1 Energy from food & sun

2 Cellular Pathways that Harvest Chemical EnergyEnergy and Electrons from Glucose Glycolysis: From Glucose to Pyruvate Pyruvate Oxidation The Citric Acid Cycle The Respiratory Chain: Electrons, Protons, and ATP Production Fermentation: ATP from Glucose, without O2 Contrasting Energy Yields Relationships between Metabolic Pathways Regulating Energy Pathways

3 Energy Maintaining order (including biological order) requires energy.

4 Energy Chemical reactions release or take up energy Anabolic reactions may make single products from many smaller units; such reactions consume energy. Catabolic reactions may reduce an organized substance (e.g., a glucose molecule) into smaller, more randomly distributed substances (e.g., carbon dioxide and water). Such reactions release energy.

5 Enzymes A catalyst is any substance that speeds up a chemical reaction without itself being used up. Living cells use biological catalysts to increase rates of chemical reactions. Most biological catalysts are proteins called enzymes. Certain RNA molecules called ribozymes also are catalysts.

6 Activation Energy Initiates ReactionsEven energetically favorable reactions require Activation Energy Figures\Chapter06\High-Res\life7e-fig jpg

7 Enzymes Lower the Energy BarrierEnzymes lower activation energy requirements and thus speed up the overall reaction. They do not change the difference in free energy (G). Thus, they do not affect the final equilibrium. At the active sites, enzymes and substrates interact by breaking old bonds and forming new ones. Enzymes catalyze reactions by: orienting substrates, adding charges to substrates, or inducing strain in substrates. Figures\Chapter06\High-Res\life7e-fig jpg

8 Energy and Electrons from GlucoseThe sugar glucose (C6H12O6) is the most common form of energy molecule. When burned in a flame, glucose reacts with oxygen and releases heat (energy), carbon dioxide, and water. C6H12O6 + 6 O2 ® 6 CO2 + 6 H2O + energy (–686 kcal/mol) The same equation applies for the biological, metabolic use of glucose except that about half the released energy is captured as chemical energy.

9 Energy and Electrons from Glucose

10 ATP: Transferring Energy in Cells ATP as energy carier ATP: Transferring Energy in Cells All living cells use adenosine triphosphate (ATP) for capture, storage and transfer of energy. Figures\Chapter06\High-Res\life7e-fig jpg

11 ATP hydrolysis releases energyATP may be thought of as the energy “currency” of the cell; some of the free energy released by certain exergonic reactions is captured in ATP, which then can release free energy to drive endergonic reactions. ATP hydrolysis releases energy Figures\Chapter06\High-Res\life7e-fig jpg

12 ATP Couples exerogenic & enderogenic reactionsCoupled reactions ATP Couples exerogenic & enderogenic reactions Figures\Chapter06\High-Res\life7e-fig jpg

13 Oxidation of Glucose The sugar glucose (C6H12O6) is the most common form of energy molecule. When burned in a flame, glucose reacts with oxygen and releases heat (energy), carbon dioxide, and water. C6H12O6 + 6 O2 ® 6 CO2 + 6 H2O + energy (–686 kcal/mol) The same equation applies for the biological, metabolic use of glucose except that about half the released energy is captured as chemical energy. Cells obtain energy from glucose by the chemical process of oxidation in a series of metabolic pathways. The reaction is highly exergonic, and it drives the endergonic formation of ATP.

14 Redox reactions Redox reactions transfer the energy of electrons. A gain of one or more electrons or hydrogen atoms is called reduction. The loss of one or more electrons or hydrogen atoms is called oxidation. Whenever one material is reduced, another is oxidized.

15 Energy and Electrons from GlucoseAn oxidizing agent accepts an electron or a hydrogen atom. A reducing agent donates an electron or a hydrogen atom. During the metabolism of glucose, glucose is the reducing agent (and is oxidized), while oxygen is the oxidizing agent (and is reduced).

16 NADH is a reducing agentThe coenzyme NAD (Nicotine Adenine di-nucleotide) is an essential electron carrier in cellular redox reactions. NAD exists in an oxidized form, NAD+, and a reduced form, NADH + H+.

17 NADH is an energy carrierGeneration of NADH (reduction) requires an input of energy: NAD+ + 2H ® NADH + H+ The oxidation reaction is exergonic: NADH + H+ + ½ O2 ® NAD+ + H2O NADH is thus an energy carrier.

18 Energy from food and sunCells need energy. Plant cells capture energy from the sun and convert it to Glucose in a process called photosynthesis. Respiration is using glucose to drive anabolic processes Plant cells are performing both photosynthesis and respiration Animal cells use food as energy source and consume it by respiration

19 Energy from food Generation of energy from food occurs in three principal stages

20 Energy from food In the first stage macromolecules in food are digested into simple molecules (monomers) Digestion happens outside the cell in the digestive system and in lysosomes

21 Energy from food In the second stage a series of reactions converts food molecules into Acetyl-CoA This step result in generation of limited amounts of ATP and NADH

22 Energy from food -GlycolysisAcetyl-CoA can be generated for various sugars, fats or amino acids For sugars, this step is called Glycolysis. It produces some usable energy and two molecules of a three- carbon sugar called pyruvate. Glycolysis begins glucose metabolism in all cells and occurs in the cytoplasm. Glycolysis does not require O2; it is an anaerobic metabolic process.

23 Energy from food -GlycolysisFor sugars, this step is called Glycolysis. It produces some usable energy and two molecules of a three-carbon sugar called pyruvate.

24 Energy from food -GlycolysisFor sugars, this step is called Glycolysis. It produces some usable energy and two molecules of a three-carbon sugar called pyruvate.

25 Energy from food The third step involves complete oxidation of Acetyl-CoA to CO2 and H2O accompanied by generation of large amounts of ATP This step requires oxygen and is called Cellular respiration Cellular respiration occurs in mitochondria

26 Oxidative phosphorylationCellular respiration uses O2 and occurs in aerobic (oxygen- containing) environments. The energy stored in covalent bonds of pyruvate is used to make ATP molecules. Pyruvate/Acetyl-CoA is used in the citric acid cycle to produce NADH NADH oxidation is used to drive ATP synthesis in a process called Oxidative phosphorylation.

27 Energy from food Generation of energy from food occurs in three principal stages

28 Organelles that Process EnergyMitochondria have an outer lipid bilayer and a highly folded inner membrane. Folds of the inner membrane give rise to the cristae, which contain large protein molecules used in cellular respiration. The region enclosed by the inner membrane is called the mitochondrial matrix.

29 Figure 4.14 A Mitochondrion Converts Energy from Fuel Molecules into ATP (Part 1)

30 Organelles that Process EnergyThe crista contain key molecules for the generation of ATP from Fuel molecule and oxigen The matrix contains ribosomes, mDNA, and several of the enzymes used for cellular respiration carbon metabolism. Site of protein synthesis The inner membrane is the primary Barrier between the cytosol and mitochondrial enzymes

31 Oxidative phosphorylationReducing agents (NADH) generated from pyruvate in the citric acid cycle, are used to generate ATP in oxidative phosphorylation

32 Oxidative phosphorylationIn oxidative phosphorylation oxidation of NADH by electron transfer is used to pump protons across the membrane. The proton gradient is used to synthesize ATP

33 Electron transport chainElectrons transported from NADH to O2 in a series of redox reactions according to redox potential and release energy.This energy is used to pump protons across the membrane. NADH + H+ yield energy upon oxidation.

34 Proton gradient The flow of electrons causes the active transport of protons across the inner mitochondrial membrane, creating a proton concentration gradient.

35 Electron transfer chainOverview ..\..\Movies\14.2-electron_transport.mov

36 Proton–motive force This transport results in a difference in electric charge and proton concentration across the membrane. The potential energy generated is called the proton- motive force.

37 Figure 7.12 A Chemiosmotic Mechanism Produces ATP (Part 2)The protons then diffuse through the ATP synthase proton channels down the concentration and electrical gradient back into the matrix of the mitochondria, creating ATP in the process. The potential energy from the proton-motive force is harnessed by ATP synthase to synthesize ATP from ADP.

38 Figure 7.12 A Chemiosmotic Mechanism Produces ATP (Part 2)In oxidative phosphorylation ATP synthesis is coupled to electron transport.

39 Uncoupling ATP synthesis from electron transferAn artificially produced proton gradient can drive ATP synthesis

40 Uncoupling ATP synthesis from electron transferIonophores (agents that make the membrane permeable to protons) cancel ATP synthesis

41 ATP synthase ATP synthase can catalyze ATP synthesis or hydrolysis Synthesis of ATP from ADP is reversible. The synthesized ATP is transported out of the mitochondrial matrix as quickly as it is made.

42 ATP Synthase ATPase mechanism of action ..\..\Movies\14.3-ATP_synthase.mov ..\..\Movies\14.4-ATP_synthase_disco.mov

43 ATPase Bacterial flagellum rotation is driven by proton gradient similar to ATPase

44 Energy from food The energy-harvesting processes in cells use different combinations of metabolic pathways. With O2 present, four major pathways operate: Glycolysis Pyruvate oxidation The citric acid cycle The respiratory chain (electron transport chain)

45 Fermentation When no O2 is available, glycolysis is followed by fermentation. Fermentation does not involve O2. It is an anaerobic process. Pyruvate is converted into lactic acid or ethanol. Breakdown of glucose is incomplete; less energy is released than by cellular respiration.

46 Fermentation When no O2 is available, glycolysis is followed by fermentation. Fermentation does not involve O2. It is an anaerobic process. Pyruvate is converted into lactic acid or ethanol. Breakdown of glucose is incomplete; less energy is released than by cellular respiration.

47 Contrasting Energy YieldsA total of 36 ATP molecules can be generated from each glucose molecule in glycolysis and cellular respiration. Fermentation has a net yield of 2 ATP molecules from each glucose molecule. The end products of fermentation contain much more unused energy than the end products of aerobic respiration.

48 Relationships between Metabolic PathwaysGlucose utilization pathways can yield more than just energy. They are interchanges for diverse biochemical traffic.

49 Energy from light Energy in food is originally captured from light energy Chloroplasts use light energy to produce sugar in a process called Photosynthesis

50 Chloroplast structureChloroplasts are surrounded by two layers, and have an internal membrane system. The internal membranes is the thylakoid membrane and is organized in piles called grana. These membranes contain chlorophyll and other pigments. The fluid in which the grana are suspended is called the stroma.

51 Figure 4.15 The Chloroplast: The Organelle That Feeds the World

52 Photosynthesis can be divided into two steps: In Photosynthesis light energy is used to produce sugar from water and Carbon dioxide C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy (686 kcal/mol) Photosynthesis occurs in the chloroplasts of green plant cells and consists of many reactions. Photosynthesis can be divided into two steps: The light reaction is driven by light energy captured by chlorophyll. It produces ATP and NADPH + H+. The Calvin–Benson cycle does not use light directly. It uses ATP, NADPH + H+, and CO2 to produce sugars.

53 Photosynthesis The light reaction is driven by light energy captured by chlorophyll. It produces ATP and NADPH + H+. The Calvin–Benson cycle does not use light directly. It uses ATP, NADPH + H+, and CO2 to produce sugars.

54 The Light Reactions ATP is produced by a chemiosmotic mechanism similar to that of mitochondria, called photophosphorylation. High-energy electrons move through a series of redox reactions, releasing energy that is used to transport protons across the membrane. Active proton transport results in the proton-motive force: a difference in pH and electric charge across the membrane.

55 The Light Reactions: Electron Transport, Reductions, and PhotophosphorylationThe electron carriers in the thylakoid membrane are oriented so as to move protons into the interior of the thylakoid, and the inside becomes acidic with respect to the outside. This difference in pH leads to the diffusion of H+ out of the thylakoid through specific protein channels, ATP synthases. The ATP synthases couple the formation of ATP to proton diffusion back across the thylakoid membrane.

56 Figure 8.11 Chloroplasts Form ATP Chemiosmotically

57 Figure 8.11 Chloroplasts Form ATP Chemiosmotically

58 Figure 7.1 Energy for Life

59 Origin of eukaryotic cellsEndosymbiosis may explain the origin of mitochondria and chloroplasts. According to the endosymbiosis theory, both organelles were formerly prokaryotic organisms that somehow became incorporated into a larger cell. Today, both mitochondria and chloroplasts have DNA and ribosomes, and are self-duplicating organelles.

60 Dual membrane support endosymbiosisFigure From Prokaryotic Cell to Eukaryotic Cell (Part 2) Dual membrane support endosymbiosis

61 The Origin of the Eukaryotic Cell

62 The Origin of the Eukaryotic CellThe evolution of eukaryotic cells included the following components: The origin of a flexible cell surface The origin of a cytoskeleton The origin of a nuclear envelope The appearance of digestive vesicles The endosymbiotic acquisition of certain organelles

63 The Origin of the Eukaryotic CellThe first step toward the eukaryotic condition may have been the loss of the cell wall by an ancestral prokaryotic cell. A surface that is flexible enough to allow for infolding lets the cell exchange materials with its environment rapidly enough to sustain a larger volume and more rapid metabolism. A flexible surface also allows endocytosis. An infolded plasma membrane attached to a chromosome within an ancestral prokaryote may have led to the formation of the nuclear envelope.

64 Figure 28.2 Membrane Infolding

65 The Origin of the Eukaryotic CellThe early steps in the evolution of the eukaryotic cell likely included three advances: The formation of ribosome-studded internal membranes, some of which surrounded the DNA The appearance of a cytoskeleton The evolution of digestive vesicles

66 Figure 28.3 From Prokaryotic Cell to Eukaryotic Cell (Part 1)

67

68 Dual membrane support endosymbiosisFigure From Prokaryotic Cell to Eukaryotic Cell (Part 2) Dual membrane support endosymbiosis

69 The Origin of the Eukaryotic CellFrom an intermediate kind of cell, the next advance was likely to have been a motile phagocyte. The first true eukaryotic cell possessed a cytoskeleton and a nuclear envelope; it also may have had an associated endoplasmic reticulum and Golgi apparatus and perhaps one or more flagella.

70 The Origin of the Eukaryotic CellDuring the early stages of eukaryotic evolution, the O2 levels in the atmosphere were increasing as a result of the photosynthetic activities of the cyanobacteria. Most living things were unable to tolerate this new aerobic, oxidizing environment, but some prokaryotes and ancient phagocytes were able to survive. One hypothesis suggests that the key to the survival of the early phagocytes was the ingestion of a prokaryote that became symbiotic and evolved into the peroxisomes of today.