1. LECTURE PRESENTATIONS For BROCK BIOLOGY OF MICROORGANISMS, THIRTEENTH EDITION Michael T. Madigan, John M. Martinko, David A. Stahl, David P. Clark Lectures by John Zamora Middle Tennessee State University © 2012 Pearson Education, Inc. Microbial Growth Chapter 5

2. I. Bacterial Cell Division 5.1Cell Growth and Binary Fission 5.2Fts Proteins and Cell Division 5.3MreB and Determinants of Cell Morphology 5.4Peptidoglycan Synthesis and Cell Division © 2012 Pearson Education, Inc.

3. 5.1 Cell Growth and Binary Fission Growth: increase in the number of cells Binary fission: cell division following enlargement of a cell to twice its minimum size (Figure 5.1) Generation time: time required for microbial cells to double in number During cell division, each daughter cell receives a chromosome and sufficient copies of all other cell constituents to exist as an independent cell © 2012 Pearson Education, Inc. Animation: Binary Fission Animation: Binary Fission Animation: Overview of Bacterial Growth Animation: Overview of Bacterial Growth

4. Figure 5.1 Septum Cell elongation Septum formation Completion of septum; formation of walls; cell separation One generation © 2012 Pearson Education, Inc.

5. 5.2 Fts Proteins and Cell Division Fts (filamentous temperature-sensitive) Proteins (Figure 5.2) –Essential for cell division in all prokaryotes –Interact to form the divisome (cell division apparatus) FtsZ: forms ring around center of cell; related to tubulin ZipA: anchor that connects FtsZ ring to cytoplasmic membrane FtsA: helps connect FtsZ ring to membrane and also recruits other divisome proteins –Related to actin © 2012 Pearson Education, Inc.

6. Figure 5.2 Cytoplasmic membrane Outer membrane Cytoplasmic membrane Divisome complex Peptidoglycan FtsZ ring © 2012 Pearson Education, Inc.

7. 5.2 Fts Proteins and Cell Division DNA replicates before the FtsZ ring forms (Figure 5.3) Location of FtsZ ring is facilitated by Min proteins –MinC, MinD, MinE FtsK protein mediates separation of chromosomes to daughter cells © 2012 Pearson Education, Inc.

8. Figure 5.3 Cell wall Min CD Cytoplasmic membrane Nucleoid MinE Divisome complex FtsZ ring Nucleoid Septum MinE Minutes 0 20 40 60 80 © 2012 Pearson Education, Inc.

9. 5.3 MreB and Determinants of Cell Morphology Prokaryotes contain a cell cytoskeleton that is dynamic and multifaceted MreB: major shape-determining factor in prokaryotes –Forms simple cytoskeleton in Bacteria and probably Archaea –Forms spiral-shaped bands around the inside of the cell, underneath the cytoplasmic membrane (Figure 5.4a and b) –Not found in coccus-shaped bacteria © 2012 Pearson Education, Inc.

10. 5.3 MreB and Determinants of Cell Morphology MreB (cont’d) –Localizes synthesis of new peptidoglycan and other cell wall components to specific locations along the cylinder of a rod-shaped cell during growth © 2012 Pearson Education, Inc.

11. Figure 5.4a Cell wall Cytoplasmic membrane MreB Sites of cell wall synthesis FtsZ © 2012 Pearson Education, Inc.

12. Figure 5.4b © 2012 Pearson Education, Inc.

13. 5.3 MreB and Determinants of Cell Morphology Crescentin: shape-determining protein produced by vibrio-shaped cells of Caulobacter crescentus –Crescentin protein organizes into filaments ~10 nm wide that localize on the concave face of the curved cells (Figure 5.4c) © 2012 Pearson Education, Inc.

14. Figure 5.4c © 2012 Pearson Education, Inc.

15. 5.3 MreB and Determinants of Cell Morphology Most archaeal genomes contain FtsZ and MreB-like proteins, thus cell morphology is similar to that seen in Bacteria © 2012 Pearson Education, Inc.

16. 5.4 Peptidoglycan Synthesis and Cell Division Production of new cell wall material is a major feature of cell division –In cocci, cell walls grow in opposite directions outward from the FtsZ ring –In rod-shaped cells, growth occurs at several points along length of the cell © 2012 Pearson Education, Inc.

17. 5.4 Peptidoglycan Synthesis and Cell Division Preexisting peptidoglycan needs to be severed to allow newly synthesized peptidoglycan to form –Beginning at the FtsZ ring, small openings in the wall are created by autolysins –New cell wall material is added across the openings –Wall band: junction between new and old peptidoglycan © 2012 Pearson Education, Inc.

18. Figure 5.5 FtsZ ring Wall bands Growth zone Septum © 2012 Pearson Education, Inc.

19. 5.4 Peptidoglycan Synthesis and Cell Division Bactoprenol: carrier molecule that plays major role in insertion of peptidoglycan precursors –C 55 alcohol (Figure 5.6) –Bonds to N-acetylglucosamine/ N-acetylmuramic acid/pentapeptide peptidoglycan precursor © 2012 Pearson Education, Inc.

20. Figure 5.6 Hydrophobic portion © 2012 Pearson Education, Inc.

21. 5.4 Peptidoglycan Synthesis and Cell Division Glycolases: enzymes that interact with bactoprenol (Figure 5.7a) –Insert cell wall precursors into growing points of cell wall –Catalyze glycosidic bond formation © 2012 Pearson Education, Inc.

22. Figure 5.7a Transglycosylase activity Growing point of cell wall Cytoplasmic membrane Autolysin activity Pentapeptide Bactoprenol Peptidoglycan Out In © 2012 Pearson Education, Inc.

23. 5.4 Peptidoglycan Synthesis and Cell Division Transpeptidation: final step in cell wall synthesis (Figure 5.7b) –Forms the peptide cross-links between muramic acid residues in adjacent glycan chains –Inhibited by the antibiotic penicillin © 2012 Pearson Education, Inc.

24. Figure 5.7b Transpeptidation © 2012 Pearson Education, Inc.

25. II. Population Growth 5.5The Concept of Exponential Growth 5.6The Mathematics of Exponential Growth 5.7The Microbial Growth Cycle 5.8Continuous Culture: The Chemostat © 2012 Pearson Education, Inc.

26. 5.5 The Concept of Exponential Growth Most bacteria have shorter generation times than eukaryotic microbes Generation time is dependent on growth medium and incubation conditions © 2012 Pearson Education, Inc.

27. 5.5 The Concept of Exponential Growth Exponential growth: growth of a microbial population in which cell numbers double within a specific time interval During exponential growth, the increase in cell number is initially slow but increases at a faster rate (Figure 5.8) © 2012 Pearson Education, Inc.

28. Figure 5.8 Time (h) Logarithmic plot Arithmetic plot Number of cells (arithmetic scale) Number of cells (logarithmic scale) 1000 500 100 0 1 2 3 4 5 1 10 10 2 10 3 © 2012 Pearson Education, Inc.

29. 5.6 The Mathematics of Exponential Growth A relationship exists between the initial number of cells present in a culture and the number present after a period of exponential growth: N = N 0 2 n N is the final cell number N 0 is the initial cell number n is the number of generations during the period of exponential growth © 2012 Pearson Education, Inc.

30. 5.6 The Mathematics of Exponential Growth Generation time (g) of the exponentially growing population is g = t/n t is the duration of exponential growth n is the number of generations during the period of exponential growth © 2012 Pearson Education, Inc.

31. 5.6 The Mathematics of Exponential Growth Specific growth rate (k) is calculated as k = 0.301/g Division rate (v) is calculated as v = 1/g © 2012 Pearson Education, Inc.

32. 5.7 The Microbial Growth Cycle Batch culture: a closed-system microbial culture of fixed volume Typical growth curve for population of cells grown in a closed system is characterized by four phases (Figure 5.10): –Lag phase –Exponential phase –Stationary phase –Death phase © 2012 Pearson Education, Inc. Animation: Bacterial Growth Curve Animation: Bacterial Growth Curve

33. Figure 5.10 Time Log 10 viable organisms/ml Optical density (OD) 10 9 8 7 6 1.0 0.75 0.50 0.25 0.1 Viable count Turbidity (optical density) Lag ExponentialStationaryDeath Growth phases © 2012 Pearson Education, Inc.

34. 5.7 The Microbial Growth Cycle Lag phase –Interval between when a culture is inoculated and when growth begins Exponential phase –Cells in this phase are typically in the healthiest state Stationary phase –Growth rate of population is zero –Either an essential nutrient is used up or waste product of the organism accumulates in the medium © 2012 Pearson Education, Inc.

35. 5.7 The Microbial Growth Cycle Death Phase – If incubation continues after cells reach stationary phase, the cells will eventually die © 2012 Pearson Education, Inc.

36. 5.8 Continuous Culture: The Chemostat Continuous culture: an open-system microbial culture of fixed volume Chemostat: most common type of continuous culture device (Figure 5.11) –Both growth rate and population density of culture can be controlled independently and simultaneously Dilution rate: rate at which fresh medium is pumped in and spent medium is pumped out Concentration of a limiting nutrient © 2012 Pearson Education, Inc.

37. Figure 5.11 Fresh medium from reservoir Sterile air or other gas Flow-rate regulator Gaseous headspace Culture vessel Culture Overflow Effluent containing microbial cells © 2012 Pearson Education, Inc.

38. 5.8 Continuous Culture: The Chemostat In a chemostat –The growth rate is controlled by dilution rate –The growth yield (cell number/ml) is controlled by the concentration of the limiting nutrient In a batch culture, growth conditions are constantly changing; it is impossible to independently control both growth parameters (Figure 5.12) © 2012 Pearson Education, Inc.

39. Figure 5.12 Only yield affected Growth rate ( ) Nutrient concentration (mg/ml) Growth yield ( ) 0 0.1 0.20.30.40.5 Rate and yield affected © 2012 Pearson Education, Inc.

40. 5.8 Continuous Culture: The Chemostat Chemostat cultures are sensitive to the dilution rate and limiting nutrient concentration (Figure 5.13) –At too high a dilution rate, the organism is washed out –At too low a dilution rate, the cells may die from starvation –Increasing concentration of a limiting nutrient results in greater biomass but same growth rate © 2012 Pearson Education, Inc.

41. Figure 5.13 5 Bacterial concentration Doubling time Doubling time (h) Steady-state bacterial concentration (g/l) Steady state Washout 4 3 2 1 0 0 0.25 0.50.75 1.0 4 6 2 0 Dilution rate (h  1 ) © 2012 Pearson Education, Inc.

42. III. Measuring Microbial Growth 5.9 Microscopic Counts 5.10 Viable Counts 5.11 Turbidimetric Methods © 2012 Pearson Education, Inc.

43. 5.9 Microscopic Counts Microbial cells are enumerated by microscopic observations (Figure 5.14) –Results can be unreliable © 2012 Pearson Education, Inc.

44. Figure 5.14 Coverslip Ridges that support coverslip To calculate number per milliliter of sample: 12 cells  25 large squares  50  10 3 Number/mm 2 (3  10 2 ) Number/mm 3 (1.5  10 4 ) Number/cm 3 (ml) (1.5  10 7 ) Sample added here. Care must be taken not to allow overflow; space between coverslip and slide is 0.02 mm ( mm. Whole grid has 25 large squares, a total area of 1 mm 2 and a total volume of 0.02 mm 3. Microscopic observation; all cells are counted in large square (16 small squares): 12 cells. (In practice, several large squares are counted and the numbers averaged.) 1 50 © 2012 Pearson Education, Inc.

45. 5.9 Microscopic Counts Limitations of microscopic counts –Cannot distinguish between live and dead cells without special stains –Small cells can be overlooked –Precision is difficult to achieve –Phase-contrast microscope required if a stain is not used –Cell suspensions of low density (<10 6 cells/ml) hard to count –Motile cells need to immobilized –Debris in sample can be mistaken for cells © 2012 Pearson Education, Inc.

46. 5.9 Microscopic Counts A second method for enumerating cells in liquid samples is with a flow cytometer –Uses laser beams, fluorescent dyes, and electronics © 2012 Pearson Education, Inc.

47. 5.10 Viable Counts Viable cell counts (plate counts): measurement of living, reproducing population –Two main ways to perform plate counts: Spread-plate method (Figure 5.15) Pour-plate method To obtain the appropriate colony number, the sample to be counted should always be diluted (Figure 5.16) © 2012 Pearson Education, Inc.

48. Figure 5.15 Sample is pipetted onto surface of agar plate (0.1 ml or less) Sample is spread evenly over surface of agar using sterile glass spreader Typical spread-plate results Surface colonies Subsurface colonies Typical pour-plate results Sample is pipetted into sterile plate Sterile medium is added and mixed well with inoculum Spread-plate method Pour-plate method Incubation Solidification and incubation © 2012 Pearson Education, Inc.

49. Figure 5.16 Sample to be counted 1 ml 9-ml broth Total dilution Plate 1-ml samples 1/10 (10  1 ) 1/100 (10  2 ) 1/10 3 (10  3 ) 1/10 4 (10  4 ) 1/10 5 (10  5 ) 1/10 6 (10  6 ) 159  10 3 1.59  10 5  Plate count Dilution factor Cells (colony-forming units) per milliliter of original sample 159 colonies 17 colonies 2 colonies 0 colonies Too many colonies to count © 2012 Pearson Education, Inc.

50. 5.10 Viable Counts Plate counts can be highly unreliable when used to assess total cell numbers of natural samples (e.g., soil and water) –Selective culture media and growth conditions target only particular species © 2012 Pearson Education, Inc.

51. 5.10 Viable Cell Counting The Great Plate Anomaly: direct microscopic counts of natural samples reveal far more organisms than those recoverable on plates Why is this? –Microscopic methods count dead cells whereas viable methods do not –Different organisms may have vastly different requirements for growth © 2012 Pearson Education, Inc.

52. 5.11 Turbidimetric Methods Turbidity measurements are an indirect, rapid, and useful method of measuring microbial growth (Figure 5.17a) –Most often measured with a spectrophotometer and measurement referred to as optical density (O.D.) © 2012 Pearson Education, Inc.

53. Figure 5.17a Light Prism Incident light, I 0 Filter Sample containing cells ( ) Unscattered light, I Photocell (measures unscattered light, I ) Spectrophotometer Optical density (OD)  Log I0I0 I © 2012 Pearson Education, Inc.

54. 5.11 Turbidimetric Methods To relate a direct cell count to a turbidity value, a standard curve must first be established (Figure 5.17c) © 2012 Pearson Education, Inc.

55. Figure 5.17c Theoretical Actual Cell numbers or mass (dry weight) Optical density 0.8 0.6 0.4 0.3 0.2 0.1 0.5 0.7 © 2012 Pearson Education, Inc.

56. 5.11 Turbidimetric Methods Turbidity measurements –Quick and easy to perform –Typically do not require destruction or significant disturbance of sample –Sometimes problematic (e.g., microbes that form clumps or biofilms in liquid medium) © 2012 Pearson Education, Inc.

57. IV. Temperature and Microbial Growth 5.12 Effect of Temperature on Growth 5.13 Microbial Life in the Cold 5.14 Microbial Life at High Temperatures © 2012 Pearson Education, Inc.

58. 5.12 Effect of Temperature on Growth Temperature is a major environmental factor controlling microbial growth Cardinal temperatures: the minimum, optimum, and maximum temperatures at which an organism grows (Figure 5.18) © 2012 Pearson Education, Inc.

59. Figure 5.18 Enzymatic reactions occurring at maximal possible rate Enzymatic reactions occurring at increasingly rapid rates Membrane gelling; transport processes so slow that growth cannot occur Protein denaturation; collapse of the cytoplasmic membrane; thermal lysis Temperature Growth rate Minimum Optimum Maximum © 2012 Pearson Education, Inc.

60. 5.12 Effect of Temperature on Growth Microorganisms can be classified into groups by their growth temperature optima (Figure 5.19) –Psychrophile: low temperature –Mesophile: midrange temperature –Thermophile: high temperature –Hyperthermophile: very high temperature © 2012 Pearson Education, Inc.

61. Figure 5.19 Temperature (°C) Growth rate Psychrophile Mesophile Thermophile Hyperthermophile 0 10 2030405060708090100110 120 4° 39° 60° 88° 106° Example: Polaromonas vacuolata Example: Escherichia coli Example: Geobacillus stearothermophilus Example: Thermococcus celer Example: Pyrolobus fumarii © 2012 Pearson Education, Inc.

62. 5.12 Effect of Temperature on Growth Mesophiles: organisms that have midrange temperature optima; found in –Warm-blooded animals –Terrestrial and aquatic environments –Temperate and tropical latitudes © 2012 Pearson Education, Inc.

63. 5.13 Microbial Life in the Cold Extremophiles –Organisms that grow under very hot or very cold conditions Psychrophiles –Organisms with cold temperature optima –Inhabit permanently cold environments (Figure 5.20) Psychrotolerant –Organisms that can grow at 0ºC but have optima of 20ºC to 40ºC –More widely distributed in nature than psychrophiles © 2012 Pearson Education, Inc.

64. Figure 5.20 © 2012 Pearson Education, Inc.

65. 5.13 Microbial Life in the Cold Molecular Adaptations to Psychrophily –Production of enzymes that function optimally in the cold; features that may provide more flexibility More  -helices than  -sheets More polar and less hydrophobic amino acids Fewer weak bonds Decreased interactions between protein domains © 2012 Pearson Education, Inc.

66. 5.13 Microbial Life in the Cold Molecular Adaptations to Psychrophily (cont’d) –Transport processes function optimally at low temperatures Modified cytoplasmic membranes –High unsaturated fatty acid content © 2012 Pearson Education, Inc.

67. 5.14 Microbial Life at High Temperatures Above ~65°C, only prokaryotic life forms exist Thermophiles: organisms with growth temperature optima between 45°C and 80°C Hyperthermophiles: organisms with optima greater than 80°C –Inhabit hot environments including boiling hot springs and seafloor hydrothermal vents that can have temperatures in excess of 100°C © 2012 Pearson Education, Inc.

68. 5.14 Microbial Life at High Temperatures Hyperthermophiles in Hot Springs –Chemoorganotrophic and chemolithotrophic species present (Figure 5.22) –High prokaryotic diversity (both Archaea and Bacteria represented) © 2012 Pearson Education, Inc.

69. Figure 5.22 © 2012 Pearson Education, Inc.

70. 5.14 Microbial Life at High Temperatures Thermal gradients form along edges of hot environments Distribution of microbial species along the gradient is dictated by organism’s biology (Figure 5.23) © 2012 Pearson Education, Inc.

71. Figure 5.23 © 2012 Pearson Education, Inc.

72. 5.14 Microbial Life at High Temperatures Studies of thermal habitats have revealed –Prokaryotes are able to grow at higher temperatures than eukaryotes –Organisms with the highest temperature optima are Archaea –Nonphototrophic organisms can grow at higher temperatures than phototrophic organisms © 2012 Pearson Education, Inc.

73. 5.14 Microbial Life at High Temperatures Molecular Adaptations to Thermophily –Enzyme and proteins function optimally at high temperatures; features that provide thermal stability Critical amino acid substitutions in a few locations provide more heat-tolerant folds An increased number of ionic bonds between basic and acidic amino acids resist unfolding in the aqueous cytoplasm Production of solutes (e.g., di-inositol phophate, diglycerol phosphate) help stabilize proteins © 2012 Pearson Education, Inc.

74. 5.14 Microbial Life at High Temperatures Molecular Adaptations to Thermophily (cont’d) –Modifications in cytoplasmic membranes to ensure heat stability Bacteria have lipids rich in saturated fatty acids Archaea have lipid monolayer rather than bilayer © 2012 Pearson Education, Inc.

75. 5.14 Microbial Life at High Temperatures Hyperthermophiles produce enzymes widely used in industrial microbiology –Example: Taq polymerase, used to automate the repetitive steps in the polymerase chain reaction (PCR) technique © 2012 Pearson Education, Inc.

76. V. Other Environmental Factors Affecting Growth 5.15 Acidity and Alkalinity 5.16 Osmotic Effects on Microbial Growth 5.17 Oxygen and Microorganisms 5.18 Toxic Forms of Oxygen © 2012 Pearson Education, Inc.

77. 5.15 Acidity and Alkalinity The pH of an environment greatly affects microbial growth (Figure 5.24) Some organisms have evolved to grow best at low or high pH, but most organisms grow best between pH 6 and 8 (neutrophiles) © 2012 Pearson Education, Inc.

78. Figure 5.24 Volcanic soils, waters Gastric fluids Lemon juice Acid mine drainage Vinegar Rhubarb Peaches Acid soil Tomatoes American cheese Cabbage Peas Corn, salmon, shrimp Pure water Seawater Very alkaline natural soil Alkaline lakes Soap solutions Household ammonia Extremely alkaline soda lakes Lime (saturated solution) Neutrality Increasing acidity Increasing alkalinity 10  7 Acidophiles Alkaliphiles pH Example Moles per liter of: HH OH  10  1 10  2 10  3 10  4 10  5 10  6 10  8 10  9 10  10 10  11 10  12 10  13 10  14 10  13 10  12 10  11 10  10 10  9 10  8 10  6 10  5 10  4 10  3 10  2 10  1 1 1 © 2012 Pearson Education, Inc.

79. 5.15 Acidity and Alkalinity Acidophiles: organisms that grow best at low pH (<6) –Some are obligate acidophiles; membranes destroyed at neutral pH –Stability of cytoplasmic membrane critical Alkaliphiles: organisms that grow best at high pH (>9) –Some have sodium motive force rather than proton motive force © 2012 Pearson Education, Inc.

80. 5.15 Acidity and Alkalinity The internal pH of a cell must stay relatively close to neutral even though the external pH is highly acidic or basic –Internal pH has been found to be as low as 4.6 and as high as 9.5 in extreme acido- and alkaliphiles, respectively © 2012 Pearson Education, Inc.

81. 5.15 Acidity and Alkalinity Microbial culture media typically contain buffers to maintain constant pH © 2012 Pearson Education, Inc.

82. 5.16 Osmotic Effects on Microbial Growth Water activity (a w ): water availability; expressed in physical terms –Defined as ratio of vapor pressure of air in equilibrium with a substance or solution to the vapor pressure of pure water © 2012 Pearson Education, Inc.

83. 5.16 Osmotic Effects on Microbial Growth Typically, the cytoplasm has a higher solute concentration than the surrounding environment, thus the tendency is for water to move into the cell (positive water balance) When a cell is in an environment with a higher external solute concentration, water will flow out unless the cell has a mechanism to prevent this © 2012 Pearson Education, Inc.

84. 5.16 Osmotic Effects on Microbial Growth Halophiles: organisms that grow best at reduced water potential; have a specific requirement for NaCl (Figure 5.25) Extreme halophiles: organisms that require high levels (15–30%) of NaCl for growth Halotolerant: organisms that can tolerate some reduction in water activity of environment but generally grow best in the absence of the added solute © 2012 Pearson Education, Inc.

85. Example: Staphylococcus aureus Example: Aliivibrio fischeri Example: Halobacterium salinarum Example: Escherichia coli NaCl (%) Growth rate 0 5 10 15 20 Nonhalophile Halotolerant Halophile Extreme halophile Figure 5.25 © 2012 Pearson Education, Inc.

86. 5.16 Osmotic Effects on Microbial Growth Osmophiles: organisms that live in environments high in sugar as solute Xerophiles: organisms able to grow in very dry environments © 2012 Pearson Education, Inc.

87. 5.16 Osmotic Effects on Microbial Growth Mechanisms for combating low water activity in surrounding environment involve increasing the internal solute concentration by –Pumping inorganic ions from environment into cell –Synthesis or concentration of organic solutes compatible solutes: compounds used by cell to counteract low water activity in surrounding environment © 2012 Pearson Education, Inc.

88. 5.17 Oxygen and Microorganisms Aerobes: require oxygen to live Anaerobes: do not require oxygen and may even be killed by exposure Facultative organisms: can live with or without oxygen Aerotolerant anaerobes: can tolerate oxygen and grow in its presence even though they cannot use it Microaerophiles: can use oxygen only when it is present at levels reduced from that in air © 2012 Pearson Education, Inc.

89. 5.17 Oxygen and Microorganisms Thioglycolate broth (Figure 5.26) –Complex medium that separates microbes based on oxygen requirements –Reacts with oxygen so oxygen can only penetrate the top of the tube © 2012 Pearson Education, Inc.

90. Figure 5.26 Oxic zone Anoxic zone © 2012 Pearson Education, Inc.

91. 5.17 Oxygen and Microorganisms Special techniques are needed to grow aerobic and anaerobic microorganisms (Figure 5.27) Reducing agents: chemicals that may be added to culture media to reduce oxygen (e.g., thioglycolate) © 2012 Pearson Education, Inc.

92. Figure 5.27 © 2012 Pearson Education, Inc.

93. 5.18 Toxic Forms of Oxygen Several toxic forms of oxygen can be formed in the cell (Figure 5.28): –Single oxygen –Superoxide anion –Hydrogen peroxide –Hydroxyl radical © 2012 Pearson Education, Inc.

94. Figure 5.28 Reactants Products Outcome: (superoxide) (hydrogen peroxide) (hydroxyl radical) (water) © 2012 Pearson Education, Inc.

95. 5.18 Toxic Forms of Oxygen Enzymes are present to neutralize most of these toxic oxygen species (Figure 5.29) –Catalase (Figure 5.30) –Peroxidase –Superoxide dismutase –Superoxide reductase © 2012 Pearson Education, Inc.

96. Figure 5.29 Peroxidase Catalase Superoxide dismutase Superoxide dismutase/catalase in combination Superoxide reductase © 2012 Pearson Education, Inc.

97. Figure 5.30 © 2012 Pearson Education, Inc.