The eukaryotic cell Flashcards

Question

Cytoskeleton filament cellular localisation

Answer 1

Most eukaryotic cells contain all 3 filaments, located in different places in different cells In absorptive epithelial cells in lumen of gut: - Actin filaments = apical region - Microtubules = oriented w the long axis of cell - Intermediate = conc along cell periphery e.g specialised junctions w neighbouring cells or lining the nuclear membrane Sensory hairs of inner ear: - Actin = conc in apical region in particular stereocilia - Microtubules = conc in apical region and kinocilia but also throughout cells Cytoskeletal filaments are organised into bundles and networks In bundles filament are closely packed in parallel arrays, in network filaments crisscross Cytoskeletal components usually attached to plasma membrane proteins and form skeleton that helps support plasma membrane

Answer 2

Confers structure Involved in cellular movement Intracellular movement – e.g cytoplasm streaming (flow of cytoplasm inside cell, driven by forces from cytoskeleton, function is to speed up transport of molecules and organelles around cell), transport of vesicles or separation chromosomes in cell division, transport of chloroplasts Whole cell movement – e.g migration, hosts defences, changes in morphology like muscle contraction, nerve axon elongation and cell protrusion and constriction Cell movement requires arrangement of microfilaments or microtubule and/or motor proteins Intermediate fibres are not involved in cell movement, instead they play role in cell adhesion Cell movements require ATP and proteins that convert the energy stored in ATP into motion Cells evolved 2 basic mechanisms for generating movement - One entails the assembly and disassembly of microfilaments and microtubules; responsible for many changes in cell shape - Other requires a special class of enzymes called motor proteins (myosins, kinesisns and dyneins) Few movements require both the action of motor proteins and cytoskeleton rearrangements

Answer 3

Made of actin = most abundant intracellular protein in most euk cells Actin - Encoded by large highly conserved gene family - Single cell organisms 1-2 actin genes - Multicellular organisms multiple genes - Different genes in the family have different functions and are found in different cells - Globular monomer called g-actin, separated into 2 lobes by deep cleft, lobes and cleft compose the ATPase fold = site where ATP and mg ion bound, cleft acts as hinge that allows lobes to flex relative to each other, g-actin monomer can assemble into chain to form filamentous polymer f-actin which can depolymerise to form g-actin - In filament the subunits are organised as helix - Filaments polarised w one end (-) end containing exposed ATP binding site - Filaments organised into bundles and networks by variety of bivalent cross-linking proteins. When cross-linked by short protein actin filaments pack side by side to form bundle, long cross-linking proteins such as filamin are flexible and can cross-link actin filaments into network Actin assembly - Within cells actin cytoskeleton dynamic w filaments able to grow and shrink rapidly - Filaments grow considerably faster at + end than – end - Initial nucleation process acts as a focal point 1. In initial nucleation phase ATP G-actin monomers slowly form stable complexes of actin 2. Nuclei rapidly elongated in second phase by addition of subunits to both ends of filament 3. After their incorporation into filament subunits slowly hydrolyse ATP and becomes stable ATP-F actin Actin treadmilling – actin subunits can flow through the filaments by attaching preferentially to the + end and dissociating preferentially from -end of filament, in this situation the length of the filament remains constant Control of actin polymerisation – regulated by proteins that bind g-actin - Thymosin β4 inhibits actin assembly. - Profilin promotes actin assembly. - Profilin binds to the G-actin coupled to ADP and catalyses the exchange of ADP with ATP - The ATP-G-actin-profilin complex can be linked to the plus end of the filament and Prolifin dissociates Polymerisation of actin causes changes in cell shape Myosin powered cell moments – myosin is large superfamily of motor proteins that bind actin filaments or microfilaments. Responsible for - Vesicular trafficking - Cytoplasmic streaming - Muscle contraction - Cytokinesis All myosins consist of one or two heavy chains and several light chains, head, neck and tail domain organisation - Myosin head domains have ATPase activity. ATP hydrolysis allows myosin movement along an actin filament. - Role of a myosin is related to its tail domain. Tail domains of myosins I, V, VI, and XI bind the plasma membrane or the membranes of intracellular organelles. - Myosin II dimers associate to form bipolar thick filaments (muscle contraction): close packing of myosin II molecules into thick filaments allows many myosin head domains to interact simultaneously with actin filaments. - In non-muscle cells the myosin II actin bundle are referred to as stress fibres and are responsible for cell and tissue migration and adhesion. ATP hydrolysis allows myosin movement along actin filament - ATP hydrolysed resulting in myosin attaching to actin subunit - Phosphate released causing head to change position pulling myofilament - ADP released and ATP attaches resulting in detachment of myosin from actin - Repeats Cell movement – initiated by formation of large, broad membrane protrusion at leading edge of a cell called lamellipodium in vertebrate cells Microfilament based cell movement occurs when 1. Actin filaments at leading edge are rapidly cross-linked into bundles and networks 2. Lamellipodium adheres to substratum moves forward then the trailing edge de-adheres

Answer 4

Tubulin dimer of monomers alpha and beta tubulin Function: - Maintenance of cell shape - Cell motility - Chromosome movement - Organelle movement Structure: - Alpha tubulin monomer bound to GTP which is nonexchangeable - Beta tubulin monomer bound to GDP which is exchangeable w GTP - GTP or GDP bound to beta tubulin modulates the addition of tubulin subunits Assembly: 1. Protofilaments assembled from alpha beta tubulin subunits 2. Protofilaments associate to form the wall of the microtubule 3. Addition of more subunits to the ends of the protofilaments elongates the microtubule Like actin filaments microtubules have a distinct polarity. Tubulin polymerizes end to end, with the β-subunits of one tubulin dimer contacting the α-subunits of the next dimer. In a protofilament, one end will have the α-subunits (-) exposed while the other end will have the β-subunits (+) exposed. As the protofilaments bundle parallel to one another with the same polarity, in a microtubule, there is one end, the (+) end, with only β-subunits exposed, while the other end, the (−) end, has only α-subunits exposed. While microtubule elongation can occur at both the (+) and (−) ends, it is significantly more rapid at the (+) end. Microtubule forms at a central site, the microtubule-organizing centre (MTOC), contains a Ɣ-tubulin which is the major site of microtubule nucleation. Animal cells - MTOC is usually a centrosome, sometimes but not always contains a pair of centrioles. Fungi - MTOC is called the spindle pole Plants - nuclear envelope appears to act as MTOC Kinesin and dynein powered cell movements: - Microtubules function as tracks in the intracellular transport of various types of “cargo.” - Two families of motor proteins, kinesins and dyneins, mediate transport. - Examples of kinesins and dyneins transport include metaphase during mitosis and meiosis and beating of cilia and flagella. Kinesins = composed of heavy chains and two light chains, most are + end directed motors (move to + end), divided into cytosolic and motitic kinesins based on cargo they transport - Cytosolic = transport of organelles and vesicles - Mitotic = participate in spindle assembly and chromosome segregation in cell division Dyneins = composed of 2 or 3 heavy chains complexes and several intermediate and light chains Dynein cannot mediate cargo transport alone requires dynactin, that links vesicles and chromosomes to the dynein light chains. Cytosolic dyneins are (-) End–Directed motor proteins that bind cargo through dynactin. - Cytosolic = role in movement of vesicles and chromosomes - Anoxemal = responsible for beating of cilia and flagella

Answer 5

Found in nearly all animals but not plants and fungi. Fibrous proteins supercoiled into thicker cables protein subunits depend on cell type Associated with the nuclear and plasma membrane principal function is structural. Formation of nuclear lamina Not involved in cell movement.

Answer 6

Site of protein synthesis in eukaryotes & prokaryotes. Thousands to millions per cell depending on activity. Found in cytosol, rER, mitochondria & chloroplasts. 2 Subunits made up of RNA molecules & proteins. Ribosomes are NOT an organelles are not bound by a membrane and are much smaller than other organelles

Answer 7

Structural layer that can be tough, flexible, and sometimes rigid. Provides structural support and protection, and acts as a filtering mechanism. Absent in animals, present in most other eukaryotes Major function prevent over-expansion of the cell when water enters. Composition depends on the cell type e.g. plants have three layers - primary layer contains cellulose, hemicellulose and pectin. Middle lamella lies between cells: Calcium and Magnesium pectates Primary cell wall consists of: Hemicellulose: polysaccharides of 5-carbon (xylose and arabinose) and 6-carbon (glucose, mannose, galactose and rhamnose) Pectin: polysaccharides of mostly galacturonic acid Cellulose microfibrils provide strength