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Chapter 6 A Tour of the Cell PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: The Fundamental Units of Life • All organisms are made of cells • Cell structure is correlated to cellular function • All cells are related by their descent from earlier cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-1 Cytoskeleton: microtubule, intermediate filament, Concept 6.1: To study cells, biologists use microscopes and the tools of biochemistry Microscopy • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The quality of an image depends on – Magnification, the ratio of an object’s image size to its real size – Resolution, the measure of the clarity of the image, or the minimum distance of two distinguishable points – Contrast, visible differences in parts of the sample Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 10 m 1m Human height Length of some nerve and muscle cells 0.1 m Chicken egg 1 cm Unaided eye Frog egg 100 µm Most plant and animal cells 10 µm Nucleus Most bacteria 1 µm 100 nm 10 nm Mitochondrion Smallest bacteria Viruses Ribosomes Proteins Lipids 1 nm Small molecules 0.1 nm Atoms Electron microscope 1 mm Light microscope Fig. 6-2 • LMs can magnify effectively to about 1,000 times the size of the actual specimen • Various techniques enhance contrast and enable cell components to be stained or labeled • Most subcellular structures, including organelles (membrane-enclosed compartments), are too small to be resolved by an LM Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings – Use different methods for enhancing visualization RESULT TECHNIQUEof cellular structures (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 (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 • 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 3-D • Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen • TEMs are used mainly to study the internal structure of cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • 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) • 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) Cross section of cilium 1 µm 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 • Biochemistry and cytology help correlate cell function with structure Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-5 TECHNIQUE Homogenization Tissue cells Homogenate 1,000 g (1,000 times the force of gravity) Differential centrifugation 10 min Supernatant poured into next tube 20,000 g 20 min Pellet rich in nuclei and cellular debris 80,000 g 60 min 150,000 g 3 hr 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 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Comparing Prokaryotic and Eukaryotic Cells • Basic features of all cells: – Plasma membrane – Semifluid substance called cytosol – Chromosomes (carry genes) – Ribosomes (make proteins) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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 Figure 6.6 A, B 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 (b) A thin section through the bacterium Bacillus coagulans (TEM) • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell • The general structure of a biological membrane is a double layer of phospholipids Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-7 Outside of cell Inside of cell 0.1 µm (a) TEM of a plasma membrane Carbohydrate side chain Hydrophilic region Hydrophobic region Hydrophilic region Phospholipid Proteins (b) Structure of the plasma membrane • The logistics of carrying out cellular metabolism sets limits on the size of cells • The surface area to volume ratio of a cell is critical • As the surface area increases by a factor of n2, the volume increases by a factor of n3 • Small cells have a greater surface area relative to volume Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-8 Surface area increases while total volume remains constant 5 1 1 Total surface area [Sum of the surface areas (height width) of all boxes sides number of boxes] Total volume [height width length number of boxes] Surface-to-volume (S-to-V) ratio [surface area ÷ volume] 6 150 750 1 125 125 6 1.2 6 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 BioFlix: Tour Of An Animal Cell BioFlix: Tour Of A Plant Cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-9a Nuclear envelope ENDOPLASMIC RETICULUM (ER) Flagellum Rough ER NUCLEUS Nucleolus Smooth ER Chromatin Centrosome Plasma membrane CYTOSKELETON: Microfilaments Intermediate filaments Microtubules Ribosomes Microvilli Golgi apparatus Peroxisome Mitochondrion Lysosome Fig. 6-9b NUCLEUS Nuclear envelope Nucleolus Chromatin Rough endoplasmic reticulum Smooth endoplasmic reticulum Ribosomes Central vacuole Golgi apparatus Microfilaments Intermediate filaments Microtubules Mitochondrion Peroxisome Chloroplast Plasma membrane Cell wall Plasmodesmata Wall of adjacent cell CYTOSKELETON 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Nucleus: Information Central • The nucleus contains most of the cell’s genes and is usually the most conspicuous organelle (additional DNA where?) • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-10 Nucleus 1 µm Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex Surface of nuclear envelope Rough ER Ribosome 1 µm 0.25 µm Close-up of nuclear envelope Pore complexes (TEM) Nuclear lamina (TEM) • Pores regulate the entry and exit of molecules from the nucleus • The shape of the nucleus is maintained by the nuclear lamina, which is composed of protein Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • In the nucleus, DNA and proteins form genetic material called chromatin • Chromatin condenses to form discrete chromosomes • The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Ribosomes: Protein Factories • 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 or the nuclear envelope (bound ribosomes) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-11 Cytosol Endoplasmic reticulum (ER) Free ribosomes Bound ribosomes Large subunit 0.5 µm TEM showing ER and ribosomes Small subunit Diagram of a ribosome Crystal Structure of the large ribosomal subunit •Harry Noller at the University of California Santa Cruz •Venki Ramakrishnan at the University of Cambridge, •Thomas Steitz at Yale University Concept 6.4: The endomembrane system regulates protein traffic and performs metabolic functions in the cell • Components of the endomembrane system: – Nuclear envelope – Endoplasmic reticulum – Golgi apparatus – Lysosomes – Vacuoles – Plasma membrane • These components are either continuous or connected via transfer by vesicles Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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 – Rough ER, with ribosomes studding its surface Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-12 Smooth ER Rough ER ER lumen Cisternae Ribosomes Transport vesicle Smooth ER Nuclear envelope Transitional ER Rough ER 200 nm Functions of Smooth ER • The smooth ER – Synthesizes lipids – Metabolizes carbohydrates – Detoxifies poison – Stores calcium Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Functions of Rough ER • The rough ER – Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates) – Distributes transport vesicles, proteins surrounded by membranes – Is a membrane factory for the cell; adding membrane proteins and phospholipids to its own membrane and other endomembrane system by transport vesicles Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-13 cis face (“receiving” side of Golgi apparatus) 0.1 µm Cisternae trans face (“shipping” side of Golgi apparatus) TEM of Golgi apparatus Lysosomes: Digestive Compartments • A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules • Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids Animation: Lysosome Formation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-14 Nucleus 1 µm Vesicle containing two damaged organelles 1 µm Mitochondrion fragment Peroxisome fragment Lysosome Lysosome Digestive enzymes Plasma membrane Lysosome Peroxisome Digestion Food vacuole Vesicle (a) Phagocytosis (b) Autophagy Mitochondrion Digestion Vacuoles: Diverse Maintenance Compartments • 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 Video: Paramecium Vacuole Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-15 Central vacuole Cytosol Nucleus Central vacuole Cell wall Chloroplast 5 µm The Endomembrane System: A Review • The endomembrane system is a complex and dynamic player in the cell’s compartmental organization Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-16-1 Nucleus Rough ER Smooth ER Plasma membrane Fig. 6-16-2 Nucleus Rough ER Smooth ER cis Golgi trans Golgi Plasma membrane Fig. 6-16-3 Nucleus Rough ER Smooth ER cis Golgi trans Golgi Plasma membrane Concept 6.5: Mitochondria and chloroplasts change energy from one form to another • Mitochondria are the sites of cellular respiration, a metabolic process that generates ATP • Chloroplasts, found in plants and algae, are the sites of photosynthesis • Peroxisomes are oxidative organelles Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Mitochondria and chloroplasts – Are not part of the endomembrane system – Have a double membrane – Have proteins made by free ribosomes – Contain their own DNA Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-17 Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix 0.1 µm 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 • Chloroplast structure includes: – Thylakoids, membranous sacs, stacked to form a granum – Stroma, the internal fluid contain chloroplast DNA & ribosome Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-18 Ribosomes Stroma Inner and outer membranes Granum Thylakoid 1 µm Peroxisomes: Oxidation • Peroxisomes – Transfer hydrogen from substrates to oxygen producing hydrogen peroxide (H2O2) as a by-product; detoxification – break fatty acids down into smaller molecules – Glyoxysome in plant seed converts faty acids to sugar – Grow larger by incorporating proteins and lipids and increase in number by splitting Chloroplast Peroxisome Mitochondrion Figure 6.19 1 µm 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-20 Microtubule 0.25 µm Microfilaments 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 © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-21 ATP Vesicle Receptor for motor protein Motor protein Microtubule (ATP powered) of cytoskeleton (a) Microtubule (b) Vesicles 0.25 µm 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Table 6-1a 10 µm Column of tubulin dimers 25 nm Tubulin dimer Table 6-1b 10 µm Actin subunit 7 nm Table 6-1c 5 µm Keratin proteins More permanent than microtubule and microfilament Fibrous subunit (keratins coiled together) 8–12 nm 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Centrosomes and Centrioles • In many cells, microtubules grow out from a centrosome near the nucleus • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-22 Centrosome Microtubule Centrioles 0.25 µm Longitudinal section Microtubules Cross section of one centriole of the other centriole Cilia and Flagella • Microtubules control the beating of cilia and flagella, locomotor appendages of some cells • Cilia and flagella differ in their beating patterns Video: Chlamydomonas Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Video: Paramecium Cilia Fig. 6-23 Direction of swimming (a) Motion of flagella 5 µm Direction of organism’s movement Power stroke Recovery stroke (b) Motion of cilia 15 µm • 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 Animation: Cilia and Flagella Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-24 Outer microtubule doublet 0.1 µm Dynein proteins Central microtubule Radial spoke Protein crosslinking outer doublets Microtubules Plasma membrane (b) Cross section of cilium Basal body 0.5 µm (a) Longitudinal section of cilium 0.1 µm Triplet (c) Cross section of basal body Plasma membrane • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-25 Microtubule doublets ATP Dynein protein (a) Effect of unrestrained dynein movement ATP Cross-linking proteins inside outer doublets Anchorage in cell (b) Effect of cross-linking proteins 1 3 2 (c) Wavelike motion 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-26 Microvillus Plasma membrane Microfilaments (actin filaments) Intermediate filaments 0.25 µm • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-27 Muscle cell Actin filament Myosin filament Myosin arm (a) Myosin motors in muscle cell contraction Cortex (outer cytoplasm): gel with actin network Inner cytoplasm: sol with actin subunits Extending pseudopodium (b) Amoeboid movement Nonmoving cortical cytoplasm (gel) Chloroplast Streaming cytoplasm (sol) Vacuole Parallel actin filaments (c) Cytoplasmic streaming in plant cells Cell wall Fig, 6-27a Muscle cell Actin filament Myosin filament Myosin arm (a) Myosin motors in muscle cell contraction Fig. 6-27bc Cortex (outer cytoplasm): gel with actin network Inner cytoplasm: sol with actin subunits Extending pseudopodium (b) Amoeboid movement Nonmoving cortical cytoplasm (gel) Chloroplast Streaming cytoplasm (sol) Vacuole Parallel actin filaments (c) Cytoplasmic streaming in plant cells Cell wall • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • 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 solgel transformations drive cytoplasmic streaming Video: Cytoplasmic Streaming Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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 © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 include: – Cell walls of plants – The extracellular matrix (ECM) of animal cells – Intercellular junctions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Cell Walls of Plants • The cell wall is an extracellular structure that distinguishes plant cells from animal cells • Prokaryotes, fungi, and some protists 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Plant cell walls may have multiple layers: – Primary cell wall: relatively thin and flexible – Middle lamella: thin layer between primary walls of adjacent cells (pectin) – Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall • Plasmodesmata are channels between adjacent plant cells Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-28 Secondary cell wall Primary cell wall 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 such as collagen, proteoglycans, and fibronectin • ECM proteins bind to receptor proteins in the plasma membrane called integrins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-30 Collagen is about half of the total protein in human body Collagen Proteoglycan complex EXTRACELLULAR FLUID Polysaccharide molecule Carbohydrates Fibronectin Core protein Integrins Proteoglycan molecule Plasma membrane Proteoglycan complex Microfilaments CYTOPLASM Integrins are the linker between microfilaments in cytoskeleton and ECM in extracellular fluid And transmit signals between the ECM and the cytoskeleton and result in changes in cell behavior • Functions of the ECM: – Support – Adhesion – Movement – Regulation of genes : information of ECM reaches the nucleus by a combination of mechanical and chemical signaling pathways. Changes in the cytoskeleton may in turn trigger chemical signaling pathways inside the cell, leading to change in proteins and functions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Intercellular Junctions • Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact • Intercellular junctions facilitate this contact • There are several types of intercellular junctions – Plasmodesmata – Tight junctions – Desmosomes – Gap junctions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-31 Cell walls Interior of cell Interior of cell 0.5 µm Plasmodesmata Plasma membranes Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells • At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid. Continuous sealing in an intestine • Desmosomes (anchoring junctions) fasten cells together into strong sheets; anchor intermediate filament in cytoplasm and attach muscle cells to each other in a muscle. “muscle tears” involve the rupture of desmosomes. • Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells, just like plasmodesmata in plants. Necessary for communication between cells in heart, muscle and in animal embryos. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 6-32 Tight junction Tight junctions prevent fluid from moving across a layer of cells 0.5 µm Tight junction Intermediate filaments Desmosome Gap junctions Space between cells Plasma membranes of adjacent cells Desmosome 1 µm Extracellular matrix Gap junction 0.1 µm Fig. 6-32a Tight junctions prevent fluid from moving across a layer of cells Tight junction Animation: Tight Junctions Intermediate filaments Desmosome Animation: Desmosomes Animation: Gap Junctions Gap junctions Space between cells Plasma membranes of adjacent cells Extracellular matrix 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings TAXOL • Paclitaxel은 Taxus brevifolia Nutt. (Taxaceae)(미국 주목나무-Western Yew)의 주피(bark)에서 추출한 diterpenoid taxane 유도체로 천연 taxane ring을 가진 alkaloid이다. (1) 이 taxane ring과 ester side chain이 cytotoxicity 작용을 가지는 것으로 알려졌다.(1) Paclitaxel은 수용성이 극히 낮아 제제화하기 위해 polyoxyethylated castor oil과 absolute ethanol의 혼합물인 Cremorphor® EL이라는 용제를 사용하고 있다. (3) 따라서 임상적으로 이러한 용제가 과량 투여되므로 심장독성과 과민반응이 발생하는 것으로 보고되고 있다. Taxol 와 -, - tubulin complex 구조 1.2 Mechanism of Action Paclitaxel은 1992년12월에 난치성 난소암의 치료제로 FDA의 승인을 받은 항암제로 세포 내 microtubule의 assembly를 증진시키고 disassembly를 저해함으로써 항암효과를 나타내는 독특한 약물이다.(2) Microtubule이란 세포질(cytoplasm)에 있는 가는 tube 같은 구조로써 세포의 골격 형성과 motility를 유지하며 유사분열 중기에서 방추사의 형성과 분열시 이동에 관여한다.(2) Microtubule은 tubulin dimer라는 고유의 subunit로 이루어졌고 이 tubulin이 assembly되어 microtubule을 형성한다. .(2) 형성된 microtubule은 유사분열시 기능수행을 위해서는 disassembly 되어야 한다. .(2) Paclitaxel은 암세포의 microtubule의 assembly를 증진 시키고 일단 형성된 microtubule을 안정화시켜 polymerization 상태로 남아있게 한다.(2) Fuchs과 Johnson의 초기 발견에 의하면, microtubule의 disassembly를 저해하여 유사분열에 필요한 방추사의 형성을 억제하므로 세포 주기상 암 세포가 G2기와 M기에 머무르게 되어 cytotoxic effect를 가진다. 이는 vincristine, vinblastine, colchicines, podophyllotoxin, maytansine과 같은 항암제와 같은 작용이나,(1) paclitaxel은 다른 항암제와 달리 microtubule을 stabilize시키고, depolymerization을 억제시킨다.(2) 또한 이 G2기와 M기 는 방사선에 매우 민감한 주기이므로 radiation therapy와 병용할 시 cytotoxicity가 증가하게 된다. (6) Fig. 6-UN1a Structure Cell Component Concept 6.3 The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes Nucleus Function Surrounded by nuclear envelope (double membrane) perforated by nuclear pores. The nuclear envelope is continuous with the endoplasmic reticulum (ER). Houses chromosomes, made of chromatin (DNA, the genetic material, and proteins); contains nucleoli, where ribosomal subunits are made. Pores regulate entry and exit os materials. Two subunits made of ribosomal RNA and proteins; can be free in cytosol or bound to ER Protein synthesis (ER) Ribosome Fig. 6-UN1b Cell Component Concept 6.4 Endoplasmic reticulum The endomembrane system (Nuclear regulates protein traffic and envelope) performs metabolic functions in the cell Golgi apparatus Lysosome Vacuole Structure Function Extensive network of membrane-bound tubules and sacs; membrane separates lumen from cytosol; continuous with the nuclear envelope. Smooth ER: synthesis of lipids, metabolism of carbohydrates, Ca2+ storage, detoxification of drugs and poisons Stacks of flattened membranous sacs; has polarity (cis and trans faces) Rough ER: Aids in sythesis of secretory and other proteins from bound ribosomes; adds carbohydrates to glycoproteins; produces new membrane Modification of proteins, carbohydrates on proteins, and phospholipids; synthesis of many polysaccharides; sorting of Golgi products, which are then released in vesicles. Breakdown of ingested subMembranous sac of hydrolytic stances cell macromolecules, enzymes (in animal cells) and damaged organelles for recycling Large membrane-bounded vesicle in plants Digestion, storage, waste disposal, water balance, cell growth, and protection Fig. 6-UN1c Cell Component Concept 6.5 Mitochondrion Mitochondria and chloroplasts change energy from one form to another Structure Bounded by double membrane; inner membrane has infoldings (cristae) Function Cellular respiration Chloroplast Typically two membranes around fluid stroma, which contains membranous thylakoids stacked into grana (in plants) Photosynthesis Peroxisome Specialized metabolic compartment bounded by a single membrane Contains enzymes that transfer hydrogen to water, producing hydrogen peroxide (H2O2) as a by-product, which is converted to water by other enzymes in the peroxisome