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Plant cells are eukaryotic cells that differ in several key aspects from the cells of other eukaryotic organisms. Their distinctive features include: * A large central vacuole, a water-filled volume enclosed by a membrane known as the ''tonoplast''〔JA Raven (1997) The vacuole: a cost-benefit analysis. Advances in Botanical Research 25, 59–86〕〔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〕 that maintains the cell's turgor, controls movement of molecules between the cytosol and sap, stores useful material and digests waste proteins and organelles. * A cell wall composed of cellulose and hemicellulose, pectin and in many cases lignin, is secreted by the protoplast on the outside of the cell membrane. This contrasts with the cell walls of fungi (which are made of chitin), and of bacteria, which are made of peptidoglycan. * Specialized cell-to-cell communication pathways known as plasmodesmata,〔Oparka, KJ (1993) Signalling via plasmodesmata-the neglected pathway. Seminars in Cell Biology 4, 131–138〕 pores in the primary cell wall through which the plasmalemma and endoplasmic reticulum〔Hepler, PK (1982) Endoplasmic reticulum in the formation of the cell plate and plasmodesmata. Protoplasma 111, 121–133〕 of adjacent cells are continuous. * Plastids, the most notable being the chloroplast, which contains chlorophyll, a green-colored pigment that absorbs sunlight, and allows the plant to make its own food in the process known as photosynthesis. Other types of plastids are the amyloplasts, specialized for starch storage, elaioplasts specialized for fat storage, and chromoplasts specialized for synthesis and storage of pigments. As in mitochondria, which have a genome encoding 37 genes,〔Anderson S, Bankier AT, et al. (1981) Sequence and organization of the human mitochondrial genome. Nature 290, 4–65〕 plastids have their own genomes of about 100–120 unique genes〔L Cui, N Veeraraghavan, et al. (2006) ChloroplastDB: the chloroplast genome database. Nucleic Acids Research, 34, D692-696〕 and, it is presumed, arose as prokaryotic endosymbionts living in the cells of an early eukaryotic ancestor of the land plants and algae.〔L. Margulis (1970) Origin of eukaryotic cells. Yale University Press, New Haven〕 * 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 Charophytes〔Lewis, LA, McCourt, RM (2004) Green algae and the origin of land plants. American Journal of Botany 91, 1535–1556〕 and the Order Trentepohliales〔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〕 * The sperm of bryophytes and pteridophytes, cycads and ''Ginkgo'' have flagella〔Silflow,CD and Lefebvre, PA (2001) Assembly and motility of eukaryotic cilia and flagella. Lessons from ''Chlamydomonas reinhardtii''. Plant Physiology 127, 1500–1507〕 similar to those in animals,〔Manton, I. and Clarke, B. (1952) An electron microscope study of the spermatozoid of ''Sphagnum''. Journal of Experimental Botany 3, 265–275〕〔D.J. Paolillo, Jr. (1967) On the structure of the axoneme in flagella of ''Polytrichum juniperinum''. Transactions of the American Microscopical Society, 86, 428–433〕 but conifers and flowering plants lack the flagella and centrioles〔PH Raven , Evert RF, Eichhorm SE (1999) Biology of Plants, 6th edition. WH Freeman, New York〕 that are present in animal cells. ==Cell types== * Parenchyma cells are living cells that have functions ranging from storage and support to photosynthesis and phloem loading (transfer cells). Apart from the xylem and phloem in their vascular bundles, leaves are composed mainly of parenchyma cells. Some parenchyma cells, as in the epidermis, are specialized for light penetration and focusing or regulation of gas exchange, but others are among the least specialized cells in plant tissue, and may remain totipotent, capable of dividing to produce new populations of undifferentiated cells, throughout their lives. Parenchyma cells have thin, permeable primary walls enabling the transport of small molecules between them, and their cytoplasm is responsible for a wide range of biochemical functions such as nectar secretion, or the manufacture of secondary products that discourage herbivory. Parenchyma cells that contain many chloroplasts and are concerned primarily with photosynthesis are called chlorenchyma cells. Others, such as the majority of the parenchyma cells in potato tubers and the seed cotyledons of legumes, have a storage function. * Collenchyma cells – collenchyma cells are alive at maturity and have only a primary wall. These cells mature from meristem derivatives that initially resemble parenchyma, but differences quickly become apparent. Plastids do not develop, and the 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.〔 Pectin and hemicellulose are the dominant constituents of collenchyma cell walls of dicotyledon angiosperms, which may contain as little as 20% of cellulose in ''Petasites''.〔PA Roelofsen (1959) 'The plant cell wall.' ''Handbuch fur Pflanzenanatomie''. Band III. Gebrüder Borntraeger, Berlin〕 Collenchyma cells are typically quite elongated, and may divide transversely 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 tough and rigid, 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. * 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 – sclereids or stone cells and fibres. 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 (hard cells that give leaves or fruits a gritty texture) include discouraging herbivory, by damaging digestive passages in small insect larval stages, and physical 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.〔EG Cutter (1977) Plant Anatomy Part 1. Cells and Tissues. Edward Arnold, London〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「plant cell」の詳細全文を読む スポンサード リンク
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