Chapter 8: Cellular Respiration (Outline) NAD+ and FAD Phases of Cellular Respiration Glycolysis Preparatory (prep) Reaction Citric Acid Cycle Electron Transport System (ETC) Fermentation Catabolic and Anabolic Reactions

Cellular Respiration A cellular process that requires oxygen and gives off carbon dioxide Usually involves breakdown of glucose to carbon dioxide and water Energy extracted from glucose molecule: Released slowly step-wise Allows ATP to be produced efficiently Oxidation-reduction enzymes include NAD+ and FAD as coenzymes

Cellular Respiration Glucose is oxidized and thus releases energy, while oxygen is reduced to form water The carbon atoms of the sugar molecule are released as carbon dioxide (CO2) Glucose is high-energy molecule; CO2 and H2O are low-energy molecules thus energy is released

Cellular Respiration Cells carry out cellular respiration in order to build up ATP molecules Energy is released slowly, step-wise, through many enzymatic reactions in different parts of the cell, why? Breakdown of glucose realizes a maximum yield of 36 or 38 ATP; this preserves 39% of energy available in glucose

NAD+ and FAD Each metabolic reaction in cellular respiration is catalyzed by its own enzyme NAD+ is a redox coenzyme that can Oxidize a metabolite by accepting two electrons and a hydrogen ion; results in NADH Reduce a metabolite by giving up electrons FAD is another redox coenzyme Sometimes used instead of NAD+ Accepts two electrons and two hydrogen ions (H+) to become FADH2

NAD+ and FAD Electrons received by NAD+ and FAD are high-energy electrons and are usually carried to the electron transport system Only a small amount of NAD+ is needed in cells, because each NAD+ molecule is used over & over again

Phases of Cellular Respiration Cellular respiration involves four phases Glycolysis: Occurs in the cytoplasm Does not require oxygen; occurs under aerobic or anaerobic conditions Glucose broken down into two 3-carbon molecules of pyruvate Enough energy is released for immediate buildup of two ATP molecules

Phases of Cellular Respiration Preparatory (Prep) reaction: Takes place inside the mitochondrion Pyruvate is oxidized to a 2-carbon (C2) acetyl group Electron energy is stored in NADH CO2 is released as waste product Occurs twice per glucose molecule

Phases of Cellular Respiration Citric acid cycle: Series of oxidation reactions occurring in the matrix of mitochondrion (i.e. aerobic) Electron energy is stored in NADH and FADH2 Produces two immediate ATP molecules per glucose molecule Four carbons are released as CO2 The cycle turns twice per glucose molecule

Phases of Cellular Respiration Electron transport chain: A series of carriers on the cristae of the mitochondria Extracts energy from NADH & FADH2 Electrons pass from higher to lower energy states, energy is released and stored for ATP production, how? Produces 32 or 34 molecules of ATP per glucose molecule

Glucose Breakdown: Overview of the Four Phases

Pyruvate Pyruvate is a pivotal metabolite in cellular respiration If O2 is not available to the cell, fermentation, an aerobic process, occurs During fermentation, glucose is incompletely metabolized to lactate or CO2 and alcohol Fermentation results in a net gain of only two ATP per glucose molecule

Glucose Breakdown: Glycolysis Proceeds in the cytosol Occurs universally in organisms (i.e. most likely evolved before the citric acid cycle and the electron transport system) Energy Investment Steps: The addition of two phosphate groups from ATP to activate glucose in 2 separate reactions Glucose splits into two (C3) G3P molecules, each with a phosphate group glucose + 2 ATP → 2 G3P + 2 ADP

Glycolysis Energy Harvesting Steps: In duplicated reactions, NAD+ accepts two electrons and one H+ ion resulting in two NADH Four ATP molecules are formed by substrate-level ATP synthesis Net gain of two ATP from glycolysis, why? Both G3Ps are oxidized to pyruvates Pyruvate enters mitochondria if oxygen is available and aerobic respiration follows If oxygen is not available, glycolysis becomes a part of fermentation and pyruvate is reduced

Inputs and Outputs of Glycolysis

Glycolysis: Substrate-level ATP synthesis A phosphate group is transferred to ADP giving one ATP molecule During glycolysis, the C3 substrate BPG gives up a phosphate group to ADP Occurs twice per glucose molecule

Glycolysis

Inside the Mitochondria Aerobic Respiration – involves the preparatory reaction, the citric acid cycle and the electron transport system Mitochondrion Structure and Function Double membrane with an intermembrane space between the outer and inner membrane Cristae - inner folds of membrane Matrix - the innermost compartment filled with a gel-like fluid Produces most of the ATP from cellular respiration (i.e. powerhouse of the cell)

Mitochondrion: Structure and Function

Glucose Breakdown: The Preparatory (Prep) Reaction Preparatory reaction connects glycolysis to the citric acid cycle End product of glycolysis (i.e. pyruvate) enters the mitochondrial matrix Pyruvate is converted to a C2 acetyl group Attached to CoA to form acetyl CoA Electrons picked up by 2 NAD+ to give 2 NADH CO2 is released and transported out of mitochondria into the cytoplasm Reaction occurs twice per glucose molecule

Preparatory Reaction

Glucose Breakdown: Citric Acid Cycle Also known as the Krebs cycle; a cyclic pathway occurring in the matrix of mitochondria Both (C2) acetyl-CoA groups from the prep reaction: Joins with a C4 molecule to give citrate (C6) Each acetyl group is oxidized to two CO2 molecules Electrons are accepted by NAD+ in three instances (forming 3 NADH) and by FAD in one instance (forming FADH2) ATP is formed (per acetyl group) by substrate-level ATP synthesis

Citric Acid Cycle

Inputs and Outputs of The Citric Acid Cycle Krebs cycle turns twice per glucose molecule

Electron Transport Chain (ETC) Location: Eukaryotes – cristae of the mitochondria Aerobic Prokaryotes – plasma membrane Series of carrier molecules: Pass energy rich electrons along Protein carriers such as cytochrome molecules Cytochromes - proteins with a central iron (heme) group; the group is the one being oxidized & reduced Receives electrons from NADH and FADH2 25

ETC (cont.) Oxygen is the final electron acceptor in the ETC Lack of oxygen blocks the entire ETC – no additional ATP is produced leading to death Some poisons also inhibit normal activity of cytochromes Example: Cyanide binds to iron in cytochrome, blocking ATP production 26

Cycling of Carries The fate of the hydrogens: H+ from NADH deliver enough energy to make 3 ATPs Those from FADH2 have only enough for 2 ATPs Recycling of coenzymes increases efficiency Once NADH delivers H+, it returns (as NAD+) to pick up more H+ However, hydrogen atoms must be combined with oxygen to make water If O2 is not present, NADH cannot release H+ No longer recycled back to NAD+ 27

ETC 28

Carriers on Cristae of a Mitochondrion The ETC consists of 3 protein complexes and 2 carriers The 3 protein complexes include: NADH-Q reductase complex Cytochrome reductase complex Cytochrome oxidase complex The other 2 carriers are coenzyme Q and cytochrome c H+ carried by NADH and FADH2 are pumped by the protein complexes into the intermembrane space; thus creating H+ gradient 29

Organization and Function of Cristae 30

ATP Production ATP synthase complex – channel protein (in cristae) that serves as an enzyme for ATP synthesis Protons diffuse from the intermembrane space (high conc.) to the matrix (low conc.) through the enzyme complex ATP synthase This catalyzes the phosphorylation of ADP to form ATP – Chemiosmosis ATP molecules them move out of the mitochondria to perform cellular work 31

Mitochondria in Active Tissue ATP production must be constant in order to sustain life Active tissues (e.g. muscles) require greater amounts of ATP and contain more mitochondria than less active cells Example – Dark meat of chickens contains more mitochondria than the white meat of the breast 32

Energy Yield from Glucose Metabolism Complete breakdown of glucose to CO2 and H2O yields 36 to 38 ATPs Substrate-level ATP synthesis 2 ATP from glycolysis 2 AP from the citric acid cycle Total of four ATP are formed outside of the electron transport system 32 to 34 ATP from the electron transport chain and chemiosmosis 33

Energy Yield from Glucose Metabolism (cont.) ETC and Chemiosmosis Per glucose molecule, 10 NADH and two FADH2 provide electrons and H+ ions to electron transport system For each NADH formed within the mitochondrion, 3 ATP are produced For each FADH2 formed by Krebs cycle, 2 ATP result since FADH2 delivers electrons after NADH For each NADH formed in the cytoplasm, 2 ATP are formed as electrons are “shuttled” across the mitochondrial membrane and delivered to FAD 34

Energy Yield from Glucose Breakdown Total NADH = 8 x 3 ATP = 24 ATP Total FADH2 = 2 x 2 ATP = 4 ATP ETC yields 28 ATP 28 (ETC) + 4 (substrate level) = 32 ATP Other 4 ATP comes from NADH produced by glycolysis which transport electrons via shuttle molecule to 2 FADH2, thus 2 x 2ATP = 4 ATP Now we have 32 + 4 = 36 ATP molecules 35

Efficiency of Cellular Respiration Energy difference (∆G) = 686 kcal One ATP phosphate bond has an energy content of 7.3 kcal 36 ATP produced during glucose breakdown (36 x 7.3) = 263 kcal Efficiency is 263/686 x 100 = 39% of available energy from glucose The rest of the energy is lost as heat 36

Overall Energy Yielded per Glucose Molecule

Fermentation When oxygen is limited: Fermentation: Spent hydrogens have no final acceptor NADH can’t recycle back to NAD+ Glycolysis stops because NAD+ is required Fermentation: “Anaerobic” pathway Can provide rapid burst of ATP in the absence of O2 Provides NAD+ for glycolysis NADH combines with pyruvate to yield NAD+ NAD+ is then free to return and pick up more e- during earlier reactions of glycolysis 38

Fermentation

Fermentation (cont.) Pyruvate is reduced by NADH to: Lactate (Animals & anaerobic bacteria) In muscles, pyruvate is reduced to lactate when it is produced faster than it can be oxidized by Krebs cycle Cheese & yogurt Industrial chemicals (i.e. isoprpanol, butyric acid, propionic acid & acetic acid) Ethanol & carbon dioxide (Yeasts) Bread & alcoholic beverages 40

Advantages and Disadvantages of Fermentation Despite the low yield of 2 ATP, it provides a quick burst of energy for muscular activity Allows glycolysis to proceed faster than O2 can be obtained Anaerobic exercise Lactic acid accumulates (toxic to cells) Causes cramping and oxygen debt When O2 restored, lactate is broken down to pyruvate; then respired or converted into glucose

Efficiency of Fermentation Two ATP produced per glucose molecule during fermentation gives (2 x 7.3)= 14.6 kcal Complete glucose breakdown to CO2 and H2O during cellular respiration = 686 kcal of energy Efficiency is 14.6/686 x 100 = 2.1% Much less efficient than complete breakdown of glucose

Catabolic and Anabolic Reactions Metabolism – sum of all chemical reactions within a living organism Catabolism – chemical reactions that result in the breakdown of complex organic compounds into simpler substances; thus releases energy Example: Cellular Respiration Anabolism – chemical reactions that build new molecules from simpler substances; thus requires energy Example: Photosynthesis