Chapter 4 A Tour of the Cell

Transcription

1 Chapter 4 A Tour of the Cell PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE TAYLOR SIMON DICKEY HOGAN Lecture by Edward J. Zalisko

2 Introduction Cells have a cytoskeleton that provides support and allows some cells to crawl and others to swim. Our understanding of nature often goes hand in hand with the invention and refinement of instruments that extend our senses.

3 Introduction In 1665, Hooke examined a piece of cork under a crude microscope and identified little rooms as cells. Leeuwenhoek, working at about the same time, used more refined lenses to describe living cells from blood, sperm, and pond water. Since the days of Hooke and Leeuwenhoek, improved microscopes have vastly expanded our view of the cell.

4 Figure 4.0-1

5 Figure Chapter 4: Big Ideas Introduction to the Cell The Nucleus and Ribosomes The Endomembrane System Energy-Converting Organelles The Cytoskeleton and Cell Surfaces

6 INTRODUCTION TO THE CELL

7 4.1 Microscopes reveal the world of the cell A variety of microscopes have been developed for a clearer view of cells and cellular structure. The first microscopes were light microscopes. In a light microscope (LM), visible light passes through a specimen, then through glass lenses, and finally is projected into the viewer s eye. Specimens can be magnified by up to 1,000 times.

8 4.1 Microscopes reveal the world of the cell Magnification is the increase in an object s image size compared with its actual size. Resolution is a measure of the clarity of an image. In other words, it is the ability of an instrument to show two nearby objects as separate.

9 4.1 Microscopes reveal the world of the cell Microscopes have limitations. The human eye and the microscope have limits of resolution the ability to distinguish between small structures. Therefore, the light microscope cannot provide the details of a small cell s structure.

10 4.1 Microscopes reveal the world of the cell Using light microscopes, scientists studied microorganisms, animal and plant cells, and some structures within cells. In the 1800s, these studies led to cell theory, which states that all living things are composed of cells and all cells come from other cells.

11 Figure 4.1a

12 Figure 4.1b

13 Figure 4.1c

14 Figure 4.1d

15 Electron microscope Light microscope Unaided eye Figure 4.1e-0 10 m 1 m 100 mm (10 cm) Human height Length of some nerve and muscle cells Chicken egg 10 mm (1 cm) 1 mm 100 μm Frog egg Paramecium Human egg 10 μm 1 μm Most plant and animal cells Nucleus Most bacteria Mitochondrion 100 nm 10 nm 1 nm 0.1 nm Smallest bacteria Viruses Ribosome Proteins Lipids Small molecules Atoms

16 Figure 4.1e-1 Unaided eye 10 m 1 m 100 mm (10 cm) 10 mm (1 cm) 1 mm 100 μm Human height Length of some nerve and muscle cells Chicken egg Frog egg Paramecium Human egg

17 Figure 4.1e-2 Light microscope Electron microscope 1 mm 100 μm 10 μm 1 μm Frog egg Paramecium Human egg Most plant and animal cells Nucleus Most bacteria Mitochondrion 100 nm 10 nm 1 nm 0.1 nm Smallest bacteria Viruses Ribosome Proteins Lipids Small molecules Atoms

18 4.1 Microscopes reveal the world of the cell Beginning in the 1950s, scientists started using a very powerful microscope called the electron microscope (EM) to view the ultrastructure of cells. Instead of light, EM uses a beam of electrons. Electron microscopes can resolve biological structures as small as 2 nanometers and magnify up to 100,000 times.

19 4.1 Microscopes reveal the world of the cell Scanning electron microscopes (SEMs) study the detailed architecture of cell surfaces. Transmission electron microscopes (TEMs) study the details of internal cell structure. Differential interference light microscopes amplify differences in density so that structures in living cells appear almost three-dimensional.

20 4.2 The small size of cells relates to the need to exchange materials across the plasma membrane Cell size must be large enough to house DNA, proteins, and structures needed to survive and reproduce, but remain small enough to allow for a surface-tovolume ratio that will allow adequate exchange with the environment.

21 Figure 4.2a Total volume Total surface area Surface-tovolume ratio 27 units 3 27 units 3 54 units units 2 2 6

22 4.2 The small size of cells relates to the need to exchange materials across the plasma membrane The plasma membrane forms a flexible boundary between the living cell and its surroundings. Phospholipids form a two-layer sheet called a phospholipid bilayer in which hydrophilic heads face outward, exposed to water, and hydrophobic tails point inward, shielded from water.

23 4.2 The small size of cells relates to the need to exchange materials across the plasma membrane Membrane proteins are embedded in the lipid bilayer. Some proteins form channels (tunnels) that shield ions and other hydrophilic molecules as they pass through the hydrophobic center of the membrane. Other proteins serve as pumps, using energy to actively transport molecules into or out of the cell.

24 Figure 4.2b Outside cell Hydrophilic heads Hydrophobic tails Phospholipid Inside cell Channel protein Hydrophilic regions of a protein Hydrophobic regions of a protein

25 Figure 4.UN01

26 4.3 Prokaryotic cells are structurally simpler than eukaryotic cells Bacteria and archaea are prokaryotic cells. All other forms of life are composed of eukaryotic cells. Eukaryotic cells are distinguished by having a membrane-enclosed nucleus and many membrane-enclosed organelles that perform specific functions. Prokaryotic cells are smaller and simpler in structure.

27 4.3 Prokaryotic cells are structurally simpler than eukaryotic cells Prokaryotic and eukaryotic cells have a plasma membrane, an interior filled with a thick, jellylike fluid called the cytosol, one or more chromosomes, which carry genes made of DNA, and ribosomes, tiny structures that make proteins according to instructions from the genes.

28 4.3 Prokaryotic cells are structurally simpler than eukaryotic cells The inside of both types of cells is called the cytoplasm. However, in eukaryotic cells, this term refers only to the region between the nucleus and the plasma membrane.

29 4.3 Prokaryotic cells are structurally simpler than eukaryotic cells In a prokaryotic cell, the DNA is coiled into a region called the nucleoid (nucleus-like) and no membrane surrounds the DNA.

30 4.3 Prokaryotic cells are structurally simpler than eukaryotic cells Outside the plasma membrane of most prokaryotes is a fairly rigid, chemically complex cell wall, which protects the cell and helps maintain its shape. Some prokaryotes have surface projections. Short projections help attach prokaryotes to each other or their substrate. Longer projections called flagella (singular, flagellum) propel a prokaryotic cell through its liquid environment.

31 Figure Fimbriae Ribosomes Nucleoid Bacterial chromosome A typical rod-shaped bacterium Plasma membrane Cell wall Capsule Flagella A colorized TEM of the bacterium Escherichia coli

32 Figure Fimbriae Ribosomes Nucleoid Bacterial chromosome A typical rod-shaped bacterium Plasma membrane Cell wall Capsule Flagella

33 Figure A colorized TEM of the bacterium Escherichia coli

34 4.4 Eukaryotic cells are partitioned into functional compartments A eukaryotic cell contains a membrane-enclosed nucleus and various other organelles ( little organs ), which perform specific functions in the cell.

35 4.4 Eukaryotic cells are partitioned into functional compartments The structures and organelles of eukaryotic cells perform four basic functions. 1. The nucleus and ribosomes are involved in the genetic control of the cell. 2. The endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and peroxisomes are involved in the manufacture, distribution, and breakdown of molecules.

36 4.4 Eukaryotic cells are partitioned into functional compartments 3. Mitochondria in all cells and chloroplasts in plant cells are involved in energy processing. 4. Structural support, movement, and communication between cells are functions of the cytoskeleton, plasma membrane, and cell wall.

37 4.4 Eukaryotic cells are partitioned into functional compartments The internal membranes of eukaryotic cells partition it into compartments. Cellular metabolism, the many chemical activities of cells, occurs within organelles.

38 4.4 Eukaryotic cells are partitioned into functional compartments Almost all of the organelles and other structures of animals cells are present in plant cells. A few exceptions exist. Lysosomes and centrosomes containing centrioles are not found in plant cells. Only the sperm cells of a few plant species have flagella.

39 4.4 Eukaryotic cells are partitioned into functional compartments Plant but not animal cells have a rigid cell wall that contains cellulose, plasmodesmata, cytoplasmic channels through cell walls that connect adjacent cells, chloroplasts, where photosynthesis occurs, and a central vacuole, a compartment that stores water and a variety of chemicals.

40 Figure 4.4a Rough endoplasmic reticulum Ribosomes NUCLEUS Nuclear envelope Nucleolus Chromatin CYTOSKELETON Microtubule Microfilament Intermediate filament Smooth endoplasmic reticulum Peroxisome Plasma membrane Golgi apparatus Lysosome Mitochondrion Centrosome with pair of centrioles

41 Figure 4.4b Smooth endoplasmic reticulum Mitochondrion Rough endoplasmic reticulum NUCLEUS Nuclear envelope Nucleolus Chromatin CYTOSKELETON Microfilament Microtubule Cell wall of adjacent cell Ribosomes Plasmodesma Golgi apparatus Peroxisome Central vacuole Chloroplast Cell wall Plasma membrane

42 THE NUCLEUS AND RIBOSOMES

43 4.5 The nucleus contains the cell s genetic instructions The nucleus contains most of the cell s DNA and controls the cell s activities by directing protein synthesis by making messenger RNA (mrna). DNA is associated with many proteins and is organized into structures called chromosomes. When a cell is not dividing, this complex of proteins and DNA, called chromatin, appears as a diffuse mass within the nucleus.

44 4.5 The nucleus contains the cell s genetic instructions The double membrane nuclear envelope has pores that regulate the entry and exit of large molecules and connect with the cell s network of membranes called the endoplasmic reticulum.

45 4.5 The nucleus contains the cell s genetic instructions The nucleolus is a prominent structure in the nucleus and the site of ribosomal RNA (rrna) synthesis.

46 Figure 4.5 Nuclear envelope Nucleolus Endoplasmic reticulum Ribosome Pore Chromatin

47 4.6 Ribosomes make proteins for use in the cell and export Ribosomes are involved in the cell s protein synthesis. Ribosomes are the cellular components that use instructions from the nucleus, written in mrna, to build proteins. Cells that make a lot of proteins have a large number of ribosomes.

48 4.6 Ribosomes make proteins for use in the cell and export Some ribosomes are free ribosomes; others are bound. Free ribosomes are suspended in the cytosol. Bound ribosomes are attached to the outside of the endoplasmic reticulum or nuclear envelope.

49 Figure 4.6 Rough ER Bound ribosome Endoplasmic reticulum Protein Ribosome Free ribosome mrna

50 THE ENDOMEMBRANE SYSTEM

51 4.7 Many organelles are connected in the endomembrane system Many of the membranes within a eukaryotic cell are part of the endomembrane system. Some of these membranes are physically connected, and others are linked when tiny vesicles (sacs made of membrane) transfer membrane segments between them.

52 4.7 Many organelles are connected in the endomembrane system Many of these organelles interact in the synthesis, distribution, storage, and export of molecules.

53 4.7 Many organelles are connected in the endomembrane system The endomembrane system includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

54 4.7 Many organelles are connected in the endomembrane system The largest component of the endomembrane system is the endoplasmic reticulum (ER), an extensive network of flattened sacs and tubules.

55 4.8 The endoplasmic reticulum is a biosynthetic workshop There are two kinds of endoplasmic reticulum, which differ in structure and function. 1. Smooth ER lacks attached ribosomes. 2. Rough ER has bound ribosomes that stud the outer surface of the membrane.

56 Figure 4.8a Rough ER Smooth ER Ribosomes Rough ER Smooth ER

57 Figure 4.8b mrna Bound ribosome Transport vesicle buds off 4 Secretory protein inside transport vesicle 3 1 Sugar chain Growing polypeptide 2 Glycoprotein Rough ER

58 4.8 The endoplasmic reticulum is a biosynthetic workshop Smooth ER is involved in a variety of metabolic processes, including the production of enzymes important in the synthesis of lipids, oils, phospholipids, and steroids, the production of enzymes that help process drugs, alcohol, and other potentially harmful substances, and the storage of calcium ions.

59 4.8 The endoplasmic reticulum is a biosynthetic workshop Rough ER makes additional membrane for itself and secretory proteins.

60 4.9 The Golgi apparatus modifies, sorts, and ships cell products The Golgi apparatus serves as a molecular warehouse and processing station for products manufactured by the ER. Products travel in transport vesicles from the ER to the Golgi apparatus. One side of the Golgi stack serves as a receiving dock for transport vesicles produced by the ER.

61 4.9 The Golgi apparatus modifies, sorts, and ships cell products Products of the ER are modified as a Golgi sac progresses through the stack. The shipping side of the Golgi functions as a depot, where products in vesicles bud off and travel to other sites Pearson Education, Inc.

62 Figure 4.9 Receiving side of Golgi apparatus Transport vesicle from the ER Golgi apparatus 4 Transport vesicle from the Golgi Shipping side of Golgi apparatus

63 4.10 Lysosomes are digestive compartments within a cell A lysosome is a membrane-enclosed sac of digestive enzymes made by rough ER and processed in the Golgi apparatus

64 4.10 Lysosomes are digestive compartments within a cell Lysosomes fuse with food vacuoles and digest food, destroy bacteria engulfed by white blood cells, or fuse with other vesicles containing damaged organelles or other materials to be recycled within a cell.

65 Animation: Lysosome Formation

66 Figure 4.10a-1 Digestive enzymes Lysosome Plasma membrane

67 Figure 4.10a-2 Digestive enzymes Lysosome Food vacuole Plasma membrane

68 Figure 4.10a-3 Digestive enzymes Lysosome Food vacuole Plasma membrane

69 Figure 4.10a-4 Digestive enzymes Lysosome Digestion Food vacuole Plasma membrane

70 Figure 4.10b-1 Lysosome Vesicle containing damaged mitochondrion

71 Figure 4.10b-2 Lysosome Vesicle containing damaged mitochondrion

72 Figure 4.10b-3 Lysosome Vesicle containing Digestion damaged mitochondrion

73 4.11 Vacuoles function in the general maintenance of the cell Vacuoles are large vesicles that have a variety of functions. Some protists have contractile vacuoles, which help to eliminate water from the protist. In plants, vacuoles may have digestive functions, contain pigments, or contain poisons that protect the plant.

74 Video: Paramecium Vacuole

75 Figure 4.11a Contractile vacuoles Nucleus

76 Figure 4.11b Central vacuole Chloroplast Nucleus

77 4.12 A review of the structures involved in manufacturing and breakdown The following figure summarizes the relationships among the major organelles of the endomembrane system.

78 Figure 4.12 Nucleus Smooth ER Rough ER Nuclear envelope Golgi apparatus Transport vesicle Plasma membrane Lysosome Transport vesicle

79 4.12 A review of the structures involved in manufacturing and breakdown Peroxisomes are metabolic compartments that do not originate from the endomembrane system. How they are related to other organelles is still unknown. Some peroxisomes break down fatty acids to be used as cellular fuel.

80 ENERGY-CONVERTING ORGANELLES

81 4.13 Mitochondria harvest chemical energy from food Mitochondria are organelles that carry out cellular respiration in nearly all eukaryotic cells. Cellular respiration converts the chemical energy in foods to chemical energy in ATP (adenosine triphosphate).

82 4.13 Mitochondria harvest chemical energy from food Mitochondria have two internal compartments. 1. The intermembrane space is the narrow region between the inner and outer membranes. 2. The mitochondrial matrix contains the mitochondrial DNA, ribosomes, and many enzymes that catalyze some of the reactions of cellular respiration.

83 4.13 Mitochondria harvest chemical energy from food Folds of the inner mitochondrial membrane, called cristae, increase the membrane s surface area, enhancing the mitochondrion s ability to produce ATP.

84 Figure 4.13 Mitochondrion Intermembrane space Outer membrane Inner membrane Crista Matrix

85 4.14 Chloroplasts convert solar energy to chemical energy Photosynthesis is the conversion of light energy from the sun to the chemical energy of sugar molecules. Chloroplasts are the photosynthesizing organelles of plants and algae.

86 4.14 Chloroplasts convert solar energy to chemical energy Chloroplasts are partitioned into compartments. Between the outer and inner membrane is a thin intermembrane space. Inside the inner membrane is a thick fluid called stroma, which contains the chloroplast DNA, ribosomes, many enzymes, and a network of interconnected sacs called thylakoids, where green chlorophyll molecules trap solar energy. In some regions, thylakoids are stacked like poker chips. Each stack is called a granum.

87 Figure 4.14 Chloroplast Granum Stroma Inner and outer membranes Thylakoid

88 4.15 EVOLUTION CONNECTION: Mitochondria and chloroplasts evolved by endosymbiosis Mitochondria and chloroplasts contain DNA and ribosomes. The structure of this DNA and these ribosomes is very similar to that found in prokaryotic cells.

89 4.15 EVOLUTION CONNECTION: Mitochondria and chloroplasts evolved by endosymbiosis The endosymbiont theory states that mitochondria and chloroplasts were formerly small prokaryotes and they began living within larger cells.

90 Figure 4.15 Endoplasmic reticulum Nucleus Ancestor of eukaryotic cells (host cell) Engulfing of oxygenusing prokaryote Engulfing of photosynthetic prokaryote Mitochondrion Mitochondrion At least one cell Nonphotosynthetic eukaryote Chloroplast Photosynthetic eukaryote

91 Figure Endoplasmic reticulum Nucleus Ancestor of eukaryotic cells (host cell)

92 Figure Endoplasmic reticulum Nucleus Ancestor of eukaryotic cells (host cell) Engulfing of oxygenusing prokaryote Mitochondrion Nonphotosynthetic eukaryote

93 Figure Endoplasmic reticulum Nucleus Ancestor of eukaryotic cells (host cell) Engulfing of oxygenusing prokaryote Engulfing of photosynthetic prokaryote Mitochondrion Mitochondrion At least one cell Nonphotosynthetic eukaryote Chloroplast Photosynthetic eukaryote

94 THE CYTOSKELETON AND CELL SURFACES

95 4.16 The cell s internal skeleton helps organize its structure and activities Cells contain a network of protein fibers, called the cytoskeleton, which organize the structures and activities of the cell.

96 Video: Cytoplasmic Streaming

97 4.16 The cell s internal skeleton helps organize its structure and activities Microtubules (made of tubulin) shape and support the cell and act as tracks along which organelles equipped with motor proteins move. In animal cells, microtubules grow out from a region near the nucleus called the centrosome, which contains a pair of centrioles, each composed of a ring of microtubules.

98 4.16 The cell s internal skeleton helps organize its structure and activities Intermediate filaments are found in the cells of most animals, reinforce cell shape and anchor some organelles, and are often more permanent fixtures in the cell.

99 4.16 The cell s internal skeleton helps organize its structure and activities Microfilaments (actin filaments) support the cell s shape and are involved in motility.

100 Figure Nucleus Nucleus 25 nm Intermediate filament 10 nm Microfilament 7 nm Microtubule

101 Figure Nucleus 25 nm Microtubule

102 Figure Nucleus 10 nm Intermediate filament

103 Figure nm Microfilament

104 Figure Nucleus

105 Figure Nucleus

106 Figure

107 4.17 SCIENTIFIC THINKING: Scientists discovered the cytoskeleton using the tools of biochemistry and microscopy In the 1940s, biochemists first isolated and identified the proteins actin and myosin from muscle cells. In 1954, scientists, using newly developed techniques of microscopy, established how filaments of actin and myosin interact in muscle contraction. In the next decade, researchers identified actin filaments in all types of cells.

108 4.17 SCIENTIFIC THINKING: Scientists discovered the cytoskeleton using the tools of biochemistry and microscopy In the 1970s, scientists were able to visualize actin filaments using fluorescent tags and in living cells. In the 1980s, biologists were able to record the changing architecture of the cytoskeleton.

109 Figure 4.17

110 4.18 Cilia and flagella move when microtubules bend The short, numerous appendages that propel protists such as Paramecium are called cilia (singular, cilium). Other protists may move using flagella, which are longer than cilia and usually limited to one or a few per cell. Some cells of multicellular organisms also have cilia or flagella.

111 Video: Paramecium Cilia

112 Video: Chlamydomonas

113 Figure 4.18a Cilia

114 Figure 4.18b Flagellum

115 4.18 Cilia and flagella move when microtubules bend A flagellum, longer than cilia, propels a cell by an undulating, whiplike motion. Cilia work more like the oars of a boat. Although differences exist, flagella and cilia have a common structure and mechanism of movement.

116 4.18 Cilia and flagella move when microtubules bend Both flagella and cilia are composed of microtubules wrapped in an extension of the plasma membrane. In nearly all eukaryotic cilia and flagella, a ring of nine microtubule doublets surrounds a central pair of microtubules. This arrangement is called the 9 2 pattern. The microtubule assembly is anchored in a basal body with nine microtubule triplets arranged in a ring.

117 Animation: Cilia and Flagella

118 Figure 4.18c-0 Outer microtubule doublet Central microtubules Cross-linking proteins Motor proteins (dyneins) Plasma membrane

119 Figure 4.18c-1 Outer microtubule doublet Central microtubules Cross-linking proteins Motor proteins (dyneins)

120 Figure 4.18c-2 Outer microtubule doublet Central microtubules Cross-linking proteins Motor proteins (dyneins) Plasma membrane

121 4.18 Cilia and flagella move when microtubules bend Cilia and flagella move by bending motor proteins called dynein feet. These feet attach to and exert a sliding force on an adjacent doublet. This walking causes the microtubules to bend.

122 4.19 The extracellular matrix of animal cells functions in support and regulation Animal cells synthesize and secrete an elaborate extracellular matrix (ECM), which helps hold cells together in tissues and protects and supports the plasma membrane.

123 4.19 The extracellular matrix of animal cells functions in support and regulation The ECM may attach to the cell through other glycoproteins that then bind to membrane proteins called integrins. Integrins span the membrane and attach on the other side to proteins connected to microfilaments of the cytoskeleton.

124 Figure 4.19 Glycoprotein complex with long polysaccharide Collagen fiber EXTRACELLULAR FLUID Connecting glycoprotein Integrin Plasma membrane Microfilaments of cytoskeleton CYTOPLASM

125 4.20 Three types of cell junctions are found in animal tissues Adjacent cells adhere, interact, and communicate through specialized junctions between them. Tight junctions prevent leakage of fluid across a layer of epithelial cells. Anchoring junctions fasten cells together into sheets. Gap junctions are channels that allow small molecules to flow through protein-lined pores between cells.

126 Animation: Desmosomes

127 Animation: Gap Junctions

128 Animation: Tight Junctions

129 Figure 4.20 Tight junctions prevent fluid from moving across a layer of cells Tight junction Anchoring junction Gap junction Plasma membranes of adjacent cells Ions or small molecules Extracellular matrix

130 4.21 Cell walls enclose and support plant cells A plant cell, but not an animal cell, has a rigid cell wall that protects and provides skeletal support that helps keep the plant upright and is primarily composed of cellulose. Plant cells have cell junctions called plasmodesmata that allow plants tissues to share water, nourishment, and chemical messages.

131 Figure 4.21 Plant cell walls Vacuole Plasmodesmata Primary cell wall Secondary cell wall Plasma membrane Cytosol

132 4.22 Review: Eukaryotic cell structures can be grouped on the basis of four main functions Eukaryotic cell structures can be grouped on the basis of four functions: 1. genetic control, 2. manufacturing, distribution, and breakdown of materials, 3. energy processing, and 4. structural support, movement, and intercellular communication.

133 Table

134 Table

135 Table

136 You should now be able to 1. Describe the importance of microscopes in understanding cell structure and function. 2. Describe the two parts of cell theory. 3. Distinguish between the structures of prokaryotic and eukaryotic cells. 4. Explain how cell size is limited. 5. Describe the structure and functions of cell membranes.

137 You should now be able to 6. Explain why compartmentalization is important in eukaryotic cells. 7. Compare the structures of plant and animal cells. Note the function of each cell part. 8. Compare the structures and functions of chloroplasts and mitochondria. 9. Describe the evidence that suggests that mitochondria and chloroplasts evolved by endosymbiosis.

138 You should now be able to 10. Compare the structures and functions of microfilaments, intermediate filaments, and microtubules. 11. Relate the structure of cilia and flagella to their functions. 12. Relate the structure of the extracellular matrix to its functions. 13. Compare the structures and functions of tight junctions, anchoring junctions, and gap junctions.

139 You should now be able to 14. Relate the structures of plant cell walls and plasmodesmata to their functions. 15. Describe the four functional categories of organelles in eukaryotic cells.

140 Table 4-1

141 Figure 4.UN02

142 Figure 4.UN03 a. b. c. d. e. f. g. h. i. l. k. j.

143 Figure 4.UN04-0 Poles of dividing cell Mark

144 Figure 4.UN04-1 Poles of dividing cell Mark

145 Figure 4.UN04-2