How Cells Harvest Chemical Energy Chapter 6

INTRODUCTION TO CELLULAR RESPIRATION

Photosynthesis and Cellular Respiration Provide Energy For Life Energy that supports life on Earth is captured from sun and used for plant, algae, protist, and bacterial photosynthesis Photosynthesis produces sugar and oxygen. Other organisms use the energy in sugar and O2 from the atmosphere in cellular respiration Cellular respiration releases CO2 and H2O Together, these two processes are responsible for the majority of life on Earth

Sunlight energy Photosynthesis in chloroplasts + + (for cellular work) ECOSYSTEM Photosynthesis in chloroplasts CO2 Glucose + + H2O O2 Cellular respiration in mitochondria Figure 6.1 The connection between photosynthesis and cellular respiration. ATP (for cellular work) Heat energy

Breathing and cellular respiration are closely related Breathing Supplies Oxygen to Our Cells For Use in Cellular Respiration and Removes Carbon Dioxide Breathing and cellular respiration are closely related Breathing is necessary for exchange of CO2 produced during cellular respiration for atmospheric O2 Cellular respiration uses O2 and produces CO2

Muscle cells carrying out Breathing O2 CO2 Lungs CO2 O2 Bloodstream Figure 6.2 The connection between breathing and cellular respiration. Muscle cells carrying out Cellular Respiration Glucose + O2 CO2 + H2O + ATP

Cellular Respiration Banks Energy in ATP Molecules Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to ATP molecules. Cellular respiration produces up to 38 ATP/glucose molecule Other foods (organic molecules) can be used as a source of energy in addition to glucose C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATPs Glucose Oxygen Carbon dioxide Water Energy Figure 6.3 Summary equation for cellular respiration: C6H12O6 + 6 O2 6 CO2 + H2O + energy

Cells Tap Energy From Electrons “falling” From Organic Fuels to Oxygen When bonds of glucose are broken, its electrons are transferred to oxygen (oxygen is electronegative, and pulls e- towards it) Cellular respiration is the controlled breakdown of organic molecules as electrons are removed from a fuel source and transferred to oxygen As electrons pass down the ETC, energy is released in small amounts, and ultimately stored in ATP

NADH ATP NAD+ + 2e– Controlled release of H+ energy for synthesis of ATP H+ Electron transport chain Figure 6.5C In cellular respiration, electrons fall down an energy staircase and finally reduce O2. 2e– H+ 1  2 O2 H2O

Cells Tap Energy From Electrons “falling” From Organic Fuels to Oxygen The cellular respiration equation explained: Glucose loses electrons (in the form of hydrogen) to oxygen and is converted to CO2 Glucose is oxidized Oxygen gains electrons (in the form of hydrogens) from glucose and is converted to H2O Oxygen is reduced The reducing agent = The molecule carrying the hydrogens (with e-) (NADH, FADH2) The oxidizing agent= The molecule that receives the hydrogens (is without e-) (NAD+, FAD)

C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy Glucose (ATP) Loss of hydrogen atoms (oxidation) C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy Glucose (ATP) Gain of hydrogen atoms (reduction) Figure 6.5A Rearrangement of hydrogen atoms (with their electrons) in the redox reactions of cellular respiration.

Enzymes are necessary to Oxidize Glucose and Other Foods Dehydrogenase enzymes remove hydrogens from organic molecules and transfer them to electron acceptors Oxidation Dehydrogenase Figure 6.5B A pair of redox reactions, occurring simultaneously. Reduction + H+ NAD+ + 2 H NADH (carries 2 electrons) 2 H+ + 2 e–

Cells tap energy from electrons “falling” from organic fuels to Oxygen The electron acceptor is a coenzyme called NAD+ (nicotinamide adenine dinucleotide) Another coenzyme that functions like NAD+ is FAD They “carry” e- from glucose to a series of proteins found along the cristae of the mitochondria called the electron transport chain or ETC Copyright © 2009 Pearson Education, Inc.

STAGES OF CELLULAR RESPIRATION AND FERMENTATION

Cellular respiration occurs in three main stages Stage 1: Glycolysis Occurs in the cytoplasm What happens ?: A single molecule of glucose is enzymatically cut in half through a series of steps to produce two molecules of pyruvate In addition to Pyruvate: Two molecules of NADH are formed (when two molecules of NAD+ are reduced) Two molecules of ATP are produced by substrate-level phosphorylation

Substrate-level Phosphorylation Substrate level phosphorylation is when an enzyme transfers a phosphate group from a substrate molecule to ADP, forming ATP Enzyme Enzyme + P ADP Figure 6.7B Substrate-level phosphorylation: transfer of a phosphate group P from a substrate to ADP, producing ATP. ATP P P Substrate Product

Figure 6.7C Details of glycolysis. ENERGY INVESTMENT PHASE Glucose ATP Steps – A fuel molecule is energized, using ATP. 1 3 Step 1 ADP P Glucose-6-phosphate 2 P Fructose-6-phosphate ATP 3 ADP P P Step A six-carbon intermediate splits Into two three-carbon intermediates. Fructose-1,6-bisphosphate 4 4 P P Glyceraldehyde-3-phosphate (G3P) Step A redox reaction generates NADH. 5 NAD+ 5 NAD+ 5 ENERGY PAYOFF PHASE NADH P NADH P + H+ + H+ P P P P 1,3-Bisphosphoglycerate ADP ADP 6 6 ATP ATP Figure 6.7C Details of glycolysis. P P 3-Phosphoglycerate 7 7 Steps – ATP and pyruvate are produced. 6 9 P P 2-Phosphoglycerate 8 8 H2O H2O P P Phosphoenolpyruvate (PEP) ADP ADP 9 9 ATP ATP Pyruvate

Pyruvate is chemically groomed for the citric acid cycle The Grooming of Pyruvate: Location: mitochondrial matrix Purpose: to prepare pyruvate for entry into Kreb’s cycle What Happens: 1. The removal of a carboxyl group (decarboxylation) which is released as CO2 2. The remaining 2-carbon fragment is oxidized into acetate NAD+ accepted the electrons forming NADH 3. Coenzyme A binds to the 2-carbon acetate forming acetyl coenzyme A Products: CO2 , NADH and Acetyl CoA

NAD+ NADH  H+ 2 CoA Pyruvate 1 Acetyl coenzyme A 3 CO2 Coenzyme A Figure 6.8 The conversion of pyruvate to acetyl CoA. Coenzyme A

Cellular respiration occurs in three main stages Stage 2: The Kreb’s Cycle (citric acid cycle) Location: mitochondrial matrix Purpose: further breakdown of what remains of glucose into CO2 ; to form electron carriers NADH, FADH2 What Happens: 2-C acetate combines with 4-C oxaloacetate forming 6-C citrate 6-C citrate then passes through a series of redox reactions that regenerate oxaloacetate (4-C molecule ) Products from one turn of the cycle: 3 NADH, 1 FADH2 , 1 ATP, 2 CO2 Products per glucose: 6 NADH, 2 FADH2 , 2 ATP, 4 CO2

CITRIC ACID CYCLE Acetyl CoA CoA CoA 2 CO2 3 NAD+ FADH2 FAD 3 NADH  Figure 6.9A An overview of the citric acid cycle: Two carbons enter the cycle through acetyl CoA, and 2 CO2, 3 NADH, 1 FADH2, and 1 ATP exit the cycle. FAD 3 NADH  3 H+ ATP ADP + P

CITRIC ACID CYCLE Acetyl CoA 2 carbons enter cycle Oxaloacetate 1 Citrate NADH + H+ NAD+ 5 NAD+ NADH + H+ 2 CITRIC ACID CYCLE Malate CO2 leaves cycle ADP  P FADH2 4 ATP Alpha-ketoglutarate Figure 6.9B Details of the citric acid cycle. FAD 3 CO2 leaves cycle Succinate NAD+ NADH + H+ Step Acetyl CoA stokes the furnace. 1 Steps – NADH, ATP, and CO2 are generated during redox reactions. 2 3 Steps – Redox reactions generate FADH2 and NADH. 4 5

Cellular respiration occurs in three main stages Stage 3: Oxidative phosphorylation (aka the ETC) Location: cristae of mitochondrion Purpose: to generate a proton gradient that is used to form ATP What Happens: Electrons are supplied by NADH & FADH2 , then passed along the proteins of the ETC. As e- are passed from protein to protein they pump H+ into the intermembrane space The potential energy of this proton gradient is used to make ATP by chemiosmosis The concentration gradient drives H+ through ATP synthases producing ATP The ETC + chemiosmosis = oxidative phosphorylation Products: 34-38 ATP/glucose

OXIDATIVE PHOSPHORYLATION Protein complex of electron carriers H+ H+ Electron carrier H+ H+ ATP synthase Intermembrane space Inner mitochondrial membrane FADH2 FAD Electron flow NADH NAD+ 2  1 O2 + 2 H+ H+ Mitochondrial matrix H+ Figure 6.10 Oxidative phosphorylation, using electron transport and chemiosmosis in the mitochondrion. ADP + P H+ ATP H2O H+ Electron Transport Chain Chemiosmosis OXIDATIVE PHOSPHORYLATION

by oxidative phosphorylation Electron shuttle across membrane Cytoplasm Mitochondrion 2 NADH 2 NADH (or 2 FADH2) 2 NADH 6 NADH 2 FADH2 GLYCOLYSIS OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) 2 Pyruvate 2 Acetyl CoA CITRIC ACID CYCLE Glucose  2 ATP  2 ATP  about 34 ATP Figure 6.12 An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration. by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation About 38 ATP Maximum per glucose:

Fermentation enables cells to produce ATP without oxygen Fermentation is an anaerobic (without oxygen) energy-generating process Includes glycolysis, and the regeneration of NAD+ Purpose: to produce ATP in the absence of oxygen What Happens?: NADH is oxidized to NAD+ when pyruvate is reduced to lactate Pyruvate serves as an “electron sink,” a place to dispose of the electrons generated by oxidation reactions in glycolysis Your muscle cells and certain bacteria can oxidize NADH through lactic acid fermentation Copyright © 2009 Pearson Education, Inc.

Lactic Acid Fermentation Steps to regenerate NAD+ Glucose 2 NAD+ 2 ADP  2 P GLYCOLYSIS Glycolysis 2 ATP 2 NADH Lactic Acid Fermentation 2 Pyruvate 2 NADH Steps to regenerate NAD+ Figure 6.13A Lactic acid fermentation oxidizes NADH to NAD+ and produces lactate. 2 NAD+ 2 Lactate Lactic acid fermentation

Fermentation enables cells to produce ATP without oxygen The baking and winemaking industry have used alcohol fermentation for thousands of years Happens in yeast cells (single-celled fungi) They convert pyruvate to CO2 and ethanol while oxidizing NADH back to NAD+

Steps to regenerate NAD+ with a release of CO2 Glucose 2 ADP 2 NAD+  2 P GLYCOLYSIS Glycolysis 2 ATP 2 NADH Alcohol Fermentation 2 Pyruvate 2 NADH 2 CO2 Steps to regenerate NAD+ with a release of CO2 released Figure 6.13B Alcohol fermentation oxidizes NADH to NAD+ and produces ethanol and CO2. 2 NAD+ 2 Ethanol Alcohol fermentation

INTERCONNECTIONS BETWEEN MOLECULAR BREAKDOWN AND SYNTHESIS

Cells use many kinds of organic molecules as fuel for cellular respiration Although glucose is considered the primary source of sugar for respiration and fermentation, there are actually three sources of molecules for generation of ATP Carbohydrates (disaccharides) Proteins (after conversion to amino acids) Fats

Food, such as peanuts Carbohydrates Fats Proteins Sugars Glycerol Fatty acids Amino acids Amino groups Figure 6.15 Pathways that break down various food molecules. OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) CITRIC ACID CYCLE Acetyl CoA Glucose G3P Pyruvate GLYCOLYSIS ATP

ATP needed to drive biosynthesis GLUCOSE SYNTHESIS CITRIC ACID CYCLE Acetyl CoA Pyruvate G3P Glucose Amino groups Amino acids Fatty acids Glycerol Sugars Proteins Fats Carbohydrates Figure 6.16 Biosynthesis of large organic molecules from intermediates of cellular respiration. Cells, tissues, organisms

Cellular respiration ATP (a) (b) (d) (c) (f) (e) generates has three stages oxidizes uses ATP (a) produce some C6H12O6 (b) (d) produces many energy for to pull electrons down to (c) (f) by process called uses H+ diffuse through ATP synthase (e) uses pumps H+ to create