Saw Tarasut
วันเสาร์ที่ 16 กุมภาพันธ์ พ.ศ. 2556
The Amazing Dots
Watch the X in the middle very closely. You should start to see a green dot that rotates around the circle - this dot is an illusion; then you should see see the purple dots disappear.... but they haven't really gone. It is an after image effect, sometimes called a 'negative retinal afterimage' - move your head slightly, and the dots will reappear... amazing or what?
Optical illusions
Head On Sand
A man lying on the sand with his head beside him in this illusionary image.
Rock Faces Illusion
Optical illusion of depressed faces in the rocks.
Cat Optical Illusion
Optical illusion of a mouse and a cat.
The Spinning Silhouette Optical Illusion
This is a very bizarre dancer optical illusion. At first glance the spinning lady appears to be circling in a clockwise motion. But if you look at the spinning dancer long enough - the lady will appear to change direction and start spinning counter-clockwise.
Mirage(ภาพลวงตา)
If you take a look at the following picture , let me tell you ... it is not animated. Your eyes are making it move. To test this, stare at one spot for a couple seconds and everything will stop moving. Or look at the black center of each circle and it will stop moving. But move your eyes to the next black center and the previous will move after you take your eyes away from it.... Weird
They look pretty normal, right? Now look at these
วันจันทร์ที่ 21 มกราคม พ.ศ. 2556
Plant cell
Plant cell
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For the scientific journal, see The Plant Cell.
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[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 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,[3] pores in the primary cell wall through which the plasmalemma and endoplasmic reticulum[4] of adjacent cells are continuous.
- Plastids, the most notable being the chloroplasts, which contain chlorophyll a green coloured pigment which is used for absorbing sunlight and is used by a plant to make its own food in the process is known as photosynthesis. Other types of plastid 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,[5] plastids have their own genomes of about 100–120 unique genes[6] and, it is presumed, arose as prokaryotic endosymbionts living in the cells of an early eukaryotic ancestor of the land plants and algae.[7]
- 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 and pteridophytes 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.
Contents[hide] |
[edit] 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 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.[13]
- 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.
[edit] 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. 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 have only 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.
Phloem is a specialised tissue for food conduction in higher plants. The conduction of food is a complex process that 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 elements required by the plant for its metabolism) through the process of osmosis. This phenomenon is called 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 the polyester cutin with a superficial layer of waxes. The epidermal cells of the primary shoot are thought to be the only plant cells with the biochemical capacity to synthesize cutin.[16] 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.
[edit] Organelles
- Cell membrane
- Cell wall
- Nuclear membrane
- Plasmodesma
- Vacuole
- Plastids
- Chloroplast
- Leucoplast
- Chromoplast
- Golgi Bodies
- Ribosome
- Endoplasmic reticulum
- Mitochondria
- Lysosome
- Cytoplasm
- Nucleus
- DNA
- Chromatin
- RNA
- Cytoskeleton
- Nucleolus
[edit] References
- ^ 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
- ^ Oparka, KJ (1993) Signalling via plasmodesmata-the neglected pathway. Seminars in Cell Biogy 4, 131–138
- ^ Hepler, PK (1982) Endoplasmic reticulum in the formation of the cell plate and plasmodesmata. Protoplasma 111, 121–133
- ^ Anderson S, Bankier AT, et al. (1981) Sequence and organization of the human mitochondrial genome. Nature 290, 4–65
- ^ L Cui, N Veeraraghavan, et al. (2006) ChloroplastDB: the chloroplast genome database. Nucleic Acids Research, 34, D692-696
- ^ L. Margulis (1970) Origin of eukaryotic cells. Yale University Press, New Haven
- ^ Lewis, LA, McCourt, RM (2004) Green algae and the origin of land plants. American Journal of Botany 91, 1535–1556
- ^ 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
- ^ 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
- ^ PH Raven , Evert RF, Eichhorm SE (1999) Biology of Plants, 6th edition. WH Freeman, New York
- ^ a b c EG Cutter (1977) Plant Anatomy Part 1. Cells and Tissues. Edward Arnold, London
- ^ PA Roelofsen (1959) 'The plant cell wall.' Handbuch fur Pflanzenanatomie. Band III. Gebrüder Borntraeger, Berlin
- ^ MT Tyree; MH Zimmermann (2003) Xylem structure and the ascent of sap, 2nd edition, Springer-Verlag, New York USA
- ^ 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
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