BIOLOGY. A Tour of the Cell CAMPBELL. Reece Urry Cain Wasserman Minorsky Jackson. Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick

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1 CAMPBELL BIOLOGY TENTH EDITION Reece Urry Cain Wasserman Minorsky Jackson 6 A Tour of the Cell Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick

2 Concept 6.2: Eukaryotic cells have internal membranes that compartmentalize their functions The basic structural and functional unit of every organism is one of two types of cells: prokaryotic or eukaryotic Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells Protists, fungi, animals, and plants all consist of eukaryotic cells

3 Comparing Prokaryotic and Eukaryotic Cells Basic features of all cells Plasma membrane Semifluid substance called cytosol Chromosomes (carry genes) Ribosomes (make proteins)

4 Prokaryotic cells are characterized by having No nucleus DNA in an unbound region called the nucleoid No membrane-bound organelles Cytoplasm bound by the plasma membrane

5 Figure 6.5 Fimbriae Nucleoid Ribosomes Plasma membrane Bacterial chromosome (a) A typical rod-shaped bacterium Cell wall Capsule Flagella (b) 0.5 µm A thin section through the bacterium Bacillus coagulans (TEM)

6 Figure 6.5a Nucleoid Ribosomes Plasma membrane Cell wall Capsule 0.5 µm (b) A thin section through the bacterium Bacillus coagulans (TEM)

7 Eukaryotic cells are characterized by having DNA in a nucleus that is bounded by a membranous nuclear envelope Membrane-bound organelles Cytoplasm in the region between the plasma membrane and nucleus Eukaryotic cells are generally much larger than prokaryotic cells

8 The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell

9 Figure 6.6 Outside of cell (a) TEM of a plasma membrane Inside of cell 0.1 µm Carbohydrate side chains Hydrophilic region Hydrophobic region Hydrophilic region (b) Phospholipid Proteins Structure of the plasma membrane

10 Figure 6.6a Outside of cell Inside of cell 0.1 µm

11 Metabolic requirements set upper limits on the size of cells The surface area to volume ratio of a cell is critical As a cell increases in size, its volume grows proportionately more than its surface area

12 Figure 6.7 Surface area increases while total volume remains constant Total surface area [sum of the surface areas (height width) of all box sides number of boxes] Total volume [height width length number of boxes] Surface-to-volume (S-to-V) ratio [surface area volume]

13 A Panoramic View of the Eukaryotic Cell A eukaryotic cell has internal membranes that partition the cell into organelles The basic fabric of biological membranes is a double layer of phospholipids and other lipids Plant and animal cells have most of the same organelles

14 Figure 6.8a ENDOPLASMIC RETICULUM (ER) Rough ER Smooth ER Nuclear envelope Nucleolus NUCLEUS Flagellum Chromatin Centrosome Plasma membrane CYTOSKELETON: Microfilaments Intermediate filaments Microtubules Ribosomes Microvilli Golgi apparatus Peroxisome Mitochondrion Lysosome

15 Figure 6.8b NUCLEUS Nuclear envelope Nucleolus Chromatin Rough ER Smooth ER Golgi apparatus Ribosomes Central vacuole Microfilaments Microtubules CYTOSKELETON Mitochondrion Peroxisome Plasma membrane Wall of adjacent cell Cell wall Plasmodesmata Chloroplast

16 Figure 6.8c Animal Cells Plant Cells 5 μm 10 μm Fungal Cells Unicellular Eukaryotes 8 μm 5 μm 1 μm Cell Parent cell Buds 1 μm Cell wall Vacuole Human cells from lining of uterus (colorized TEM) Nucleus Nucleolus Yeast cells budding (colorized SEM) A single yeast cell (colorized TEM) Nucleus Mitochondrion Cells from duckweed (colorized TEM) Cell Cell wall Chloroplast Mitochondrion Nucleus Nucleolus Chlamydomonas (colorized SEM) Chlamydomonas (colorized TEM) Flagella Nucleus Nucleolus Vacuole Chloroplast Cell wall

17 Figure 6.8ca 10 μm Cell Nucleus Nucleolus Human cells from lining of uterus (colorized TEM)

18 Figure 6.8cb 5 μm Parent cell Buds Yeast cells budding (colorized SEM)

19 Figure 6.8cc 1 μm Cell wall Vacuole A single yeast cell (colorized TEM) Nucleus Mitochondrion

20 Figure 6.8cd 5 μm Cell Cell wall Chloroplast Mitochondrion Nucleus Nucleolus Cells from duckweed (colorized TEM)

21 Figure 6.8ce 8 μm Chlamydomonas (colorized SEM)

22 1 μm Figure 6.8cf Flagella Nucleus Nucleolus Vacuole Chloroplast Cell wall Chlamydomonas (colorized TEM)

23 BioFlix: Tour of a Plant Cell

24 BioFlix: Tour of an Animal Cell

25 Concept 6.3: The eukaryotic cell s genetic instructions are housed in the nucleus and carried out by the ribosomes The nucleus contains most of the DNA in a eukaryotic cell Ribosomes use the information from the DNA to make proteins

26 The Nucleus: Information Central The nucleus contains most of the cell s genes and is usually the most conspicuous organelle The nuclear envelope encloses the nucleus, separating it from the cytoplasm The nuclear membrane is a double membrane; each membrane consists of a lipid bilayer

27 0.5 μm 0.25 μm Figure μm Surface of nuclear envelope (TEM) Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex Ribosome Nucleus Nucleolus Chromatin Nucleus Rough ER Close-up of nuclear envelope Chromatin Pore complexes (TEM) Nuclear lamina (TEM)

28 Figure 6.9a Nucleus Nuclear envelope: Inner membrane Outer membrane Nucleolus Chromatin Nuclear pore Pore complex Ribosome Rough ER Close-up of nuclear envelope Chromatin

29 Figure 6.9b 1 μm Nuclear envelope: Inner membrane Outer membrane Nuclear pore Surface of nuclear envelope (TEM)

30 Figure 6.9c 0.25 μm Pore complexes (TEM)

31 Figure 6.9d 0.5 μm Nuclear lamina (TEM)

32 Pores regulate the entry and exit of molecules from the nucleus The nuclear size of the envelop is lined by the nuclear lamina, which is composed of proteins and maintains the shape of the nucleus

33 In the nucleus, DNA is organized into discrete units called chromosomes Each chromosome is composed of a single DNA molecule associated with proteins The DNA and proteins of chromosomes are together called chromatin Chromatin condenses to form discrete chromosomes as a cell prepares to divide The nucleolus is located within the nucleus and is the site of ribosomal RNA (rrna) synthesis

34 Ribosomes: Protein Factories Ribosomes are complexes made of ribosomal RNA and protein Ribosomes carry out protein synthesis in two locations In the cytosol (free ribosomes) On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes)

35 Figure 6.10 Ribosomes ER 0.25 μm Free ribosomes in cytosol Endoplasmic reticulum (ER) Ribosomes bound to ER Large subunit TEM showing ER and ribosomes Diagram of a ribosome Small subunit Computer model of a ribosome

36 Figure 6.10a 0.25 μm Free ribosomes in cytosol Endoplasmic reticulum (ER) Ribosomes bound to ER Large subunit TEM showing ER and ribosomes Diagram of a ribosome Small subunit

37 Figure 6.10b 0.25 μm Free ribosomes in cytosol Endoplasmic reticulum (ER) Ribosomes bound to ER TEM showing ER and ribosomes

38 Figure 6.10c Large subunit Small subunit Computer model of a ribosome

39 Concept 6.4: The endomembrane system regulates protein traffic and performs metabolic functions in the cell The endomembrane system consists of Nuclear envelope Endoplasmic reticulum Golgi apparatus Lysosomes Vacuoles Plasma membrane These components are either continuous or connected via transfer by vesicles

40 The Endoplasmic Reticulum: Biosynthetic Factory The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells The ER membrane is continuous with the nuclear envelope There are two distinct regions of ER Smooth ER, which lacks ribosomes Rough ER, whose surface is studded with ribosomes

41 Figure 6.11 Smooth ER Rough ER Nuclear envelope Smooth ER Rough ER ER lumen Cisternae Ribosomes Transport vesicle Transitional ER 0.20 μm

42 Figure 6.11a Smooth ER Rough ER 0.20 μm

43 Video: ER and Mitochondria in Leaf Cells

44 Video: Staining of Endoplasmic Reticulum

45 Functions of Smooth ER The smooth ER Synthesizes lipids Metabolizes carbohydrates Detoxifies drugs and poisons Stores calcium ions

46 Functions of Rough ER The rough ER Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates) Distributes transport vesicles, secretory proteins surrounded by membranes Is a membrane factory for the cell

47 The Golgi Apparatus: Shipping and Receiving Center The Golgi apparatus consists of flattened membranous sacs called cisternae Functions of the Golgi apparatus Modifies products of the ER Manufactures certain macromolecules Sorts and packages materials into transport vesicles

48 Figure 6.12 Golgi apparatus cis face ( receiving side of Golgi apparatus) Cisternae 0.1 μm trans face ( shipping side of Golgi apparatus) TEM of Golgi apparatus

49 Figure 6.12a 0.1 μm TEM of Golgi apparatus

50 Video: Golgi Complex in 3-D

51 Lysosomes: Digestive Compartments A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules Lysosomal enzymes work best in the acidic environment inside the lysosome Hydrolytic enzymes and lysosomal membranes are made by rough ER and then transferred to the Golgi apparatus for further processing

52 Some types of cell can engulf another cell by phagocytosis; this forms a food vacuole A lysosome fuses with the food vacuole and digests the molecules Lysosomes also use enzymes to recycle the cell s own organelles and macromolecules, a process called autophagy

53 Figure 6.13 Nucleus 1 μm Vesicle containing two damaged organelles 1 μm Lysosome Digestive enzymes Lysosome Mitochondrion fragment Peroxisome fragment Lysosome Plasma membrane Food vacuole (a) Phagocytosis: lysosome digesting food Digestion Peroxisome Mitochondrion Vesicle (b) Autophagy: lysosome breaking down damaged organelles Digestion

54 Figure 6.13a Nucleus 1 μm Lysosome Digestive enzymes Plasma membrane Lysosome Food vacuole Digestion (a) Phagocytosis: lysosome digesting food

55 Figure 6.13aa Nucleus 1 μm Lysosome

56 Figure 6.13b Vesicle containing two damaged organelles 1 μm Mitochondrion fragment Peroxisome fragment Peroxisome Lysosome (b) Mitochondrion Vesicle Autophagy: lysosome breaking down damaged organelles Digestion

57 Figure 6.13ba Vesicle containing two damaged organelles 1 μm Mitochondrion fragment Peroxisome fragment

58 Animation: Lysosome Formation

59 Video: Phagocytosis in Action

60 Vacuoles: Diverse Maintenance Compartments Vacuoles are large vesicles derived from the ER and Golgi apparatus Vacuoles perform a variety of functions in different kinds of cells

61 Food vacuoles are formed by phagocytosis Contractile vacuoles, found in many freshwater protists, pump excess water out of cells Central vacuoles, found in many mature plant cells, hold organic compounds and water

62 Figure 6.14 Central vacuole Cytosol Nucleus Central vacuole Cell wall Chloroplast 5 μm

63 Figure 6.14a Cytosol Nucleus Cell wall Chloroplast Central vacuole 5 μm

64 Video: Paramecium Vacuole

65 The Endomembrane System: A Review The endomembrane system is a complex and dynamic player in the cell s compartmental organization

66 Figure 6.15 Nucleus Rough ER Smooth ER cis Golgi trans Golgi Plasma membrane

67 Video: ER to Golgi Traffic

68 Video: Secretion from the Golgi

69 Concept 6.5: Mitochondria and chloroplasts change energy from one form to another Mitochondria are the sites of cellular respiration, a metabolic process that uses oxygen to generate ATP Chloroplasts, found in plants and algae, are the sites of photosynthesis Peroxisomes are oxidative organelles

70 The Evolutionary Origins of Mitochondria and Chloroplasts Mitochondria and chloroplasts have similarities with bacteria Enveloped by a double membrane Contain free ribosomes and circular DNA molecules Grow and reproduce somewhat independently in cells These similarities led to the endosymbiont theory

71 The endosymbiont theory suggests that an early ancestor of eukaryotes engulfed an oxygen-using nonphotosynthetic prokaryotic cell The engulfed cell formed a relationship with the host cell, becoming an endosymbiont The endosymbionts evolved into mitochondria At least one of these cells may have then taken up a photosynthetic prokaryote, which evolved into a chloroplast

72 Figure 6.16 Endoplasmic reticulum Nucleus Nuclear envelope Ancestor of eukaryotic cells (host cell) Engulfing of oxygenusing nonphotosynthetic prokaryote, which becomes a mitochondrion Engulfing of Mitochondrion photosynthetic prokaryote Chloroplast Mitochondrion At least one cell Nonphotosynthetic eukaryote Photosynthetic eukaryote

73 Mitochondria: Chemical Energy Conversion Mitochondria are in nearly all eukaryotic cells They have a smooth outer membrane and an inner membrane folded into cristae The inner membrane creates two compartments: intermembrane space and mitochondrial matrix Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix Cristae present a large surface area for enzymes that synthesize ATP

74 Figure 6.17 Mitochondrion Intermembrane space Outer membrane Mitochondria 10 μm Free ribosomes in the mitochondrial matrix DNA Inner membrane Cristae Matrix (a) Diagram and TEM of mitochondrion 0.1 μm Mitochondrial DNA (b) Nuclear DNA Network of mitochondria in Euglena (LM)

75 Figure 6.17a Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix (a) DNA Inner membrane Cristae Matrix Diagram and TEM of mitochondrion 0.1 μm

76 Figure 6.17aa Outer membrane Inner membrane Cristae Matrix 0.1 μm

77 Figure 6.17b Mitochondria 10 μm Mitochondrial DNA Nuclear DNA (b) Network of mitochondria in Euglena (LM)

78 Video: Mitochondria in 3-D

79 Chloroplasts: Capture of Light Energy Chloroplasts contain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis Chloroplasts are found in leaves and other green organs of plants and in algae

80 Figure 6.18 Chloroplast Ribosomes Stroma Inner and outer membranes Granum 50 μm DNA Thylakoid Intermembrane space (a) Diagram and TEM of chloroplast 1 μm Chloroplasts (red) (b) Chloroplasts in an algal cell

81 Figure 6.18a Ribosomes Stroma Inner and outer membranes Granum (a) DNA Thylakoid Intermembrane space Diagram and TEM of chloroplast 1 μm

82 Figure 6.18aa Stroma Inner and outer membranes Granum 1 μm

83 Figure 6.18b 50 μm Chloroplasts (red) (b) Chloroplasts in an algal cell

84 Chloroplast structure includes Thylakoids, membranous sacs, stacked to form a granum Stroma, the internal fluid The chloroplast is one of a group of plant organelles, called plastids

85 Peroxisomes: Oxidation Peroxisomes are specialized metabolic compartments bounded by a single membrane Peroxisomes produce hydrogen peroxide and convert it to water Peroxisomes perform reactions with many different functions How peroxisomes are related to other organelles is still unknown

86 Figure 6.19 Peroxisome Mitochondrion Chloroplasts 1 μm

87 Concept 6.6: The cytoskeleton is a network of fibers that organizes structures and activities in the cell The cytoskeleton is a network of fibers extending throughout the cytoplasm It organizes the cell s structures and activities, anchoring many organelles It is composed of three types of molecular structures Microtubules Microfilaments Intermediate filaments

88 Figure μm

89 Video: The Cytoskeleton in Neuron Growth Cone

90 Video: Interphase Microtubule Dynamics

91 Video: Microtubule Dynamics

92 Video: Actin Visualization in Dendrites

93 Video: Cytoskeletal Protein Dynamics

94 Roles of the Cytoskeleton: Support and Motility The cytoskeleton helps to support the cell and maintain its shape It interacts with motor proteins to produce motility Inside the cell, vesicles can travel along tracks provided by the cytoskeleton

95 Figure 6.21 ATP Vesicle Receptor for motor protein Microtubule Motor protein (ATP powered) (a) Motor proteins walk vesicles along cytoskeletal fibers. Vesicles Microtubule of cytoskeleton 0.25 μm (b) SEM of a squid giant axon

96 Figure 6.21a Microtubule Vesicles 0.25 μm (b) SEM of a squid giant axon

97 Video: Movement of Organelles In Vitro

98 Video: Movement of Organelles In Vivo

99 Video: Transport Along Microtubules

100 Components of the Cytoskeleton Three main types of fibers make up the cytoskeleton Microtubules are the thickest of the three components of the cytoskeleton Microfilaments, also called actin filaments, are the thinnest components Intermediate filaments are fibers with diameters in a middle range

101 Table 6.1

102 Table 6.1a

103 Table 6.1b

104 Table 6.1ba

105 Table 6.1c

106 Table 6.1ca

107 Table 6.1d

108 Table 6.1da

109 Microtubules Microtubules are hollow rods about 25 nm in diameter and about 200 nm to 25 microns long Functions of microtubules Shaping the cell Guiding movement of organelles Separating chromosomes during cell division

110 Centrosomes and Centrioles In animal cells, microtubules grow out from a centrosome near the nucleus In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring

111 Figure 6.22 Centrosome Microtubule Centrioles 0.25 μm Longitudinal section of one centriole Microtubules Cross section of the other centriole

112 Figure 6.22a 0.25 μm Longitudinal section of one centriole Microtubules Cross section of the other centriole

113 Cilia and Flagella Microtubules control the beating of flagella and cilia, microtubule-containing extensions that project from some cells Cilia and flagella differ in their beating patterns

114 Figure 6.23 (a) Motion of flagella Direction of swimming 5 μm (b) Motion of cilia Direction of organism s movement Power stroke Recovery stroke 15 μm

115 Figure 6.23a 5 μm

116 Video: Chlamydomonas

117 Video: Flagellum Movement in Swimming Sperm

118 Video: Motion of Isolated Flagellum

119 Video: Paramecium Cilia

120 Figure 6.23b 15 μm

121 Video: Ciliary Motion

122 Cilia and flagella share a common structure A core of microtubules sheathed by the plasma membrane A basal body that anchors the cilium or flagellum A motor protein called dynein, which drives the bending movements of a cilium or flagellum

123 Figure 6.24 Microtubules Plasma membrane Basal body 0.1 μm (b) Cross section of motile cilium 0.1 μm Triplet Outer microtubule doublet Motor proteins (dyneins) Central microtubule Radial spoke Cross-linking proteins between outer doublets Plasma membrane 0.5 μm (a) Longitudinal section of motile cilium (c) Cross section of basal body

124 Figure 6.24a Microtubules Plasma membrane Basal body (a) 0.5 μm Longitudinal section of motile cilium

125 Figure 6.24b 0.1 μm (b) Cross section of motile cilium Outer microtubule doublet Motor proteins (dyneins) Central microtubule Radial spoke Cross-linking proteins between outer doublets Plasma membrane

126 Figure 6.24ba 0.1 μm Outer microtubule doublet Motor proteins (dyneins) Central microtubule Radial spoke (b) Cross section of motile cilium Cross-linking proteins between outer doublets

127 Figure 6.24c 0.1 μm Triplet (c) Cross section of basal body

128 Figure 6.24ca 0.1 μm Triplet (c) Cross section of basal body

129 Animation: Cilia and Flagella

130 Video: Microtubule Sliding in Flagellum Movement

131 How dynein walking moves flagella and cilia Dynein arms alternately grab, move, and release the outer microtubules Protein cross-links limit sliding Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum

132 Microfilaments (Actin Filaments) Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits The structural role of microfilaments is to bear tension, resisting pulling forces within the cell They form a 3-D network called the cortex just inside the plasma membrane to help support the cell s shape Bundles of microfilaments make up the core of microvilli of intestinal cells

133 0.25 µm Figure 6.25 Microvillus Plasma membrane Microfilaments (actin filaments) Intermediate filaments

134 Microfilaments that function in cellular motility contain the protein myosin in addition to actin In muscle cells, thousands of actin filaments are arranged parallel to one another Thicker filaments composed of myosin interdigitate with the thinner actin fibers

135 Figure 6.26 Muscle cell 0.5 µm Actin filament Myosin filament Myosin head (a) Myosin motors in muscle cell contraction Chloroplast (c) Cytoplasmic streaming in plant cells 30 µm Cortex (outer cytoplasm): gel with actin network 100 µm Inner cytoplasm (more fluid) (b) Amoeboid movement Extending pseudopodium

136 Figure 6.26a Muscle cell 0.5 µm Actin filament Myosin filament Myosin head (a) Myosin motors in muscle cell contraction

137 Figure 6.26aa 0.5 µm

138 Figure 6.26b Cortex (outer cytoplasm): gel with actin network Inner cytoplasm (more fluid) 100 µm (b) Amoeboid movement Extending pseudopodium

139 Video: Actin Network in Crawling Cells

140 Figure 6.26c Chloroplast (c) Cytoplasmic streaming in plant cells 30 µm

141 Video: Cytoplasmic Streaming

142 Video: Chloroplast Movement

143 Localized contraction brought about by actin and myosin also drives amoeboid movement Cells crawl along a surface by extending pseudopodia (cellular extensions) and moving toward them

144 Cytoplasmic streaming is a circular flow of cytoplasm within cells This streaming speeds distribution of materials within the cell In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming

145 Intermediate Filaments Intermediate filaments range in diameter from 8 12 nanometers, larger than microfilaments but smaller than microtubules They support cell shape and fix organelles in place Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes

146 Concept 6.7: Extracellular components and connections between cells help coordinate cellular activities Most cells synthesize and secrete materials that are external to the plasma membrane These extracellular structures are involved in a great many cellular functions

147 Cell Walls of Plants The cell wall is an extracellular structure that distinguishes plant cells from animal cells Prokaryotes, fungi, and some unicellular eukaryotes also have cell walls The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein

148 Plant cell walls may have multiple layers Primary cell wall: Relatively thin and flexible Middle lamella: Thin layer between primary walls of adjacent cells Secondary cell wall (in some cells): Added between the plasma membrane and the primary cell wall Plasmodesmata are channels between adjacent plant cells

149 Figure 6.27 Secondary cell wall Primary cell wall Middle lamella 1 μm Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata

150 Figure 6.27a Secondary cell wall Primary cell wall Middle lamella 1 μm

151 The Extracellular Matrix (ECM) of Animal Cells Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM) The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin ECM proteins bind to receptor proteins in the plasma membrane called integrins

152 Figure 6.28 EXTRACELLULAR FLUID A proteoglycan complex Collagen Fibronectin Polysaccharide molecule Carbohydrates Core protein Plasma membrane Microfilaments CYTOPLASM Integrins Proteoglycan molecule

153 Figure 6.28a Collagen EXTRACELLULAR FLUID A proteoglycan complex Fibronectin Plasma membrane Integrins Microfilaments CYTOPLASM

154 Figure 6.28b Polysaccharide molecule Carbohydrates Core protein Proteoglycan molecule

155 Video: Cartoon Model of a Collagen Triple Helix

156 Video: E-Cadherin Expression

157 Video: Fibronectin Fibrils

158 Video: Staining of the Cell-Cell Junctions

159 The ECM has an influential role in the lives of cells ECM can regulate a cell s behavior by communicating with a cell through integrins The ECM around a cell can influence the activity of gene in the nucleus Mechanical signaling may occur through cytoskeletal changes, that trigger chemical signals in the cell

160 Cell Junctions Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact

161 Plasmodesmata in Plant Cells Plasmodesmata are channels that perforate plant cell walls Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell

162 Figure 6.29 Cell walls Interior of cell Interior of cell 0.5 μm Plasmodesmata Plasma membranes

163 Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells Three types of cell junctions are common in epithelial tissues At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid Desmosomes (anchoring junctions) fasten cells together into strong sheets Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells

164 TEM Figure 6.30 Tight junctions prevent fluid from moving across a layer of cells. Tight junction TEM 0.5 μm Tight junction Intermediate filaments Desmosome Gap junction Desmosome (TEM) 1 μm Ions or small molecules Plasma membranes of adjacent cells Space between cells Extracellular matrix 0.1 μm Gap junctions

165 Figure 6.30a Tight junctions prevent fluid from moving across a layer of cells. Intermediate filaments Tight junction Desmosome Space between cells Plasma membranes of adjacent cells Gap junction Ions or small molecules Extracellular matrix

166 Figure 6.30b Tight junction TEM 0.5 μm

167 Figure 6.30c Desmosome (TEM) 1 μm

168 Figure 6.30d TEM 0.1 μm Gap junctions

169 Animation: Desmosomes

170 Animation: Gap Junctions

171 Animation: Tight Junctions

172 The Cell: A Living Unit Greater Than the Sum of Its Parts Cells rely on the integration of structures and organelles in order to function For example, a macrophage s ability to destroy bacteria involves the whole cell, coordinating components such as the cytoskeleton, lysosomes, and plasma membrane

173 Figure μm

174 Figure 6.UN01a Budding cell Mature parent cell 1 μm

175 Figure 6.UN01b V = 4 _ πr 3 3 r d

176 Figure 6.UN02 Nucleus 5 μm

177 Figure 6.UN03

178 Figure 6.UN04

179 Figure 6.UN05

180 Figure 6.UN06 Epithelial cell