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Cytoskeleton Microtubules are cylindrical structures composed of tubulin subunits Microtubules and Actin Filaments Microtubules and Actin Filaments • Two components of the cytoskeleton—microtubules and actin filaments—are formed from globular protein subunits • (a) Longitudinal view and • (b) Transverse section (made at a right angle to the long axis) of a microtubule • Microtubules are hollow tubules composed of two different types of molecules, alpha (α) tubulin and beta (β) tubulin • These tubulin molecules first come together to form soluble dimers ("two parts") which then self-assemble into insoluble hollow tubules • The arrangement results in 13 "protofilaments" around a hollow core • (c) Actin filaments consist of two linear chains of identical molecules coiled around one another to form a helix Cortical Microtubules Cortical Microtubules • (a) A longitudinal view of cortical microtubules (indicated by arrows) in leaf cells of the fern Botrychium virginianum • The microtubules occur just inside the wall and plasma membrane • (b) A transverse view of cortical microtubules (arrows), which can be seen to be separated from the wall by the plasma membrane • Cortical microtubules play a role in the alignment of cellulose microfibrils in the cell wall Cortical Microtubules Botrychium virginianum (rattlesnake fern) • • • Widely distributed (U.S., Mexico, Australia, Asia, Norway, Finland, Russia, etc.) Used in India to treat dysentery (an inflammation of the intestine causing diarrhea with blood and caused by bacteria, viruses, parasitic worms, or protozoa) (image and text: Wikipedia) Actin filaments consist of two linear chains of actin molecules in the form of a helix Actin Filaments Actin Filaments • (a) A bundle of actin filaments as revealed in an electron micrograph of a leaf cell of maize (Zea mays) • (b) Several bundles of actin filaments as revealed in a fluorescence micrograph of a stem hair of tomato (Solanum lycopersicum) • Actin filaments are involved in a variety of activities, including cytoplasmic streaming Cytoplasmic Streaming in Giant Algal Cells Cytoplasmic Streaming in Giant Algal Cells • • • • (a) A track followed by the streaming cytoplasm in a giant algal cell (e.g., Chara and Nitella) (b) A longitudinal section through part of the cell, showing the arrangement of stationary and streaming layers of cytoplasm The proportions have been distorted in both diagrams for clarity (image: Wikipedia) Chara globularis Cytoplasmic Streaming in Giant Algal Cells, continued • • • • • • Chara corallina is a freshwater plant that inhabits temperate zone ponds and lakes It consists of alternating nodes and internodes Each internodal segment is a single large cell, up to 10 cm in length Because the cell is so large, it uses cytoplasmic streaming to distribute organelles and nutrients throughout the cytoplasm, which surrounds a large central vacuole (Johnson, Wyttenbach, Wayne, & Hoy, 2002) (image: Wikipedia) A similar species, Nitella mucronata (Wikipedia) Chara corallina cell (Johnson, et al., 2002) Flagella and Cilia Structure of a Flagellum • (a) Diagram of a flagellum with its underlying basal body • (b) Electron micrograph of the flagellum of Chlamydomonas, as seen in transverse section • Virtually all eukaryotic flagella have this same internal structure • Consists of an outer cylinder of nine pairs of microtubules surrounding two additional microtubules in the center • The "arms," the radial spokes, and the connecting links are formed from different types of protein • The basal bodies from which flagella arise have nine outer triplets, with no microtubules in the center • The "hub" of the wheel in the basal body is not a microtubule, although it has about the same diameter Structure of a Flagellum Structure of a Flagellum Cell Wall Cellulose is the principal component of plant cell walls Primary Walls Primary Walls • (a) Surface view of the primary wall of a carrot (Daucus carota) cell, prepared by a fast-freeze, deep-etch technique, showing cellulose microfibrils, cross-linked by an intricate web of matrix molecules • (b) Schematic diagram showing how the cellulose microfibrils are cross-linked into a complex network by hemicelllose molecules • The hemicellulose molecules are linked to the surface of the microfibrils by hydrogen bonds • The cellulose-hemicellulose network is permeated by a network of pectins, which are highly hydrophilic polysaccharides • Both hemicellulose and pectin are matrix substances • The middle lamella is a pectin-rich layer that cements together the primary walls of adjacent cells Primary Walls Stone Cells Stone Cells • Stone cells (sclereids) from the flesh of a pear (Pyrus communis) seen in polarized light • Clusters of such stone cells are responsible for the gritty texture of this fruit • The stone cells have very thick secondary walls traversed by numerous simple pits, which appear as lines in the walls • The walls appear bright in polarized light because of the crystalline properties of their principal component, cellulose Many plant cells have a secondary wall in addition to a primary wall Detailed Structure of a Cell Wall Detailed Structure of a Cell Wall Source: Wikipedia Detailed Structure of a Cell Wall • (a) Portion of wall showing, from the outside in, the middle lamella, primary wall, and three layers of secondary wall • Cellulose, the principal component of the cell wall, exists as a system of fibrils of different sizes • (b) The largest fibrils, macrofibrils, can be seen with the light microscope • (c) With the aid of an electron microscope, the macrofibrils can be resolved into microfibrils about 10 to 25 nanometers wide • (d) Parts of the microfibrils, the micelles, are arranged in an orderly fashion and impart crystalline properties to the wall • (e) A fragment of a micelle shows parts of the chainlike cellulose molecules in a lattice arrangement • The middle lamella joins adjacent cells • The primary wall is deposited while the cell is increasing in size • The secondary wall is deposited after the primary wall has stopped increasing in size • Whereas the primary wall has pit-fields, the secondary wall has pits Primary Pit-fields, Pits, and Plasmodesmata Primary Pit-fields, Pits, and Plasmodesmata • (a) Cells with primary walls and primary pit-fields, which are thin areas in the walls • As shown here, plasmodesmata commonly traverse the wall at the primary pit-fields • (b) Cells with secondary walls and numerous simple pits • (c) A simple pit-pair • (d) A bordered pit-pair Primary Pit-fields, Pits, and Plasmodesmata Primary Pit-fields, Pits, and Plasmodesmata Primary Pit-fields, Pits, and Plasmodesmata The Layers of Secondary Walls The Layers of Secondary Cell Walls • Diagram showing the organization of the cellulose microfibrils and the three layers (S1, S2, S3) of the secondary wall • The different orientations of the three layers strengthen the secondary wall Growth of the cell wall involves interactions among plasma membrane, secretory vesicles, and microtubules Cell Expansion Cell Expansion • The orientation of cellulose microfibrils within the primary wall influences the direction of cell expansion • (a) If the cellulose microfibrils are randomly oriented in all walls, the cell will expand equally in all directions, tending to become spherical in shape • (b) If the microfibrils are oriented at right angles to the ultimate long axis of the cell, the cell will expand longitudinally along that axis Plasmodesmata are cytoplasmic strands that connect the protoplasts of adjacent cells Synthesis of Cellulose Microfibrils Synthesis of Cellulose Microfibrils • Cellulose microfibrils are synthesized by enzyme complexes that move within the plane of the plasma membrane • (a) The enzymes are complexes of cellulose synthase that, in seed plants, form rosettes embedded in the plasma membrane • Each enzyme rosette, shown here in a longitudinal section, synthesizes cellulose from the glucose derivative UDP-glucose (uridine diphosphate glucose) • The UDP-glucose molecules enter the rosette on the inner (cytoplasmic) face of the membrane, and a cellulose microfibril is extruded rom the outer face of the membrane • (b) As the far ends of the newly formed microfibrils become integrated into the cell wall, the rosettes continue synthesizing cellulose, moving along a route (arrows) that parallels the cortical microtubules in the underlying cytoplasm Synthesis of Cellulose Microfibrils Plasmodesmata Plasmodesmata in Diospyros • • • • Previous slide: Light micrograph of plasmodesmata in the thick primary walls of persimmon (Diospyros) endosperm, the nutritive tissue within the seed The plasmodesmata appear as fine lines extending from cell to cell across the walls The middle lamella appears as a light line between these cells Plasmodesmata generally are not discernible with the light microscope, but the extreme thickness of the persimmon endosperm cell walls greatly increases the length of the plasmodesmata, making them more visible Images: Wikipedia Plasmodesmata and Desmotubules Plasmodesmata and Desmotubules • (a) Plasmodesmata connecting two leaf cells from the cottonwood (Populus deltoides) tree • (b) As seen with the electron microscope, plasmodesmata appear as narrow, plasma-membrane-lined channels in the walls, each traversed by a modified tubule of endoplasmic reticulum known as the desmotubule • Note the continuity of the endoplasmic reticulum on either side of the wall with the desmotubules of the plasmodesmata • The middle lamella between the adjacent primary walls is not discernible in the electron micrographs, as is common in such preparations Plasmodesmata and Desmotubules The Cell Cycle Virtual Cell: Mitosis http://vcell.ndsu.nodak.edu/animations/mitosis/index.htm Virtual Cell: Meiosis http://vcell.ndsu.nodak.edu/animations/meiosis/index.htm The Cell Cycle The Cell Cycle • Mitosis (the division of the nucleus) and cytokinesis (the division of the cytoplasm), which together constitute the M phase, take place after completion of the three preparatory phases (G1, S, and G2) of interphase • Progression of the cell cycle is mainly controlled at two checkpoints, one at the end of G1 and the other at the end of G2 • In cells of different species or even of different tissues within the same organism, the various phases occupy different proportions of the total cycle Cell Division in a Cell With a Large Vacuole Cell Division in a Cell With a Large Vacuole • (a) Initially, the nucleus lies along one wall of the cell, which contains a large central vacuole • (b) Strands of cytoplasm penetrate the vacuole, providing a pathway for the nucleus to migrate to the center of the cell • (c) The nucleus has reached the center of the cell and is suspended there by numerous cytoplasmic strands • Some of the strands have begun to merge to form the phragmosome through which cell division will take place • (d) The phragmosome, which forms a layer that bisects the cell, is fully formed • (e) When mitosis is completed, the cell will divide in the plane occupied by the phragmosome Mitosis and Cytokinesis During prophase, the chromosomes shorten and thicken Mitosis, a Diagrammatic Representation Mitosis, a Diagrammatic Representation • • • • • • • • • • (a) During early prophase, the four chromosomes shown here become visible as long threads scattered throughout the nucleus (b) As prophase continues the chromosomes shorten and thicken until each can be seen to consist of two threads (chromatids) attached to each other at their centromeres (c) By late prophase, kinetochores develop on both sides of each chromosome at the centromere Finally, the nucleolus and nuclear envelope disappear (d) Metaphase begins with the appearance of the spindle in the area formerly occupied by the nucleus During metaphase, the chromosomes migrate to the equatorial plane of the spindle At full metaphase (shown here), the centromeres of the chromosomes lie on that plane (e) Anaphase begins as the centromeres of the sister chromatids separate The sister chromatids, now called daughter chromosomes, then move to opposite poles at the spindle (f) Telophase begins when the daughter chromosomes have completed their migration Mitosis in a Living Cell Mitosis in a Living Cell • • • • • • • • Phase-contrast optics of a cell of the African blood lily (Haemanthus katherinae) show the stages of mitosis The spindle is barely discernible in these cells, which have been flattened to show all of the chromosomes more clearly (a) Late prophase: the chromosomes have condensed; A clear zone has developed around the nucleus (b) Late prophase – early metaphase: the nuclear envelope has disappeared, and the ends of some of the chromosomes are protruding into the cytoplasm (c) Metaphase: the chromosomes are arranged with their centromeres on the equatorial plane (d) Mid-anaphase: the sister chromatids (now called daughter chromosomes) have separated and are moving to opposite poles of the spindle (e) Late anaphase (f) Telophase – cytokinesis: the daughter chromosomes have reached the opposite poles, and the two chromosome masses have begun the formation of two daughter nuclei; Cell plate formation is nearly complete Dividing Cells in a Root Tip Dividing Cells in a Root Tip • By comparing these cells with the phases of mitosis illustrated in Figures 3—40 and 3—41, you should be able to identify the various mitotic phases shown in this photomicrograph of an onion (Allium) root tip Fully Condensed Chromosome Fully Condensed Chromosome • The chromosomal DNA was replicated during the 5 phase of the cell cycle • Each chromosome now consists of two identical parts, called sister chromatids, which are attached at the centromere, the constricted area in the center • The kinetochores are protein-containing structures, one on each chromatid, associated with the centromere • Attached to the kinetochores are microtubules that form part of the spindle During metaphase, the chromosomes become aligned on the equatorial plane of the mitotic spindle The mitotic spindle consists of a highly organized array of kinetochore microtubules and polar microtubules During anaphase, the sister chromatids separate and, as daughter chromosomes, move to opposite poles of the spindle During telophase, the chromosomes lengthen and become indistinct Mitotic Spindle at Metaphase Mitotic Spindle at Metaphase • The spindle consists of kinetochore microtubules and overlapping polar microtubules • Note that the minus ends of the microtubules are at or near the poles and the plus ends away from the poles • Following a tug-of-war, the chromosomes have come to lie on the equatorial plane Cytokinesis in plants occurs by the formation of a phragmoplast and a cell plate Cell Plate Formation Cell Plate Formation • In plant cells, separation of the daughter chromosomes is followed by formation of a cell plate, which completes the separation of the dividing cells • Here numerous Golgi vesicles can be seen fusing in an early stage of cell plate formation • The two groups of chromosomes on either side of the developing cell plate are at telophase • Arrows point to portions of the nuclear envelope reorganizing around the chromosomes Progressive Stages of Cell Formation Progressive Stages of Cell Formation • These electron micrographs of root cells of lettuce (Lactuca sativa) show the association of the endoplasmic reticulum with the developing cell plate and the origin of plasmodesmata • (a) A relatively early stage of cell plate formation, with numerous small, fusing Golgi vesicles and loosely arranged elements of tubular (smooth) endoplasmic reticulum • (b) An advanced stage of cell plate formation, revealing a persistent close relationship between the endoplasmic reticulum and fusing vesicles • Strands of tubular endoplasmic reticulum become trapped during cell plate consolidation • (c) Mature plasmodesmata, which consist of a plasma-membrane-lined channel and a tubule, the desmotubule, of endoplasmic reticulum Progressive Stages of Cell Formation Progressive Stages of Cell Formation Fluorescence Micrographs of Microtubular Arrays in Root Tip Cells of Onion (Allium cepa) Fluorescence Micrographs of Microtubular Arrays in Root Tip Cells of Onion (Allium cepa) • (a) Prior to formation of the preprophase band of microtubules, most of the microtubules lie just beneath the plasma membrane • (b) A preprophase band of microtubules (arrowheads) encircles the nucleus at the site of the future cell plate • Other microtubules (arrows), forming the prophase spindle, outline the nuclear envelope, which itself is not visible • The lower right-had cell is at a later stage than that above • (c) The mitotic spindle at metaphase • (d) During telophase, new microtubules form a phragmoplast, in which cell plate formation takes place Microtubule Arrays and the Cell Cycle Microtubule Arrays and the Cell Cycle • Changes in the distribution of microtubules during the cell cycle and cell wall formation during cytokinesis • (a) During interphase, and in enlarging and differentiating cells, the microtubules lie just inside the plasma membrane • (b) Just before prophase, a ringlike band of microtubules, the preprophase band, encircles the nucleus in a plane corresponding to the equatorial plane of the future mitotic spindle, and microtubules of the prophase spindle begin to assemble on opposite sides of the nucleus • (c) During metaphase, the microtubules form the mitotic spindle • (d) During telophase, microtubules are organized into a phragmoplast between the two daughter nuclei • The cell plate, made up of fusing Golgi vesicles guided into position by the phragmoplast microtubules, forms at the equator of the phragmoplast • (e) As the cell plate matures in the center of the phragmoplast, the phragmoplast and developing cell plate grow outward until they reach the wall of the dividing cell Microtubule Arrays and the Cell Cycle, continued • (f) During early interphase, microtubules radiate outward from the nuclear envelope into the cytoplasm • (g) Each sister cell forms its own primary wall • (h) With enlargement of the daughter cells (only the upper one is shown here), the mother cell wall is torn • In (g) and (h) the microtubules once more lie just inside the plasma membrane, where they play a role in the orientation of newly forming cellulosic microfibrils Summary of Main Points of this Chapter • The cell is the fundamental unit of life • Cells are of two fundamentally different types: prokaryotic and eukaryotic • Plant cells typically consist of a cell wall and a protoplast • The nucleus is surrounded by a nuclear envelope and contains nucleoplasm, chromatin, and one or more nucleoli • Ribosomes are the sites of protein synthesis • There are three main types of plastids: chloroplasts, chromoplasts, and leucoplasts • Mitochondria are the sites of respiration • Plastids and mitochondria share certain features with prokaryotic cells • Peroxisomes are surrounded by a single membrane Summary of Main Points of this Chapter, continued • Vacuoles perform a variety of functions • The endoplasmic reticulum is an extensive three-dimensional system of membranes with a variety of roles • The Golgi apparatus is a highly polarized membrane system involved in secretion • The cytoskeleton is composed of microtubules and actin filaments • The cell wall is the major distinguishing feature of the plant cell • Dividing eukaryotic cells pass through a regular sequence of events known as the cell cycle • During prophase, the duplicated chromosomes shorten and thicken • Metaphase, anaphase, and telophase followed by cytokinesis result in two daughter cells References • • • Heinze, M., Reichelt, R., Kleff, S., & Eising, R. (2000). High resolution scanning electron microscopy of protein inclusions (cores) purified from peroxisomes of sunflower (Helianthus annuus L.) cotyledons. Crystal Research and Technology, 35(6-7). http://dx.doi.org/10.1002/1521-4079(200007)35:6/7<877::AIDCRAT877>3.0.CO;2-S Hu, J., Baker, A., Bartel, B., Linka, N., Mullen, R. T., Reumann, S., & Zolman, B. K. (2012). Plant peroxisomes: Biogenesis and function. The Plant Cell, 24(6), 2279-2303. http:/?/?dx.?doi.?org/?10.?1105/?tpc.?112.?096586 Also: • Wikipedia, as indicated throughout