The Production of ATP Catabolic Pathways Cellular respiration is a catabolic pathway that consumes organic molecules and yields ATP. Carbohydrates, fats, and proteins can all fuel cellular respiration. We'll look first at the simplest case, the breakdown of the sugar glucose. But before doing that we have to learn about two molecules that are essential to respiration. 36
NAD + and FAD The molecules NAD + and FAD are used to store, and later release, energy during respiration; they are key to respiration. Each molecule has two forms, each form stores a different amount of energy. So moving between those two forms either stores chemical potential energy or releases it. Here are the reactions: NAD + + 2H + + 2e + Energy NADH + H + FAD + 2H + + 2e + Energy FADH 2 The double arrows indicate that each reaction is reversible, they can proceed in either direction. When the reaction goes to the right, energy is stored. When it goes to the left, energy is released 37
NAD + and FAD NAD + + 2H + + 2e + Energy NADH + H + FAD + 2H + + 2e + Energy FADH 2 The amount of energy that is useable when the reaction goes to the left, depends on the availability of electron acceptors . Without a molecule, such as O 2 , to accept the excess electrons the energy stored in NADH and FADH 2 cannot be used to make ATP. 38
Electron Acceptors Oxygen is the best electron acceptor because it generates the greatest free energy change ( ∆ G) and produces the most energy. In the absence of oxygen, other molecules, such as nitrate, sulfate, and carbon dioxide can be used as electron acceptors. If O 2 is present , • 1 NADH stores enough energy to create about 3 ATPs • 1 FADH 2 stores enough energy to make about 2 ATPs 39
7 NADH is converted to NAD + . During this process, A energy is released B energy is stored C no energy is stored or released Answer 40
8 FADH 2 is converted to FAD. During this process, A energy is stored B energy is released C no energy is stored or released Answer 41
Reduction and Oxidation NAD + + 2H + + 2e + Energy NADH + H + FAD + 2H + + 2e + Energy FADH 2 When we go from left to right we are adding electrons to a molecule. That is called reducing the molecule, or the process of reduction . Going from right to left, we are taking electrons from a molecule. That is called oxidizing the molecule, or the process of oxidation . 42
Oxidation The reason for the term oxidation is that this is the effect that oxygen usually has: it takes electrons from a molecule, oxidizing the molecule The rusting of iron is an example of oxidation: oxygen is taking electrons from the metal, oxidizing it. 4 Fe + 3 O 2 → 2 Fe 2 O 3 43
Reduction and Oxidation LEO says GER Since it doesn't seem right that adding L osing electrons is called E lectrons is "reduction"; here's a O xidation way to remember these two terms. G aining E lectrons is R eduction 44
9 Which of the following cannot act as an electron acceptor? A sulfate B oxygen C ammonia D nitrate Answer 45
10 The loss of an electron is __________ and the gain of an electron is ____________. A oxidation, reduction B reduction, oxidation C catalysis, phosphorylation Answer D phosphoroylation, catalysis 46
11 NADH is the reduced form of NAD + . True False Answer 47
Types of Cellular Respiration Cells follow different paths of cellular respiration depending on the presence or absence of oxygen. Cells can be classified into 3 categories based on their response to oxygen. • Obligate Anaerobes which cannot survive in the presence of oxygen • Obligate Aerobes which require oxygen • Facultative Anaerobes which can survive in the presence or absence of oxygen. 48
The Stages of Respiration Cellular respiration consists of four stages: • Glycolysis • Pyruvate Decarboxylation • The Citric Acid Cycle (Krebs Cycle) • Oxidative Phosphorylation 49
Glycolysis Glycolysis is the first stage of cellular respiration. It involves the breakdown of glucose , a 6 carbon sugar, into 2 molecules of pyruvate , a 3 carbon sugar. C 6 H 12 O 6 Glycolysis means the Glycolysis means the (Glucose) splitting of glucose splitting of glucose 2 NAD + 2 ATP Some ATP is needed Some ATP is needed Gycolysis to start the process to start the process (E a ) (E a ) 4 ATP 2 NADH The net result is: The net result is: 2 C 3 H 4 O 3 (Pyruvate) a net of 2 ATPs are a net of 2 ATPs are formed along with 2 formed along with 2 NADHs and the 2 NADHs and the 2 pryuvates. pryuvates. 50
12 Until 2.5 billon years ago there was no oxygen in the Earth's atmosphere. Which of the following was also not present? A facultative anaerobes B obligate anaerobes Answer C obligate aerobes D bacteria 51
13 How much activation energy is required to start glycolysis? A 0 ATP B 1 ATP C 2 ATP Answer D 4 ATP 52
14 The net products of glycolysis are: A 2 pyruvate B 2 NADH and 2 pyruvate C 2 ATP, 2 NADH, and 2 pyruvate D 4 ATP, 2 NADH, and 2 pyruvate Answer 53
Pyruvate Decarboxylation (PD) The Citric Acid Cycle can only process 2carbon molecules, and pyruvate is a 3carbon molecule: C 3 H 4 O 3 PD is an enzyme catalyzed reaction that takes the 2 2 C 3 H 4 O 3 (Pyruvate) pyruvate molecules and 2 NAD + converts them to 2 Acetyl Co A molecules: these are PDC 2carbon molecules. 2 NADH 2 CO 2 Energy is stored during PD by the converting 2 NAD + to 2 2 Acetyl CoA NADH and the extra pyruvate carbons are expelled as CO 2 . 54
The Citric Acid Cycle This shows one cycle, which is due to one Acetyl CoA molecule. To account for one glucose molecule, two cycles are needed. Let's tally up the output for one cycle to confirm our results. 55
The Citric Acid Cycle This is one turn of the cycle, due to 1 Acetyl Co A. Note the production of: 1 ATP 1 ATP 3 NADH 3 NADH 1 FADH 2 1 FADH 2 But 1 glucose molecule, yields 2 Acetyl CoA molecules, (therefore, 2 turns of the cycle) yielding : 2 ATP 6 NADH Click here for a video of the Citric Acid Cycle 2 FADH 2 56
The Citric Acid Cycle The citric acid cycle is sometimes called the Krebs cycle . The cycle breaks down one AcetylCoA for each turn, generating 1 ATP, 3 NADH, 2 CO 2 and 1 FADH 2 per AcetylCoA. Since 2 AcetylCoA molecules were created from each glucose, the Citric Acid Cycle creates 2 ATP; 6 NADH; 4CO 2 , and 2 FADH 2 for each glucose molecule. 57
15 Glycolysis produces ____ ATP. Pyruvate Decarboxylation produces ____ ATP. The Citric Acid Cycle produces _____ ATP. Answer A 1, 1, 2 B 4, 0, 2 C 4, 0, 4 D 2, 0, 2 58
16 During pyruvate decarboxylation, 3carbon pyruvate is converted to 2carbon AcetylCoA. What happens to the excess carbons atoms in this process? A They are expelled in molecules of CH 4 B They are expelled in molecules of CO 2 C They are covalently bonded to NADH Answer D They are recycled to reform glucose 59
17 In total, the first 3 stages of cellular respiration produce how many molecules of carbon dioxide? A 1 B 2 C 3 Answer D 6 60
Oxidative Phosphorylation (OP) So far we've done a lot of work to just get a net gain of 4 ATPs. But we have stored a lot of potential energy in the form of NADH and FADH 2 . The big energy payoff is in oxidative phosphorylation , where we convert the energy stored in those molecules to ATP. 61
Oxidative Phosphorylation (OP) We're now going to convert all the NADH and FADH 2 into ATP, so the energy can be stored throughout the cell. Here's what we start this cycle with. Stage NADH FADH2 ATP Glycolysis 2 0 2 2 0 PD 0 6 2 CAC 2 Total 10 2 4 When O 2 is present, we get about 3 ATPs per NADH and 2 ATPs per FADH 2 . So how many ATPs would we have at the end of this next stage? 62
Electron Transport Chain (ETC) Oxidative phosphorylation is powered by the electron transport chain. One way to think of the ETC is as a proton pump . The ETC transports electrons, through chemical reactions, out and then back through a plasma membrane. The net effect is to pump protons from the inside to the outside of a plasma membrane, creating a proton gradient which is used to power oxidative phosphorylation. 63
Electron Transport Chain (ETC) The proton path in red. The electron path is shown in black. The ETC generates no ATP, but enables Oxidative Phosphorylation, which accounts for most of the ATP produced. 64
Anaerobic ETC For the first 2 billion years of life on Earth, anaerobic (no O 2 ) respiration was the only means of obtaining energy from food. , SO 4 2 , or These organisms used the electron acceptors, NO 3 CO 2 to pull the electrons through the ETC. These molecules would accept the electrons at the end of the chain forming N 2 , H 2 S, and CH 4 respectively. 65
Aerobic ETC But then, the Oxygen Revolution occurred about 2.5 BYA, flooding the planet with oxygen. In aerobic respiration, the final electron acceptor of the electron transport chain is O 2 ; forming water (H 2 O) . Oxygen strongly attracts electrons in order to fill its outer shell. This stronger pull makes much more energy available to life, enabling the more complex food chains we see today. Click here for a video of the ETC 66
18 Which of the following is created during the electron transport chain in human cells? A I, II, III, IV I ATP B I, II only Answer II NADH C III only D III, IV only III proton gradient IV H 2 O 67
19 Obligate aerobes use which of the following as their final electron acceptor? A CO 2 B NO 3 C O 2 Answer D SO 4 2 68
Oxidative Phosphorylation (OP) The ETC creates a positive electrostatic potential outside the plasma membrane and a negative potential inside. The excess protons outside, are strongly attracted to the inside, but are blocked by the membrane. One path is open to the protons, but they must do work to use it. ATP Synthase is essentially a motor, constructed of proteins. The protons must travel through that motor in order to return to the cell, creating an electric current that powers the motor. As the motor turns, it adds a phosphate group to ADP, creating ATP. Electrical energy is transformed to chemical energy. Click here for a video of ATP Synthase 69
Oxidative Phosphorylation The Hydroelectric Analogy The Hoover Dam is a massive structure that holds back the potential energy of 9 trillion gallons of water 70
Oxidative Phosphorylation The Hydroelectric Analogy Like oxidative phosphorylation, it creates a gradient then exploits the stored energy by allowing water to pass through a small pipeline, transforming it to kinetic energy. 71
Oxidative Phosphorylation The Hydroelectric Analogy Massive turbines are spun, causing the kinetic energy to be turned into mechanical energy which is utilized to make electrical energy. 72
Aerobic Respiration We calculated earlier that we would expect to get 38 ATP molecules by the time we'd converted all the NADH and FADH 2 to ATP. The actual yield is between 36 38 ATP molecules per glucose molecule. The reason for the small variance is that in some cases energy is needed to transport the NADH molecules to the site of the ETC. 73
20 ATP synthase... A synthesizes ATP B is an enzyme C is a protein complex D all of the above Answer 74
21 Energy released by the electron transport chain is used to pump H+ ions into which location? A Outside the membrane B Inside the membrane Answer 75
22 What is the maximum number of ATP produced from a breakdown of a glucose molecule? A 4 B 18 C 36 Answer D 38 76
The Versatility of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration. · Glycolysis accepts a wide range of carbohydrates · Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle · Fats are digested to glycerol which is used in glycolysis. An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate 77
The Versatility of Catabolism 78
Fermentation Return to Table of Contents 79
Fermentation When no electron acceptors are available, obligate anaerobes and facultative anaerobes can still break down glucose to release energy through a process called fermentation. Fermentation begins just as cellular respiration does, with glycolysis. 80
Fermentation C 6 H 12 O 6 Glycolysis results in 2 pyruvate (Glucose) molecules and 2 NADH 2 molecules. Without an electron 2 NAD + 2 ATP acceptor, the energy stored in these molecules can't be used. Gycolysis The net energy gain is just 2 ATPs. 4 ATP 2 NADH (Remember 2 were invested and 4 were produced, netting 2) 2 C 3 H 4 O 3 (Pyruvate) 81
Fermentation However, the Pyruvate still needs C 6 H 12 O 6 to be cleared from the cell, and the (Glucose) NADH converted back to NAD + to 2 NAD + 2 ATP begin another cycle. The process of doing this is called Gycolysis fermentation . 4 ATP 2 NADH No additional energy is released 2 C 3 H 4 O 3 during this process. (Pyruvate) 82
Types of Fermentation There are two types of fermentation: 2 C 3 H 4 O 3 (Pyruvate) • Lactic acid fermentation 2 NADH • Ethanol fermentation Fermentation 2 NAD + OR Ethanol Lactic Acid Fermentation Fermentation 2 Lactic Acid CO 2 & 2 Ethanol 83
Fermentation Fermentation breaks down the products of glycolysis so that glycolysis can be repeated with another glucose molecule. 1 glucose molecule had yielded 1 glucose molecule had yielded 2 C 3 H 4 O 3 (Pyruvate) 2 NADH 2 ATPs, 2 Pyruvates and 2 2 ATPs, 2 Pyruvates and 2 NADHs. That is the input to the NADHs. That is the input to the Fermentation fermentation stage of anaerobic fermentation stage of anaerobic respiration. respiration. 2 NAD + OR Lactic Acid Ethanol Fermentation Fermentation The pyruvates and NADHs are The pyruvates and NADHs are fermented into 2 NAD + and fermented into 2 NAD + and 2 Lactic Acid either Lactic Acid or CO 2 & Ethanol. CO 2 & Ethanol. either Lactic Acid or CO 2 & 2 Ethanol 84
Fermentation The result of the combined steps of glycolysis and fermentation is: • The input is 1 Glucose + 2 ATP molecules • The output is 4 ATP molecules (for a net gain of 2 ATP's) In addition, • Lactic Acid fermentation results in lactic acid • Ethanol fermentation results in ethanol and CO 2 85
Cellular Respiration vs. Fermentation The big difference is that for each glucose molecule: aerobic cellular respiration yields 36 to 38 ATPs fermentation yields only 2 ATPs 86
Examples of Fermentation • Some anaerobic bacteria rely soley on fermentation, such as lactobacillus, which is used to make cheese and yogurt. • The alcohol in wine, beer, etc. results from yeast (a facultative anaerobe) undergoing ethanol fermentation. • Bread rises due to the release of CO 2 bubbles by fermenting yeast. • Your muscles burn after a strenuous workout because they can't get enough O 2 , so they perform lactic acid fermentation. Lactic acid results in soreness. 87
23 When a cell has completed glycolysis and lactic acid fermentation, the final products are: I Lactic acid Answer II Ethanol I, II, III, IV, V A I, II, III, V B III Carbon dioxide C I, IV, V I, V D IV NADH V ATP 88
24 Bread rises due to the production of _______ during fermentation. A ethanol B carbon dioxide C lactic acid Answer D pyruvate 89
25 Muscles produce lactic acid during strenuous exercise. Therefore, muscles are an example of what kind of cell? A facultative anaerobe B facultative aerobe C obligate anaerobe Answer D obligate aerobe 90
Photosynthesis Return to Table of Contents 91
Photosynthesis Respiration gets energy from glucose and stores it as ATP. But what is the source of glucose? And, where did the oxygen that flooded Earth 2.5 BYA come from? 92
Aerobic Respiration vs. Photosynthesis Here's the balanced chemical equation for aerobic respiration : C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + ATP And here's the balanced chemical equation for photosynthesis : 6CO 2 + 6H 2 O + Light Energy C 6 H 12 O 6 + 6O 2 93
Aerobic Respiration vs. Photosynthesis C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + ATP Aerobic respiration uses oxygen (O 2 ) and glucose (C 6 H 12 O 6 ) to create carbon dioxide (CO 2 ) and water (H 2 O)...and release energy. 6CO 2 + 6H 2 O + Light Energy C 6 H 12 O 6 + 6O 2 Photosynthesis is the exact opposite, it takes carbon dioxide (CO 2 ) and water (H 2 O) plus energy to make glucose (C 6 H 12 O 6 ) and oxygen (O 2 ) 94
Photosynthesis and Respiration Summing these two equations reveals that the ATP used by cells is derived from light energy, from the sun. That is the source of energy for most life on Earth. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + ATP (Energy) 6CO 2 + 6H 2 O + Light Energy C 6 H 12 O 6 + 6O 2 Light Energy ATP (Energy) 95
Photosynthesis and Respiration Light Energy ATP (Energy) Except for a small number of bacteria that live on chemical reactions in challenging environments, the energy for all life on Earth comes from these processes...from the energy of sunlight. Even though not every organism undergoes photosythesis, the products that plants produce are used in reactions that consumers use. In this way, you can say that . . . You are solar powered! 96
26What are the reactants of cellular respiration? A Oxygen and Water B Glucose and Carbon Dioxide Answer C Glucose and Water D Glucose and Oxygen 97
27What are the products of photosynthesis? A Glucose and Oxygen B Oxygen and Water Answer C Glucose and Carbon Dioxide D Carbon Dioxide and Water 98
28What are the reactants of photosynthesis? A Carbon Dioxide and Water B Oxygen and Water C Glucose and Oxygen Answer D Glucose and Carbon Dioxide 99
29Photosynthesis ____________ energy, whereas cellular respiration __________ energy. A consumes, produces B produces, consumes Answer C produces, produces D consumes, consumes 100
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