Pathways that Harvest and Store Chemical Energy

Transcription

1 Pathways that Harvest and Store Chemical Energy

2 Chapter 6 Pathways that Harvest and Store Chemical Energy Key Concepts 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy

3 Chapter 6 Pathways that Harvest and Store Chemical Energy 6.4 Catabolic and Anabolic Pathways Are Integrated 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO 2 to Carbohydrates

4 Chapter 6 Opening Question Why does fresh air inhibit the formation of alcohol by yeast cells?

5 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Energy is stored in chemical bonds and can be released and transformed by metabolic pathways. Chemical energy available to do work is termed free energy (G).

6 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Five principles governing metabolic pathways: 1. Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway. 2. Each reaction is catalyzed by a specific enzyme. 3. Most metabolic pathways are similar in all organisms.

7 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism 4. In eukaryotes, many metabolic pathways occur inside specific organelles. 5. Each metabolic pathway is controlled by enzymes that can be inhibited or activated.

8 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism In cells, energy-transforming reactions are often coupled: An energy-releasing (exergonic) reaction is coupled to an energy-requiring (endergonic) reaction.

9 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Adenosine triphosphate (ATP) is a kind of energy currency in cells. Energy released by exergonic reactions is stored in the bonds of ATP. When ATP is hydrolyzed, free energy is released to drive endergonic reactions.

10 Figure 6.1 The Concept of Coupling Reactions

11 Figure 6.2 ATP

12 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Hydrolysis of ATP is exergonic: ATP H2O ADP P i freeenergy ΔG is about 7.3 kcal

13 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Free energy of the bond between phosphate groups is much higher than the energy of the O H bond that forms after hydrolysis. text art pg 102 here (1 st one, in left-hand column)

14 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Phosphate groups are negatively charged, so energy is required to get them near enough to each other to make the covalent bonds in the ATP molecule. ATP can be formed by substrate-level phosphorylation or oxidative phosphorylation.

15 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Energy can also be transferred by the transfer of electrons in oxidation reduction, or redox reactions. Reduction is the gain of one or more electrons. Oxidation is the loss of one or more electrons.

16 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Oxidation and reduction always occur together.

17 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Transfers of hydrogen atoms involve transfers of electrons (H = H + + e ). When a molecule loses a hydrogen atom, it becomes oxidized.

18 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism The more reduced a molecule is, the more energy is stored in its bonds. Energy is transferred in a redox reaction. Energy in the reducing agent is transferred to the reduced product.

19 Figure 6.3 Oxidation, Reduction, and Energy

20 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Coenzyme NAD + is a key electron carrier in redox reactions. NAD + (oxidized form) NADH (reduced form)

21 Figure 6.4 A NAD + /NADH Is an Electron Carrier in Redox Reactions

22 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Reduction of NAD + is highly endergonic: NAD H 2e NADH Oxidation of NADH is highly exergonic: NADH H 1 O NAD H O 2 2 2

23 Figure 6.4 B NAD + /NADH Is an Electron Carrier in Redox Reactions

24 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism In cells, energy is released in catabolism by oxidation and trapped by reduction of coenzymes such as NADH. Energy for anabolic processes is supplied by ATP. Oxidative phosphorylation transfers energy from NADH to ATP.

25 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Oxidative phosphorylation couples oxidation of NADH: NADH NAD H 2e energy with production of ATP: energy ADP P i ATP

26 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism The coupling is chemiosmosis diffusion of protons across a membrane, which drives the synthesis of ATP. Chemiosmosis converts potential energy of a proton gradient across a membrane into the chemical energy in ATP.

27 Figure 6.5 A Chemiosmosis

28 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism ATP synthase membrane protein with two subunits: F 0 is the H + channel; potential energy of the proton gradient drives the H + through. F 1 has active sites for ATP synthesis.

29 Figure 6.5 B Chemiosmosis

30 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Chemiosmosis can be demonstrated experimentally. A proton gradient can be introduced artificially in chloroplasts or mitochondria in a test tube. ATP is synthesized if ATP synthase, ADP, and inorganic phosphate are present.

31 Figure 6.6 An Experiment Demonstrates the Chemiosmotic Mechanism

32 Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism Cellular respiration is a major catabolic pathway. Glucose is oxidized: carbohydrate 6O2 6CO2 6H 2O chemical energy Photosynthesis is a major anabolic pathway. Light energy is converted to chemical energy: 6CO 6H 2O light energy O2 2 6 carbohydra te

33 Figure 6.7 ATP, Reduced Coenzymes, and Metabolism

34 Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Cellular Respiration A lot of energy is released when reduced molecules with many C C and C H bonds are fully oxidized to CO 2. Oxidation occurs in a series of small steps in three pathways: 1. glycolysis 2. pyruvate oxidation 3. citric acid cycle

35 Figure 6.8 Energy Metabolism Occurs in Small Steps

36 Figure 6.9 Energy-Releasing Metabolic Pathways

37 Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Glycolysis: ten reactions. Takes place in the cytosol. Final products: 2 molecules of pyruvate (pyruvic acid) 2 molecules of ATP 2 molecules of NADH

38 Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 1)

39 Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 2)

40 Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 3)

41 Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Examples of reaction types common in metabolic pathways: Step 6: Oxidation reduction Step 7: Substrate-level phosphorylation text art pg 107 here

42 Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Pyruvate Oxidation: Products: CO 2 and acetate; acetate is then bound to coenzyme A (CoA)

43 Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Citric Acid Cycle: 8 reactions, operates twice for every glucose molecule that enters glycolysis. Starts with Acetyl CoA; acetyl group is oxidized to two CO 2. Oxaloacetate is regenerated in the last step.

44 Figure 6.11 The Citric Acid Cycle

45 Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Final reaction of citric acid cycle:

46 Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy Electron transport/atp Synthesis: NADH is reoxidized to NAD + and O 2 is reduced to H 2 O in a series of steps. Respiratory chain series of redox carrier proteins embedded in the inner mitochondrial membrane. Electron transport electrons from the oxidation of NADH and FADH 2 pass from one carrier to the next in the chain.

47 Figure 6.12 Electron Transport and ATP Synthesis in Mitochondria

48 Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy The oxidation reactions are exergonic; the energy is used to actively transport H + ions out of the mitochondrial matrix, setting up a proton gradient. ATP synthase in the membrane uses the H + gradient to synthesize ATP by chemiosmosis.

49 Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy About 32 molecules of ATP are produced for each fully oxidized glucose. The role of O 2 : most of the ATP produced is formed by oxidative phosphorylation, which is due to the reoxidation of NADH.

50 Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Under anaerobic conditions, NADH is reoxidized by fermentation. There are many different types of fermentation, but all operate to regenerate NAD +. The overall yield of ATP is only two the ATP made in glycolysis.

51 Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Lactic acid fermentation: End product is lactic acid (lactate). NADH is used to reduce pyruvate to lactic acid, thus regenerating NAD +.

52 Figure 6.13 A Fermentation

53 Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Alcoholic fermentation: End product is ethyl alcohol (ethanol). Pyruvate is converted to acetaldehyde, and CO 2 is released. NADH is used to reduce acetaldehyde to ethanol, regenerating NAD + for glycolysis.

54 Figure 6.13 B Fermentation

55 Concept 6.4 Catabolic and Anabolic Pathways Are Integrated Metabolic pathways are linked. Carbon skeletons (molecules with covalently linked carbon atoms) can enter catabolic or anabolic pathways.

56 Figure 6.14 Relationships among the Major Metabolic Pathways of the Cell

57 Concept 6.4 Catabolic and Anabolic Pathways Are Integrated Catabolism: Polysaccharides are hydrolyzed to glucose, which enter glycolysis. Lipids break down to fatty acids and glycerol. Fatty acids can be converted to acetyl CoA. Proteins are hydrolyzed to amino acids that can feed into glycolysis or the citric acid cycle.

58 Concept 6.4 Catabolic and Anabolic Pathways Are Integrated Anabolism: Many catabolic pathways can operate in reverse. Gluconeogenesis citric acid cycle and glycolysis intermediates can be reduced to form glucose. Acetyl CoA can be used to form fatty acids. Some citric acid intermediates can form nucleic acids.

59 Concept 6.4 Catabolic and Anabolic Pathways Are Integrated Amounts of different molecules are maintained at fairly constant levels the metabolic pools. This is accomplished by regulation of enzymes allosteric regulation, feedback inhibition. Enzymes can also be regulated by altering the transcription of genes that encode the enzymes.

60 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosynthesis involves two pathways: Light reactions convert light energy into chemical energy (in ATP and the reduced electron carrier NADPH). Carbon-fixation reactions use the ATP and NADPH, along with CO 2, to produce carbohydrates.

61 Figure 6.15 An Overview of Photosynthesis

62 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Light is a form of electromagnetic radiation, which travels as a wave but also behaves as particles (photons). Photons can be absorbed by a molecule, adding energy to the molecule it moves to an excited state.

63 Figure 6.16 The Electromagnetic Spectrum

64 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Pigments: molecules that absorb wavelengths in the visible spectrum. Chlorophyll absorbs blue and red light; the remaining light is mostly green. Absorption spectrum plot of light energy absorbed against wavelength. Action spectrum plot of the biological activity of an organism against the wavelengths to which it is exposed

65 Figure 6.17 Absorption and Action Spectra

66 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy In plants, two chlorophylls absorb light energy chlorophyll a and chlorophyll b. Accessory pigments absorb wavelengths between red and blue and transfer some of that energy to the chlorophylls.

67 Figure 6.18 The Molecular Structure of Chlorophyll (Part 1)

68 Figure 6.18 The Molecular Structure of Chlorophyll (Part 2)

69 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The pigments are arranged into lightharvesting complexes, or antenna systems. A photosystem spans the thylakoid membrane in the chloroplast; it consists of antenna systems and a reaction center.

70 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron. Chl* gives the excited electron to an acceptor and becomes oxidized to Chl +. The acceptor molecule is reduced. Chl * acceptor Chl acceptor

71 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy The electron acceptor is first in an electron transport system in the thylakoid membrane. Final electron acceptor is NADP +, which gets reduced: NADP H 2e NADPH ATP is produced chemiosmotically during electron transport (photophosphorylation).

72 Figure 6.19 Noncyclic Electron Transport Uses Two Photosystems

73 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Two photosystems: Photosystem I absorbs light energy at 700 nm, passes an excited electron to NADP +, reducing it to NADPH. Photosystem II absorbs light energy at 680 nm, produces ATP, and oxidizes water molecules.

74 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosystem II When Chl* gives up an electron, it is unstable and grabs an electron from another molecule, H 2 O, which splits the H O H bonds. 2Chl * H Chl H O O

75 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosystem I When Chl* gives up an electron, it grabs another electron from the end of the transport system of Photosystem II. This electron ends up reducing NADP + to NADPH.

76 Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy ATP is needed for carbon-fixation pathways. Cyclic electron transport uses only photosystem I and produces ATP; an electron is passed from an excited chlorophyll and recycles back to the same chlorophyll.

77 Figure 6.20 Cyclic Electron Transport Traps Light Energy as ATP

78 Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO 2 to Carbohydrates The Calvin cycle: CO 2 fixation. It occurs in the stroma of the chloroplast. Each reaction is catalyzed by a specific enzyme.

79 Figure 6.21 The Calvin Cycle

80 Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO 2 to Carbohydrates 1. Fixation of CO 2 : CO 2 is added to ribulose 1,5-bisphosphate (RuBP). Ribulose bisphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction. A 6-carbon molecule results, which quickly breaks into two 3-carbon molecules: 3- phosphoglycerate (3PG).

81 Figure 6.22 RuBP Is the Carbon Dioxide Acceptor

82 Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO 2 to Carbohydrates 2. 3PG is reduced to form glyceraldehyde 3- phosphate (G3P).

83 Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO 2 to Carbohydrates 3. The CO 2 acceptor, RuBP, is regenerated from G3P. Some of the extra G3P is exported to the cytosol and is converted to hexoses (glucose and fructose). When glucose accumulates, it is linked to form starch, a storage carbohydrate.

84 Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO 2 to Carbohydrates The C H bonds generated by the Calvin cycle provide almost all the energy for life on Earth. Photosynthetic organisms (autotrophs) use most of this energy to support their own growth and reproduction. Heterotrophs cannot photosynthesize and depend on autotrophs for chemical energy.

85 Answer to Opening Question Pasteur s findings: Catabolism of the beet sugar is a cellular process, so living yeast cells must be present. With air (O 2 ) yeasts used aerobic metabolism to fully oxidize glucose to CO 2. Without air, yeasts used alcoholic fermentation, producing ethanol, less CO 2, and less energy (slower growth).

86 Figure 6.23 Products of Glucose Metabolism