The process by which cells break down organic molecules (food) to make ATP is called cellular respiration

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2 Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis makes O 2 and organic molecules (like sugars and proteins), which are used in cellular respiration Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work The process by which cells break down organic molecules (food) to make ATP is called cellular respiration Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3 Fig. 9-2 Light energy ECOSYSTEM CO 2 + H 2 O Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O 2 ATP ATP powers most cellular work Heat energy

4 Catabolic Pathways and Production of ATP The breakdown of organic molecules such as glucose is exergonic (releases free energy used to power ADP + P i ATP) Aerobic respiration breaks down organic molecules completely with the help of O 2 and generates lots of energy for making ATP Fermentation is a partial breakdown of sugars that occurs without O 2 and makes only a small amount of ATP Aerobic exercise makes you breathe hardyou re using up a lot of oxygen! Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

5 Cellular respiration technically includes both aerobic and anaerobic respiration but usually refers to aerobic respiration Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C 6 H 12 O O 2 6 CO H 2 O + Energy Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

6 Redox Reactions: Oxidation and Reduction The transfer of electrons during chemical reactions releases energy stored in organic molecules This released energy is ultimately used to synthesize ATP Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

7 The Principle of Redox Chemical reactions that transfer electrons between reactants are called oxidationreduction reactions, or redox reactions In oxidation, a substance loses electrons, or is oxidized In reduction, a substance gains electrons, or is reduced (its charge becomes reduced, more negative) Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

8 Fig. 9-UN1 becomes oxidized (loses electron) becomes reduced (gains electron)

9 The reactant that gives away electrons is called the reducing agent (because it reduces the charge of whatever it gives the electrons to) The electron receptor is called the oxidizing agent (it increases the charge of whatever it gives electrons to) Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds An example is the reaction between methane and O 2 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

10 Fig. 9-3 A molecule can also be considered oxidized or reduced when its electrons move closer to or farther away from the central atom. Reactants becomes oxidized Products becomes reduced Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water

11 Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O 2 is reduced: becomes oxidized becomes reduced Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

12 Fig. 2-8 Transferring electrons can release Third shell (highest energy level) energy Second shell (higher energy level) Energy absorbed First shell (lowest energy level) Energy lost (b) Atomic nucleus When an electron moves to a more electronegative atom, it is held closer to the nucleus of that atom, so it loses potential energy.

13 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Stepwise Energy Harvest via NAD + and the Electron Transport Chain In cellular respiration, glucose and other organic molecules are broken down in a series of steps Electrons from organic compounds (like sugars) are usually first transferred to NAD +, an electron carrier When 2 electrons and 2 H + are added to NAD +, it becomes NADH (H + always travels with the electrons) NAD + + 2e- + H + NADH

14 NADH passes the electrons to the electron transport chain where their energy is harvested The electron transport chain passes electrons in a series of small steps instead of one explosive reaction The energy yielded is used to power the endergonic reaction ADP + P i ATP Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

15 The Stages of Cellular Respiration: A Preview Cellular respiration has three stages: Glycolysis (breaks down glucose into two molecules of pyruvate) In between step: Conversion of Pyruvate into Acetyl CoA The citric acid cycle (completes the breakdown of glucose) Oxidative phosphorylation (accounts for most of the ATP synthesis) Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

16 Fig Electrons carried via NADH Glycolysis Glucose Pyruvate Cytosol ATP Substrate-level phosphorylation

17 Fig Electrons carried via NADH Electrons carried via NADH and FADH 2 Glucose Glycolysis Pyruvate Citric acid cycle Cytosol Mitochondrion ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation

18 Fig Electrons carried via NADH Electrons carried via NADH and FADH 2 Glucose Glycolysis Pyruvate Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Cytosol Mitochondrion ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation ATP Oxidative phosphorylation

19 The process that generates most (~ 90%) of the ATP is called oxidative phosphorylation because it is powered by redox reactions Oxidative = things are being oxidized Phosphorylation = Phosphate groups are being added to ADP to make ATP BioFlix: Cellular Respiration Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

20 1 st stage: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis ( splitting of sugar ) breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases: Energy investment phase Energy payoff phase Glucose + 2ATP + 2 NAD + 2 Pyruvate+ 4 ATP + 2NADH Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

21 Fig. 9-8 Energy investment phase Glucose 2 ADP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 4 ATP formed 2 NAD e + 4 H + 2 NADH + 2 H + 2 Pyruvate + 2 H 2 O Net Glucose 2 Pyruvate + 2 H 2 O 4 ATP formed 2 ATP used 2 ATP 2 NAD e + 4 H + 2 NADH + 2 H +

22 Fig Glucose ATP 1 Hexokinase ADP Glucose Glucose-6-phosphate ATP 1 Hexokinase ADP Glucose-6-phosphate

23 Fig Glucose ATP 1 Hexokinase ADP Glucose-6-phosphate Fructose-6-phosphate 2 Phosphoglucoisomerase Glucose-6-phosphate 2 Phosphoglucoisomerase Fructose-6-phosphate

24 Fig Glucose ATP ADP 1 Hexokinase Fructose-6-phosphate Glucose-6-phosphate 2 Phosphoglucoisomerase Fructose-6-phosphate ATP 3 Phosphofructokinase ADP ATP ADP Fructose- 1, 6-bisphosphate 3 Phosphofructokinase Fructose- 1, 6-bisphosphate

25 Fig Glucose ATP 1 Hexokinase ADP Glucose-6-phosphate 2 Phosphoglucoisomerase Fructose-6-phosphate ATP 3 Phosphofructokinase Fructose- 1, 6-bisphosphate 4 Aldolase ADP 5 Isomerase Fructose- 1, 6-bisphosphate 4 Aldolase 5 Isomerase Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate

26 Fig NAD + 2 NADH + 2 H + 6 Triose phosphate dehydrogenase 2 P i 2 1, 3-Bisphosphoglycerate Glyceraldehyde- 3-phosphate 2 2 NAD + NADH 2 P i + 2 H + 6 Triose phosphate dehydrogenase 2 1, 3-Bisphosphoglycerate

27 Fig NAD + 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H + 2 1, 3-Bisphosphoglycerate 2 ADP 2 ATP 7 Phosphoglycerokinase 2 1, 3-Bisphosphoglycerate 2 ADP 2 3-Phosphoglycerate 2 ATP 7 Phosphoglycerokinase 2 3-Phosphoglycerate

28 Fig NAD + 6 Triose phosphate dehydrogenase 2 NADH + 2 H + 2 P i 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 3-Phosphoglycerate 8 Phosphoglyceromutase 2 3-Phosphoglycerate 2 2-Phosphoglycerate 8 Phosphoglyceromutase 2 2-Phosphoglycerate

29 Fig NAD + 2 NADH + 2 H + 6 Triose phosphate dehydrogenase 2 P i 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 3-Phosphoglycerate 2 2-Phosphoglycerate 8 Phosphoglyceromutase 2 2-Phosphoglycerate 9 Enolase 2 H 2 O 2 H 2 O 9 Enolase 2 Phosphoenolpyruvate 2 Phosphoenolpyruvate

30 Fig NAD + 2 NADH + 2 H + 6 Triose phosphate dehydrogenase 2 P i 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 3-Phosphoglycerate 8 Phosphoglyceromutase 2 2 ADP 2 ATP Phosphoenolpyruvate 10 Pyruvate kinase 2 2-Phosphoglycerate 9 Enolase 2 H 2 O 2 Phosphoenolpyruvate 2 ADP 2 ATP 10 Pyruvate kinase 2 Pyruvate 2 Pyruvate

31 In between step: pyruvate is converted into Acetyl-CoA. If O 2 is present, pyruvate enters the mitochondrion Before the citric acid cycle (also called the Kreb s cycle) can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis In- between step turning pyruvate into Acetyl-CoA: 2 Pyruvate + 2NAD + 2 Acetyl Co-A + 2 NADH + 2Co2 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

32 Fig CYTOSOL MITOCHONDRION NAD + NADH + H + 2 Pyruvate Transport protein 1 3 CO 2 Coenzyme A Acetyl CoA

33 Remember the parts of the mitochondrion?

34 The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix Each time you add an Acetyl CoA, the cycle turns once, generating 1 ATP, 3 NADH, and 1 FADH 2 per turn Since each glucose molecule generates 2 pyruvates, and thus 2 Acetyl CoAs, the cycle turns twice for each glucose molecule. 2 Acetyl CoA 2ATP + 6NADH + 2FADH 2 + 4CO 2 FADH 2 is an electron carrier that can hold 2 electrons, very similar to NADH. When it s not holding electrons, it is called FAD (like NAD + ) Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

35 Fig Pyruvate NAD + NADH CO 2 CoA + H + Acetyl CoA CoA CoA Citric acid cycle 2 CO 2 FADH 2 FAD 3 3 NAD + NADH + 3 H + ADP + P i ATP

36 The citric acid cycle has 8 steps, each catalyzed by a specific enzyme The acetyl part of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate (the CoA part is removed) The next seven steps change the citrate back to oxaloacetate, making the process a cycle The carbons that originally came from glucose are turned into CO 2 during the cycle NADH and FADH 2 carry electrons to the last stage Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

37 Fig Acetyl CoA CoA SH NADH +H + 1 H 2 O NAD + 8 Oxaloacetate 2 H 2 O 7 Malate Citric acid cycle Citrate Isocitrate NAD + 3 CO 2 NADH + H + Fumarate 6 CoA SH CoA SH 4 -Ketoglutarate FADH 2 FAD 5 NAD + CO 2 Succinate GTP GDP P i Succinyl CoA NADH + H + ADP ATP

38 During oxidative phosphorylation, electron transport creates an H+ concentration gradient, which generates ATP Most of the energy from glucose is still stored inside the electrons that are being carried by NADH and FADH 2 These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis in the oxidative phosphorylation stage of cellular respiration Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

39 The Pathway of Electron Transport The electron transport chain is in the cristae of the mitochondrion Most of the chain s components are proteins Each protein is more electronegative than the last Electrons drop in free energy as they go down the chain and are finally passed to O 2, forming H 2 O Image from: Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

40 Fig NADH 50 2 e NAD + FADH 2 40 FMN Fe S e 2 FAD FAD Fe S Multiprotein complexes Q Cyt b 30 Fe S Cyt c 1 Cyt c I V Cyt a 20 Cyt a e (from NADH or FADH 2 ) 0 2 H / 2 O 2 H 2 O

41 Electrons are transferred from NADH or FADH 2 to the electron transport chain Electrons are passed through a number of proteins and finally to O 2 (which came from breathing) The electron transport chain itself generates no ATP The chain s function is to break the large freeenergy drop from food to O 2 into smaller steps that release energy in manageable amounts that can be used to do work Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

42 The Energy-Coupling Mechanism Electron transfer in the electron transport chain releases energy, causing proteins to pump H + from the mitochondrial matrix to the intermembrane space by active transport H + then moves back across the membrane, passing through channels in ATP synthase ATP synthase is like a turbine. The energy released by H + as it flows through the synthase is added onto ADP and P i to allow them to form ATP. Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

43 Fig INTERMEMBRANE SPACE H + ADP + P i ATP MITOCHONDRIAL MATRIX

44 As the H+ ions go down their concentration gradient through the ATP synthase, ATP synthase turns and phosphorylates ADP, making ATP. Each NADH makes about 2.5 ATP Each FADH 2 makes about 1.5 ATP, because FADH 2 donates its electrons in the middle of the electron transport chain, not at the beginning, so not as many H+ are pumped across the membrane for each FADH 2 Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

45 Fig H + H + Protein complex of electron carriers H + H + FADH 2 FAD 2 H / 2 O 2 H 2 O ATP synthase NADH (carrying electrons from food) NAD + ADP + P i H + ATP 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation

46 An Accounting of ATP Production by Cellular Respiration During cellular respiration, most energy flows with the electrons, which move in this order: glucose NADH electron transport chain H + gradient force ATP About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP What happens to the rest of the energy from glucose? Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

47 Fig CYTOSOL Electron shuttles span membrane 2 NADH or MITOCHONDRION 2 FADH 2 2 NADH 2 NADH 6 NADH 2 FADH 2 Glycolysis 2 Glucose Pyruvate 2 Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis + 2 ATP + 2 ATP + about 26 or 28 ATP Maximum per glucose: About 30 or 32 ATP

48 Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Most cellular respiration requires O 2 to produce ATP Glycolysis can produce its 2 ATP with or without O 2 (in aerobic or anaerobic conditions) In the absence of O 2, glycolysis is used to make ATP, followed by fermentation or anaerobic respiration Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

49 Anaerobic respiration uses an electron transport chain with an electron acceptor other than O 2, for example sulfate Mostly used by bacteria that live in oxygen-free environments Fermentation uses glycolysis, but not the citric acid cycle or electron transport chain, to make ATP Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

50 Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD +, which can be reused by glycolysis Two common types are alcohol fermentation and lactic acid fermentation Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

51 In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO 2 Alcohol fermentation by yeast is used in brewing, winemaking, and baking Pyruvate + NADH Alcohol + CO 2 + NAD + Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

52 Fig. 9-18a 2 ADP + 2 P i 2 ATP Glucose Glycolysis 2 Pyruvate 2 NAD + 2 NADH 2 CO H + 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation

53 In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO 2 Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O 2 is scarce Pyruvate + NADH Lactate + NAD + Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

54 Fig. 9-18b 2 ADP + 2 P i 2 ATP Glucose Glycolysis 2 NAD + 2 NADH + 2 H + 2 Pyruvate 2 Lactate (b) Lactic acid fermentation

55 Why do fermentation? Why not just keep making ATP through glycolysis? Without oxygen, pyruvate builds up and all the NAD + carriers get full of electrons. NAD + Eventually glycolysis will stop! Fermentation regenerates the NAD + carriers so glycolysis can continue.

56 Fermentation and Aerobic Respiration Compared Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate Aerobic respiration uses O 2 ; fermentation doesn t. Aerobic respiration produces ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

57 Fig Glucose CYTOSOL Glycolysis No O 2 present: Fermentation Pyruvate O 2 present: Aerobic cellular respiration Ethanol or lactate Acetyl CoA MITOCHONDRION Citric acid cycle

58 Glucose is not the only fuel for respiration Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration, including carbohydrates, fats, and proteins These molecules may be partially broken down before entering glycolysis or the citric acid cycle Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

59 Fig Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P NH 3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation

60 Remember! Why do we need carbohydrates? CARBOHYDRATES SUPPLY ENERGY Cells burn GLUCOSE for their energy needs (easier than burning fats or proteins) Images from:

61 EXERCISE and ENERGY (Short term energy) Cells normally contain small amounts of ATP produced by glycolysis & cellular respiration (only enough for a few seconds of activity) Once this ATP is used up, lactic acid fermentation can provide enough ATP to last about 90 seconds of intense exercise.

62 EXERCISE and ENERGY (Short term energy) Once the race is over, lactic acid must be broken down using oxygen. A quick sprint builds up an oxygen debt that must be repaid by heavy breathing. Image from:

63 Well conditioned athletes must pace themselves during a long race. EXERCISE and ENERGY (LONGER term energy) For exercise longer than 90 seconds Aerobic respiration is the only way to make enough ATP. Cellular respiration releases energy more slowly than fermentation and requires plenty of oxygen.

64 What happens in a long race when the body s glucose is all used up? Animal cells store glucose as glycogen to use later. Image from:

65 EXERCISE and ENERGY (LONGER term energy) Muscle cells store glycogen which can be broken down into glucose to supply energy for minutes of activity.

66 EXERCISE and ENERGY (LONGER term energy) After glycogen stores are used up the body breaks down fat. That s why aerobic exercise must continue for longer than 20 minutes if you want to lose fat!

67 ALL CELLS NEED ENERGY All eukaryotes (including plant and animal cells) have mitochondria for cellular respiration All prokaryotes (bacteria and archaea) have their electron transport enzymes attached to their cell membranes

68 How do cells control how much ATP is produced? Feedback inhibition is the most common mechanism for control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of cellular respiration is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway ATP can act as an inhibitor for these enzymes Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

69 Fig Glucose Inhibits Glycolysis Fructose-6-phosphate Phosphofructokinase Fructose-1,6-bisphosphate AMP Stimulates + Inhibits Pyruvate ATP Acetyl CoA Citrate Citric acid cycle Oxidative phosphorylation