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Chapter 6 A tour of the cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Why study cells? • Cell Theory – All organisms are made of cells – The cell is the basic unit of organization for all organisms • Cell = simplest collection of matter that can live – All cells come from pre-existing cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Why study cells? • Biological diversity & unity – Underlying the diversity of life is a striking unity: • DNA is the universal genetic language • Cells are the basic units of structure & function You are here Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Why study cells? • Activities of life – Almost everything you think of a whole organism needing to do, must be done at the cellular level: • Reproduction • Growth & development • Energy utilization • Response to environment • Homeostasis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Why study cells? • Cell structure is correlated to cellular function 10 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How do we study cells? • Microscopes opened up the world of cells – Robert Hooke (1665) • the 1st cytologist Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How do we study cells? • Microscopes – Light microscope (LM) – Electron microscope (EM) • Transmission electron microscope (TEM) • Scanning electron microscope (SEM) 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Light microscopes • visible light passes through specimen – use lenses to magnify & focus cellular structures – 0.2 m resolution (~size of a bacterium) – can be used to study live cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Light microscopes – 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Light microscopes (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. with confocal enhancement without confocal enhancement 50 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electron microscope (EM) – 1950s – 2.0 nm resolution – 100 times > light microscope – reveals organelles – for the most part, can only use on dead cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electron microscope (EM) • Uses a beam of electrons – focused using electromagnets; generates computer image – beam passes: • through a specimen (TEM) • onto its surface (SEM) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings transmission electron microscope (TEM) • allows detailed study of the internal ultrastructure of cells • electron beam passes through thin section of specimen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Longitudinal section of cilium Cross section of cilium 1 µm scanning electron microscope (SEM) • allows detailed study of the surface of a specimen • sample surface covered with thin film of gold • beam excites surface electrons • great depth of field = image that seems 3-D Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 µm Cilia SEM images Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Isolating Organelles • Cell fractionation – takes cells apart & separates major organelles – separation based on variable density of organelles • ultracentrifuge Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Homogenization Tissue cells 1000 g Homogenate (1000 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 Pellet rich in plant) “microsomes” (pieces of plasma membranes and Pellet rich in cells’ internal ribosomes membranes) ultracentifuge • spins up to 130,000 rpm – force > 1 million X gravity (1,000,000g) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings microcentrifuge • used in biotechnology research – study cells at protein & DNA level Figure 6.5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings General cell characteristics • All cells: – are surrounded by plasma membrane – have cytosol • semi-fluid substance within the membrane • cytoplasm = cytosol + organelles – contain chromosomes which have genes in the form of DNA – have ribosomes • tiny “organelles” that make proteins using instructions contained in genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Types of cells • Prokaryotic vs. eukaryotic cells – location of chromosomes Prokaryotic cell • DNA in nucleoid region, without a membrane separating it from rest of cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Eukaryotic cell • Chromosomes in nucleus, membraneenclosed organelle Prokaryotic cells 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) A thin section through the bacterium Bacillus coagulans (TEM) Eukaryotic Cells • Have extensive and elaborately arranged internal membranes, which form organelles • Generally larger than prokaryotic cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Eukaryotic cells – 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 Mitochondrion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysosome Not in animal cells: Chloroplasts Central vacuole & tonoplast Cell wall Plasmodesmata Eukaryotic cells – plant cell Nuclear envelope Nucleolus Chromatin NUCLEUS Rough endoplasmic reticulum Smooth Centrosome 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Not in plant cells: Lysosomes Centrioles Flagella (in some plant sperm) Limits to cell size • The logistics of carrying out cellular metabolism sets limits on the size of cells • Lower size limit: need enough DNA, enzymes, & “equipment” to maintain homeostasis & reproduce – Smallest bacterial cells are 0.1 – 1.0 m (most bacteria are 1-10 m) • Upper size limit: surface area to volume ratio – Largest eukaryotic cells are generally 1-100 m Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Size limits?? • Thiomargarita namibiensis ("Sulfur pearl of Namibia") – gram-negative coccus Proteobacterium – found in ocean sediments – largest bacterium ever discovered – up to 750 µm (0.75 mm); easily visible to the naked eye. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Limits to cell size • As a cell gets bigger, its volume increases faster than its surface area Surface area increases while total volume remains constant – Smaller objects have a greater ratio of surface area to volume – facilitates the exchange of materials into & out of the cell 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) 6:1 ~1:1 6:1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Limits to cell size • A large cell cannot move material in & out fast enough to support life • How to get bigger?? Become multicellular! Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell Membrane • Exchange organelle – aka plasma membrane Outside of cell – Functions as a selective barrier Carbohydrate side chain – Allows sufficient passage of O2, nutrients & waste Hydrophilic region Inside of cell 0.1 µm Hydrophobic region (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 Nucleus – Genetic Library of the Cell • about 5 m in diameter • contains most of the genes in the eukaryotic cell • Where else are genes found in the cell? mitochondria & chloroplasts Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nucleus structure Nucleus Nucleus 1 µm Nucleolus Chromatin 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 Pore complexes (TEM). Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nuclear lamina (TEM). Nuclear envelope • double membrane (separated by 20-40 nm space) • encloses the nucleus, separating its contents from the cytoplasm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nuclear envelope • Perforated by pores (~100 nm diam.) • Pores lined with pore complex (protein structure) that regulates entry & exit of large molecules & particles • Examples? RNA, ribosomal subunits Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nuclear contents • Within nucleus, DNA organized into fibrous material, chromatin – Appears as diffuse mass in non-dividing cell • When cell prepares to divide, chromatin fibers coil up as distinct structures, chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nuclear envelope • Except at pores, nuclear side of envelope is lined with nuclear lamina – netlike array of protein filaments (intermediate filaments; lamin polymers) embedded in inner membrane – maintains nuclear shape – mutations in lamins & lamin-binding proteins cause a wide range of human diseases, collectively termed laminopathies including muscular dystrophy & accelerated aging Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nucleolus • prominent in non-dividing cells – number of nucleoli depends on species • composed of densely staining granules & fibers • synthesis of ribosomal RNA • assembly with imported proteins to form ribosomal subunits (exit through pores) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ribosomes • Function – protein synthesis – cells that make many proteins have more ribosomes (and nucleoli!) • Ex. pancreas (digestive enzymes; insulin) • Structure – contain ribosomal RNA (rRNA) & protein • rRNA (not protein) catalyzes peptide bond formation in protein synthesis! – composed of 2 subunits Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ribosomes Ribosomes Cytosol Free ribosomes Bound ribosomes Large subunit 0.5 µm TEM showing ER and ribosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Small subunit Diagram of a ribosome Types of ribosomes • Free ribosomes – suspended in cytosol – synthesize proteins that function within cytosol • Bound ribosomes – attached to outside of endoplasmic reticulum or nuclear envelope – synthesize proteins for export or for membranes • Structurally identical! Can switch roles as needed Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Eukaryotic vs. prokaryotic ribosomes • Prokaryotes & eukaryotes have different ribosomes – different size subunits – different proteins – evolutionary significance? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endomembrane system • Includes many different structures: – Endoplasmic reticulum, Golgi apparatus, Lysosomes, Vacuoles, Vesicles • Related either thru direct contact or by transfer of vesicles • Plays key role in synthesis (& hydrolysis) of macromolecules in cell (biosynthesis) • Various “players” modify macromolecules for various functions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endomembrane system • Relationships among organelles of the endomembrane system 1 Nuclear envelope is connected to rough & smooth ER Nucleus Rough ER Membranes & proteins 2 Made by ER flow to Golgi in transport vesicles Smooth ER cis Golgi Golgi pinches off transport Vesicles3& other vesicles that give rise to lysosomes & Vacuoles trans Golgi Lysosome can fuse with Another vesicle for digestion Transport vesicle carries proteins to plasma membrane for secretion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plasma membrane Plasma membrane expands by fusion of vesicles; proteins are secreted from cell Endoplasmic Reticulum: Biosynthetic Factory • Function – Makes membranes – Performs many biosynthesis functions • Structure – Membrane connected to nuclear envelope & extends throughout cell – Accounts for ~50% of total membrane in many eukaryotic cells • Rough ER = w/ bound ribosomes • Smooth ER = w/o bound ribosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endoplasmic reticulum • Consists of membranous tubules & sacs called cisternae Smooth ER • ER separates internal compartment from cytosol (cisternal space) Nuclear envelope Rough ER ER lumen Cisternae Ribosomes Transport vesicle Smooth ER Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transitional ER Rough ER 200 µm Endoplasmic reticulum • Functions of smooth ER – Synthesizes lipids • Oils, phospholipids, steroids – STF: ovaries & testes are rich in smooth ER . . . Why?? – Metabolizes carbohydrates • Hydrolysis of glycogen in liver cells – 1st produces glucose phosphate which can’t cross membrane; ER enzymes remove PO4 so it can enter bloodstream Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endoplasmic reticulum • Functions of smooth ER (cont’d) – Detoxifies drugs & poisons • Usually by adding OH–; makes toxin more soluble – Cell responds to increased drug use by making more smooth ER = tolerance – Stores calcium ions for muscle contraction • Pumps Ca2+ from cytosol to cisternal space – When Ca 2+ rushes back out, muscle contracts Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endoplasmic reticulum • Functions of rough ER – Membrane factory • Grows in place by adding proteins & phospholipids (assemble by enzymes in ER) – as ER membrane expands, it buds off & transfers to other cell parts that need it • Membrane bound proteins synthesized directly into membrane – anchored by hydrophobic portions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Endoplasmic reticulum • Functions of rough ER (contd.) – Produces proteins to be distributed by transport vesicles • mainly secretory proteins (for export) – mostly glycoproteins (= proteins that are covalently bonded to carbohydrates) • carbohydrate (oligosaccharides) attached to protein in ER membrane • growing polypeptide chain threads into cisternal space (through pore) where it folds into native shape Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Golgi Apparatus : receiving, processing, & shipping • first reported by Camillo Golgi (1843-1926) – at a meeting of the Medical Society of Pavia on 19 April 1898 – he named it the 'internal reticular apparatus' Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Golgi apparatus cisGolgi face apparatus (“receiving” side) 1 Vesicles move 2 Vesicles coalesce to 6 Vesicles also 0.1 0 µm from ER to Golgi form new cis Golgi cisternae transport certain Cisternae proteins back to ER 3 Cisternal maturation: move from cisto-trans 4 Vesicles bud & carry proteins to other locations or to plasma membrane for secretion 5 Vesicles transport some trans face proteins backward to newer (“shipping”) Golgi cisternae Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TEM of Golgi apparatus Golgi Apparatus • Structure – consists of flattened membranous sacs called cisternae • look like stack of pita bread • usually 3-8 sacs per set – can be as few as 1 in animal cells – can be hundreds in some plant cells • specialized secretory cells have more sets Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Golgi Apparatus • Structure (contd.) – Usually near nucleus – 2 sides = 2 functions (polarity!) • Cis = “receiving” = receives material by fusing with vesicles from ER • Trans = “shipping” = buds off vesicles that travel to other sites (transport) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Golgi Apparatus • Functions – Finishes, sorts, & ships cell products – Extensive in cells specialized for secretion – Which cells have a lot of Golgi? Secretory cells (pancreatic, intestinal goblet, salivary) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Golgi apparatus • Functions (contd.) – modification of rough ER products • done by changing sugar monomers • allows wide variety of oligosaccharides – manufacture of certain macromolecules • pectins & non-cellulose polysaccharides – sorting macromolecules & targeting for proper destination • done by adding molecular ID tags (ex. phosphate groups) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysosomes: Digestive Compartments • membranous sacs of hydrolytic enzymes – can digest all kinds of macromolecules • examples? Polysaccharides Polypeptides/proteins Lipids/fatty acids Nucleic acids – enzymes & membrane of lysosomes are synthesized by rough ER & transferred to Golgi Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings lysosomes • fuse with vacuoles containing ingested food of cell parts to be recycled • polymers are digested into monomers – pass to cytosol to become nutrients of cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysomes • Help carry out intracellular digestion of engulfed particles. What is this process called? 1 µm Nucleus Phagocytosis • what kinds of cells do this? Lysosome Protists (ameba) Macrophages (WBCs) Lysosome contains active hydrolytic enzymes Food vacuole fuses with lysosome Hydrolytic enzymes digest food particles Digestive enzymes Lysosome Plasma membrane Digestion Food vacuole (a) Phagocytosis: lysosome digesting food Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings lysosomes • Also use hydrolytic enzymes to recycle cell’s own organic material . . . called? Lysosome containing two damaged organelles 1µm Autophagy A human liver cell recycles half of its macromolecules each week! Mitochondrion fragment Peroxisome fragment Lysosome fuses with vesicle containing damaged organelle Hydrolytic enzymes digest organelle components Lysosome Vesicle containing damaged mitochondrion Digestion (b) Autophagy: lysosome breaking down damaged organelle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysosomal enzymes • Work best at pH 5 (cytosol pH ~7.3) – organelle creates custom pH • How? Proteins in lysosomal membrane pump H+ ions from cytosol into lysosome • Why? • Enzymes are proteins • Proteins are sensitive to pH – affects structure • Digestive enzymes won’t work well if they leak into cytosol – helps prevent autodigestion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings When things go wrong . . . • If lysosomal enzymes don’t function, – biomolecules collect & accumulate in lysosomes • Lysosomes grow larger & larger, eventually disrupting cell & organ function • “Lysosomal storage diseases” are usually fatal • Ex. Tay-Sachs disease – Lipids build up in brain cells – Child dies before age 5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sometimes, it’s supposed to work that way . . • Apoptosis = programmed cell death – Auto-destruct mechanism – Some cells must die in an organized fashion, especially during development • Ex. Space between your fingers • Ex. Self-destruction of damaged cells before they can become cancerous Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Vacuoles • Diverse maintenance compartments – Food vacuoles • formed by phagocytosis – Contractile vacuoles • Pump excess water out of protist cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings vacuoles • Central vacuoles (in plant cells) – Functions: • Storage, waste disposal, protection, growth – hold reserves of important organic compounds & water – increase surface area (internally) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Central vacuole Cytosol Tonoplast Nucleus Central vacuole Cell wall Chloroplast 5 µm Peroxisomes: Oxidation • Peroxisomes – Do not bud from endomembrane system • Membrane grows in place using proteins & lipids in cytosol – Produce hydrogen peroxide & convert it to water – (2H2O2 2H2O + O2) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplast Peroxisome Mitochondrion 1 µm Mitochondria & chloroplasts • Energy conversion! – not “creation” of energy; 1st law of thermodynamics! • Mitochondria – sites of cellular respiration • Chloroplasts – sites of photosynthesis – found only in plants (& algae) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mitochondria: Chemical Energy Conversion • found in nearly all eukaryotic cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings mitochondria • enclosed by two membranes • smooth outer membrane • inner membrane folded into cristae – STF: what is the value of christae? • interior filled with matrix – contains enzymes, mitochondrial DNA, ribosomes, proteins – Example of compartmentalization! Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings mitochondria Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix Mitochondrial DNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 100 µm Chloroplast: Capture of Light Energy • specialized member of a family of closely related plant organelles called plastids • contains chlorophyll Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings chloroplasts • found in leaves and other green organs of plants and in algae Chloroplast Ribosomes Stroma Chloroplast DNA Inner and outer membranes Granum 1 µm Thylakoid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings chloroplast • structure includes – Thylakoids, membranous disc-shaped sacs • Stacked to form granum (pl. = grana) – Stroma, the internal fluid surrounding thylakoids • Contains chloroplast DNA, ribosomes, proteins, enzymes • Example of compartmentalization! Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings cytoskeleton • network of fibers extending throughout the cytoplasm Microtubule 0.25 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Microfilaments Cytoskeleton • functions – mechanical support – motility – regulation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings cytoskeleton – motor proteins interact with cytoskeleton to provide motility 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 (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 Components of the Cytoskeleton • three main types of fibers make up the cytoskeleton Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Microtubules • Microtubules – Hollow; resist compression – Shape the cell – Guide movement of organelles – Help separate the chromosome copies in dividing cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Centrosomes & Centrioles • centrosome – “microtubule-organizing center” Centrosome – in animal cells, contains a pair of centrioles Microtubule Centrioles 0.25 µm Longitudinal section of one centriole Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Microtubules Cross section of the other centriole Cilia and Flagella • Cilia and flagella – 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 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). 15 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cilia & flagella • share common ultrastructure 9+2 doublets Outer microtubule doublet Dynein arms 0.1 µm Plasma membrane Central microtubule Outer doublets cross-linking proteins inside Microtubules Radial spoke Plasma membrane Basal body (b) 0.5 µm 0.1 µm (a) Triplet 9 triplets (c) Cross section of basal body Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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. 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.) 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Microfilaments (actin) • Solid; tension-bearing • Network gives cortex (outer cytoplasmic layer) gel consistency – vs. sol consistency of cytosol • Alternate with myosin to contract – muscle contraction – contracting band forms cleavage furrow in cytokinesis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings microfilaments – found in microvilli Microvillus Plasma membrane Microfilaments (actin filaments) Intermediate filaments 0.25 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings microfilaments Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings microfilaments • Those that function in cellular motility – contain the protein myosin in addition to actin Muscle cell Actin filament Myosin filament Myosin arm (a) Myosin motors in muscle cell contraction. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Amoeboid movement – Involves the contraction of actin and myosin filaments Cortex (outer cytoplasm): gel with actin network Inner cytoplasm: sol with actin subunits Extending pseudopodium (b) Amoeboid movement Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Cytoplasmic streaming – another form of locomotion created by microfilaments Nonmoving cytoplasm (gel) Chloroplast Streaming cytoplasm (sol) Parallel actin filaments (b) Cytoplasmic streaming in plant cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell wall Intermediate fibers • Solid; intermediate diameter • Tension-bearing • Diverse class of elements – in keratin family of proteins • More permanent • Maintain overall shape & anchor some organelles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell Walls of Plants – extracellular structure of plant cells that distinguishes them from animal cells – made of cellulose fibers embedded in other polysaccharides and protein – may have multiple layers Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plant cell walls Plasma membrane Central vacuole of cell Secondary cell wall Primary cell wall Central vacuole of cell Middle lamella 1 µm Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Extracellular matrix (ECM) • Main ingredients are glycoproteins (collagen, proteoglycans, & fibronectins) – Fibronectins bind to integrins (receptor proteins) EXTRACELLULAR FLUID Collagen A proteoglycan complex Polysaccharide molecule Carbohydrates Core protein Fibronectin Plasma membrane Integrin Integrins Microfilaments Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CYTOPLASM Proteoglycan molecule Exctracellular matrix • Functions of the ECM include – Support – Adhesion – Movement Collagen fibers – Regulation • New research suggests that changes in ECM can alter internal cell structure & function Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Collagen fibrils Intercellular Junctions • Integrate cells into higher levels of structure & function – Organize animal & plant cells into tissues, organs, and organ systems • Special patches of direct physical contact – Allow neighboring cells adhere, interact, & communicate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intercellular Junctions in plants • plasmodesmata – channels that perforate cell walls – plasma membrane lines channels – cytosol flows between cells Cell walls – unifies most of plant into one living continuum 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 • integrate cells in various ways • three types: – Tight junctions – Desmosomes – Gap junctions • structure fits function! Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intercellular junctions in animals TIGHT JUNCTIONS Tight junctions prevent fluid from moving across a layer of cells Tight junction •Neighboring cells fused •Form continuous belts •Prevent leakage 0.5 µm DESMOSOMES •Anchoring junctions •Rivets fasten cells into sheets •Reinforced w/ intermediate fibers Tight junctions Intermediate filaments Desmosome Gap junctions Extracellular matrix Plasma membranesGap junction of adjacent cells Space between cells 1 µm GAP JUNCTIONS •Communication junctions •Provide cytoplasmic channels •Common in animal embryos 0.1 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cell functions are emergent properties • the cell is a living unit greater than the sum of its parts 5 µm • How do cell parts interact in this function? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Any questions?? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings