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Plant cell
Plant cell
Plant cells are eukaryotic cells that
differ in several key respects 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[1]
[2]
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
Plant cell structure
many cases lignin, are 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.
• Specialised cell-cell communication pathways known as plasmodesmata[3] , pores in the primary cell wall through
which the plasmalemma and endoplasmic reticulum[4] of adjacent cells are continuous.
• Plastids, notably the chloroplasts which contain chlorophyll and the biochemical systems for light harvesting and
photosynthesis, but also 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[5] plastids have their own genomes of about 100-120 unique genes[6] and probably arose as
prokaryotic endosymbionts living in the cells of an early eukaryotic ancestor of the land plants and algae.[7]
• Unlike animal cells, plant cells are stationary.
• 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[8] and the Order
Trentepohliales[9]
• The sperm of bryophytes have flagellae similar to those in animals,[10] [11] but higher plants, (including
Gymnosperms and flowering plants) lack the flagellae and centrioles[12] that are present in animal cells.
Cell types
• Parenchyma cells are living cells that have diverse functions ranging from storage and support to photosynthesis
and phloem loading (transfer cells). Apart from the xylem and phloem in its vascular bundles, leaves are mainly
composed 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 which contain many chloroplasts
and are concerned primarily with photosynthesis are called chlorenchyma cells. Others, such as the majority of
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Plant cell
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 and a secondary 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[13] .
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[14] . 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.[13] 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
The major classes of cells differentiate from undifferentiated
meristematic cells (analogous to the stem cells of animals) to form the
tissue structures of roots, stems, leaves, flowers and reproductive
structures.
Xylem cells[15] are elongated cells with lignified secondary thickening
of the cell walls. Xylem cells are specialised for conduction of water,
and first appeared in plants during their transition to land in the
Silurian period more than 425 million years ago (see Cooksonia). The
possession of xylem defines the vascular plants or Tracheophytes.
cells of Arabidopsis thaliana epidermis
Xylem tracheids are pointed, elongated xylem cells, the simplest of
which have continuous primary cell walls and lignified secondary wall
thickenings in the form of rings, hoops or reticulate networks. More complex tracheids with valve-like perforations
called bordered pits characterise the gymnosperms. The ferns and other pteridophytes and the gymnosperms only
have xylem tracheids, while the angiosperms also have xylem vessels. Vessel members are hollow xylem cells
aligned end-to-end, without end walls that are assembled into long continuous tubes. The bryophytes lack true xylem
cells, but their sporophytes have a water conducting tissue known as the hydrome that is composed of elongated cells
of simpler construction.
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Plant cell
Phloem is a specialised tissue for food conduction in higher plants.The conduction of food is a complex process
which is carried in the plant with the help of special cell called phloem cells. These cells conduct inter and intra
cellular fluid (food:- proteins and other essential element required by the plant for its metabolism) through the
process of osmosis. This phenomena is called as ascent of sap in plants. Phloem consists of two cell types, the sieve
tubes and the intimately-associated companion cells. The sieve tube elements lack nuclei and ribosomes, 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 plates, 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 sugars. The bryophytes lack phloem, but moss sporophytes have a simpler tissue with analogous
function known as the leptome.
Plant epidermal cells are specialised parenchyma cells covering the external surfaces of leaves, stems and roots. The
epidermal cells of aerial organs arise from the superficial layer of cells known as the tunica (L1 and L2 layers) that
covers the plant shoot apex[13] , whereas the cortex and vascular tissues arise from innermost layer of the shoot apex
known as the corpus (L3 layer). 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 cuticle made of 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 trichomes, and the root hairs of primary roots. In the shoot epidermis of most plants, only the 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.[16]
Organelles
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Cell membrane
Cell wall
Nuclear membrane
Plasmodesma
Vacuole
Plastids
Chloroplast
Leucoplast
Chromoplast
Golgi Bodies
Ribosome
Endoplasmic reticulum
Mitochondrion
Lysosome
Cytoplasm
Nucleus
DNA
Chromatin
RNA
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Plant cell
References
[1] JA Raven (1997) The vacuole: a cost-benefit analysis. Advances in Botanical Research 25, 59–86
[2] 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
[3] Oparka, KJ (1993) Signalling via plasmodesmata-the neglected pathway. Seminars in Cell Biogy 4, 131-138
[4] Hepler, PK (1982) Endoplasmic reticulum in the formation of the cell plate and plasmodesmata. Protoplasma 111, 121-133
[5] Anderson S, Bankier AT, et al. (1981) Sequence and organization of the human mitochondrial genome. Nature 290, 4–65
[6] L Cui, N Veeraraghavan, et al. (2006) ChloroplastDB: the chloroplast genome database. Nucleic Acids Research, 34, D692-696
[7] L. Margulis (1970)Origin of eukaryotic cells. Yale University Press, New Haven
[8] Lewis, LA, McCourt, RM (2004) Green algae and the origin of land plants. American Journal of Botany 91, 1535-1556
[9] 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
[10] Manton, I. and Clarke, B. (1952) An electron microscope study of the spermatozoid of Sphagnum. Journal of Experimental Botany 3,
265-275
[11] 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
[12] PH Raven , Evert RF, Eichhorm SE (1999) Biology of Plants, 6th edition. WH Freeman, New York
[13] EG Cutter (1977) Plant Anatomy Part 1. Cells and Tissues. Edward Arnold, London
[14] PA Roelofsen (1959) 'The plant cell wall.' Handbuch fur Pflanzenanatomie. Band III. Gebrüder Borntraeger, Berlin
[15] MT Tyree; MH Zimmermann (2003) Xylem structure and the ascent of sap, 2nd edition, Springer-Verlag, New York USA
[16] 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
External links
• Parts of a plant cell (http://www.smartymaps.com/map.php?s=plantEukaryote)
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