Bioenergetics. Finding adequate sources of energy is a constant challenge for all living organisms, including this bear.

Transcription

1 33 Bioenergetics Finding adequate sources of energy is a constant challenge for all living organisms, including this bear. Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena

2 Course 33.1 Energy Changes in Living Organisms 33.2 Metabolism and Cell Structure 33.3 Biological Oxidation-Reduction: Energy Delivery 33.4 Molecular Oxygen and Metabolism 33.5 High-Energy Phosphate Bonds 33.6 Phosphorylation: Energy Conversion 33.7 Photosynthesis 33 Summary 2

3 Energy Changes in Living Organisms One basic requirement for life is a source of energy. Bioenergetics is the study of the transformation, distribution, and utilization of energy by living organisms. The major source of biological energy is the chemical reactions occurring inside cells. The bioenergetics of a cell can be compared to the energetics of a manufacturing plant as seen on the following slide... 3

4 Energy Changes in Living Organisms First, energy is delivered to the cell or manufacturing plant. Second, this energy is converted to a more usable form. Third, work is done. 4

5 Metabolism and Cell Structure The sum of all chemical reactions that occur within a living organism is defined as metabolism. Metabolism is subdivided into two contrasting categories: anabolism and catabolism... 5

6 Metabolism and Cell Structure Anabolism is the process by which simple substances are synthesized (built up) into complex substances. Catabolism is the process by which complex substances are broken down into simpler substances. Anabolic reactions usually involve carbon reduction and consume cellular energy. Catabolic reactions usually involve carbon oxidation and produce energy for the cell. 6

7 Metabolism and Cell Structure Cells segregate many of their metabolic reactions into specific, subcellular locations. The simple procaryotes (cells without internal membranebound bodies) have a minimum amount of spatial organization. Metabolic reactions in the cells of higher plants and animals are often segregated into specialized compartments. These cells, the eucaryotes, contain internal, membrane-bound bodies called organelles as seen on the following slide... 7

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9 Metabolism and Cell Structure In procaryotes the anabolic processes of DNA and RNA synthesis are localized in the nuclear material, whereas most other metabolic reactions are spread throughout the cytoplasm. In the eucaryotic cell, most of the DNA and RNA syntheses are localized in the nucleus. Anabolism of proteins takes place in the ribosomes, whereas that of carbohydrates and lipids occurs primarily in the cytoplasm. 9

10 Your Turn! The formation of carbohydrates from carbon dioxide and water is anabolic or catabolic? 10

11 Your Turn! The formation of carbohydrates from carbon dioxide and water is anabolic or catabolic? This process is anabolic since this reaction requires energy and produces a more complex molecule. 11

12 Biological Oxidation-Reduction: Energy Delivery The ultimate source of biological energy on Earth is sunlight. Plants capture light energy and transform it to chemical energy by a process called photosynthesis. This chemical energy is stored in the form of reduced carbon atoms in carbohydrate molecules. The energy contained in carbohydrates, lipids, and proteins comes from sunlight. 12

13 Biological Oxidation-Reduction: Energy Delivery Humans draw most of their energy from foodstuffs that contain reduced carbons. Fats are more reduced than carbohydrates and are more energy rich. 13

14 Biological Oxidation-Reduction: Energy Delivery The oxidation of food molecules result in a release of energy. This is similar to the release of energy when wood is burned. However the energy released by cells when food is oxidized occurs in a stepwise fashion. The energy is not released all at once. Carbohydrate carbons are oxidized to carbon dioxide during oxidation. This gas is exhaled as a waste product. 14

15 Your Turn! Which molecule is likely to provide more energy when oxidized? CHO COOH H C OH H C H HO C H H C H H C OH H C H H C OH H C H CH 2 OH D-glucose CH 3 hexanoic acid 15

16 Your Turn! Hexanoic acid is less oxidized than D-glucose. Notice that hexanoic acid contains few oxygen atoms. Hexanoic acid would provide more energy upon oxidation since it is less oxidized to start. CHO COOH H C OH H C H HO C H H C H H C OH H C H H C OH H C H CH 2 OH D-glucose CH 3 hexanoic acid 16

17 Biological Oxidation-Reduction: Energy Delivery When energy levels change, energy must be released or absorbed as shown by the arrows in the diagram. Photosynthesis causes carbons to move to a higher energy level, as they are reduced to carbohydrate carbons. The carbohydrate carbons are then oxidized to a lower energy level with the release of energy to do work. 17

18 Biological Oxidation-Reduction: Energy Delivery In eucaryotic cells, specific organelles are present that specialize in redox reactions. The mitochondria are the sites for most of the catabolic redox reactions. 18

19 Biological Oxidation-Reduction: Energy Delivery Chloroplasts are organelles found in higher plants and contain an electron-transport system that is responsible for the anabolic redox reactions in photosynthesis. 19

20 Biological Oxidation-Reduction: Energy Delivery To move electrons from one place to another the cell uses a set of redox coenzymes. The redox coenzymes act as temporary storage places for electrons. The three most common redox coenzymes are nicotinamide adenine dinucleotide, NAD + nicotinamide adenine dinucleotide phosphate, NADP + flavin adenine dinucleotide, FAD. Humans synthesize NAD + and NADP + from the vitamin niacin, while FAD is made from the vitamin riboflavin. 20

21 Biological Oxidation-Reduction: Energy Delivery Structures of NAD + and NADP + are shown here. 21

22 Biological Oxidation-Reduction: Energy Delivery The structure of FAD is shown here. 22

23 Your Turn! Where is the adenosine diphosphate portion on this NAD + molecule? 23

24 Your Turn! Where is the adenosine diphosphate portion on this NAD + molecule? 24

25 Your Turn! What is the structural difference between NAD + and NADP +. 25

26 Your Turn! NADP + has a phosphate group attached to the ribose ring. 26

27 Your Turn! Which structure in the equation below is the oxidized form of the nicotinamide ring and which is the reduced form? 27

28 Your Turn! oxidized form reduced form The addition of electrons and hydrogen is reduction. The structure with less electrons and hydrogen (the oxidized form) is reduced by the addition of electrons and 28 hydrogen.

29 Biological Oxidation-Reduction: Energy Delivery A very important function of the redox coenzymes is to carry electrons to the mitochondrial electron-transport system. As the coenzymes are oxidized, molecular oxygen is reduced. 29

30 Biological Oxidation-Reduction: Energy Delivery Energy is released with the movement of electrons. Over 85% of a typical cell s energy is derived from this redox process. Energy released by the redox process is not used immediately by the cell but is instead stored, usually in high-energy phosphate bonds such as those in ATP. 30

31 Your Turn! What is wrong with the following equation? O O C O - C O + NAD + H C O - C OH + NADH CH 3 CH 3 31

32 Your Turn! O O C O - C O + NAD + oxidized coenzyme H C O - C OH + NADH reduced coenzyme CH 3 oxidized organic molecule CH 3 reduced organic molecule NAD + is the oxidized form of the NADH coenzyme and NADH is the reduced form. NAD + is being reduced so the organic reactant should be oxidized, but in this equation the organic reactant is also being reduced. Here organic product is more reduced than the organic 32 reactant. The organic product contains more hydrogen.

33 Your Turn! O O C O - C O + NADH reduced coenzyme H C O - C OH + NAD + oxidized coenzyme CH 3 oxidized organic molecule CH 3 reduced organic molecule The correct way to show the equation is as follows where the organic reactant is reduced and the coenzyme is oxidized. 33

34 Your Turn! Place FAD and FADH 2 in the equation below to correctly show the oxidation of the organic reactant. COO - CH 2 CH 2 COO - COO - CH CH COO - 34

35 Your Turn! The FAD gains hydrogen to form FADH 2. The coenzyme with more hydrogen is the reduced form. The organic reactant has more hydrogen than the organic product. The organic reactant is more reduced than the product. COO - CH 2 CH 2 COO - Reduced organic molecule COO - CH + FAD + FADH 2 Oxidized Reduced coenzyme CH coenzyme COO - Oxidized organic molecule 35

36 Molecular Oxygen and Metabolism Molecular oxygen plays a critical role in energy production. It acts as the final receptacle for electrons in the mitochondrial electron-transport system. Aerobic metabolism (metabolism in the presence of molecular oxygen) is the best way to produce energy for most cells. 36

37 Molecular Oxygen and Metabolism The chemical reactions inside mitochondria are specifically designed to carry out a four-electron redox reaction with diatomic oxygen. Other reduced products of O 2 are dangerous. They are known as reactive oxygen species (ROS), and can react with and destroy many vital cell molecules. For example sometimes a two-electron redox reaction occurs which makes hydrogen peroxide (an ROS). 37

38 Molecular Oxygen and Metabolism Since cells have to live with the danger of ROS they have developed defense mechanisms including two important protective enzymes. Most cells carry an enzyme, superoxide dismutase, that destroys superoxides, O 2 - (another ROS) by making hydrogen peroxide. A second enzyme, catalase, can convert the hydrogen peroxide into water. 38

39 High-Energy Phosphate Bonds Cells need an energy-delivery system. Most cellular energy is produced in the mitochondria, but this energy must be transported throughout the cell. Such a delivery system must carry relatively large amounts of energy and be easily accessible to cellular reactions. Molecules that contain high-energy phosphate bonds meet this need. 39

40 High-Energy Phosphate Bonds The most common high-energy phosphate bond within the cell is the phosphate anhydride bond (or phosphoanhydride bond). 40

41 High-Energy Phosphate Bonds The phosphate anhydride bond is an important component of the nucleotide triphosphates, the most important of which is adenosine triphosphate (ATP). 41

42 High-Energy Phosphate Bonds Adenosine triphosphate functions by storing and transporting the energy in its high-energy phosphate bonds to the places in the cell where energy is needed. ATP is the common intermediary in energy metabolism. The cell realizes several advantages by storing energy in ATP... 42

43 High-Energy Phosphate Bonds First, the stored energy is easily accessible to the cell and readily released by a hydrolysis reaction yielding adenosine diphosphate (ADP) and an inorganic phosphate ion (P i ). 43

44 High-Energy Phosphate Bonds Second, ATP serves as the common energy currency for the cell. Energy from catabolism of many different kinds of molecules is stored in ATP. In the cell, energy utilization is greatly simplified by converting stored energy to ATP, the common energy currency. 44

45 Your Turn! ATP stands for adenosine triphosphate and ADP stands for adenosine diphosphate. What does AMP stand for? Would the conversion of ADP to AMP require energy or release energy? 45

46 Your Turn! ATP stands for adenosine triphosphate and ADP stands for adenosine diphosphate. What does AMP stand for? Would the conversion of ADP to AMP require energy or release energy? AMP stands for adenosine monophosphate. The conversion of ADP to AMP would release energy because it would entail the breaking of a high-energy phosphate anhydride bond. AMP contains one less phosphate than ADP. 46

47 Phosphorylation: Energy Conversion There are two forms for chemical storage of biological energy. reduced carbon atoms high-energy phosphate bonds The energy obtained from oxidation of carbon atoms is converted to high-energy phosphate bonds in molecules like ATP. 47

48 Phosphorylation: Energy Conversion The energy flow from nutrients with reduced carbons (energy-yielding nutrients) to high-energy phosphate bonds that are used to do work is summarized below. 48

49 Phosphorylation: Energy Conversion Energy is stored in phosphate anhydride bonds through two biological processes. substrate-level phosphorylation oxidative phosphorylation 49

50 Phosphorylation: Energy Conversion Substrate-level phosphorylation is the process whereby energy derived from oxidation is used to form highenergy phosphate bonds on various biochemical molecules (substrates). 50

51 Phosphorylation: Energy Conversion Three biological molecules that contain high-energy phosphate bonds (phosphorylated substrates) are shown here. 51

52 Phosphorylation: Energy Conversion The phosphorylated substrates often transfer the phosphate to ADP to form ATP. This process is called substratelevel phosphorylation because ADP gains a phosphate from a cellular substrate. 52

53 Phosphorylation: Energy Conversion Oxidative phosphorylation uses energy from redox reactions to form ATP. This process is found in the mitochondria and starts by oxidizing the two coenzymes, NADH and FADH 2, using molecular oxygen in a process called mitochondrial electron transport. 53

54 Phosphorylation: Energy Conversion The process involving FADH 2 oxidation is shown here. Notice that two moles of ATP are formed from one mole of FAD. 54

55 Phosphorylation: Energy Conversion The process involving NADH oxidation is shown here. Notice that three moles of ATP are formed from one mole of NAD +. 55

56 Phosphorylation: Energy Conversion Energy is released when NADH and FADH 2 are oxidized. This energy is trapped when ADP is phosphorylated to ATP. Thus, oxidative phosphorylation combines oxidative release of energy with phosphorylation of ADP. 56

57 Your Turn! FADH 2 and NADH are oxidized to form ATP during oxidative phosphorylation. The oxidation of which coenzyme produces more energy? 57

58 Your Turn! FADH 2 and NADH are oxidized to form ATP during oxidative phosphorylation. The oxidation of which coenzyme produces more energy? The oxidation of one mole of FADH 2 results in the formation of two moles of ATP. The oxidation of NADH results in the formation of three moles of ATP. The oxidation of NADH produces more energy. 58

59 Your Turn! How many moles of ATP can be produced from 0.75 mol of NADH and 1.25 mol of FADH 2 during mitochondrial electron transport and oxidative phosphorylation? 59

60 Your Turn! 2.25 moles of ATP produced from the NADH and 2.50 moles of ATP produced from the FADH 2. The total amount of ATP produced is 4.75 moles. 3 mole ATP 0.75 mol NADH x = 1mol NADH 2.25 mol ATP 1.25 mol FADH 2 mole ATP x 1mol FADH 2 = mol ATP 2.25 mol ATP moleatp = 4.75 mol ATP 60

61 Your Turn! How would you classify the reaction shown below (substrate-level phosphorylation or oxidative phosphorylation)? O O - C O O - C C O PO ADP + H + C O + ATP H C CH 3 H 61

62 Your Turn! O O - C O O - C C O PO ADP + H + C O + ATP H C H CH 3 This is substrate-level phosphorylation in which a phosphorylated substrate transfers a phosphate to ADP to form ATP. 62

63 Photosynthesis Light from the sun is the original source of nearly all energy for biological systems. Photosynthesis is a process by which energy from the sun is converted to chemical energy that is stored in chemical bonds. Photosynthesis is performed by a wide variety of organisms, both eucaryotic and procaryotic. 63

64 Photosynthesis Photosynthesis in higher plants is a complex series of reactions in which carbohydrates are synthesized from atmospheric carbon dioxide and water. Sunlight provides the large energy requirement for this process. An important side benefit of photosynthesis is the generation of oxygen, which is crucial to all aerobic metabolism. 64

65 Photosynthesis Photosynthesis traps light energy by reducing carbons. For eucaryotes the necessary electron-transfer reactions used in this process are segregated in the chloroplast. The chloroplast contains an electron-transport system within its internal membranes. Unlike the mitrochondrial system which oxidizes coenzymes to liberate energy, chloroplast electron-transport system reduces coenzymes with an input of energy. 65

66 Photosynthesis The photosynthetic mechanism is complex, but it can be divided into two general components: the dark reactions and the light reactions. The dark reactions produce glucose from carbon dioxide, reduced coenzymes, and ATP. The light reactions of photosynthesis form the ATP and NADPH needed to produce glucose. 66

67 Photosynthesis During the light reactions, light is absorbed by colored compounds (pigments) located in the chloroplasts. The most abundant of these pigments is chlorophyll. Once the light energy is absorbed, it is transferred to specific molecules (special chlorophylls) that lose electrons. These energized electrons travel through the chloroplast electron-transport system. 67

68 Photosynthesis The electrons lost by these special chlorophylls are moved to higher energy levels until they can reduce molecules of the coenzyme NADP + to form NADPH. Water is the source of electrons (the electron donor), giving up electrons and producing oxygen gas and hydrogen ions in the process. 68

69 Photosynthesis The movement of electrons is shown here. Notice that ATP is produced from ADP and NADPH is produced from NADP +. 69

70 Photosynthesis The overall redox reaction moves four electrons from two water molecules to produce two molecules of NADPH. The NADPH and ATP produced during the light reactions of photosynthesis are used to make glucose from carbon dioxide and water. 70

71 Your Turn! During the dark reactions of photosynthesis glucose is produced from carbon dioxide, reduced coenzymes, and ATP. Why are these reactions called dark reactions? 71

72 Your Turn! During the dark reactions of photosynthesis glucose is produced from carbon dioxide, reduced coenzymes, and ATP. Why are these reactions called dark reactions? These reactions are called dark reactions because they don t require sunlight. 72

73 33 Summary Bioenergetics is the study of the transformation, distribution, and utilization of energy by living organisms. Metabolism is the sum of all chemical reactions that occur within a living organism. Metabolism is divided into anabolism and catabolism. The mitochondrion is the most important catabolic organelle in eukaryotes. It provides the majority of energy for most cells. 73

74 33 Summary Plants capture light energy during photosynthesis. This energy is stored in the form of reduced carbons in carbohydrates. Animals draw most of their energy from foodstuffs containing reduced carbons. Chloroplasts are organelles in which photosynthesis occurs. Mitochondria are the cell s powerhouses where most 74 of the catabolic redox reactions take place.

75 33 Summary The redox coenzymes NADH, NADPH and FADH 2 carry electrons from one place to another inside cells. These redox coenzymes reduce molecular oxygen in the mitochondrial electron transport system. Aerobic metabolism is the most efficient metabolic energy production. The most common high-energy phosphate bond in the cell is the phosphate anhydride (or phosphoanhydride) bond. 75

76 33 Summary Phosphate anhydride bonds are important components of nucleotide triphosphates, the most common of which is adenosine triphosphate (ATP). ATP is the common energy currency of the cell. Two common forms of chemical energy storage in cells are reduced carbon atoms in molecules such as carbohydrates and phosphate anhydride bonds in molecules like ATP. 76

77 33 Summary Substrate-level phosphorylation uses energy derived from oxidation to form high-energy phosphate bonds on substrates. Oxidative phosphorylation uses energy from redox reactions to form ATP. Photosynthesis is a process by which energy from the sun is converted to chemical energy that is stored in chemical bonds. Photosynthesis produces both carbohydrates and molecular oxygen. 77