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					How Cells Harvest Energy Chapter 9

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Chemical Energy to Drive Metabolism
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Autotrophs harvest sunlight and convert radiant energy into chemical energy. Heterotrophs live off the energy produced by autotrophs. – extract energy from food via digestion and catabolism C6H12O6 + 6O2 ----> 6CO2 + 6 H20 + energy

Cellular Respiration

Plants and Animals
Only in Plants
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Photosynthesis

Cellular Respiration
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Cells harvest energy by breaking bonds and shifting electrons from one molecule to another. – aerobic respiration - final electron acceptor is oxygen – anaerobic respiration - final electron acceptor is inorganic molecule other than oxygen – fermentation - final electron acceptor is an organic molecule
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ATP
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Adenosine Triphosphate (ATP) is the energy currency of the cell. – used to drive movement – used to drive endergonic reactions

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ATP
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Most of the ATP produced in cells is made by the enzyme ATP synthase. – Enzyme is embedded in the membrane and provides a channel through which protons can cross the membrane down their concentration gradient.  ATP synthesis is achieved by a rotary motor driven by a gradient of protons.

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Protons move Intermembrane + H across the + space H+ H+ H membrane + H+ H+ H+ down their H conc. gradient. Rotor The energy released causes the rotor and the Rod rod structures to rotate. This Catalytic mechanical head energy is ADP + Pi converted to chemical ATP energy with the formation of H+ ATP Mitochondrial matrix
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Glucose Catabolism
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Cells catabolize organic molecules and produce ATP in two ways: – substrate-level phosphorylation - glycolysis – aerobic respiration  in most organisms, both are combined  glycolysis  pyruvate oxidation  Krebs cycle  electron transport chain
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Aerobic Respiration

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Glycolysis info – Glucose converted to two 3-C chains – Occurs in the cytoplasm – Anaerobic - no oxygen – Results in net generation of 2 ATP's – Products include :  Pyruvate (3 C) proceeds to Citric Acid Cycle  Lactic Acid (accumulates)  Every living critter capable of Glycolysis  Inefficient - net yield only 2 ATPs  Not discarded by evolution but used as starting point for energy production 9  If no O2 - Fermentation occurs

Stage One - Glycolysis
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For each molecule of glucose that passes through glycolysis, the cell nets two ATP molecules. Priming – glucose priming – cleavage and rearrangement Substrate-level phosphorylation – oxidation – ATP generation
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Priming Reactions

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Cleavage Reactions

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Energy-Harvesting Reactions

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Recycling NADH
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As long as food molecules are available to be converted into glucose, a cell can produce ATP. – Continual production creates NADH accumulation and NAD+ depletion. +  NADH must be recycled into NAD .  aerobic respiration  Oxygen as electron acceptor  Fermentation  Organic molecule
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Recycling NADH

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H H C OH O C

C O

C
C H

H C OH H C OH

ATP PO 4 C
O ADP

Glucose

Glycolysis
(Anaerobic)
C

ATP
PO 4 C O PO4 ADP

4ADP
2 X Pyruvate
C C

Fermentation +CO2
Ethyl Alcohol

4ATP
Lactic Acid

Acetyl-CoA

Citric Acid Cycle

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Stage Two - Oxidation of Pyruvate
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If oxygen present can extract more energy Within mitochondria, pyruvate is decarboxylated, yielding acetyl-CoA, NADH, and CO2.

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Stage Three - Krebs Cycle
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Acetyl-CoA is oxidized in a series of nine reactions. – two steps:  priming  energy extraction

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Krebs Cycle
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1: Condensation 2-3: Isomerization 4: First oxidation 5: Second oxidation 6: Substrate-level phosphorylation 7: Third oxidation 8-9: Regeneration and oxaloacetate

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Oxidation of pyruvate Pyruvate The cycle begins when a 2C unit from acetylCoA reacts with a 4C molecule (oxaloacetate) to produce citrate (6C). NAD+ CO2 Glucose

NADH

Glycolysis
S CoA Pyruvate oxidation

C O CH3 Acetyl-CoA (2C)

Krebs cycle
Krebs COO– CoA-SH The COO– cycle dehydrogenation of O C CH2 malate produces a Electron transport CH2 third NADH, and the HO C COO– Citrate chain cycle returns to its COO– 1 synthetase CH 2 starting point. Oxaloacetate (4C) Citrate (6C) COO– NADH COO– 2 Aconitase CH2 Malate COO – 9 dehydrogenase 3 NAD + HC COO– HO CH Isocitrate (6C) HO CH CH2 Malate (4C) COO– – COO 21

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Krebs cycle

8 Fumarase

Isocitrate 4 dehydrogenase CO2

COO–
CH HC

H2O Fumarate (4C) Succinate 7 dehydrogenase

Oxidative NAD+ decarboxylation produces NADH with the release NADH of CO2.

a-Ketoglutarate (5C) COO– COO FADH NAD + CH2 5 CO2 2 a-Ketoglutarate CoA-SH CH2 dehydrogenase 6 NADH C O FAD Succinyl-CoA CoA-SH Succinate (4C) synthetase COO– The oxidation of Succinyl-CoA (4C) COO– succinate COO– produces FADH2. A second oxidative CH2 CH2 decarboxylation GTP GDP + Pi CH2 produces a second NADH CH2 – with the release of a COO C O second CO2. S CoA ADP ATP

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Products of Glycolysis and Krebs Cycle
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Glucose consumed Produces 6-CO2 4 ATPs 12 electron carriers – 10 NADH – 2 FADH2

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Harvesting Energy by Extracting Electrons
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Glucose catabolism involves a series of oxidation-reduction reactions that release energy by repositioning electrons closer to oxygen atoms. – Energy is harvested from glucose molecules in gradual steps, using NAD+ as an electron carrier.

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Electron Transport

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Stage Four: The Electron Transport Chain
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NADH molecules carry electrons to the inner mitochondrial membrane, where they transfer electrons to a series of membraneassociated proteins.

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Electron Transport Chain

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Chemiosmosis

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Theoretical ATP Yield of Aerobic Respiration

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Regulating Aerobic Respiration
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Control of glucose catabolism occurs at two key points in the catabolic pathway. – glycolysis - phosphofructokinase – Krebs cycle - citrate synthetase

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Control of Glucose Catabolism

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Catabolism of Proteins and Fats
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Proteins are utilized by deaminating their amino acids, and then metabolizing the product. Fats are utilized by beta-oxidation.

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Cellular Extraction of Chemical Energy

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Fermentation
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Electrons that result from the glycolytic breakdown of glucose are donated to an organic molecule. + – regenerates NAD from NADH  ethanol fermentation  lactic acid fermentation

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Evolution of Cellular Respiration
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degradation glycolysis anaerobic photosynthesis oxygen-forming photosynthesis nitrogen fixation aerobic respiration

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