Plant cell 23977 224144956 2008-07-07T14:57:12Z Alexf 55327 rvv {{for|the scientific journal|The Plant Cell}} [[Image:Plant cell structure svg.svg|thumb|323px|Plant cell structure]] '''Plant cells''' are [[eukaryote|eukaryotic]] cells that differ in several key respects from the [[cell (biology)|cells]] of other [[eukaryote|eukaryotic]] [[organism]]s. Their distinctive features include: * A large central [[vacuole]], a sap-filled volume enclosed by a membrane known as the ''[[tonoplast]]''<ref name=JRaven>JA Raven (1997) The vacuole: a cost-benefit analysis. Advances in Botanical Research 25, 59–86</ref><ref name=Leigh&Sanders>RA Leigh and D Sanders (1997) Advances in Botanical Research, Vol 25: The Plant Vacuole. Academic Press, California and London. ISBN 0 12 441870-8</ref> maintains the cell's [[turgor]], controls movement of [[molecule]]s between the [[cytosol]] and [[plant sap|sap]], stores useful material and digests waste [[protein]]s and [[organelle]]s. * A [[cell wall]] composed of [[cellulose]] and [[hemicellulose]], [[pectin]] and in many cases [[lignin]], and secreted by the [[protoplast]] on the outside of the [[cell membrane]]. This contrasts with the cell walls of [[fungus|fungi]] (which are made of [[chitin]]), and of [[bacteria]], which are made of [[peptidoglycan]]. * Specialised cell-cell communication pathways known as [[plasmodesmata]]<ref name=Oparka>Oparka, KJ (1993) Signalling via plasmodesmata-the neglected pathway. Seminars in Cell Biogy 4, 131-138</ref>, pores in the primary cell wall through which the [[plasmalemma]] and [[endoplasmic reticulum]]<ref name=Hepler>Hepler, PK (1982) Endoplasmic reticulum in the formation of the cell plate and plasmodesmata. Protoplasma 111, 121-133</ref> of adjacent cells are continuous. * [[Plastid]]s, notably the [[chloroplast]]s which contain [[chlorophyll]] and the biochemical systems for light harvesting and [[photosynthesis]], but also [[amyloplast]]s specialized for [[starch]] storage, [[elaioplast]]s specialized for [[fat]] storage and [[chromoplast]]s specialized for synthesis and storage of [[pigment]]s. As in [[mitochondria]], which have a genome encoding 37 genes<ref name=Andersonetal1981>Anderson S, Bankier AT, et al. (1981) Sequence and organization of the human mitochondrial genome. Nature 290, 4–65</ref> plastids have their own [[genome]]s of about 100-120 unique [[gene]]s<ref name-Cui2006>L Cui, N Veeraraghavan, et al. (2006) ChloroplastDB: the chloroplast genome database. Nucleic Acids Research, 34, D692-696</ref> and probably arose as [[prokaryote|prokaryotic]] [[endosymbiont]]s living in the cells of an early [[eukaryote|eukaryotic]] ancestor of the [[embryophyte|land plants]] and [[algae]].<ref name=Margulis>L. Margulis (1970)Origin of eukaryotic cells. Yale University Press, New Haven</ref> * Cell division by construction of a [[phragmoplast]] as a template for building a [[cell plate]] late in [[cytokinesis]] is characteristic of land plants and a few groups of algae, notably the [[Charophyta|Charophytes]]<ref name=Lewis2004>Lewis, LA, McCourt, RM (2004) Green algae and the origin of land plants. American Journal of Botany 91, 1535-1556</ref> and the Order [[Trentepohliales]]<ref name=LopezBautista>López-Bautista, JM, Waters, DA and Chapman, RL (2003) Phragmoplastin, green algae and the evolution of cytokinesis. International Journal of Systematic and Evolutionary Microbiology 53, 1715-1718</ref> * The sperm of [[Bryophyta|Bryophytes]] have [[flagella]]e similar to those in animals<ref name=Manton>Manton, I. and Clarke, B. (1952) An [[electron microscope]] study of the [[spermatozoid]] of ''[[Sphagnum]]''. Journal of Experimental Botany 3, 265-275</ref><ref name=Paolillo>D.J. Paolillo, Jr. (1967) On the structure of the [[axoneme]] in flagella of ''[[Polytrichaceae|Polytrichum juniperinum]]''. Transactions of the American Microscopical Society, 86, 428-433</ref>, but higher plants, (including [[gymnospermae|Gymnosperms]] and [[flowering plant]]s) lack the [[flagellum|flagella]]e and [[centriole]]s<ref name=raven>PH Raven , Evert RF, Eichhorm SE (1999) Biology of Plants, 6th edition. WH Freeman, New York</ref> that are present in [[Eukaryote#Animal cell|animal cell]]s. ==Cell types== * [[Parenchyma|Parenchyma cells]] are living cells that have diverse functions ranging from storage (storage cell) and support to photosynthesis (chlorenchyma cell) and phloem loading (transfer cell). Some parenchyma cells, as in the epidermis, are specialized for light penetration and focusing or regulation of [[gas exchange]]. Parenchyma cells have thin primary walls which serves for the mediation of materials, and their cytoplasm is responsible for a wide range of biochemical functions such as [[nectar]] [[secretion]], or the manufacture of [[secondary metabolite|secondary products]] that discourage [[herbivory]]. Apart from the xylem and phloem in its vascular bundles, leaves are composed of parenchyma cells. Parenchyma cells which contain many chloroplasts and are concerned primarily with photosynthesis are called chlorenchyma cells. * [[Ground tissue|Collenchyma cells]] - Collenchyma cells are also alive at maturity and have only a primary wall. These cells mature from meristem derivatives. They pass briefly through a stage resembling parenchyma, however they are destined to differentiate into collenchyma, and this fact is quite obvious from the very earliest stages. Plastids do not develop and secretory apparatus (ER and Golgi) proliferates to secrete additional primary wall. The wall is most commonly thickest at the corners, where three or more cells come in contact and thinnest where only two cells come in contact, though other arrangements of the wall thickening are possible<ref name=Cutter/>. Pectin and hemicellulose are the dominant constituents of collenchyma cell walls which may contain as little as 20% of cellulose in ''Petasites''<ref name=Roelofsen>PA Roelofsen (1959) ''The plant cell wall.'' Handbuch fur Pflanzenanatomie. Band III. Gebrüder Borntraeger, Berlin</ref>. Collenchyma cells are typically quite elongated, and may divide transversly to give a septate appearance. The role of this cell type is to support the plant in axes still growing in length, and to confer flexibility and tensile strength on tissues. The primary wall lacks lignin that would make it rigid and brittle, so this cell type provides what could be called plastic support. Support that can hold a young stem or petiole into the air, but in cells that can be stretched as the cells around them elongate. Stretchable support (without elastic snap-back) is a good way to describe what collenchyma does. Parts of the strings in celery are collenchyma. * [[Ground tissue|Sclerenchyma cells]] - Sclerenchyma cells (from the Greek skleros, hard) are hard and tough cells with a function in mechanical support. They are of two broad types - [[sclereid]]s or stone cells and [[fibre]]s. The cells develop an extensive secondary cell wall that is laid down on the inside of the [[primary cell wall]]. The secondary wall is impregnated with [[lignin]], making it hard and impermeable to water. Thus, these cells cannot survive for long as they cannot exchange sufficient material to maintain active metabolism. Sclerenchyma cells are typically dead at functional maturity, and the cytoplasm is missing, leaving an empty central cavity. Functions for sclereid cells include discouraging herbivory (hard cells that give leaves a gritty texture, or rip open digestive passages in small insect larval stages), protection (a solid tissue of hard sclereid cells form the pit wall in a peach and many other fruits). Functions of fibres include provision of load-bearing support and tensile strength to the leaves and stems of herbaceous plants.<ref name=Cutter>EG Cutter (1977) Plant Anatomy Part 1. Cells and Tissues. Edward Arnold, London</ref> Sclerenchyma fibres are not involved in conduction, either of water and nutrients (as in the [[xylem]]) or of carbon compounds (as in the [[phloem]]), but it is likely that they may have evolved as modifications of xylem and phloem initials in early land plants. ==Tissue types== [[Image:Arabidopsis-epiderm-conidiospore-hyaloperonospora-parasitica.jpg|thumb|right|cells of ''[[Arabidopsis]]'' [[Epidermis (botany)|epidermis]]]] The major classes of cells differentiate from undifferentiated [[meristem]]atic cells (analogous to the stem cells of animals) to form the tissue structures of [[root]]s, [[plant stem|stems]], [[leaf|leaves]], [[flower]]s and reproductive structures. [[Xylem]] cells<ref name=Tyree>MT Tyree; MH Zimmermann (2003) Xylem structure and the ascent of sap, 2nd edition, Springer-Verlag, New York USA</ref> are elongated cells with lignified secondary thickening of the cell walls. The bryophytes lack true xylem cells, but the [[sporophyte]]s of bryophytes have a water conducting tissue known as the hydrome that is composed of elongated cells of simpler construction. Xylem cells are specialised for conduction of water, and first appeared in plants during their transition to land in the [[Silurian]] period (see ''[[Cooksonia]]''). They characterise the [[vascular plants]] or [[Tracheophyte]]s. Xylem tracheids are pointed, elongated xylem cells that have continuous primary cell walls. The [[fern]]s and other [[pteridophyte]]s and the [[gymnosperm]]s only have xylem [[tracheid]]s, while the [[angiosperms]] also have [[vessel element|xylem vessel]]s. Vessel members are hollow xylem cells aligned end-to-end, without end walls or perforate that may be assembled into long continuous tubes. [[Phloem]] is a specialised tissue for food conduction in higher plants. The [[bryophyte]]s lack phloem, but [[moss]] [[sporophyte]]s have a simpler tissue with analogous function known as the leptome. Phloem consists of two cell types, the [[sieve tube element|sieve tubes]] and the intimately-associated [[companion cell]]s. The [[sieve tube element]]s lack [[cell nucleus|nuclei]] and [[ribosome]]s, and their metabolism and functions are regulated by the adjacent nucleate companion cells. Sieve tubes are joined end to end with perforate end-plates between known as ''[[sieve plate]]s'', which allow transport of photosynthate between the sieve elements. The companion cells, connected to the sieve tubes via [[plasmodesmata]], are responsible for loading the phloem with [[sugar]]s. Plant [[Epidermis (botany)|epidermal cells]] are specialised parenchyma cells covering the external surfaces of stems and other aerial organs and roots. The epidermal cells arise from the superficial layer of cells known as the ''tunica'' that covers the plant [[meristem|shoot apex]]<ref name=Cutter/>, whereas the cortex and vascular tissues arise from innermost layers of the shoot apex. The epidermis of roots originates from the layer of cells immediately beneath the root cap. The epidermis of all aerial organs, but not roots, is covered with a [[plant cuticle|cuticle]] made of [[epicuticular wax|waxes]] and the [[polyester]] [[cutin]]. Several cell types may be present in the epidermis. Notable among these are the stomatal guard cells, glandular and clothing hairs or [[trichome]]s, and the [[root hair]]s of primary roots. In the epidermis of most plants, only the [[stomata|guard cells]] have chloroplasts. The epidermal cells of the primary shoot are thought to be the only plant cells with the biochemical capacity to synthesize cutin.<ref name="Kolattukudy 1996">Kolattukudy, PE (1996) Biosynthetic pathways of cutin and waxes, and their sensitivity to environmental stresses. In: Plant Cuticles. Ed. by G. Kerstiens, BIOS Scientific publishers Ltd., Oxford, pp 83-108</ref> ==Parts== *[[Cell membrane]] *[[Cell wall]] *[[Plasmodesma]] *[[Vacuole]] *Tonoplast *Crystal *[[Plastids]] *[[Chloroplast]] *[[Leucoplast]] *[[Chromoplast]] *[[Golgi apparatus|Golgi Complex]] *[[Ribosome]] *[[Endoplasmic reticulum]] *[[Mitochondrion]] *Microtubule *Microfilament *[[Lysosome]] *Microbody *[[Cytoplasm]] *[[Cell nucleus|Nucleus]] *Nuclear envelope (membrane) *Nuclear pore *[[DNA]] *[[Chromatin]] *[[RNA]] *Messenger RNA *Transfer RNA ==References== {{Reflist}} {{Botany}} [[Category:Plant anatomy]] [[ar:خلية نباتية]] [[es:Célula vegetal]] [[fr:Cellule végétale]] [[gl:Célula eucariota vexetal]] [[id:Sel tumbuhan]] [[it:Cellula vegetale]] [[lt:Augalinė ląstelė]] [[nl:Plantaardige cel]] [[pl:Komórka roślinna]] [[pt:Célula vegetal]] [[ro:Celulă vegetală]] [[sk:Rastlinná bunka]] [[sr:Биљна ћелија]] [[sv:Växtcell]] [[vi:Tế bào thực vật]]