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Chapter 6 A Tour of the Cell PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Importance of Cells Cell Theory: 1. All organisms are composed of one or more cells 2. Cells are the smallest living things 3. Cells arise only by division of a previously existing cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell Size 1. Cell size – from a few micrometers to several centimeters 2. Most cells are small because larger cells do not function efficiently 3. It is advantageous to have a large surface-to-volume ratio – Smaller sizes have greater Surface for volume Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A smaller cell – Has a higher surface to volume ratio, which facilitates the exchange of materials into and out of the cell Surface area increases while total volume remains constant 5 1 1 Total surface area (height width number of sides number of boxes) 6 150 750 Total volume (height width length number of boxes) 1 125 125 Surface-to-volume ratio (surface area volume) 6 12 6 Figure 6.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Cell structure is correlated to cellular function Figure 6.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 10 µm Visualizing Cells Unaided eye • Different types of microscopes – Can be used to visualize different sized cellular structures 10 m 0.1 m Human height Length of some nerve and muscle cells Chicken egg 1 cm Light microscope 1m 10 µ m 1µm 100 nm Most plant and Animal cells Nucleus Most bacteria Mitochondrion Smallest bacteria Viruses 10 nm Ribosomes Proteins 1 nm Lipids Small molecules Figure 6.2 0.1 nm Atoms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electron microscope 100 µm Electron microscope Frog egg 1 mm Measurements 1 centimeter (cm) = 102 meter (m) = 0.4 inch 1 millimeter (mm) = 10–3 m 1 micrometer (µm) = 10–3 mm = 10–6 m 1 nanometer (nm) = 10–3 mm = 10–9 m – Use different methods for enhancing visualization of cellular structures TECHNIQUE RESULT (a) Brightfield (unstained specimen). Passes light directly through specimen. Unless cell is naturally pigmented or artificially stained, image has little contrast. [Parts (a)–(d) show a human cheek epithelial cell.] 50 µm (b) Brightfield (stained specimen). Staining with various dyes enhances contrast, but most staining procedures require that cells be fixed (preserved). (c) Phase-contrast. Enhances contrast in unstained cells by amplifying variations in density within specimen; especially useful for examining living, unpigmented cells. Figure 6.3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (d) Differential-interference-contrast (Nomarski). Like phase-contrast microscopy, it uses optical modifications to exaggerate differences in density, making the image appear almost 3D. (e) Fluorescence. Shows the locations of specific molecules in the cell by tagging the molecules with fluorescent dyes or antibodies. These fluorescent substances absorb ultraviolet radiation and emit visible light, as shown here in a cell from an artery. 50 µm (f) Confocal. Uses lasers and special optics for “optical sectioning” of fluorescently-stained specimens. Only a single plane of focus is illuminated; out-of-focus fluorescence above and below the plane is subtracted by a computer. A sharp image results, as seen in stained nervous tissue (top), where nerve cells are green, support cells are red, and regions of overlap are yellow. A standard fluorescence micrograph (bottom) of this relatively thick tissue is blurry. 50 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electron Microscopes • 1. The scanning electron microscope (SEM) – Provides for detailed study of the surface of a specimen TECHNIQUE RESULTS 1 µm Cilia (a) Scanning electron microscopy (SEM). Micrographs taken with a scanning electron microscope show a 3D image of the surface of a specimen. This SEM shows the surface of a cell from a rabbit trachea (windpipe) covered with motile organelles called cilia. Beating of the cilia helps move inhaled debris upward toward the throat. Figure 6.4 (a) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electron Microscopes 2. The transmission electron microscope (TEM) – Provides for detailed study of the internal ultrastructure of cells Longitudinal section of cilium (b) Transmission electron microscopy (TEM). A transmission electron microscope profiles a thin section of a specimen. Here we see a section through a tracheal cell, revealing its ultrastructure. In preparing the TEM, some cilia were cut along their lengths, creating longitudinal sections, while other cilia were cut straight across, creating cross sections. Figure 6.4 (b) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cross section of cilium 1 µm Types of Cells Two types: 1. Prokaryotic – NO NUCLEUS 2. Eukaryotic - NUCLEUS Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Common features Prokaryotic & Eukaryotic Cells share: 1. Bound by plasma membrane 2. Contain cytosol – semifluid substance where organelles found 3. Have ribosomes 4. Contain chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prokaryotes Eukaryotes 1. Contain a membrane bound nucleus No membrane bound NUCLEUS DNA is found is specialized region of the cytoplasm called NUCLEOID region Includes nucleoplasm, DNA, & DNA associated enzymes 2. No membrane bound ORGANELLES Many membrane organelles Sub cellular structure that contains components which have SPECIFIC functions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prokaryotes Eukaryotes 3. Size – very small (.2 to 50 micrometers) Size – much larger (100 micrometers) 4. DNA is present as a SINGLE CIRCULAR MOLECULE (called genophore) Multiple molecules of LINEAR DNA (called chromosomes) 5. No HISTONE proteins DNA organized around HISTONE proteins 6. Reproduce by BINARY FISSION Reproduce by MITOSIS Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prokaryotes Eukaryotes 7. No cholesterol in plasma membrane (less stable) Cholesterol to stabilize 8. 70 S ribosomes (refers to mass) 80 S ribosomes 9. Cell wall of PEPTIDOGLYCAN No Peptidoglycan May have no cell wall If cell wall (composed of cellulose or chitin) 10. Bacteria & Archaea Protista (primitive) Fungi, Plantae, Animalia Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prokaryotes • 1. Plasma Membrane • 2. Ribosomes • 3. Cytoplasm • 4. Cell wall of peptidoglycan • 5. Some have flagella (for movement) • 6. Pili (small hair-like appendages found on surface of bacteria) • Aid in ADHERANCE Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pili: attachment structures on the surface of some prokaryotes Nucleoid: region where the cell’s DNA is located (not enclosed by a membrane) Ribosomes: organelles that synthesize proteins Bacterial chromosome (a) A typical rod-shaped bacterium Plasma membrane: membrane enclosing the cytoplasm Cell wall: rigid structure outside the plasma membrane Capsule: jelly-like outer coating of many prokaryotes 0.5 µm Flagella: locomotion organelles of some bacteria Figure 6.6 A, B Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) A thin section through the bacterium Bacillus coagulans (TEM) Eukaryotic sub cell structures 1. NUCLEUS a. Surrounded by a double membrane called NUCLEAR MEMBRANE or Nuclear envelope Studded with nuclear PORES (mRNA exit here) b. Nucleolus – dark staining region within the nucleus * site for ribosome assembly c. DNA - usually in the for of chromatin (with histone proteins) - DNA in this state is uncondensed Chromosomes – condensed DNA only visualized during MITOSIS (cell division that makes exact copies) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The nuclear envelope – Encloses the nucleus, separating its contents from the cytoplasm Nucleus 1 µm Nucleolus Chromatin Nucleus Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex Rough ER Surface of nuclear envelope. 1 µm Ribosome 0.25 µm Close-up of nuclear envelope Figure 6.10 Pore complexes (TEM). Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nuclear lamina (TEM). Eukaryotic Sub Structures d. Enzymes that interact w/DNA (transcription & DNA replication) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2. Endoplasmic Reticulum (ER) a. Huge collection of membrane bound “tubes” that snake throughout the cytoplasm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The ER membrane – Is continuous with the nuclear envelope Smooth ER Rough ER Nuclear envelope ER lumen Cisternae Ribosomes Transitional ER Transport vesicle Smooth ER Rough ER 200 µm Figure 6.12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings b. Types of ER 1. Rough ER covered with Ribosomes (little circles) Main site for protein synthesis Newly made proteins are pushed into the LUMEN (opening or empty center region or the Rough ER) carry out initial modifications of newly manufactured protein Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2. Smooth ER No Ribosomes – looks exactly like rough ER w/o ribo site for synthesis of lipids & carbohydrates site for toxin inactivation ex. Alcohol dehydrogenase Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3. Ribosomes a. Composed of ribosomal RNA and protein 2 Sub units (large and small) b. Carry out protein synthesis (aka protein factories) c. Two types 1. Free ribosomes – located in cytosol (proteins will function within cytosol) 2. Bound ribosomes - located on Rough ER (proteins will be packaged for insertion into membranes, packaging within organelles) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – Carry out protein synthesis Ribosomes ER Cytosol Endoplasmic reticulum (ER) Free ribosomes Bound ribosomes Large subunit 0.5 µm TEM showing ER and ribosomes Figure 6.11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Small subunit Diagram of a ribosome 4. Golgi (complex, body, appartus) a. stack of flattened membrane sacs called cisternae completes all modifications to newly made macromolecules b. * Packages molecules into vessicles & places a molecular address onto package *Sends vessicle to ultimate destination (w/in cell or to be secreted) c. Two faces 1. cis face – incoming/receiving 2. trans face - shipping Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Functions of the Golgi apparatus Golgi apparatus cis face (“receiving” side of Golgi apparatus) 1 Vesicles move 2 Vesicles coalesce to 6 Vesicles also form new cis Golgi cisternae from ER to Golgi transport certain Cisternae proteins back to ER 3 Cisternal maturation: Golgi cisternae move in a cisto-trans direction Figure 6.13 5 Vesicles transport specific proteins backward to newer Golgi cisternae 4 Vesicles form and leave Golgi, carrying specific proteins to other locations or to the plasma membrane for secretion trans face (“shipping” side of Golgi apparatus) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0.1 0 µm TEM of Golgi apparatus 5. Lysosomes (membranous sacs) Contain hydrolytic enzymes that “digest” macromolecules Recycle of “worn out” cell components Apoptosis – cell suicide in cells infected w/viruses or transformed into cancer Can arise from “budding” off the Golgi apparatus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Lysosomes carry out intracellular digestion by – Phagocytosis Nucleus 1 µm Lysosome Lysosome contains active hydrolytic enzymes Food vacuole fuses with lysosome Hydrolytic enzymes digest food particles Digestive enzymes Lysosome Plasma membrane Digestion Food vacuole Figure 6.14 A Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (a) Phagocytosis: lysosome digesting food • Autophagy Lysosome containing two damaged organelles 1µm Mitochondrion fragment Peroxisome fragment Lysosome fuses with vesicle containing damaged organelle Hydrolytic enzymes digest organelle components Lysosome Vesicle containing damaged mitochondrion Figure 6.14 B Digestion (b) Autophagy: lysosome breaking down damaged organelle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Relationships among organelles of the endomembrane system 1 Nuclear envelope is connected to rough ER, which is also continuous with smooth ER Nucleus Rough ER 2 Membranes and proteins produced by the ER flow in the form of transport vesicles to the Golgi Smooth ER cis Golgi Nuclear envelop 3 Golgi pinches off transport Vesicles and other vesicles that give rise to lysosomes and Vacuoles Plasma membrane trans Golgi 4 Lysosome available for fusion with another vesicle for digestion Figure 6.16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 Transport vesicle carries 6 proteins to plasma membrane for secretion Plasma membrane expands by fusion of vesicles; proteins are secreted from cell 6. Peroxisomes (type of vessicle) Contains collection of enzymes that inactivate toxic byproducts of oxygen presence ex. Catalase Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Peroxisomes: Oxidation • Peroxisomes – Produce hydrogen peroxide and convert it to water Chloroplast Peroxisome Mitochondrion Figure 6.19 1 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 7. Vacuoles – they are like your garage and often look like balloons! Found in plant or fungal cells Three types: 1. Food Vacuole – form by phagocytosis 2. Contractile Vacuole – pump out excess water (freshwater protists) 3. Central Vacuole – found in PLANTS hold reserves of important organic molecules can contain by-products or pigments can also be protection – hold poisonous compounds Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A animal cell ENDOPLASMIC RETICULUM (ER) Rough ER Smooth ER Nuclear envelope Nucleolus NUCLEUS Chromatin Flagelium Plasma membrane Centrosome CYTOSKELETON Microfilaments Intermediate filaments Ribosomes Microtubules Microvilli Golgi apparatus Peroxisome Figure 6.9 Mitochondrion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysosome In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm) • A plant cell Nuclear envelope Nucleolus Chromatin NUCLEUS Centrosome Rough endoplasmic reticulum Smooth endoplasmic reticulum Ribosomes (small brwon dots) Central vacuole Tonoplast Golgi apparatus Microfilaments Intermediate filaments CYTOSKELETON Microtubules Mitochondrion Peroxisome Plasma membrane Chloroplast Cell wall Plasmodesmata Wall of adjacent cell Figure 6.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In plant cells but not animal cells: Chloroplasts Central vacuole and tonoplast Cell wall Plasmodesmata 8. Mitochondria – Powerhouse of the cell a. Location for aerobic respiration (cellular respiration) Use oxygen to produce ATP Three parts: inner membrane, outer membrane, matrix 1. Krebs cycle occurs in matrix biochem reactions that converts energy into usable forms for the cell 1. Electron Transport system in the inner membrane last biochemical step in production of ATP *oxygen is the FINAL ELECTRON RECEPTOR Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mitochondria are enclosed by two membranes – A smooth outer membrane – An inner membrane folded into cristae Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix Figure 6.17 Mitochondrial DNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 100 µm • Mitochrondria – Have their own DNA (circular and single stranded) – Replicate INDEPENDENTLY of the cell – Endosymbiotic theory – scientists think mitochrondria and chloroplasts were once a cell and then engulfed by eukaryotes about a billion years ago Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 9. Chloroplasts Found only in plants ( also in a few protists) Site for photosynthesis Parts: Stroma – light independent rxns occur Grana – composed of thylakoid membrane light dependent rxns occur here via electron transport chain Contain own DNA, reproduce independently Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Chloroplasts – Are found in leaves and other green organs of plants and in algae Chloroplast Ribosomes Stroma Chloroplast DNA Inner and outer membranes Granum 1 µm Figure 6.18 Thylakoid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The cytoskeleton – Is a network of fibers extending throughout the cytoplasm Microtubule Figure 6.20 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0.25 µm Microfilaments 10. Cyotskeleton – the FRAMEWORK collection of proteins fibers that confer shape & mvmt to the cell Parts: 1. Microtublules fat hollow polymers of the protein TUBULIN *main support of cell & form cilia & flagella *can function as “tracks” along which other subcellular structures can be transported Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Parts: 2. Intermediate filaments Composed of strong proteins like KERRATIN Functions to hold other cytoskeletal components together (like rope) Found in large quantities in nucleoplasm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Parts: 3. Microfilaments Composed of polymers of the protein ACTIN Usually involved in movement Found under plasma membrane Cell shape, muscle movement Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • There are three main types of fibers that make up the cytoskeleton Table 6.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Microfilaments that function in cellular motility – Contain the protein myosin in addition to actin Muscle cell Actin filament Myosin filament Myosin arm Figure 6.27 A (a) Myosin motors in muscle cell contraction. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – Is involved in cell motility, which utilizes motor proteins ATP Vesicle Receptor for motor protein Motor protein Microtubule (ATP powered) of cytoskeleton (a) Motor proteins that attach to receptors on organelles can “walk” the organelles along microtubules or, in some cases, microfilaments. Vesicles Microtubule 0.25 µm Figure 6.21 A, B (b) Vesicles containing neurotransmitters migrate to the tips of nerve cell axons via the mechanism in (a). In this SEM of a squid giant axon, two vesicles can be seen moving along a microtubule. (A separate part of the experiment provided the evidence that they were in fact moving.) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Centrosomes and Centrioles • The centrosome – Is considered to be a “microtubule-organizing center” Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – Contains a pair of centrioles Centrosome Microtubule Centrioles 0.25 µm Figure 6.22 Longitudinal section of one centriole Microtubules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cross section of the other centriole Cilia and Flagella 11. Cilia (short and many) Flagella (few & long) – Contain specialized arrangements of microtubules – Are locomotor appendages of some cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Flagella beating pattern (a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellatelocomotion (LM). Direction of swimming Figure 6.23 A 1 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Ciliary motion (b) Motion of cilia. Cilia have a backand-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a freshwater protozoan (SEM). Figure 6.23 B 15 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Cilia and flagella share a common ultrastructure (9 + 2 arrangement) Outer microtubule doublet Dynein arms 0.1 µm Central microtubule Outer doublets cross-linking proteins inside Microtubules Radial spoke Plasma membrane Basal body (b) 0.5 µm (a) 0.1 µm Triplet (c) Figure 6.24 A-C Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cross section of basal body Plasma membrane • The protein dynein – Is responsible for the bending movement of cilia and flagella Microtubule doublets ATP Dynein arm (a) Powered by ATP, the dynein arms of one microtubule doublet grip the adjacent doublet, push it up, release, and then grip again. If the two microtubule doublets were not attached, they would slide relative to each other. Figure 6.25 A Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ATP Outer doublets cross-linking proteins Anchorage in cell (b) In a cilium or flagellum, two adjacent doublets cannot slide far because they are physically restrained by proteins, so they bend. (Only two of the nine outer doublets in Figure 6.24b are shown here.) Figure 6.25 B Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 3 2 (c) Localized, synchronized activation of many dynein arms probably causes a bend to begin at the base of the Cilium or flagellum and move outward toward the tip. Many successive bends, such as the ones shown here to the left and right, result in a wavelike motion. In this diagram, the two central microtubules and the cross-linking proteins are not shown. Figure 6.25 C Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Cytoplasmic streaming – Is another form of locomotion created by microfilaments Nonmoving cytoplasm (gel) Chloroplast Streaming cytoplasm (sol) Parallel actin filaments Figure 6.27 C (b) Cytoplasmic streaming in plant cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell wall Extracellular Extracellular components and connections between cells help coordinate cellular activities Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell Walls of Plants • The cell wall – Is an extracellular structure of plant cells that distinguishes them from animal cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Plant cell walls – Are made of cellulose fibers embedded in other polysaccharides and protein – May have multiple layers Central vacuole of cell Plasma membrane Secondary cell wall Primary cell wall Central vacuole of cell Middle lamella 1 µm Central vacuole Cytosol Plasma membrane Plant cell walls Figure 6.28 Plasmodesmata Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Extracellular Matrix (ECM) of Animal Cells • Animal cells – Lack cell walls – Are covered by an elaborate matrix, the ECM Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The ECM – Is made up of glycoproteins and other macromolecules EXTRACELLULAR FLUID Collagen A proteoglycan complex Polysaccharide molecule Carbohydrates Core protein Fibronectin Plasma membrane Integrin Integrins Microfilaments Figure 6.29 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CYTOPLASM Proteoglycan molecule • Functions of the ECM include – Support – Adhesion – Movement – Regulation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants: Plasmodesmata • Plasmodesmata – Are channels that perforate plant cell walls Cell walls Interior of cell Interior of cell Figure 6.30 0.5 µm Plasmodesmata Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plasma membranes Animals: Tight Junctions, Desmosomes, and Gap Junctions • In animals, there are three types of intercellular junctions – Tight junctions – Desmosomes – Gap junctions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Types of intercellular junctions in animals TIGHT JUNCTIONS Tight junction Tight junctions prevent fluid from moving across a layer of cells 0.5 µm At tight junctions, the membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continuous seals around the cells, tight junctions prevent leakage of extracellular fluid across A layer of epithelial cells. DESMOSOMES Desmosomes (also called anchoring junctions) function like rivets, fastening cells Together into strong sheets. Intermediate Filaments made of sturdy keratin proteins Anchor desmosomes in the cytoplasm. Tight junctions Intermediate filaments Desmosome Gap junctions Space between Plasma membranes cells of adjacent cells 1 µm Extracellular matrix Gap junction Figure 6.31 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0.1 µm GAP JUNCTIONS Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for communication between cells in many types of tissues, including heart muscle and animal embryos. The Cell: A Living Unit Greater Than the Sum of Its Parts 5 µm • Cells rely on the integration of structures and organelles in order to function Figure 6.32 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The plasma membrane – Functions as a selective barrier – Allows sufficient passage of nutrients and waste Outside of cell Carbohydrate side chain Hydrophilic region Inside of cell 0.1 µm Hydrophobic region Figure 6.8 A, B (a) TEM of a plasma membrane. The plasma membrane, here in a red blood cell, appears as a pair of dark bands separated by a light band. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hydrophilic region Phospholipid Proteins (b) Structure of the plasma membrane