis occurs in the cytoplasm
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watch citric acid cycle videos
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D.) The Citric Acid Cycle - Pathway #2
Glycolysis releases less than a quarter of the chemical energy stored in glucose. Most of the energy remains stockpiled in the 2 pyruvates If molecular O2 is present, pyruvate enters the mitochondrion via active transport. Pyruvate is oxidized further into acetyl coenzyme A (Acetyl CoA) NOW WE ARE READY FOR THE CITRIC ACID CYCLE! Discovered by Hans Krebs 1930’s
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Pyruvate Enters the Mitochondrion
CYTOSOL MITOCHONDRION NADH + H+ NAD+ active transport Pyruvate Acetyl CoA CO2 Coenzyme A Acetyl CoA Enters the Citric Acid Cycle
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Remember** 2 Acetyl CoA molecules enter the
Citric Acid Cycle because there were 2 pyruvates from the original Glucose that was split during Glycolysis** The Citric Acid Cycle is an 8 step metabolic pathway each step catayzed by a specific enzyme. 6. The Acetyl CoA is oxidized during the 8 reactions 7. Each turn of the Citric Acid Cycle yields: 3 NADH 1 FADH2 (another electron acceptor) (flavin adenine dinucleotide) 1 ATP (via substrate-level phosphorylation) 2 CO2 How many times does the cycle turn per glucose? TWICE
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Occurs in the mitochondrion Acetyl CoA enters the Cycle Yield per turn: 3 NADH 1 FADH2 1 ATP 2 CO2 It’s oxidized… and oxidized… and oxidized… and oxidized..
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E.) Oxidative Phosphorylation - Pathway #3
So far, glycolysis and the citric acid cycle produce only 4 ATP molecules per glucose. All 4 ATP produced by Substrate-level phosphorylation At this point, most of the energy is extracted from glucose is contained in NADH and FADH2 Both NADH and FADH2 go into pathway #3 Oxidative phosphorylation 3. Oxidative phoshorylation can be broken up into 2 parts: Electron Transport Chain and Chemiosmosis Oxidative Phosphorylation occurs in the inner membranes of the mitochondrion (cristae - folds of membranes that increase the surface area inside the mitochondrion)
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watch oxidative phophorylation videos
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Electron Transport Chain Chemiosmosis
Inside the mitochondrion H+ H+ Intermembrane space H+ H+ Cyt c Q IV I III Inner Mitochondrial membrane II FADH2 FAD+ 2H+ + 1/2 O2 H2O NADH + H+ NAD+ ADP + Pi ATP H+ Mitochondral Matrix Electron Transport Chain Chemiosmosis
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4. The Electron Chain - What is it??
The Electron Transport Chain is an array of proteins built into the membranes of the cristae. Most of these proteins are cytochromes (proteins with a prosthetic group - iron) NADH gives up its 2 high energy electrons to the first protein in the complex. The electrons are passed to the next protein in the chain, giving up some of their potential energy. As the electrons pass from protein to protein, they cause the membrane to become permeable to H+. A high concentration of H+ in the intermembrane space results. Oxygen catches the electrons at the very last step in the chain. The last complex adds 2 H+ to the Oxygen forming H2O The high concentration of H+ in the intermembrane space acts like water behind a dam.
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5. Chemiosmosis and ATP Synthesis
The backed up H+ start moving through a special protein in the membrane called ATP SYNTHASE As the H+ diffuse down their concentration gradient, ATP SYNTHASE captures their energy and uses is to convert ADP molecules to ATP molecules. This mode of ATP synthesis is called oxidative phosphorylation because its powered by the transfer of electrons to oxygen. d) Oxidative Phosphorylation produces about 34 ATP No, it is not an EXACT number of ATP WHY??????
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The ratio of NADH to number of ATP produced is NOT a
a whole number. 1 NADH = ATP The NADH produced in the cytosol lose electrons when the are transported through the mitochondrial membrane. It depends on the “shuttle” that brings them through. (see diagram) Some of the proton motive force in the mitochondrion may be used to drive other work. So, oxidative phosphorylation produces between ATP
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H+ ATP Synthase H+ H+ H+ H+ H+ H+ H+ ATP ADP + Pi
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Glycolysis 2 Oxidative Citric Acid Cycle 2ATP Via substrate-level
2 NADH 2 FADH2 2 NADH 6 NADH 2 FADH2 2 NADH Oxidative Phosphorylation Electron Transport and Chemiosmosis Glycolysis 2 Acetyl CoA Citric Acid Cycle Glucose Pyruvate 2ATP Via substrate-level phosphorylation 2ATP Via substrate-level phosphorylation About 32 or 34 ATP depending on which shuttle Transports electrons from NADH in cytosol About 36-38 ATP
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H+ ATP Synthase ATP + Pi ATP H+ Chemiosmosis
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F.) Fermentation Fermentation enables some cells to produce ATP without the help of oxygen. (Anaerobic Respiration) Fermentation consists of Glycolysis + reactions that regenerate NAD+. Remember ** NAD+ is needed to oxidize glucose, especially If oxygen is NOT present** (anaerobic respiration) 3. There are 2 basic types of Fermentation: Alcoholic Fermentation Pyruvate is converted into ethanol Lactic Acid Fermentation Pyruvate is reduced into lactic acid
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Alcohol Fermentation 2 ATP 2 ADP + 2 Pi Glucose Glycolysis 2 Pyruvate
2 NAD+ 2 NADH CO2 + 2 H+ 2 Ethanol 2 Acetylaldehyde Alcohol Fermentation
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2 ATP Lactic Acid Fermentation 2 ADP + 2 Pi Glucose Glycolysis 2 NAD+
2 NADH + 2 H+ 2 Pyruvate 2 Lactate Lactic Acid Fermentation
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Alcohol Fermentation is carried out by many bacteria under
anaerobic conditions. Yeast (a fungus) also carries out alcohol fermentation. Lactic Acid Fermentation is carried out by certain fungi and bacteria and used in the dairy industry to make cheese and yogurt. Human muscle cells make ATP by lactic acid fermentation when oxygen is scarce. This occurs during the early stages of strenuous exercise, when sugar catabolism for ATP production outpaces the muscle supply of oxygen from blood. The lactic acid that accumulates may cause muscle fatigue and pain. OUCH! NO PAIN, NO GAIN In case you were wondering……. The lactic acid is gradually carry away by the blood to The liver where it is converted back to pyruvate
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Glucose Glycolysis Cytosol Pyruvate No O2 present O2 present
Mitochondrion Ethanol or Lactic Acid Acetyl CoA Citric Acid Cycle
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G.) Other “FUELS” Besides Glucose (other catabolic pathways)
Proteins Carbohydrates Fats hydrolysis hydrolysis hydrolysis Amino Acids sugars Fatty Acids Glycerol Glycolysis Glucose Glyceraldehyde-3 P NH3 Pyruvate Acetyl CoA Citric Acid Cycle Oxidative Phophorylation
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