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Chapter 6: The Importance of Cells • All organisms are made of cells • The cell is the simplest collection of matter that can support life • Cell structure is correlated to cellular function • All cells are related by their descent from earlier cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cells are the key to life! • Outline: • A) How can we look at cells? Microscopes! • B) How can we study the components of cells? • C) Prokaryotic and Eukaryotic cell structure. • D) External cellular structures Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings To study cells, biologists use microscopes and the tools of biochemistry • Though usually too small to be seen by the unaided eye, cells can be extremely complex • Scientists use microscopes to visualize cells too small to see with the naked eye • In a light microscope (LM), visible light passes through a specimen and then through glass lenses, which magnify the image • The minimum resolution of an LM is about 200 nanometers (nm), the size of a small bacterium Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 10 m Microscopy Human height Length of some nerve and muscle cells 0.1 m Chicken egg •Most subcellular structures, or organelles, are too small to be resolved by a LM Frog egg 1 mm 1 centimeter (cm) = 10–2 meter (m) = 0.4 inch 100 µm 1 millimeter (mm) = 10–3 m 1 micrometer (µm) = 10–3 mm = 10–6 m 1 nanometer (nm) = 10–3 µm = 10–9 m 10 µm Most plant and animal cells Nucleus Most bacteria 1 µm 100 nm Mitochondrion Smallest bacteria Viruses Ribosomes 10 nm Proteins Lipids 1 nm Small molecules 0.1 nm Atoms Electron microscope •Various techniques enhance contrast and enable cell components to be stained or labeled 1 cm Light microscope •LMs can magnify effectively to about 1,000 times the size of the actual specimen Measurements Unaided eye 1m A compound, optical microscope is most commonly used in teaching and research laboratories. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Brightfield (unstained specimen) Techniques to enhance contrast: Staining 50 µm Brightfield (stained specimen) Phase-contrast Differentialinterferencecontrast (Nomarski) Techniques to enhance contrast: Labeling some cellular components for visualization Fluorescence 50 µm Confocal https://www.youtube.com/watch ?v=qvGVoxdy-yM 50 µm Electron microscopes are used to study the VERY SMALL 1 µm organelles of a cell, or other small particles Cilia • Two basic types of electron microscopes (EMs) are used to study subcellular structures • Scanning electron microscopes (SEMs) focus a beam of electrons onto the surface of a specimen, providing images that look 3D • Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen • TEMs are used mainly to study the internal ultrastructure of cells Scanning electron microscopy (SEM) Transmission electron microscopy (TEM) Longitudinal section of cilium Cross section of cilium 1 µm Comparison of optical and electron microscopes. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Questions: • What types of microscopes would you use if you would like to visualize the following structures: – A. A human (eukaryotic) cell? – B. A bacterial (prokaryotic) cell? – C. A virus? – D. A plant (eukaryotic) cell? – E. An organelle of a cell? – F. A protein? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Studying cells by biochemical techniques: Isolating Organelles by Cell Fractionation • Cell fractionation takes cells apart and separates the major organelles from one another • Ultracentrifuges fractionate cells into their component parts • Cell fractionation enables scientists to determine the functions of organelles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Isolating Organelles by Cell Fractionation Homogenization Tissue cells Differential centrifugation Homogenate Ultracentrifuges are used To fractionate the organelles From a cellular extract 1000 g (1000 times the force of gravity) 10 min Supernatant poured into next tube bucket with tube 20,000 g 20 min 80,000 g 60 min Pellet rich in nuclei and cellular debris 150,000 g 3 hr rotor with buckets centrifuge Pellet rich in mitochondria (and chloroplasts if cells are from a plant) Pellet rich in “microsomes” (pieces of plasma membranes and cells’ internal membranes) Pellet rich in ribosomes 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparing Prokaryotic and Eukaryotic Cells • Basic features of all cells (both prokaryotic and eukaryotic cells): – Plasma membrane – Semifluid substance called the cytosol – Chromosomes (carry genes) – Ribosomes (make proteins) • Prokaryotic cells have no nucleus • In a prokaryotic cell, DNA is in an unbound region called the nucleoid • Prokaryotic cells lack membrane-bound organelles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An example of a prokaryotic cell Pili Nucleoid Ribosomes Plasma membrane Bacterial chromosome Cell wall Capsule 0.5 µm Flagella A typical rod-shaped bacterium A thin section through the bacterium Bacillus coagulans (TEM) Eukaryotic cells • Eukaryotic cells have DNA in a nucleus that is bounded by a membranous nuclear envelope • Eukaryotic cells have membrane-bound organelles • Eukaryotic cells are generally much larger than prokaryotic cells • The logistics of carrying out cellular metabolism sets limits on the size of cells • The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of the cell • The general structure of a biological membrane is a double layer of phospholipids Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plasma Membranes surround the cell Outside of cell Carbohydrate side chain Hydrophilic region Inside of cell 0.1 µm Hydrophobic region Hydrophilic region TEM of a plasma membrane Phospholipid Proteins Structure of the plasma membrane A Panoramic View of the Eukaryotic Cell • A eukaryotic cell has internal membranes that partition the cell into organelles • Plant and animal cells have most of the same organelles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An animal cell In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm) A plant cell In plant cells but not animal cells: Chloroplasts Central vacuole and tonoplast Cell wall Plasmodesmata Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The eukaryotic cellular nucleus houses the DNA genome of the cell Nucleus Nucleus 1 µm Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex Rough ER Surface of nuclear envelope Ribosome 1 µm 0.25 µm Close-up of nuclear envelope Pore complexes (TEM) Nuclear lamina (TEM) Ribosomes: Protein Factories in the Cell • Ribosomes are particles 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 (ER) or the nuclear envelope (bound ribosomes) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ribosomes: Protein Factories in the Cell Ribosomes ER Cytosol Endoplasmic reticulum (ER) Free ribosomes Bound ribosomes Large subunit Small subunit 0.5 µm TEM showing ER and ribosomes Diagram of a ribosome The endomembrane system regulates protein traffic and performs metabolic functions in the cell • • Components of the endomembrane system – which are regions of the cell that are composed of a membrane that transport things between different compartments of a cell: – Nuclear envelope – surrounds the nucleus – Endoplasmic reticulum – Golgi apparatus – Lysosomes – Vacuoles – Vesicles – Plasma membrane – encases the entire cell and contains the cytoplasm holding all of the organelles These components are either continuous or connected via transfer by vesicles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endomembrane System Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 • Synthesizes lipids • Metabolizes carbohydrates • Stores calcium • Detoxifies poison Rough ER, with ribosomes binding to and studding its surface • Produces proteins and membranes, which are distributed by transport vesicles • Is a membrane factory for the cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Smooth ER The Endoplasmic Reticulum: biosynthetic factory Rough ER Nuclear envelope ER lumen Cisternae Ribosomes Transport vesicle Smooth ER Transitional ER Rough ER 200 nm 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 – Many post-translational modifications to proteins are performed in the cisternae of the Golgi – Manufactures certain macromolecules – Sorts and packages materials into transport vesicles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Golgi Apparatus: Shipping and Receiving Center Golgi apparatus cis face (“receiving” side of Golgi apparatus) Vesicles also transport certain proteins back to ER Vesicles move from ER to Golgi Vesicles coalesce to form new cis Golgi cisternae 0.1 µm Cisternae Cisternal maturation: Golgi cisternae move in a cisto-trans direction Vesicles form and leave Golgi, carrying specific proteins to other locations or to the plasma membrane for secretion Vesicles transport specific proteins backward to newer Golgi cisternae trans face (“shipping” side of Golgi apparatus) TEM of Golgi apparatus Lysosomes: Digestive Compartments • A lysosome is a membranous sac of hydrolytic enzymes • Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids • Lysosomes can digest material taken into the cell from outside by a process know as phagocytosis • Lysosomes also use enzymes to recycle organelles and macromolecules from within the cell, a process called autophagy • Plant cells do not have lysosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 µm Nucleus Lysosomes: digestive compartments within the cell Phagocytosis Lysosome Lysosome contains Food vacuole Hydrolytic active hydrolytic enzymes digest fuses with enzymes food particles lysosome Digestive enzymes Plasma membrane Lysosome Digestion Food vacuole Phagocytosis: lysosome digesting food Lysosome containing two damaged organelles Lysosomes: digestive compartments within the cell 1 µm Mitochondrion fragment Peroxisome fragment Autophagy Lysosome fuses with vesicle containing damaged organelle Hydrolytic enzymes digest organelle components Lysosome Digestion Vesicle containing damaged mitochondrion Autophagy: lysosome breaking down damaged organelle Vacuoles: Diverse Maintenance Compartments • Vacuoles are membrane-bound sacs with varied functions • A plant cell or fungal cell may have one or several vacuoles • 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 • Animal cells do not have central vacuoles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Vacuoles: Diverse Maintenance Compartments Central vacuole Cytosol Tonoplast Nucleus Central vacuole Cell wall Chloroplast 5 µm Vesicles perform many different intracellular functions • Vesicles, like vacuoles, are membrane-bound sacs with varied functions • They are much smaller than vacuoles • The perform many functions within the cell, including moving material between the ER and Golgi and moving things between the cisternae of the Golgi • They also function in delivering things into the cell from the extracellular environment in a process known as endocytosis • They also deliver things to be excreted such as waste and hormones to the exterior of the cell in a process known as exocytosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A diagram of how things are transported out of a cell by vesicle movement Nucleus Rough ER Smooth ER Nuclear envelope cis Golgi Transport vesicle Plasma membrane trans Golgi Work in groups and try to put these cellular processes in order for a protein destined for secretion • A piece of mRNA is synthesized by transcription from a DNA gene • A vesicle containing a protein fuses with the plasma membrane to release the protein out of the cell • The ribosome synthesizes a protein by translation • The protein enters into the endoplasmic reticulum (ER) to be modified • The protein is folded to form secondary protein structures • The protein first enters into a vesicle • The protein is folded to form tertiary and quaternary protein structures • The protein enters into the cis Golgi apparatus for modifications • The protein leaves the Golgi apparatus in a vesicle • The protein is transported in transport vesicles between the cisternae of the Golgi apparatus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mitochondria and chloroplasts change energy from one form to another • Mitochondria are the sites of cellular respiration • Chloroplasts, found only in plants and algae, are the sites of photosynthesis • Mitochondria and chloroplasts are not part of the endomembrane system • Peroxisomes are oxidative organelles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 as the final steps of cellular respiration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mitochondria: Chemical Energy Conversion Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix Mitochondrial DNA 100 nm Chloroplasts: Capture of Light Energy • The chloroplast is a member of a family of organelles called plastids • 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 ANIMALS DO NOT HAVE CHLOROPLASTS • Chloroplast structure includes: – Thylakoids, membranous sacs – Stroma, the internal fluid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplasts: Capture of Light Energy Chloroplast Ribosomes Stroma Chloroplast DNA Inner and outer membranes Granum 1 µm Thylakoid Peroxisomes: Oxidation • Peroxisomes are specialized metabolic compartments bounded by a single membrane • Peroxisomes produce hydrogen peroxide and convert it to water Chloroplast Peroxisome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mitochondrion 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The cytoskeleton is a network of fibers that organizes structures and activities in the cell Microtubule Microfilaments 0.25 µm Roles of the Cytoskeleton: Support, Motility, and Regulation • 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 “monorails” provided by the cytoskeleton • Recent evidence suggests that the cytoskeleton may help regulate biochemical activities Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The role of the cytoskeleton in intracellular vesicle movement Vesicle ATP Receptor for motor protein Motor protein (ATP powered) Microtubule of cytoskeleton An EM image of vesicles being transported Along a microtubule (the cytoskeleton) Microtubule Vesicles 0.25 µm Components of 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Components of the Cytoskeleton: Microtubules Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Microtubules: Centrosomes and Centrioles •In many cells, microtubules grow out from a centrosome near the nucleus Centrosome Microtubule Centrioles 0.25 µm •The centrosome is a “microtubule-organizing center” •In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring Longitudinal section Microtubules of one centriole Cross section of the other centriole Microtubules: Cilia and Flagella • Microtubules control the beating of cilia and flagella, which are the locomotor appendages of some cells • Cilia and flagella share a common ultrastructure: – 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Structure of Flagella and cilia Outer microtubule doublet Dynein arms Central microtubule 0.1 µm Cross-linking proteins inside outer doublets Microtubules Plasma membrane Basal body 0.5 µm Radial spoke 0.1 µm Triplet Cross section of basal body Plasma membrane Microtubule doublets Flagella and Cilia movements via dynein Dynein arms alternately grab, move, and release the outer microtubules such that dynein appears to be “walking” and pulling one microtubule past another. Dynein “walking” ATP Dynein arm Flagella and Cilia movements via dynein •Protein cross-links limit sliding and holds doublets of microtubules together Cross-linking proteins inside outer doublets Anchorage in cell Effect of cross-linking proteins •Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum Wavelike motion ATP Cilia and Flagella differ in their beating pattern FLAGELLA Direction of swimming Motion of flagella 5 µm Cilia and Flagella differ in their beating pattern Cilia Direction of organism’s movement Direction of active stroke Motion of cilia Direction of recovery stroke 15 µm Components of the Cytoskeleton: Microfilaments Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 3D network just inside the plasma membrane to help support the cell’s shape • Bundles of microfilaments make up the core of microvilli of intestinal cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Microvilli in intestinal cells are made of microfilaments Microvillus Plasma membrane Microfilaments (actin filaments) Intermediate filaments 0.25 µm Microfilaments (Actin Filaments) in muscle cells •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 Muscle cell Actin filament Myosin filament Myosin arm Myosin motors in muscle cell contraction Microfilaments (Actin Filaments) in amoeba movement •Localized contraction brought about by actin and myosin also drives amoeboid movement •Pseudopodia (cellular extensions) extend and contract through the reversible assembly and contraction of actin subunits into microfilaments Cortex (outer cytoplasm): gel with actin network Inner cytoplasm: sol with actin subunits Extending pseudopodium Amoeboid movement Microfilaments (Actin Filaments) in cytoplasmic movement •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 Nonmoving cytoplasm (gel) Chloroplast Streaming cytoplasm (sol) Vacuole Parallel actin filaments Cytoplasmic streaming in plant cells Cell wall Components of the Cytoskeleton: Intermediate Filaments Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An animal cell ENDOPLASMIC RETICULUM (ER Nuclear envelope Flagellum Rough ER Smooth ER NUCLEUS Nucleolus Chromatin In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm) Centrosome Plasma membrane CYTOSKELETON Microfilaments Intermediate filaments Microtubules Ribosomes: Microvilli Golgi apparatus Peroxisome Mitochondrion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysosome A plant cell Nuclear envelope NUCLEUS Nucleolus Chromatin Centrosome Rough endoplasmic reticulum Smooth endoplasmic reticulum Ribosomes (small brown dots) In plant cells but not animal cells: Chloroplasts Central vacuole and tonoplast Cell wall Plasmodesmata Central vacuole Golgi apparatus Microfilaments Intermediate filaments Microtubules Mitochondrion Peroxisome Chloroplast Plasma membrane Cell wall Plasmodesmata Wall of adjacent cell CYTOSKELETON Extracellular components and connections between cells help coordinate cellular activities • Most cells synthesize and secrete materials that are to be released to the external sides of the plasma membrane • These extracellular structures include: – Cell walls of plants – The extracellular matrix (ECM) of animal cells – Intercellular junctions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell Walls of Plants • The cell wall is an extracellular structure that distinguishes plant cells from animal cells • 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 • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Central vacuole of cell Cell Walls of Plants Plasma membrane Secondary cell wall Primary cell wall Central vacuole of cell Middle lamella 1 µm Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata 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 and other macromolecules • Functions of the ECM: – – – – Support Adhesion Movement Regulation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Extracellular Matrix (ECM) of Animal Cells Collagen fiber EXTRACELLULAR FLUID Fibronectin Plasma membrane Integrin CYTOPLASM Microfilaments Proteoglycan complex Proteoglycan complex The Extracellular Matrix (ECM) of Animal Cells Polysaccharide molecule Carbohydrates Core protein Proteoglycan molecule Intercellular Junctions • Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact • Intercellular junctions facilitate this contact Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intercellular junctions in Plants: Plasmodesmata • 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 Cell walls Interior of cell Interior of cell 0.5 µm Plasmodesmata Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plasma membranes Intercellular junctions in Animals: Tight Junctions, Desmosomes, and Gap Junctions • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intercellular junctions Tight junctions prevent in Animals: fluid from moving across a layer of cells Tight Junctions, Desmosomes, and Gap Junctions Tight junction 0.5 µm Tight junction Intermediate filaments Desmosome 1 µm Space between cells Gap junctions Plasma membranes of adjacent cells Gap junction Extracellular matrix 0.1 µm The Cell: A Living Unit Greater Than the Sum of Its Parts • Cells rely on the integration of all of the different 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings