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Cellular Respiration: Harvesting Chemical Energy

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1 Cellular Respiration: Harvesting Chemical Energy
Chapter 9 Cellular Respiration: Harvesting Chemical Energy

2 Overview: Life Is Work Living cells Ex: The giant panda
Require transfusions of energy from outside sources to perform their many tasks Ex: The giant panda Obtains (chemical, potential) energy for its cells by eating plants Figure 9.1

3 powers most cellular work
Energy Flows into an ecosystem as sunlight and leaves as heat Light energy ECOSYSTEM CO2 + H2O Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O2 ATP powers most cellular work Heat energy Figure 9.2

4 Review: 1st law- Energy cannot be created or destroyed. Energy can be converted from one form to another. The sum of the energy before the conversion is equal to the sum of the energy after the conversion. 2nd law- Some usable energy dissipates during transformations and is lost. During changes from one form of energy to another, some usable energy dissipates, usually as heat. The amount of usable energy therefore decreases.

5 Cellular Respiration Glucose + O2  energy (ATP) + CO2 + H2O
This is exergonic –DG and converts one form of chemical energy (glucose) into another (ATP), with heat dissipates Consumes oxygen and organic molecules (such as glucose) Glucose Oxidation is the most prevalent and efficient catabolic pathway Yields lots of ATP An alternate catabolic process, fermentation Is a partial degradation of sugars that occurs without oxygen (in anaerobic conditions)

6 More on ATP ATP (adenosine triphosphate)
ATP is a nucleotide.  Nucleotides are the building blocks of nucleic acids such as DNA and RNA.  They contain a nitrogen-containing base, a 5-carbon sugar, and phosphate groups. The energy in one glucose molecule is used to produce up to 38 ATP.  ATP has approximately the right amount of energy for most cellular reactions.

7 ATP, ADP, AMP The bonds between the phosphate groups are high-energy bonds. Energy is required (and stored) to form the bonds and energy is released when the bonds are broken. (below)

8 Phosphorylation ATP is produced and used continuously. The entire amount of ATP in an organism is recycled once per minute. Most cells maintain only a few seconds supply of ATP. To keep working, cells must regenerate ATP When a phosphate group is transferred to another molecule, it is called PHOSPHORYLATION. ADP + Pi + energy ---->  ATP It takes about 7.3 kcals of energy to phosphorylate ADP into ATP Enzyme that catalyzes this reaction is ATP synthase

9 Phosphorylation Phosphate groups from ATP can also be transferred to other molecules. Enzymes that catalyze this reaction are called KINASES. In these phosphorylation reactions, energy is transferred from the phosphate group in ATP to the phosphorylated compound. This newly energized compound will participate in other reactions.

10 Redox Reactions: Oxidation and Reduction
Catabolic pathways also yield energy due to the transfer of electrons In oxidation A substance loses electrons, or is oxidized In reduction A substance gains electrons, or is reduced “Redox” reactions Transfer electrons from one reactant to another by oxidation and reduction Na Cl Na Cl– becomes oxidized (loses electron) becomes reduced (gains electron)

11 “Redox” reactions Transfer of electrons from one reactant to another by oxidation and reduction When the electron moves to a lower energy level, energy is released.

12 Redox cont’d Some redox reactions
Do not completely exchange electrons, instead they change the degree of electron sharing in covalent bonds CH4 H C O Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water + 2O2 CO2 Energy 2 H2O becomes oxidized becomes reduced Reactants Products Figure 9.3

13 Oxidation of Organic Fuel Molecules During Cellular Respiration
Glucose is oxidized and oxygen is reduced C6H12O6 + 6O CO2 + 6H2O + Energy becomes oxidized becomes reduced Glucose + Oxygen ----> Carbon dioxide + water + Energy Yields DG = -686 Kcal/mole This energy is used to add phosphates to ADP + P  ATP There is a need to convert glucose (sugar) which is stored chemical energy into ATP which is usable cell energy

14 Electron carriers Some compounds can accept and donate electrons readily, and these are called electron carriers in organisms. There are a number of molecules that serve as electron carriers. NAD (nicotinamide adenine dinucleotide) is used in respiration. Reduced to NADH. NADP (nicotinamide adenine dinucleotide phosphate) is another used in photosynthesis. Reduced to NADPH These molecules readily give up electrons (oxidized) and gain electrons (reduced). FAD (flavin adenine dinucleotide) is reduced to FADH2 H often represents one electron, one proton.

15 The electron transport chain
NADH, the reduced form of NAD+ Passes the electrons along an electron transport chain Electron are passed in a series of steps instead of in one explosive reaction If electron transfer is not stepwise A large release of energy occurs = dangerous to living cells Uses the energy from the electron transfer to form ATP 2 H 1/2 O2 (from food via NADH) 2 H e– 2 H+ 2 e– H2O Controlled release of energy for synthesis of ATP ATP Electron transport chain Free energy, G (b) Cellular respiration + Figure 9.5 B

16 ATP originates in cellular respiration
What happens is that sugar is broken down into smaller molecules and energy is released. This energy is used to generate ATP from ADP and P. Sugar > smaller molecules >ADP + Pi > ATP

17 The Stages of Cellular Respiration: A Preview
Respiration is a cumulative function of three metabolic stages Glycolysis Breaks down glucose into two molecules of pyruvate The citric acid (Krebs) cycle Completes the breakdown of glucose Oxidative phosphorylation Is driven by the electron transport chain (ETC) Generates ATP

18 Oxidative phosphorylation: electron transport and chemiosmosis
An overview of cellular respiration Figure 9.6 Electrons carried via NADH Glycolsis Glucose Pyruvate ATP Substrate-level phosphorylation Electrons carried via NADH and FADH2 Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Oxidative Mitochondrion Cytosol

19 Both glycolysis and the citric acid cycle
Can generate ATP by substrate-level phosphorylation Figure 9.7 Enzyme ATP ADP Product Substrate P +

20 Glycolysis Means “splitting of sugar”
Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate Glycolysis Means “splitting of sugar” Breaks down glucose into pyruvate Occurs in the cytoplasm of the cell

21 Energy investment phase
Glycolysis consists of two major phases Energy investment phase Energy payoff phase Glycolysis Citric acid cycle Oxidative phosphorylation ATP 2 ATP 4 ATP used formed Glucose 2 ATP + 2 P 4 ADP + 4 2 NAD+ + 4 e- + 4 H + 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Energy investment phase Energy payoff phase 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H + Figure 9.8 exergonic, energy liberating  -DG                                                     endergonic, energy intake +DG 

22 A closer look at the energy investment phase:
[Step 1] Energy input required    Terminal P from an ATP (-->ADP) is bonded to the C6 of a glucose DG = (enzyme: hexokinase) [Step 2] glucose phosphate------> fructose-6-phosphate atoms rearranged DG = (enzyme: phosphoglucoisomerase) [Step 3] fructose-6-phosphate------> fructose 1,6 biphosphate Phosphate added to C1 DG = (enzyme: phosphofructokinase)  splits into two products [Step 4] fructose -1, 6 - biphosphate---> dihydroxyacetone phosphate AND glyceraldehyde phosphate* DG = ( enzyme: aldolase) Our 6-carbon sugar is split into TWO  3-carbon products [Step 5]    *Ultimately, all of the dihydroxyacetone phosphate will be converted into glyceraldehyde-3-phosphate, so each step is actually x 2 from here (because each of the two molecules will proceed through Steps 5- 9 ) dihydroxyacetone phosphate > glyceraldehyde-3-phosphate (enzyme: isomerase) Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate H OH HO CH2OH O P CH2O CH2 C CHOH ATP ADP Hexokinase Glucose Glucose-6-phosphate Fructose-6-phosphate Phosphoglucoisomerase Phosphofructokinase Fructose- 1, 6-bisphosphate Aldolase Isomerase Glycolysis 1 2 3 4 5 Oxidative phosphorylation Citric acid cycle Figure 9.9 A

23 A closer look at the energy payoff phase
[Step 6]     2 Glyceraldehyde-3-phosphate molecules are then oxidized 2 hydrogens (with e-) are removed, and NAD+ is reduced to NADH and H+ Also, a free phosphate attaches to each of the glyceraldehyde-3-phosphates 2 Glyceraldehyde-3-phosphate > 1, 3 biphosphoglycerate (x2) NAD+ is reduced to NADH and H+ Pi = free phosphates, not taken from another molecule such as ATP NAD = nicotinamide adenine dinucleotide (niacin derivative) DG = (enzyme: triose phosphate dehydrogenase) [Step 7] a P is released from each 1,3 biphosphoglycerate molecule and is used to "recharge" 2 ADPs----> 2 ATPs thus, converting the 1,3 biphosphoglycerate to 3-phosphoglycerate (highly exergonic= large DG to pull preceding reactions forward) (x2) 1,3 Diphosphoglycerate > 3-phosphoglycerate ADP---> ATP DG = (enzyme: phosphoglycerokinase) (x 2) [Step 8]   The remaining P group is enzymatically transferred from the 3C to the 2C 3- phosphoglycerate > 2 - phosphoglycerate DG = (enzyme: phosphoglyceromutase) (x 2) [Step 9] A molecule of H2O removed 2 - phosphoglycerate > phosphoenolpyruvate - H2O DG = (enzyme: enolase) (x 2) [Step 10] Another phosphate P is transferred to ADP ( highly exergonic, pulls 2 preceding rxns!!!) phosphoenolpyruvate > pyruvate (pyruvic acid) DG = ( enzyme: pyruvate kinase) 2 NAD+ NADH 2 + 2 H+ Triose phosphate dehydrogenase P i P C CHOH O CH2 O– 1, 3-Bisphosphoglycerate 2 ADP 2 ATP Phosphoglycerokinase 3-Phosphoglycerate Phosphoglyceromutase CH2OH H 2-Phosphoglycerate 2 H2O Enolase Phosphoenolpyruvate Pyruvate kinase CH3 6 8 7 9 10 Pyruvate

24 Mitochondria Review The mitochondrion is surrounded by two membranes. The outer is smooth and the inner folds inwards. The inner folds are called cristae. Within the inner compartment of the mitochondrion, surrounding the cristae, there is a dense solution known as the matrix. The matrix contains enzymes, co-enzymes, water, phosphates, and other molecules needed in respiration. The outer membrane is permeable to most small molecules, but the inner one permits the passage of only certain molecules, such as pyruvic acid and ATP. Proteins are built into the membrane of the cristae. These proteins are involved with the Electron Transport Chain. The inner membrane is about 80% protein and 20% lipids. 95% of the ATP generated by the heterotrophic cell is produced by the mitochondrion.

25 The Fate of the Pyruvate
The pyruvate passes from the cytoplasm (where its produced thru glycolysis) and crosses the outer & inner membranes of the mitochondria Before entering the Krebs Cycle, the 3-C pyruvate molecule is oxidized: the carbon and oxygen atoms of the carboxyl group are removed (into CO2) and a 2-C acetyl group is left (CH3CO) In the course of this rxn, the carboxyl hydrogen reduces a molecule of NAD+ to NADH The acetyl is momentarily accepted by a "coenzyme A" molecule (a large, complex molecule formed from pantothenic acid (vitamin B)), forming Acetyl-CoA From here the Acetyl CoA can enter the Krebs (aka, Citric Acid) Cycle.  Acetyl CoA moves into the mitochondria and is completely dismantled by the enzymes in the mitochondria.

26 Before the citric acid cycle can begin
Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis CYTOSOL MITOCHONDRION NADH + H+ NAD+ 2 3 1 CO2 Coenzyme A Pyruvate Acetyle CoA S CoA C CH3 O Transport protein O– Figure 9.10

27 An overview of the citric acid cycle
Discovered 1937 by British biochemist Sir Hans Adolf Krebs CO2 is produced. Acetyls are dismantled and reconfigured. The electrons are what's important. The Krebs's cycle only gives us two molecules of ATP. Added with the two molecules of ATP made in Glycolysis, the total is now a meager four molecules of ATP. The remainder of the ATPs come from the Electron Transport System, which takes the electrons produced in the Krebs's cycle and makes ATP. ATP 2 CO2 3 NAD+ 3 NADH + 3 H+ ADP + P i FAD FADH2 Citric acid cycle CoA Acetyle CoA NADH CO2 Pyruvate (from glycolysis, 2 molecules per glucose) Glycolysis Oxidative phosphorylation Figure 9.11

28 A closer look at the citric acid cycle
In general: 2-C acetyl combines with 4-C oxaloacetic acid to form a 6-C citric acid. Two carbons (per cycle) are oxidized to CO2 which regenerates a molecule of oxaloacetic acid Each turn of the cycle uses up one acetyl group and regenerates a oxaloacetic acid, then begins the cycle again. Energy is released by breaking C-H and C-C bonds and is stored by transforming ADP to ATP (1 molecule per turn of the cycle) and to convert NAD+ to NADH and H+ (3x per cycle) Also, FAD (flavin adenine dinucleotide) is converted to FADH2 (one molecule per cycle) 1. No oxygen is required in Krebs Cycle 2. All e- and p+ are accepted by either NAD+ or FAD There are 8 steps in the cycle and the aim is to totally dismantle the Acetyl CoA, using only its electrons. We have totally taken apart the glucose molecule. Only four ATPs have resulted, 2 from glycolysis and 2 from the CAC. But we still have a lot of ELECTRONS captured in the form of NADH and FADH2 = potential energy Acetyl CoA NADH Oxaloacetate Citrate Malate Fumarate Succinate Succinyl CoA a-Ketoglutarate Isocitrate Citric acid cycle S SH FADH2 FAD GTP GDP NAD+ ADP P i CO2 H2O + H+ C CH3 O COO– CH2 HO HC CH 1 2 3 4 5 6 7 8 Glycolysis Oxidative phosphorylation ATP Figure 9.12 Figure 9.12

29 Oxidative phosphorylation
Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis NADH and FADH2 Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

30 The Pathway of Electron Transport
In the electron transport chain Electrons from NADH and FADH2 lose energy in several steps At the end of the chain Electrons are passed to oxygen, forming water Transport Chain carriers are called CYTOCHROMES (consist of protein and a heme group = atom of iron enclosed in a porphyrin ring (note the similarity to hemoglobin!) electrons are "passed down" a "staircase" of cytochromes and the energy released in lowering the energy level is stored in additional ATP molecules...called Oxidative Phosphorylation remember, the energy from lowering electrons can do this: ADP + P = ATP ---> for every  e- passed down the chain from NADH, 3 ATPs are formed ---> for every 2e- from FADH2, 2 ATP's are formed H2O O2 NADH FADH2 FMN Fe•S O FAD Cyt b Cyt c1 Cyt c Cyt a Cyt a3 2 H + + 12 I II III IV Multiprotein complexes 10 20 30 40 50 Free energy (G) relative to O2 (kcl/mol) Figure 9.13

31 Chemiosmosis: The Energy-Coupling Mechanism
ATP synthase Is the enzyme that actually makes ATP INTERMEMBRANE SPACE H+ P i + ADP ATP A rotor within the membrane spins clockwise when H+ flows past it down the H+ gradient. A stator anchored in the membrane holds the knob stationary. A rod (for “stalk”) extending into the knob also spins, activating catalytic sites in the knob. Three catalytic sites in the stationary knob join inorganic Phosphate to ADP to make ATP. MITOCHONDRIAL MATRIX Figure 9.14

32 Chemiosmosis Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work At certain steps along the electron transport chain, electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space The resulting H+ gradient Stores energy Drives chemiosmosis in ATP synthase Is referred to as a “proton-motive force”

33 Electron transport chain Oxidative phosphorylation
Chemiosmosis and the electron transport chain Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP Inner Mitochondrial membrane H+ P i Protein complex of electron carners Cyt c I II III IV (Carrying electrons from, food) NADH+ FADH2 NAD+ FAD+ 2 H+ + 1/2 O2 H2O ADP + Electron transport chain Electron transport and pumping of protons (H+), which create an H+ gradient across the membrane Chemiosmosis ATP synthesis powered by the flow Of H+ back across the membrane synthase Q Oxidative phosphorylation Intermembrane space mitochondrial matrix Figure 9.15

34 There are three main processes in this metabolic enterprise
Summary There are three main processes in this metabolic enterprise Electron shuttles span membrane CYTOSOL 2 NADH 2 FADH2 6 NADH Glycolysis Glucose 2 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis MITOCHONDRION by substrate-level phosphorylation by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol Maximum per glucose: About 36 or 38 ATP + 2 ATP + about 32 or 34 ATP or Figure 9.16

35 The energy tally sheet Glycolysis: 2 ATP =2 ATP
2 NADH ets  6 ATP =6 ATP Oxidation: pyruvate  acetyl CoA  1 NADH  ets  3 ATP (x2) = 6 ATP CAC/Krebs 1 ATP x2 turns  2 ATP 3 NADH  ets  9 ATP x2 turns  18 ATP 1 FADH2  ets  2 ATP x2 turns  4 ATP = 24 ATP TOTAL = 38 ATP

36 efficiency Less than 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making approximately 38 ATP DG = ADP + Pi --> ATP = about 7 kcal/mole X 38 = 266 kcal/mole (captured in P bonds) DG = -686 kcal/mole (free energy change during glycolysis and oxidation for glucose) = about 39% efficiency

37 Oxidative cellular respiration
FERMENTATION Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen Oxidative cellular respiration Relies on oxygen to produce ATP Involves Krebs cycle and ETS In the absence of oxygen Cells can still produce a little bit of ATP through fermentation

38 Without oxygen: Glycolysis
Can produce ATP with or without oxygen, in either aerobic or anaerobic conditions Pyruvate is produced, some ATP is made In anaerobic respiration (fermentation) pyruvate must be processed in the absence of oxygen The breakdown of the sugar still takes place through a series of chemical reactions, but with a different outcome.

39 The Evolutionary Significance of Glycolysis
Occurs in the cytoplasm of nearly all organisms Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

40 Pyruvate is a key juncture in catabolism
Glucose CYTOSOL Pyruvate No O2 present Fermentation O2 present Cellular respiration Ethanol or lactate Acetyl CoA MITOCHONDRION Citric acid cycle Figure 9.18

41 Fermentation Living organisms have developed numerous and different fermentation pathways; however, most organisms use the following Embden-Meyerhoff pathway, named for the two discoverers. The pyruvate can take two pathways in anaerobic respiration: a. Pyruvate will be converted to ethyl alcohol (ethanol) and carbon dioxide. This is called alcoholic fermentation and is the basis of our wine, beer and liquor industry. b. The pyruvate will be converted to lactic acid. This is called lactic acid fermentation. Lactic acid is what makes your muscles burn during prolonged exercise, this process is also used to make yogurt.

42 Fermentation consists of
Types of Fermentation Fermentation consists of Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis In alcohol fermentation Pyruvate is converted to ethanol in two steps, one of which releases CO2 During lactic acid fermentation Pyruvate is reduced directly to NADH to form lactate as a waste product

43 (a) Alcohol fermentation (b) Lactic acid fermentation
2 ADP + 2 P1 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH 2 Pyruvate 2 Acetaldehyde 2 Ethanol (a) Alcohol fermentation 2 Lactate (b) Lactic acid fermentation H OH CH3 C O – O O– CO2 2 Figure 9.17

44 Fermentation and Cellular Respiration Compared
Both fermentation and cellular respiration Use glycolysis to oxidize glucose and other organic fuels to pyruvate Fermentation and cellular respiration Differ in their final electron acceptor Oxidative cellular respiration Produces more ATP

45 The Versatility of Catabolism
Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Catabolic pathways Funnel electrons from many kinds of organic molecules into cellular respiration

46 The catabolism of various molecules from food
Amino acids Sugars Glycerol Fatty Glycolysis Glucose Glyceraldehyde-3- P Pyruvate Acetyl CoA NH3 Citric acid cycle Oxidative phosphorylation Fats Proteins Carbohydrates Figure 9.19

47 Regulation of Cellular Respiration via Feedback Mechanisms
Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Starting molecule Product

48 Feedback Control Regulates Biological Processes
The control of cellular respiration Glucose Glycolysis Fructose-6-phosphate Phosphofructokinase Fructose-1,6-bisphosphate Inhibits Pyruvate ATP Acetyl CoA Citric acid cycle Citrate Oxidative phosphorylation Stimulates AMP + Figure 9.20

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