How Cells Harvest Chemical Energy Chapter 6 How Cells Harvest Chemical Energy

INTRODUCTION TO CELLULAR RESPIRATION 6.1 Photosynthesis and cellular respiration provide energy for life All living organisms require energy to maintain homeostasis, to move, and to reproduce Photosynthesis converts energy from the sun to glucose and O2 Cellular respiration breaks down glucose and releases energy in ATP Energy flows through an ecosystem; chemicals are recycled

Photosynthesis in chloroplasts Cellular respiration in mitochondria Figure 6.1 Sunlight energy ECOSYSTEM Photosynthesis in chloroplasts CO2 Glucose H2O O2 Cellular respiration in mitochondria (for cellular work) ATP Heat energy

6.2 Breathing supplies oxygen to our cells and removes carbon dioxide 6.2 Breathing supplies oxygen to our cells and removes carbon dioxide Breathing and cellular respiration are closely related Breathing brings O2 into the body from the environment O2 is distributed to cells in the bloodstream In cellular respiration, mitochondria use O2 to harvest energy and generate ATP Breathing disposes of the CO2 produced as a waste product of cellular respiration

Muscle cells carrying out Figure 6.2 Breathing O2 CO2 Lungs CO2 Bloodstream O2 Muscle cells carrying out Cellular Respiration Glucose  O2 CO2  H2O  ATP

6.3 Cellular respiration banks energy in ATP molecules 6.3 Cellular respiration banks energy in ATP molecules Cellular respiration : the aerobic harvesting of energy from food molecules by cells The process uses O2 and releases CO2 and H2O Food molecules -polysaccharide -protein -lipid Cellular respiration Monosaccharide Amino acid Fatty acid, glycerol +O2CO2+H2O ATPs

The most common fuel for living cells is the sugar glucose LE 6-3 The most common fuel for living cells is the sugar glucose C6H12O6 6 O2 6 CO2 6 H2O ATP Glucose Oxygen Carbon dioxide Water  Heat

6.4 The human body uses energy from ATP for all its activities CONNECTION 6.4 The human body uses energy from ATP for all its activities The body needs a continual supply of energy to maintain basic functioning In addition, ATP supplies energy (kilocalories) for voluntary activities An average adult human needs about 2,200 kcal of energy each day

kcal consumed per hour by a 67.5-kg (150-lb) person* Figure 6.4 Activity kcal consumed per hour by a 67.5-kg (150-lb) person* Running (8–9 mph) 979 Dancing (fast) 510 Bicycling (10 mph) 490 Swimming (2 mph) 408 Walking (4 mph) 341 Walking (3 mph) 245 Dancing (slow) 204 Driving a car 61 Sitting (writing) 28 *Not including kcal needed for body maintenance

Oxidation and reduction reactions (Redox reactions) Lose electrons Gain electrons Lose hydrogen atoms Gain hydrogen atoms Gain oxygen atoms Lose oxygen atoms Oxidation is an energy-releasing process AH2 A + 2H Reduction is an energy-required process A + 2H AH2 Energy

Oxidation/reduction occurs in one of four different ways ① Direct electron transfer Fe2+ + Cu2+ ↔ Fe3+ + Cu+ ② Hydrogen atom transfer: H = H+ + e- AH2 + B ↔ A + BH2 (dehydrogenation) catalyzed by a dehydrogenase ③ Hydride ion transfer: :H- = H+ + 2e- AH2 + NAD+ ↔ A + NADH + H+ (dehydrogenation) catalyzed by a NAD+-linked dehydrogenase ④ Direct combination with oxygen R-CH3 + 1/2O2  R-CH2-OH (oxygenation) catalyzed by oxygenase

Reduction potential이 작으면 작을수록 - 전자를 줄 수 있는 능력이 강하고 Reduction potential : the ability to give electrons to another molecule Reduction potential이 작으면 작을수록 - 전자를 줄 수 있는 능력이 강하고 - 전자를 받을 수 있는 능력은 약하다 -0.5 AH2/A BH2/B CH2/C 2e- Energy Reduction potential 2e- Energy +0.5

AH2 + B  A + BH2 Oxidation and reduction are coupled Reductant (reducing agent): electron-donating molecule, e.g., AH2 Oxidant (oxidizing agent): electron accepting molecule, e.g., B Conjugate redox pair: AH2-A, BH2-B If the reduction potential of AH2/A pair < that of BH2/B, The reaction is exergonic reaction (energy-releasing reaction) If the reduction potential of AH2/A pair > that of BH2/B, The reaction is endergonic reaction

6.5 Cells tap energy from electrons “falling” from organic fuels to oxygen Loss of hydrogen atoms (becomes oxidized) C6H12O6 6 O2 6 CO2 6 H2O ATP Glucose  Heat Gain of hydrogen atoms (becomes reduced)

C6H12O6 6CO2 NAD+ or FAD NADH or FADH2 6H2O 6O2 NAD (nicotinamide adenine dinucleotide) NAD+ + 2H  NADH + H+ (2H = 2e- + 2H+) NAD+ accepts 2 electrons and 1 proton (H-, hydride ion) NAD+ is a hydride ion carrier FAD (flavin adenine dinucleotide) FAD + 2H  FADH2 Hydogen carrier

Nicotinamide adenine dinucleotide Nicotinamide ring Nicotinamide adenine dinucleotide

Becomes oxidized 2H Becomes reduced NAD 2H NADH H Figure 6.5B Becomes oxidized 2H Becomes reduced NAD 2H NADH H (carries 2 electrons) 2 H 2

NADH passes electrons to an electron transport chain As electrons “fall” from carrier to carrier and finally to O2, energy is released in small quantities The energy released is used by the cell to make ATP

Controlled release of energy for synthesis of ATP H Figure 6.5C NADH NAD ATP 2 Controlled release of energy for synthesis of ATP H Electron transport chain 2 2 1 2 H O2 H2O

Electron transport chain Two mechanisms generate ATP 1. Chemiosmotic phosphorylation Cells use the energy released by “falling” electrons to pump H+ ions across a membrane The energy of the gradient is harnessed to make ATP by the process of chemiosmosis High H+ concentration ATP synthase uses gradient energy to make ATP Membrane Electron transport chain ATP synthase Energy from Low H+ concentration By Peter Mitchell

Organic molecule (substrate) New organic molecule (product) 2. Substrate level phosphorylation ATP can also be made by transferring phosphate groups from organic molecules to ADP Enzyme Adenosine Organic molecule (substrate) This process is called substrate-level phosphorylation Adenosine New organic molecule (product) Figure 6.7B

STAGES OF CELLULAR RESPIRATION AND FERMENTATION 4 stages of cellular respiration Glycolysis : C6H12O6  2 pyruvate : 2NADH, 2ATP Pyruvate oxidation : 2 pyruvate  2 acetyl-CoA : 2NADH Krebs cycle (TCA cycle) : 2 acetyl-CoA  4 CO2 6 NADH, 2FADH2, 2 ATP Oxidative phosphorylation 1/2O2 + 2H+ NADH or FADH2 H2O 2e- 3ATP or 2ATP

High-energy electrons NADH High-energy electrons carried by NADH NADH FADH2 and GLYCOLYSIS OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) CITRIC ACID CYCLE Glucose Pyruvate Cytoplasm Mitochondrion CO2 CO2 ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

6.7 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis Glucose+2NAD++2ADP+Pi 2pyruvate+2NADH+2H++2ATP+2H2O ATP is produced by substrate-level phosphorylation In cytoplasm

Figure 6.7A Glucose 2 ADP 2 NAD 2 P 2 NADH 2 ATP 2 H 2 Pyruvate

Dihydroxyacetone phosphate

6.8 Pyruvate is chemically groomed for the citric acid cycle A large, multienzyme complex catalyzes three reactions in the mitochondrial matrix A carbon atom is removed from pyruvate and released in CO2 The remaining two-carbon compound is oxidized, and a molecule of NAD+ is reduced to NADH Coenzyme A joins with the 2-carbon group to produce acetyl CoA

Pyruvate + CoA + NAD+  Acetyl-CoA + NADH + H+ + CO2 LE 6-8 Pyruvate oxidation Pyruvate + CoA + NAD+  Acetyl-CoA + NADH + H+ + CO2 In mitochondrial matrix NAD NADH H 2 CoA Pyruvate 1 Acetyl coenzyme A 3 CO2 Coenzyme A

6.9 The citric acid cycle completes the oxidation of organic fuel, generating many NADH and FADH2 molecules In mitochondrial matrix The role of TCA cycle Synthesis of NADH, FADH2, ATP Supply precussors for many biosynthetic pathways

Acetyl CoA Citric Acid Cycle CoA CoA 2 CO2 3 NAD FADH2 FAD 3 NADH Figure 6.9A Acetyl CoA CoA CoA 2 CO2 Citric Acid Cycle 3 NAD FADH2 FAD 3 NADH 3 H ATP ADP P

Substrate-level phosphorylation LE 6-9b CoA Acetyl CoA CoA 2 carbons enter cycle Oxaloacetate Citrate NADH +H+ leaves cycle CO2 NAD+ NAD+ CITRIC ACID CYCLE Malate Substrate-level phosphorylation NADH + H+ ADP + P FADH2 ATP Alpha-ketoglutarate FAD leaves cycle CO2 Succinate NADH +H+ NAD+

6.10 Most ATP production occurs by oxidative phosphorylation 6.10 Most ATP production occurs by oxidative phosphorylation An electron transport chain in the mitochondrial membrane creates a H+ gradient Electrons from NADH and FADH2 travel down the chain to O2, which picks up H+ H2O is formed as a product Energy released by redox reactions actively transports H+ across the membrane from the mitochondrial matrix to the intermembrane space

In chemiosmosis, ATP synthases drive the synthesis of ATP Exergonic reactions of the electron transport chain produce an H+ gradient that stores potential energy ATP synthases harness the energy by acting like turbines Help the H+ diffuse back against the gradient through the inner membrane Attach phosphate groups to ADP, producing ATP

Figure 6.10 H H H H Intermem- brane space H Mobile electron carriers H H Protein complex of electron carriers H H ATP synthase III IV I Inner mito- chondrial membrane II Electron flow FADH2 FAD 1 2 2 H O2 H2O NADH NAD Mito- chondrial matrix H ADP P ATP H Electron Transport Chain Chemiosmosis Oxidative Phosphorylation

Mobile electron carriers in the electron transport chain Ubiquinone (Q) + 2H Ubiquinol (QH2) Oxidized cytochrome c + e- Reduced cytochrome c 전자를 줄수있는 능력 NADH > QH2 > reduced cytochrome c > O2 Enzyme complexes in the electron transport chain NADH dehydrogenase NADH + H+ + Q  NAD+ + QH2 Cytochrome bc1 complex QH2 + 2cyt. cox  Q + 2cyt.cred + 2H+ Cytochrome c oxidase 2cyt.cred + 2H+ + 1/2O2  2cyt.cox + H2O

6.11 Certain poisons interrupt critical events in cellular respiration CONNECTION 6.11 Certain poisons interrupt critical events in cellular respiration Rotenone, cyanide, and carbon monoxide block parts of the electron transport chain Oligomycin blocks the passage of H+ through ATP synthase Uncouplers such as DNP destroy the H+ gradient by making the membrane leaky to H+

Cyanide, carbon monoxide Figure 6.11 https://www.youtube.com/watch?v=xbJ0nbzt5Kw Rotenone Cyanide, carbon monoxide Oligomycin H H H ATP synthase H H H H DNP FADH2 FAD 2 1 O2 2 H NADH NAD H H2O ADP P ATP

6.12 Review: Each molecule of glucose yields many molecules of ATP Glycolysis and the citric acid cycle together yield four ATP per glucose molecule Oxidative phosphorylation, using electron transport and chemiosmosis, yields 28 ATP per glucose These numbers are maximums Some cells may lose a few ATP to NAD+ or FAD shuttles

   CYTOPLASM Electron shuttles across membrane Mitochondrion 2 NADH Figure 6.12 CYTOPLASM Electron shuttles across membrane Mitochondrion 2 NADH 2 or NADH 2 FADH2 6 NADH 2 2 NADH FADH2 Pyruvate Oxidation 2 Acetyl CoA Glycolysis Oxidative Phosphorylation (electron transport and chemiosmosis) 2 Pyruvate Citric Acid Cycle Glucose Maximum per glucose: ATP  2 ATP  2 about  28 ATP About ATP 32 by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation

6.13 Fermentation is an anaerobic alternative to cellular respiration 6.13 Fermentation is an anaerobic alternative to cellular respiration Fermentation Generates two ATP molecules from glycolysis in the absence of oxygen Recycles NADH to NAD+ anaerobically Muscle cells use lactic acid fermentation NADH is oxidized to NAD+ as pyruvate is reduced to lactate

+ LE 6-13a 2 Lactate NAD NADH NADH NAD P ATP 2 ADP 2 2 2 2 GLYCOLYSIS 2 ADP + 2 P 2 ATP 2 Pyruvate 2 Lactate Glucose

Animation: Fermentation Overview Alcohol fermentation occurs in brewing, wine making, and baking NADH is oxidized to NAD+ while converting pyruvate to CO2 and ethanol Animation: Fermentation Overview

+ LE 6-13b NAD NADH NADH NAD 2 ADP P ATP CO2 released 2 Ethanol 2 2 GLYCOLYSIS 2 ADP + 2 P 2 ATP 2 CO2 released Glucose 2 Pyruvate 2 Ethanol 2

Strict anaerobes Require anaerobic conditions to generate ATP by fermentation Are poisoned by oxygen Facultative anaerobes Can make ATP by fermentation or oxidative phosphorylation depending on whether O2 is available

INTERCONNECTIONS BETWEEN MOLECULAR BREAKDOWN AND SYNTHESIS 6.14 Cells use many kinds of organic molecules as fuel for cellular respiration Cells use three main kinds of food molecules to make ATP Carbohydrates Hydrolyzed by enzymes to glucose, which enters glycolysis

Proteins Digested to constituent amino acids, which are transformed into various compounds Become intermediates in glycolysis or the citric acid cycle

Fats Digested to glycerol and free fatty acids Glycerol becomes an intermediate in glycolysis Fatty acids are broken into 2-carbon fragments that enter the citric acid cycle as acetyl CoA

Pyruvate Oxidation Acetyl CoA Oxidative Phosphorylation Figure 6.15 Food, such as peanuts Carbohydrates Fats Proteins Sugars Glycerol Fatty acids Amino acids Amino groups Citric Acid Cycle Pyruvate Oxidation Acetyl CoA Oxidative Phosphorylation Glucose G3P Pyruvate Glycolysis ATP

6.15 Food molecules provide raw materials for biosynthesis 6.15 Food molecules provide raw materials for biosynthesis Some raw materials from food can be incorporated directly into an organism’s molecules Cells can also make molecules not found in food Intermediate compounds of glycolysis and the citric acid cycle act as raw materials Biosynthetic pathways consume ATP rather than generate it Biosynthesis is not always the direct reverse of breakdown pathways

Pyruvate Oxidation Acetyl CoA Figure 6.16 ATP needed to drive biosynthesis ATP Citric Acid Cycle Pyruvate Oxidation Acetyl CoA Glucose Synthesis Pyruvate G3P Glucose Amino groups Amino acids Fatty acids Glycerol Sugars Proteins Fats Carbohydrates Cells, tissues, organisms