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Transcript
Cells: Organelles Chapter 4 Overview Eukaryotic cells Cytoplasm Organelles Cytoskeleton How cells move Prokaryotic cells Eukaryotic Cells Membrane-bound organelles = metabolic activities occur within controlled environment DNA contained within true nucleus Protein cytoskeleton Cytoplasm Cellular material inside cell where cellular activities are carried out 3 components: – Cytosol – Inclusions – Cytoplasmic organelles Cytosol Thick semi-translucent fluid in which organelles are suspended Composed of water with variety of solutes e.g. proteins, salts, sugars, etc. Has both colloidal & suspension properties: = sol-gel transformation = visible particles that settle out Inclusions Chemical substances contained within cell Vary depending on type of cell e.g. glycogen in liver & muscle cells e.g. lipid droplets in adipose cells e.g. melanin pigment in skin & hair Organelles “Little organs” = cellular compartments Individual structures within cells that perform specific functions Some are non-membranous Some are membrane-bound = have own internal environments Nucleus: A Cell’s Control Centre Largest cytoplasmic organelle Contains genetic material & protein-building instructions in DNA Keeps DNA separate from cytoplasmic components a. Nuclear Envelope & Nucleoplasm Nuclear envelope: – Semi-permeable lipid bilayer with membrane proteins – Outer membrane: continuous with endoplasmic reticulum – Inner membrane: fibrous proteins that help organize DNA molecules Nucleoplasm: – Semi-fluid matrix b. Nucleolus Builds ribosomes used for protein synthesis Contains proteins & ribosomal RNA c. Chromosomes & Chromatin Chromosome = 1 DNA molecule & associated proteins Chromatin = all DNA molecules & associated proteins The Endomembrane System Consists of ribosomes, endoplasmic reticulum, Golgi apparatus, & vesicles Makes, stores, & transports biological molecules Destroys potentially harmful materials All proteins bound for export or plasma membrane pass through Ribosomes Small granules of proteins & ribosomal RNA Synthesize all proteins secreted by cells Can be free-floating or membrane-bound Endoplasmic Reticulum: Where Proteins Are Born Site of protein & lipid synthesis Tubules & membranes connect to form fluidfilled cavities that adjoin nuclear envelope a. Rough ER Surface studded with ribosomes Where all proteins are made, regardless of fate Cells that make, store, & secrete proteins have lots of RER e.g. liver cells, antibodyproducing plasma cells, etc. b. Smooth ER No ribosomes No role in protein synthesis Enzymes catalyze many reactions e.g. plasma membrane lipid synthesis e.g. detoxification in liver & kidney e.g. breakdown of glycogen in liver After proteins have been synthesized in the ER, they are packaged in transport vesicles Vesicles bud off & proceed to the Golgi apparatus for further processing Golgi Apparatus “Traffic director” for proteins Modifies, packages, & transports proteins & lipids from RER to export vesicles Transport vesicles from ER fuse with Golgi apparatus Proteins & lipids are modified (sugar side chains added, etc.) & repackaged into vesicles Export vesicles transport products away to be: – Digested – Incorporated into plasma membrane – Expelled from cell via exocytosis Vesicles Sacs that are used for storage or transport Bud off from ER, Golgi apparatus, plasma membrane • Peroxisomes • Lysosomes a. Peroxisomes Rid cells of toxic substances & neutralize free radicals Contain enzymes that digest: – Fatty acids – Amino acids – H2O2 e.g. break-down of alcohol in liver & kidney cells b. Lysosomes Contain acidic digestive enzymes = can digest almost all biological materials Abundant in phagocytes Functions include: – Degrading old, dying, & non-functional organelles & tissues – Digesting foreign materials – Breaking down bone to release Ca2+ into blood Mitochondrion: A Cell’s Powerplant Site of aerobic respiration (found only in aerobic eukaryotic cells) Allow cell to produce lots of ATP # differs depending on cell type Have own DNA & RNA so can replicate themselves when cellular ATP needs increase Mitochondrion Structure Outer membrane • Selectively permeable Inner membrane • Highly impermeable • Contains ATP synthase • Has membrane potential Cristae • ↑ surface area of inner membrane, which ↑ capacity to generate ATP Matrix • Contains 100s of enzymes which oxidize pyruvate & fatty acids, & control the Krebs cycle Possible Endosymbiotic Origins of Mitochondria Endosymbiont Theory: One prokaryotic cell entered another cell (ingested or parasitic) & escaped digestion = able to reproduce inside host Over generations, evolved to life on the inside & lost traits necessary for independent life Specialized Plant Organelles Plastids = Organelles used in photosynthesis or for storage Central vacuole = Fluid-filled compartment a. Plastids Chloroplasts – Store chlorophyll a & b – Capture light E – Endosymbiotic origins? Chromoplasts – Store carotenoids – Capture light E Amyloplasts – Starch storage b. Central Vacuole 50%-90% of cell interior Stores amino acids, sugars, ions, toxins Expansion of vacuole prompts cell SA to ↑ = ↑ absorption of H2O & nutrients Cytoskeleton: A Cell’s Support System Supportive protein filaments between nucleus & cell membrane Form cell skeleton that supports cellular structure & allows for cell movement (like skeletal & muscular systems in humans) Found in eukaryotic cells (prokaryotic cells have poorly-developed cytoskeletons, although reinforcing filaments are similar) Elements include: – Microfilaments – Microtubules – Intermediate filaments Centrosomes & Centrioles Centrosome = where microtubules are produced = each contains a pair of centrioles Centriole = barrel-shaped organelle important in cell division More on these later! 1. Microtubules Anchor / move organelles & cell structures Globular tubulin monomers – “Minus” end anchored in centrosomes – “Plus” end grows as tubulin added Not permanently stable – Fall apart in controlled ways – Plant toxins & animal microtubules 2. Microfilaments Strengthen cell structure, anchor membrane proteins, & aid in muscle contraction 2 polypeptide chains of globular actin monomers – Form bundles & networks Form & fall apart in controlled ways Responsible for cytoplasmic streaming Cytoplasmic Streaming Microfilaments loosen up Allows fluid motion of cytoplasm Rearranges substances & structures within cell interior 3. Intermediate Filaments Very stable fibrous cytoskeletal elements Types of filaments vary depending on cell type e.g. lamins: support nucleus, aid in muscle contraction e.g. desmins & vimentins: anchor contractile units e.g. cytokeratins: strengthen cells that produce nails, claws, horns, etc. Eukaryotic Cell Walls Plants, some protists & fungi Protective/supportive layer around plasma membrane Porous = allows free movement of H2O & solutes between external environment & plasma membrane Waxes build up at surfaces exposed to air = ↓ H2O loss Plants often have 2 cell walls 1° cell wall: = flexible = adheres adjoining cells 2° cell wall: = provides rigidity & structural support = very abundant in woody plants How Cells Move Cells can move: – Elements within themselves – Themselves (parts or whole self) Internal Cellular Movement Movement of organelles, etc. Uses motor proteins e.g. kinesins, dyneins, myosins, etc. Move along cytoskeletal elements Fueled by ATP How Cells Get Around Cilia Flagella Pseudopodia Cilia Hair-like extensions of cell surface Beat in synchrony Move substances or cell itself e.g. Paramecium e.g. cells in respiratory tract that move mucus away from lungs Flagella Singular = flagellum Whip-like extension of cell Propels cell along e.g. sperm cell, many protists Cilium & Flagellum Structure 9+2 array down length = 9 pairs of microtubules form ring around central pair Centriole sits beneath array as basal body Flagellum / cilium moves by sliding mechanism Powered by ATP Dynein arms in 1 pair of microtubules grab pair in front, tilt downward, & let go As 1 pair is being grabbed, it grabs pair in front Creates bending motion in each cilium / flagellum Pseudopodia “False feet” Temporary extensions of cytoplasm Microfilaments elongate in 1 direction Attached motor proteins drag plasma membrane along in that direction e.g. amoebas, macrophages Prokaryotic Cells Outwardly similar in appearance to eukaryotic cells Archaea – All polypeptide chains begin with methionine – Make histones to stabilize DNA • Bacteria – All polypeptide chains begin with formylmethionine – Make histone-like proteins to stabilize DNA Prokaryotic Cell Membranes Semi-permeable cell membrane Many have cell wall (structurally different from eukaryotic cell walls) Sticky polysaccharides on outer surface of cell wall Contain no membrane bound-organelles Photosynthetic bacteria harbour pigments within inpouchings of plasma membrane DNA is free-floating in cytoplasm DNA is circular Some have plasmids =small circular DNA molecules with few genes Many have ≥ 1 flagellum Many have pili = protein filaments = allow adhesion to surfaces = allow transfer of genetic material to other cells Harvard cell video Musical version: http://aimediaserver.com/studiodaily/videopla yer/?src=harvard/harvard.swf&width=640&h eight=520 Narrated version: http://multimedia.mcb.harvard.edu/media.html