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Anthocerophyta

Anthocerophyta

:This is an article about the non-vascular plants. The name Hornwort is also often applied to the aquatic plant Ceratophyllum demersum in the family Ceratophyllaceae Anthocerotaceae
- Anthoceros
- Folioceros
- Leiosporoceros
- Phaeoceros
- Sphaerosporoceros Dendrocerotaceae
- Dendroceros
- Megaceros
- Notoceros Notothyladaceae
- Notothylas Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant. Hornworts may be found world-wide, though they tend to grow only in places that are damp or humid. Some species grow in large numbers as tiny weeds in the soil of gardens and cultivated fields. Large tropical and sub-tropical species of Dendroceros may be found growing on the bark of trees.

Description

The plant body of a hornwort is a haploid gametophyte stage. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter. Each cell of the thallus usually contains just one chloroplast per cell. In most species, this chloroplast is fused with other organelles to form a large pyrenoid that both manufactures and stores food. This particular feature is very unusual in land plants, but is common among algae. Many hornworts develop internal mucilage-filled cavities when groups of cells break down. These cavities are invaded by photosynthetic cyanobacteria, especially species of Nostoc. Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color. There may also be small slime pores on the underside of the thallus. These pores superficially resemble the stomata of other plants. stomata The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. Hornworts sporophytes are unusual in that the sporophyte grows from a meristem near its base, instead of from its tip the way other plants do. Unlike liverworts, most hornworts have true stomata on the sporophyte as mosses do. The exceptions are the genera Notothylas and Megaceros, which do not have stomata. When the sporophyte is mature, it has a multicellular outer layer, a central rod-like columella running up the center, and a layer of tissue in between that produces spores and pseudo-elaters. The pseudo-elaters are multi-cellular, unlike the elaters of liverworts. They have helical thickenings that change shape in response to drying out, and thereby twist in and thereby help to disperse the spores. Hornwort spores are relatively large for bryophytes, measuring between 30 and 80 um in diameter or more. The spores are polar, usually with a distinctive Y-shaped tri-radiate ridge on the proximal surface, and with a distal surface ornamented with bumps or spines.

Life cycle

The life of a hornwort starts from a haploid spore. In most species, there is a single cell inside the spore, and a slender extension of this cell called the germ tube germinates from the proximal side of the spore. The tip of the germ tube divides to form an octant of cells, and the first rhizoid grows as an extension of the original germ cell. The tip continues to divide new cells, which produces a thalloid protonema. By contrast, species of the family Dendrocerotaceae may begin dividing within the spore, becoming multicellular and even photosynthetic before the spore germinates. In either case, the protonema is a transitory stage in the life of a hornwort. photosynthetic From the protonema grows the adult gametophyte, which is the persistent and independent stage in the life cycle. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter, and several layers of cells in thickness. It is green or yellow-green from the chlorophyll in its cells, or bluish-green when colonies of cyanobacteria grow inside the plant. When the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants are monoicous, with both sex organs on the same plant, but some plants (even within the same species) are dioicous, with separate male and female gametophytes. The female organs are known as archegonia (singular archegonium) and the male organs are known as antheridia (singular antheridium). Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells. The biflagellate sperm must swim from the antheridia, or else be splashed to the archegonia. When this happens, the sperm and egg cell fuse to form a zygote, the cell from which the sporophyte stage of the life cycle will develop. Unlike all other bryophytes, the first cell division of the zygote is longitudinal. Further divisions produce three basic regions of the sporophyte. At the bottom of the sporophyte (closest to the interior of the gametophyte), is a foot. This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence. In the middle of the sporophyte (just above the foot), is a meristem that will continue to divide and produce new cells for the third region. This third region is the capsule. Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce pseudo-elaters and spores. These are released from the capsule when it splits lengthwise from the tip.

Classification of Hornworts

Hornworts were traditionally considered a class within the Division Bryophyta (bryophytes). However, it now appears that this group is paraphyletic, so the hornworts tend to be given their own division, called Anthocerotophyta. The Bryophyta is now restricted to include only mosses. There is a single class of hornworts, called Anthocerotopsida, or traditionally Anthocerotae. This class includes a single order of hornworts (Anthocerotales) in this classification scheme. In some other classification schemes, a second order Notothyladales (containing only the genus Notothylas) is recognized because of the unique and unusual features present in that group. Among land plants, hornworts appear to be one of the oldest surviving groups. There are only about 100 species known, but new species are still being discovered. The number and names of genera are a current matter of investigation, and several competing classification schemes have been published since 1988. genera

Families and genera

Anthocerotaceae
- Anthoceros
- Folioceros
- Leiosporoceros
- Phaeoceros
- Sphaerosporoceros Dendrocerotaceae
- Dendroceros
- Megaceros
- Notoceros Notothyladaceae
- Notothylas

References

- Chopra, R. N. & Kumra, P. K. (1988). Biology of Bryophytes. New York: John Wiley & Sons. ISBN 0-470-21359-0.
- Grolle, Riclef (1983). "Nomina generica Hepaticarum; references, types and synonymies". Acta Botanica Fennica 121, 1-62.
- Hasegawa, J. (1994). "New classification of Anthocerotae". J. Hattori Bot. Lab 76: 21-34.
- Renzaglia, Karen S. (1978). "A comparative morphology and developmental anatomy of the Anthocerotophyta". J. Hattori Bot. Lab 44: 31-90.
- Renzaglia, Karen S. & Vaughn, Kevin C. (2000). Anatomy, development, and classification of hornworts. In A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology, pp. 1-20. Cambridge: Cambridge University Press. ISBN 0-521-66097-1.
- Schofield, W. B. (1985). Introduction to Bryology. New York: Macmillan.
- Schuster, Rudolf M. (1992). The Hepaticae and Anthocerotae of North America, East of the Hundredth Meridian, Volume VI. Chicago: Field Museum of Natural History.
- Smith, Gilbert M. (1938). Cryptogamic Botany, Volume II: Bryophytes and Pteridophytes. New York: McGraw-Hill Book Company.
- Watson, E. V. (1971). The Structure and Life of Bryophytes (3rd ed.). London: Hutchinson University Library. ISBN 0-09-109301-5.

External links

- [http://www3.uakron.edu/biology/hornworts/hornworts.html Hornwort Web Portal]
- [http://koning.ecsu.ctstateu.edu/Plant_Biology/hornwort.html Hornwort biology information]
- [http://www.ucmp.berkeley.edu/plants/anthocerotophyta.html Anthocerotophyta description and fossil history at UCMP]
- [http://www.natureserve.org/explorer/speciesIndex/Class_Anthocerotopsida_106589_1.htm Hornwort species in the United States and Canada]
- [http://www.peripatus.gen.nz/Taxa/Bryophyta/NZAnthocerotae.html New Zealand Anthocerotae] ---- Category: Plants Category: Bryophytes Category: cryptogams

Ceratophyllaceae

C. demersum
C. submersum Ceratophyllum is a cosmopolitan genus of flowering plants, commonly found in ponds, marshes, and quiet streams in tropical and in temperate regions. They are usually called hornworts, although this name is also used for unrelated plants of the division Anthocerotophyta. Ceratophyllum grows completely submerged, usually though not always floating on the surface. They do not tolerate drought. At intervals along nodes of the stem they produce rings of bright green leaves, which are narrow and often branched. The forked leaves feel brittle and stiff to the touch. The plants have no roots at all, but sometimes they develop modified leaves with a rootlike appearance, which anchor the plant to the bottom. The flowers are small and don't attract the attention, with the male and female flowers on the same plant. Because of their appearance and their high oxygen production, they are often used in freshwater aquaria. Hornwort plants float in great numbers just under the surface. They offer excellent protection to fish-spawn, but also to snails, infected with bilharzia. By screening the lighting, they obstruct algal growth. Hornwort, also known as coontail, is a very easy to grow plant that is tolerant of most water conditions and temperatures. It makes an excellent, tough to kill beginner's plant. Ceratophyllum is unique enough to warrant its own family, the Ceratophyllaceae, and in newer systems its own order, the Ceratophyllales. In APG it has a place of its own, just basal to the eudicots. The division into species is not completely settled. There are two main species:
- Ceratophyllum demersum - Common Hornwort or Rigid Hornwort
- Ceratophyllum submersum - Tropical Hornwort More than 30 other species have been described, but many of them are probably mere variants of these.
eudicots eudicots Category:Magnoliopsida Category:Aquatic plants

Anthoceros

see text. Anthoceros is a genus of hornworts in the family Anthocerotaceae. The genus is global in its distribution. Its name means 'flower horn', and refers to the characteristic horn-shaped sporophytes that all hornworts produce. The dark color of the spores is the easiest way to distinguish Anthoceros from the related genus Phaeoceros, which produces spores that are yellow. The genus is distinguished by having spores that are dark brown to black, a relatively frilly thallus when compared to Phaeoceros, and larger and more internal cavities than Phaeoceros.

References

- Proskauer, J. (1951). "Studies on Anthocerotales. III". Bull. Torrey Bot. Club 78: 331-349.

External links

---- Category: Bryophytes Category: cryptogams

Folioceros

Folioceros appendiculatus
Folioceros assamicus
Folioceros dixitianus
Folioceros fuciformis
Folioceros glandulosus
Folioceros indicus
Folioceros kashyapii
Folioceros mamillisporus
Folioceros mangaloreus
Folioceros paliformis
Folioceros physocladus
Folioceros satpurensis
Folioceros spinisporus
Folioceros udarii
Folioceros vesiculosus Folioceros is a genus of hornworts in the family Anthocerotaceae. The genus is common locally in the tropical and subtropical regions of Asia, growing on moist rocks, in fallow fields, and near waterfalls. It has a yellow-green gametophyte thallus that is crispy and translucent, with short branchings that are almost pinnate. Plants are usually less than a centimeter wide and 3 centimeters long. They may be monoicous or dioicous. The genus Folioceros was formally diagnosed by the botanist D. C. Bharadwaj (1971) and based on the type species F. assamicus. Some features that he cited as distinguishing the genus were:
- Pseudoelaters less than 7 um wide and more than 300 um long.
- Spore ornamentation that is spinose or baculate, rather than reticulate.
- Thallus with large cavities formed by splitting of the internal tissue. The classification system of Hässel de Menendez (1988) places Folioceros in its own family Foliocerotaceae and order Foliocerotales. This classification is based on a cladistic morphological analysis, but has not been generally accepted or supported by additional research in the literature. For the present, Folioceros is usually placed in the Anthocerotaceae.

References

- Asthana, A. K. & Srivastava, S. C. (1991). Indian Hornworts (A Taxonomic Study). J. Cramer: Bryophytorum Bibliotheca, Band 42. ISBN 3-443-62014-0.
- Bharadwaj, D. C. (1971). On Folioceros, A New Genus of Anthocerotales. Geophytology 1 (1): 6-15.
- Hässel de Menendez, G. G. (1988). A proposal for a new classification of the genera within the Anthocerotophyta. J. Hattori Bot. Lab. 64: 71-86.
- Zhu, R. L. & So, M. L. (1996). Mosses and Liverworts of Hong Kong, volume 2. Hong Kong: Heavenly People Depot. ISBN 962-7350-80-X.

External links

[http://bryophytes.plant.siu.edu/folipic.html Images of Folioceros] ---- Category: Bryophytes Category: cryptogams

Dendroceros

See text. Dendroceros is a genus in family Dendrocerotaceae of order Anthocerotales, a group of hornworts. The genus contains about 51 species in tropical and sub-tropical regions of the world.

Description of Dendroceros

The gametophyte is yellowish-green and usually less than one-half cm wide. The thallus branches in a bifurcating pattern. In the subgenus Apoceros, there are cavities in the central strand of the thallus. The edges of the thallus are only a single layer of cells thick and have an undulating margin. It is common to find symbiotic colonies of the blue-green bacteria Nostoc growing among the cells. Under a microscope, the epidermal cells have trigones. The sporophyte is erect when mature, growing up to 5 cm tall. Like other hornworts, its surface has stomata. The interior of the sporophyte differentiates into a central column and a surrounding mass of spores and elater cells, with a distinct spiral. The spores are both green and relatively large with an ornamented surface.

Habitat

Dendroceros grows on humid ground, rocky outcrops, and on the sides of trees. Its name literally means "tree horn". ---- Category: Bryophytes Category: cryptogams

Notothyladaceae

Notothylas anaporata
Notothylas breutellii
Notothylas chaudhurii
Notothylas dissecta
Notothylas flabellata
Notothylas himalayensis
Notothylas indica
Notothylas japonicus
Notothylas javanicus
Notothylas khasiana
Notothylas levieri
Notothylas orbicularis
Notothylas pandei
Notothylas pfleidereri Notothylas is a genus of hornworts in the family Notothyladaceae. The genus is found globally, but is usually overlooked. It is the smallest of all the hornworts, with a yellow-green gametophyte thallus that is seldom more than a centimeter in diameter, and usually much smaller. The genus Notothylas is also unusual among hornworts in that the sporophyte is bullet-shaped and does not grow very large (less than two millimeters). The sporophytes grow outwards rather than upwards, and like Megaceros, there are no stomata on the surface of the sporophyte. The elater cells do not grow helical thickenings. A number of classification systems place Notothylas in its own order Notothyladales (frequently misspelled Notothylales in the literature). This classification is based on the assumption that the unique physical characteristics of the genus reflect an early divergence from other hornworts. However, this assumption has not yet been rigorously tested or supported by either phylogenetic analysis or fossil evidence. For the present, Notothylas is usually placed in the Anthocerotales with all other species of hornworts.

References

- Asthana, A. K. & Srivastava, S. C. (1991). Indian Hornworts (A Taxonomic Study). J. Cramer: Bryophytorum Bibliotheca, Band 42. ISBN 3-443-62014-0.
- Hässel de Menendez, G. G. (1988). A proposal for a new classification of the genera within the Anthocerotophyta. J. Hattori Bot. Lab. 64: 71-86.
- Hicks, Marie L. (1992). Guide to the Liverworts of North Carolina. Durham, NC: Duke University Press. ISBN 0-8223-1175-5.
- Prihar, N. S. (1961). An Introduction to Embryophyta, Volume I. Allahbad: Central Book Depot. ---- Category: Bryophytes Category: cryptogams

Bryophyte

Bryophytes are embryophyte plants ('land plants') that are nevertheless non-vascular: they have tissues and enclosed reproductive systems, but they lack vascular tissue that circulates liquids. They neither flower nor produce seeds, reproducing via spores.

Bryophyte classification

spore There are three groups, the Marchantiophyta (liverworts), Anthocerotophyta (hornworts), and Bryophyta (mosses). Modern studies generally show one of two patterns. In one of these patterns, the liverworts were the first to diverge, followed by the hornworts, while the mosses are the closest living relatives of the vascular plants. In the other pattern, the hornworts were the first to diverge, followed by the vascular plants, while the mosses are the closest living relatives of the liverworts. Originally the three groups were brought together as the three classes of division Bryophyta. However, since the three groups of bryophytes form a paraphyletic group, they now are placed in three separate divisions.

Bryophyte sexuality

These plants are generally gametophyte-oriented; that is, the normal plant is the haploid gametophyte, with the only diploid structure being the sporangium in season. As a result, bryophyte sexuality is very different from that of other plants. There are two basic categories of sexuality in bryophytes:
- dioicous - These plants produce only antheridia (male organs) or archegonia (female organ) on a single plant body.
- monoicous - These plants produce both antheridia and archegonia on the same plant body. Some bryophyte species may be either monoicous or dioicous depending on environmental conditions. Other species grow exclusively with one type of sexuality. Notice that these terms are not the same as monoecious and dioecious, which refer to whether or not a sporophyte plant bears one or both kinds of gametophyte. Those terms apply only to seed plants.

References

- Chopra, R. N. & Kumra, P. K. (1988). Biology of Bryophytes. New York: John Wiley & Sons. ISBN 0-470-21359-0.
- Crum, Howard (2001). Structural Diversity of Bryophytes. Ann Arbor: University of Michigan Herbarium. ISBN 0-9620733-4-2.
- Goffinet, Bernard. (2000). Origin and phylogenetic relationships of bryophytes. In A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology, pp. 124-149. Cambridge: Cambridge University Press. ISBN 0-521-66097-1.
- Oostendorp, Cora (1987). The Bryophytes of the Palaeozoic and the Mesozoic. Bryophytorum Bibliotheca, Band 34. Berlin & Stuttgart: J. Cramer. ISBN 3-443-62006-X.
- Prihar, N. S. (1961). An Introduction to Embryophyta: Volume I, Bryophyta (4th ed.). Allahabad: Central Book Depot.
- Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (2005). Biology of Plants (7th ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1007-2.
- Schofield, W. B. (1985). Introduction to Bryology. New York: Macmillan. ISBN 0-02-949660-8.
- Watson, E. V. (1971). The Structure and Life of Bryophytes (3rd ed.). London: Hutchinson University Library. ISBN 0-09-109301-5. sort11 Bryophyta Category: Bryophytes Category: cryptogams ja:センタイ類 simple:Bryophyta

Sporophyte

A sporophyte is the diploid structure or phase of life of a sexually reproducing plant. Each living cell of the sporophyte contains two complete sets of chromosomes. The sporophyte is the dominant life form in ferns, gymnosperms, and angiosperms (flowering plants). In plants that undergo alternation of generations, the sporophyte produces haploid spores that develop into a gametophyte. Through mitosis, the gametophyte produces a zygote that becomes the sporophyte. In some plants, the sporophyte is initially parasitic on the gametophyte for a time. Category:Plant morphology Category:Botany

Haploid

Ploidy indicates the number of copies of the basic number of chromosomes. The number of basic sets of chromosomes in an organism is called the monoploid number (x). The ploidy of cells can vary within an organism. In humans, most cells are diploid (containing one set of chromosomes from each parent), though sex cells (sperm and oocytes) are haploid. In contrast, tetraploidy (four sets of chromosomes), a type of polyploidy, is not uncommon in healthy plant species. Euploidy, or the euploid number, is a species' normal number of chromosomes per cell. For example, the euploid number of chromosomes in a human cell is 46.

Haploid

Haploid (meaning simple in Greek) cells bear one copy of each chromosome. Most fungi, and a few algae exist as haploid organisms, male bees, wasps and ants are also haploid. For organisms that only ever have one set of chromosomes, the term monoploid can be used interchangeably with haploid. Plants and other algae switch between a haploid and a diploid or polyploid state, with one of the stages emphasized over the other. This is called alternation of generations. Most diploid organisms produce haploid sex cells that can combine to form a diploid zygote, for example animals are primarily diploid but produce haploid gametes. During meiosis, germ cell precursors have their number of chromosomes halved by randomly "choosing" one homologue, resulting in haploid germ cells (sperm and ovum).

Diploid

Diploid cells (meaning double in Greek) have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. Most somatic cells (body cells) of higher organisms are diploid.

Haplodiploidy

A haplodiploid species is one in which one of the sexes has haploid cells and the other has diploid cells. Most commonly, the male is haploid and the female is diploid. In such species, the male develops from unfertilized eggs, a process called arrhenotokous parthenogenesis or simply arrhenotoky, while the female develops from fertilized eggs: the sperm provides a second set of chromosomes when it fertilizes the egg. Haplodiploidy is found in many species of insects from the order Hymenoptera, particularly ants, bees, and wasps. It increases the significance of kin selection, which may explain the eusociality of these sorts of insects. In some Hymenopteran species, worker insects are also able to produce diploid (and therefore female) fertile offspring, which develop as normal queens. The second set of chromosomes comes not from sperm, but from one of the three polar bodies during anaphase II of meiosis. This process is called thelytokous parthenogenesis or simply thelytoky.

Haploidisation

Haploidisation (from the Greek απλοποίηση = simplification) is the process of creating a haploid cell from a diploid cell. This is a laboratory procedure that forces a normal cell to spit out half of its chromosome content, leaving just one set. In mammals this renders this cell equal to sperm or egg. This was one of the procedures used by Japanese researchers to produce Kaguya the fatherless mouse.

Aneuploidy

Aneuploidy is when a cell contains an abnormal or non-integer ploidy number. This may lead to problems in cell development. Most forms of aneuploidy in humans are lethal, but trisomy (three copies) of the sex chromosome (the cause of Klinefelter's syndrome and others) and of chromosome 21 (the cause of Down syndrome) are relatively common. Many forms of cancer have incorrect ploidy numbers, due to the accumulation of mutations which increase chromosome missegregation.

Polyploidy

Polyploidy is the state where all cells have multiple pairs of chromosomes beyond the basic set. These may be from the same species or from closely related species. In the later case these are known as allopolyploids (also known as amphidiploids or allotetraploids), that are formed from the hybridisation of two separate species followed by their subsequent chromosome doubling. A good examples is the so called Brassica triangle where three different parent species have hybridized in each pair combination to form three different allopolyploid species. Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms many somatic cells are polyploid due to a process called endoreduplication where duplication of the genome occurs without mitosis (cell division).

Variable or Indefinite Ploidy

Depending on growth conditions, prokaryotes such as bacteria may have a chromosome copy number of 1 to 4, and that number is commonly fractional, counting portions of the chromosome partly replicated at a given time. This is because such organisms tend to multiply continuously.

References

Category:Classical genetics

Gametophyte

A gametophyte is the haploid structure or phase of life of a sexually-reproducing plant. Each cell of a gametophyte contains one complete set of chromosomes. The gametophyte can be the dominant part of the plant's life cycle as in mosses, or very reduced as in ferns and flowering plants (angiosperms), where the female form (ovule) is known as a megagametophyte and the male form (pollen) is called a microgametophyte, the dominant phase of life for bryophytes (nonvascular plants). An early developmental stage in the gametophyte of both mosses and ferns (immediately following the meiospore) is called the Protonema. See also : Sporophyte, Alternation of generations, Archegonium, Antheridium Category:Botany

Rosette (botany)

In botany, a rosette indicates a circular arrangement of the leaves, with all the leaves at a single height. Often, perennial plants whose regular foliage dies down to the ground in winter retain a rosette of leaves at the soil's surface. Category: plant morphology

Centimeter

To help compare different orders of magnitude this page lists lengths between 10^-2 m and 10^-1 m (1 cm and 10 cm).
- Distances shorter than 1 cm
- 1.0 cm is equal to
- 10 millimetres
- .39 inches
- edge of square of area 1 cm²
- edge of cube of volume 1 ml
- 1.5 cm — length of an average mosquito.
- 2.54 cm — 1 inch
- 3.1 cm — 1 attoparsec (10^-18 parsecs)
- 35 mm — width of film commonly used in motion pictures and still photography
- 42.7 mm — diameter of a golfball
- Distances longer than 10 cm

Chloroplast

Chloroplasts are organelles found in plant cells and eukaryotic algae which conduct photosynthesis. Chloroplasts capture light energy from the sun to produce the free energy stored in ATP and NADPH through a process called photosynthesis.

Origins

Chloroplasts are one of the forms a plastid may take, and are generally considered to have originated as endosymbiotic cyanobacteria. In this respect they are similar to mitochondria, but are found only in plants and protista. Both organelles are surrounded by a double membrane with an intermembrane space; both have their own DNA and are involved in energy metabolism; and both have reticulations, or many infoldings, filling their inner spaces. In green plants chloroplasts are surrounded by two lipid-bilayer membranes. The inner membrane is now thought to correspond to the outer membrane of the ancestral cyanobacterium. The chloroplast genome is considerably reduced compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities. Many of the missing genes are encoded in the nuclear genome of the plant, algae or protist. It is interesting to note that in some algae (such as the heterokonts and other protists such as Euglenozoa and Cercozoa), chloroplasts seem to have arisen through a secondary event of endosymbiosis, in which a eukaryotic cell engulfed a second eukaryotic cell containing chloroplasts, forming chloroplasts with three or four membrane layers. In some cases, such secondary endosymbionts have themselves been engulfed by still other eukaryotes, forming tertiary endosymbionts.

Structure

Cercozoa The fluid within the chloroplast is called the stroma, corresponding to the cytoplasm of the bacterium, and contains tiny circular DNA and ribosomes, though most of their proteins are encoded by genes contained in the cell nucleus, with the protein products trafficked to the chloroplast. Within the stroma are stacks of thylakoids, the sub-organelles where photosynthesis actually takes place. A stack of thylakoids is called a granum. A thylakoid looks like a flattened disk, and inside is an empty area called the thylakoid space or lumen. The photosynthesis reaction takes place on the membrane of the thylakoid, and, as is also the case with mitochondria, involves the coupling of cross-membrane fluxes with biosynthesis.

Biochemistry

The photosynthetic proteins in the membrane bind chlorophyll, which is present with various accessory pigments. These give chloroplasts their green color. During autumn, the removal of chlorophyll from plant leaves exposes red and yellow pigments (such as xanthophyll) which were previously masked. Algal chloroplasts may be golden, brown, or red and show variation in the number of membranes and the presence of thylakoids. Pigments undergo electronic excitations driven by the absorption of sunlight — red and blue for chlorophyll. The green we see is the color not absorbed. The energy released by the electronically-excited pigments as they return to their ground state is the basis for the energy captured by photosynthesis to produce ATP and NADPH and the ultimate formation of sugars. Energy of the absorbed photons not used to produce chemical energy is eventually given off to the surroundings. Thus, chloroplasts are small heat engines operating between the hot light from the sun and the lower ambient molecular temperature. (Photovoltaic cells do likewise.)

References

External links

- [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chloroplasts.html Chloroplasts] and [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LightReactions.html Photosynthesis: The Role of Light] from [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/ Kimball's Biology Pages]
- [http://reference.allrefer.com/encyclopedia/C/chloropl.html Chloroplast, Botany]
- [http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=8041699 Use of chloroplast DNA in studying plant phylogeny and evolution] Category:Organelles Category:Photosynthesis ko:엽록체 ja:葉緑体

Organelle

(2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles]] In cell biology, an organelle is one of several structures with specialized functions, suspended in the cytoplasm of a eukaryotic cell. Organelles were historically identified through the use of microscopy, and were also identified through the use of cell fractionation. A few large organelles probably originated from endosymbiont bacteria:
- chloroplast
- Other plastids, such as leucoplasts, amyloplasts, Etioplasts, Elaioplast, rhodoplasts, leukoplasts, and chromoplasts.
- mitochondrion Other organelles include:
- acrosome
- centriole
- endoplasmic reticulum
- golgi apparatus
- lysosome
- myofibril
- nucleus
- peroxisome
- ribosome
- vacuole
- vesicle
- melanosome
- cilium/flagellum
- parenthesome Other related structures:
- cytosol
- endomembrane system
- nucleosome
- microtubule
- cell membrane

Pyrenoid

In cell biology, pyrenoids are centers of carbon dioxide fixation. They are not membrane-bound organelles, but specialized areas in algal plastids and contain high amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO). RubisCO takes carbon dioxide and adds it to the sugar ribulose-1,5-bisphosphate. It needs six molecules of carbon dioxide and six molecules of ribulose-1,5-bisphosphate = six cycles of the Calvin cycle to make one new molecule of glucose. In some organisms, the concentration of RubisCO in the pyrenoid is so high that the contents of the organelle assume a crystalline appearance. Complex pyrenoids are highly differentiated areas of chloroplast surrounded by a thick starch sheath. This is assumed to aid the concentration of dissolved carbon dioxide by preventing diffusion away from the site of fixation while simultaneously reducing the level of oxygen in the pyrenoid which can inhibit the carbon fixation reaction catalysed by RubisCO. Pyrenoids are not found in higher plants and it is thought that the slower rate of diffusion of carbon dioxide in water compared to air (1:1000) necessitates their use by small aquatic organisms. Category:Cell biology Category:molecular biology Category:biochemistry

Pyrenoid in Botany

Differentiated region of the chloroplast that may be the center of starch formation and depositions, or may be the site of certain photosynthetic enzymes.

Plant

- Land plants (embryophytes)
- Non-vascular plants (bryophytes)
- Marchantiophyta - liverworts
- Anthocerotophyta - hornworts
- Bryophyta - mosses
- Vascular plants (tracheophytes)
- Lycopodiophyta - clubmosses
- Equisetophyta - horsetails
- Pteridophyta - "true" ferns
- Psilotophyta - whisk ferns
- Ophioglossophyta - adderstongues
- Seed plants (spermatophytes)
- †Pteridospermatophyta - seed ferns
- Pinophyta - conifers
- Cycadophyta - cycads
- Ginkgophyta - ginkgo
- Gnetophyta - gnetae
- Magnoliophyta - flowering plants Magnoliophyta Plants are a major group of living things (about 300,000 species), including familiar organisms such as trees, flowers, herbs, and ferns. Aristotle divided all living things between plants, which generally do not move or have sensory organs, and animals. In Linnaeus' system, these became the Kingdoms Vegetabilia (later Plantae) and Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these are still often considered plants in many contexts. Indeed, any attempt to match "plant" with a single taxon is doomed to fail, because plant is a vaguely defined concept unrelated to the presumed phylogenic concepts on which modern taxonomy is based.

Embryophytes

:See main article at Embryophytes Most familiar are the multicellular land plants, called embryophytes. They include the vascular plants, plants with full systems of leaves, stems, and roots. They also include a few of their close relatives, often called bryophytes, of which mosses and liverworts are the most common. All of these plants have eukaryotic cells with cell walls composed of cellulose, and most obtain their energy through photosynthesis, using light and carbon dioxide to synthesize food. About three hundred plant species do not photosynthesize but are parasites on other species of photosynthetic plants. Plants are distinguished from green algae, from which they evolved, by having specialized reproductive organs protected by non-reproductive tissues. Bryophytes first appeared during the early Palaeozoic. They can only survive where moisture is available for significant periods, although some species are desiccation tolerant. Most species of bryophyte remain small throughout their life-cycle. This involves an alternation between two generations: a haploid stage, called the gametophyte, and a diploid stage, called the sporophyte. The sporophyte is short-lived and remains dependent on its parent gametophyte. Vascular plants first appeared during the Silurian period, and by the Devonian had diversified and spread into many different land environments. They have a number of adaptations that allowed them to overcome the limitations of the bryophytes. These include a cuticle resistant to desiccation, and vascular tissues which transport water throughout the organism. In most the sporophyte acts as a separate individual, while the gametophyte remains small. Devonians (Pteridophyta) more closely allied to seed plants than they are to clubmosses (Lycopodiophyta)]] The first primitive seed plants, Pteridosperms (seed ferns) and Cordaites, both groups now extinct, appeared in the late Devonian and diversified through the Carboniferous, with further evolution through the Permian and Triassic periods. In these the gametophyte stage is completely reduced, and the sporophyte begins life inside an enclosure called a seed, which develops while on the parent plant, and with fertilisation by means of pollen grains. Whereas other vascular plants, such as ferns, reproduce by means of spores and so need moisture to develop, some seed plants can survive and reproduce in extremely arid conditions. Early seed plants are referred to as gymnosperms (naked seeds), as the seed embryo is not enclosed in a protective structure at pollination, with the pollen landing directly on the embryo. Four surviving groups remain widespread now, particularly the conifers, which are dominant trees in several biomes. The angiosperms, comprising the flowering plants, were the last major group of plants to appear, emerging from within the gymnosperms during the Jurassic and diversifying rapidly during the Cretaceous. These differ in that the seed embryo is enclosed, so the pollen has to grow a tube to penetrate the protective seed coat; they are the predominant group of flora in most biomes today.

Algae and Fungi

The algae comprise several different groups of organisms that produce energy through photosynthesis. However, they are not classified within the kingdom plantae but in the kingdom protista instead. The most conspicuous are the seaweeds, multicellular algae that often closely resemble terrestrial plants, but as stated above are not plants, found among the green, red, and brown algae. These and other algal groups also include various single-celled creatures and forms that are simple collections of cells, without differentiated tissues. Many can move about, and some have even lost their ability to photosynthesize; when first discovered, these were considered as both plants and animals. Now they are considered neither, but protists. The embryophytes developed from green algae; the two are collectively referred to as the green plants or Viridiplantae. The kingdom Plantae is now usually taken to mean this monophyletic group, as shown above. With a few exceptions among the green algae, all such forms have cell walls containing cellulose and chloroplasts containing chlorophylls a and b, and store food in the form of starch. They undergo closed mitosis without centrioles, and typically have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. The same is true of the red algae, and the two groups are generally believed to have a common origin. In contrast, most other algae have chloroplasts with three or four membranes. They are not in general close relatives of the green plants, acquiring chloroplasts separately from ingested or symbiotic green and red algae. Unlike embryophytes and algae, fungi are not photosynthetic, but are saprophytes: they obtain their food by breaking down and absorbing surrounding materials. Most fungi are formed by microscopic tubes called hyphae, which may or may not be divided into cells but contain eukaryotic nuclei. Fruiting bodies, of which mushrooms are the most familiar, are actually only the reproductive structures of fungi. They are not related to any of the photosynthetic groups, but are close relatives of animals. Therefore, fungus has a kingdom of its own.

Importance

The photosynthesis and carbon fixation conducted by land plants and algae are the ultimate source of energy and organic material in nearly all habitats. These processes also radically changed the composition of the Earth's atmosphere, which as a result contains a large proportion of oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that do not are confined to relatively few, anaerobic environments. Much of human nutrition depends on cereals. Other plants that are eaten include fruits, vegetables, herbs, and spices. Some vascular plants, referred to as trees and shrubs, produce woody stems and are an important source of building material. A number of plants are used decoratively, including a variety of flowers.

Growth

It is a common misconception that most of the solid material in a plant is taken from the soil, when in fact almost all of it is actually taken from the air. Through a process known as photosynthesis, plants use the energy in sunlight to convert carbon dioxide from the air into simple sugars. These sugars are then used as building blocks and form the main structural component of the plant. Plants rely on soil primarily for water (in quantitative terms), but also obtain nitrogen, phosphorus and other crucial nutrients. phosphorus Simple plants like algae may have short life spans as individuals, but their populations are commonly seasonal. Other plants may be organized according to their seasonal growth pattern:
- Annual: live and reproduce within one growing season.
- Biennial: live for two growing seasons; usually reproduce in second year.
- Perennial: live for many growing seasons; continue to reproduce once mature. Among the vascular plants, perennials include both evergreens that keep their leaves the entire year, and deciduous plants which lose their leaves for some part. In temperate and boreal climates, they generally lose their leaves during the winter; many tropical plants lose their leaves during the dry season. The growth rate of plants is extremely variable. Some mosses grow less than 0.001 mm/h, while most trees grow 0.025-0.250 mm/h. Some climbing species, such as kudzu, which do not need to produce thick supportive tissue, may grow up to 12.5 mm/h.

Fossils

Plant fossils include roots, wood, leaves, seeds, fruit, pollen, spores, phytoliths, and amber (the fossilized resin produced by some plants). Fossil land plants are recorded in terrestrial, lacustrine, fluvial and nearshore marine sediments. Pollen, spores and algae (dinoflagellates and acritarchs) are used for dating sedimentary rock sequences. The remains of fossil plants are not as common as fossil animals, although plant fossils are locally abundant in many regions worldwide. Early fossils of these ancient plants show the individual cells within the plant tissue. The Devonian period also saw the evolution of what many believe to be the first modern tree, Archaeopteris. This fern-like tree combined a woody trunk with the fronds of a fern, but produced no seeds. Archaeopteris The Coal Measures are a major source of Palaeozoic plant fossils, with many groups of plants in existence at this time. The spoil heaps of coal mines are the best places to collect; coal itself is the remains of fossilised plants, though structural detail of the plant fossils is rarely visible in coal. In the Fossil Forest at Victoria Park in Glasgow, Scotland, the stumps of Lepidodendron trees are found in their original growth positions. The fossilized remains of conifer and angiosperm roots, stems and branches may be locally abundant in lake and inshore sedimentary rocks from the Mesozoic and Caenozoic eras. Sequoia and its allies, magnolia, oak, and palms are often found. Petrified wood is common in some parts of the world, and is most frequently found in arid or desert areas were it is more readily exposed by erosion. Petrified wood is often heavily silicified (the organic material replaced by silicon dioxide), and the impregnated tissue is often preserved in fine detail. Such specimens may be cut and polished using lapidary equipment. Fossil forests of petrified wood have been found in all continents. Fossils of seed ferns such as Glossopteris are widely distributed throughout several continents of the southern hemisphere, a fact that gave support to Alfred Wegener's early ideas regarding Continental drift theory.

Distribution

References and further reading

- Kenrick, Paul & Crane, Peter R. (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. ISBN 1-56098-730-8.
- Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (2005). Biology of Plants (7th ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1007-2.
- Taylor, Thomas N. & Taylor, Edith L. (1993). The Biology and Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-651589-4.

External links

- [http://tolweb.org/tree?group=Green_plants&contgroup=Eukaryotes Tree of Life]
- Chaw, S.-M. et al. [http://mbe.library.arizona.edu/data/1997/1401/7chaw.pdf Molecular Phylogeny of Extant Gymnosperms and Seed Plant Evolution: Analysis of Nuclear 18s rRNA Sequences (pdf file)] Molec. Biol. Evol. 14 (1): 56-68. 1997.
- [http://florabase.calm.wa.gov.au/phylogeny/cronq88.html Interactive Cronquist classification]

Botanical and vegetation databases

- [http://www.efloras.org/index.aspx e-Floras (Flora of China, Flora of North America and others)]
- [http://plants.usda.gov/ United States of America]
- [http://rbg-web2.rbge.org.uk/FE/fe.html Flora Europaea]
- [http://www.anbg.gov.au/cpbr/databases/ Australia]
- [http://davesgarden.com/pdb/ 'Dave's Garden' horticultural plant database]
- [http://www.chilebosque.cl Chilean plants at Chilebosque] Category:Plants Category:Plant_taxonomy zh-min-nan:Si̍t-bu̍t ko:식물 ms:Tumbuhan ja:植物 simple:Plant th:พืช

Mucilage

Mucilage is a thick gluey substance, often produced by plants. Mucilage is another term for so called exopolysaccharides. These are sugar substances that cover the outside of, for example, unicellular or filamentous green algae and cyanobacteria. Amongst the green algae especially the group Volvocales are known to produce exopolysaccharides at least in a certain part of their life cycle. Exopolysaccharides are the most stabilising factor for microaggregates and are widely distributed in soils. Therefore exopolysaccharide-producing "soil algae" play a vital role in the ecology of the world's soils. Mucilage is edible, but tastes rather bland. It is used in medicine for its demulcent properties. Mucilage is also a term for an adhesive composed of a solution of a sticky vegetable product -- sometimes the above, but more typically a vegetable gum -- in water, used primarily to seal paper (e.g. postage stamps and envelope flaps)). Category:Adhesives

Photosynthetic

Photosynthesis is an important biochemical process in which plants, algae, and some bacteria harness the energy of sunlight to produce food. Ultimately, nearly all living things depend on energy produced from photosynthesis for their nourishment, making it vital to life on Earth. It is also responsible for producing the oxygen that makes up a large portion of the Earth's atmosphere. Organisms that produce energy through photosynthesis are called photoautotrophs.

Plant photosynthesis

Plants are photoautotrophs, which means they are able to synthesize food directly from inorganic compounds using light energy, instead of eating other organisms or relying on material derived from them. This is distinct from chemoautotrophs that do not depend on light energy, but use energy from inorganic compounds. The energy for photosynthesis ultimately comes from absorbed photons and involves a reducing agent, which is water in the case of plants, releasing oxygen as a waste product. The light energy is converted to chemical energy, in the form of ATP and NADPH, using the light-dependent reactions and is then available for carbon fixation. Most notably plants use the chemical energy to fix carbon dioxide into carbohydrates and other organic compounds through light-independent reactions. The overall equation for photosynthesis in green plants is: :n CO₂ + 2n H₂O + light energy → (CH₂O)_n + n O₂ + n H₂O Where n is defined according to the structure of the resulting carbohydrate. However, hexose sugars and starch are the primary products, so the following generalised equation is often used to represent photosynthesis: :6 CO₂ + 12 H₂O + light energy → C₆H₁₂O₆ + 6 O₂ + 6 H₂O More specifically, photosynthetic reactions usually produces an intermediate product, which is then converted to the final hexose carbohydrate products. These carbohydrate products are then variously used to form other organic compounds, such as the building material cellulose, as precursors for lipid and amino acid biosynthesis or as a fuel in cellular respiration. The latter not only occurs in plants, but also in animals when the energy from plants get passed through a food chain. In general outline, cellular respiration is the opposite of photosynthesis: glucose and other compounds are oxidised to produce carbon dioxide, water, and chemical energy. However, both processes actually take place through a different sequence of reactions and in different cellular compartments. food chain Plants capture light primarily using the pigment chlorophyll, which is the reason that most plants have a green color. The function of chlorophyll is often supported by other accessory pigments such as carotenoids and xanthophylls. Both chlorophyll and accessory pigments are contained in organelles (compartments within the cell) called chloroplasts. Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the leaves. The cells in the interior tissues of a leaf, called the mesophyll, contain about half a million chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant, waxy cuticle, that protects the leaf from excessive evaporation of water as well as decreasing the absorption of ultraviolet or blue light to reduce heating. The transparent, colourless epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.

Photosynthesis in algae and bacteria

Algae range from multicellular forms like kelp to microscopic, single-celled organisms. Although they are not as complex as land plants, photosynthesis takes place biochemically the same way. Light is absorbed by chlorophyll, although various accessory pigments to give them a wide variety of colours, located inside chloroplasts. All algae produce oxygen, and many are autotrophic. However, some are heterotrophic, relying on materials produced by other organisms. Photosynthetic bacteria do not have chloroplasts. Instead, photosynthesis takes place directly within the cell. The cyanobacteria contain chlorophyll and oxygen, in the same way that chloroplasts do, in fact chloroplasts are now considered to have evolved from cyanobacteria by endosymbiosis. The other photosynthetic bacteria have a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen.

Molecular production

Light-dependent reaction

bacteriochlorophyll bacteriochlorophyll The products of the light dependent reactions are ATP from photophosphorylation and NADPH from photoreduction. Both are then utilized as an energy source for the light-independent reactions.

Z scheme

In plants, the light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy to sythesize ATP and NADPH. The photons are captured in the antenna complexes of photosystem I and II by chlorophyll and accessory pigments (see diagram at right). When a chorophyll a molecule at a photosystems reaction center absorbs energy, an electron is excited and transferred to an electron-acceptor molecule through a process called Photoinduced charge separation. These electrons are shuttled through an electron transport chain that initially functions to generate a chemiosmotic potential across the membrane, the so called Z-scheme shown in the diagram. An ATP synthase enzyme uses the chemiosmotic potential to make ATP during photophosphorylation while NADPH is a product of the terminal redox reaction in the Z-scheme.

Water photolysis

The NADPH is the main reducing agent in chloroplasts, providing a source of energetic electrons to other reactions. Its production leaves chlorophyll with a deficit of electrons (oxidized), which must be obtained from some other reducing agent. The excited electrons lost from chlorophyll in photosystem I are replaced from the electron transport chain by plastocyanin. However, since photosystem II includes the first steps of the Z-scheme, an external source of electrons is required to reduce its oxidized chlorophyll a molecules. This role is played by water during a reaction known as photolysis and results in water being split to give electrons, oxygen and hydrogen ions. Photosystem II is the only known biological enzyme that carries out this oxidation of water. Initially, the hydrogen ions from photolysis contribute to the chemiosmotic potential but eventually they combine with the hydrogen carrier molecule NADP⁺ to form NADPH. Oxygen is a waste product of photosynthesis but it has a vital role for all organisms that use it for cellular respiration.

Oxygen and photosynthesis

With respect to oxygen and photosynthesis, there are two important concepts.
- Plant and algal cells also use oxygen for cellular respiration, although they have a net output of oxygen since much more is produced during photosynthesis.
- Oxygen is a product of the photolysis reaction not the fixation of carbon dioxide during the light-independent reactions. Consequently, the source of oxygen during photosynthesis is water, NOT carbon dioxide.

Bacterial variations

The second concept was first proposed by Cornelis Bernadus van Neil in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, bacteria only have one photosystem and use reducing agents other than water. They get electrons from a variety of different inorganic chemicals including sulfide or hydrogen, so for most of these bacteria oxygen is not produced. Others, such as the halophiles (an Archeae) produced so called purple membranes where the bacteriorhodopsin could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the chemiosmotic theory: light hit the membranes and the pH of the solution that contained the purple membranes dropped as protons were pumping out of the membrane.

Light-independent reaction

The fixation of carbon dioxide is a light-independent process in which carbon dioxide combines with a five-carbon sugar, ribulose bisphosphate (RuBP), to give two molecules of a three-carbon compound, glycerate 3-phosphate (GP). This compound is also sometimes known as 3-phosphoglycerate (PGA). GP, in the presence of ATP and NADPH from the light-dependent stages, is reduced to glyceraldehye 3-phosphate (G3P). This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or even as triose phosphate (a three-carbon sugar). This is the point at which carbohydrates are produced during photosynthesis. Some of the triose phosphates condense to form hexose phosphates, sucrose, starch and cellulose or are converted to acetylcoenzyme A to make amino acids and lipids. Others go on to regenerate RuBP so the process can continue (see Calvin Cycle).

Discovery

Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 1800s. Jan van Helmont began the research of the process in the mid-1600s when he carefully measured the mass of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from the water, the only substance he added to the potted plant. This was a partially accurate hypothesis - much of the gained mass also comes from carbon dioxide as well as water. However, this was a signalling point to the idea that the bulk of a plant's biomass comes from the inputs of photosynthesis, not the soil itself. Joseph Priestley, a chemist and minister, discovered that when he isolated a volume of air under an inverted jar, and burned a candle in it, the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly "injure" air. He then showed that the air that had been "injured" by the candle and the mouse could be restored by a plant. In 1778, Jan Ingenhousz, court physician to the Austrian Empress, repeated Priestley's experiments. He discovered that it was the influence of sun and light on the plant that could cause it to rescue a mouse in a matter of hours. In 1796, Jean Senebier, a French pastor, showed that CO₂ was the "fixed" or "injured" air and that it was taken up by plants in photosynthesis. Soon afterwards, Nicolas-Théodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO₂, but also to the incorporation of water. Thus the basic reaction by which photosynthesis is used to produce food (such as glucose) was outlined. Modern scientists built on the foundation of knowledge from those scientists centuries ago and were able to discover many things. Cornelius Van Niel made key discoveries explaining the chemistry of photosynthesis. By studying purple sulfur bacteria and green bacteria he was the first scientist to demonstrate that photosynthesis is a light-dependent redox reaction, in which hydrogen reduces carbon dioxide. Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by Robert Hill in 1937 and 1939. He showed that isolated chloroplasts give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. The Hill reaction is as follows: :2 H₂O + 2 A + (light, chloroplasts) → 2 AH₂ + O₂ where A is the electron acceptor. Therefore, in light the electron acceptor is reduced and oxygen is evolved. Samuel Ruben and Martin Camen used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water. Melvin Calvin and his partner Benson were able to puzzle out each stage in the dark or light-independent phase of photosynthesis, known as the Calvin Cycle. A Nobel Prize winning scientist, Rudolph A. Marcus, was able to discover the function and significance of the electron transport chain.

Bioenergetics of Photosynthesis

Photosynthesis is a physiological phenomenon that coverts solar energy into photochemical energy. This physiological phenomenon may be described thermodynamically in terms of changes in energy, entropy and free energy. The energetics of photosynthesis, driven by light, causes a change in entropy that in turn yields a usable source of energy for the plant. The following equation summarizes the products and reactants of photosynthesis in the typical green photosynthesizing plant: CO₂ + H₂O → O₂ + (CH₂O) + 112 kcal/mol CO₂ On earth, there are two sources of free energy: light energy from the sun, and terrestrial sources, including volcanoes, hot springs and radioactivity of certain elements. The biochemical value of electromagnetic radiation has led plants to use the free energy from the sun in particular. Visible light, which is used specifically by green plants to photosynthesize, may result in the formation of electronically excited states of certain substances called pigments (Gregory). For example, Chl a is a pigment which acts as a catalyst, converting solar energy into photochemical energy that is necessary for photosynthesis (Govindjee). With the presence of solar energy, the plant has a usable source of energy, which is termed the free energy (F) of the system. However, thermal energy is not completely interconvertible, which means that the character of the solar energy may lead to the limited convertibility of it into forms that may be used by the plant. This relates back to the work of Josiah Willard Gibbs: the change in free energy (ΔF) is related to both the change in entropy (ΔS) and the change in enthalpy (ΔH) of the system (Rabinowitch). Gibbs energy equation: ΔF = ΔH – TΔS Past experiments have shown that the total energy produced by photosynthesis is 112 kcal/mol. However in the experiment, the free energy due to light was 120 kcal/mol. An overall loss of 8 kcal/mol was due to entropy, as described by Gibbs equation (Gonindjee). In other words, since the usable energy of the system is related directly to the entropy and temperature of the system, a smaller amount of thermal energy is available for conversion into usable forms of energy (including mechanical and chemical) when entropy is great (Rabinowitch). This concept relates back to the second law of thermodynamics in that an increase in entropy is needed to convert light energy into energy suitable for the plant. Overall, in conjunction with the oxidation-reduction reaction nature of the photosynthesis equation, and the interrelationships between entropy and enthalpy, energy in a usable form will be produced by the photosynthesizing green plant. References Govindjee. Bioenergetics of Photosynthesis. New York: Academic Press, 1975. Gregory, R.P.F. Biochemistry of Photosynthesis. Belfast: Universities Press, 1971. Rabinowitch, Eugene and Govindjee. Photosynthesis. New York: John Wiley & Sons, Inc., 1969.

Factors affecting photosynthesis

There are three main factors affecting photosynthesis and several corollary factors. The three main are:
- Light irradiance and wavelength
- Carbon dioxide concentration
- Temperature

Light Intensity (Irradiance), Wavelength and Temperature

In the early 1900s F.F. Blackman investigated the effects of light intensity (irradiance) and temperature on the rate of photosynthesis. At constant temperature the rate of photosynthesis varies with irradiance, initially increasing as the irradiance increases. However at higher irradiance this relationship no longer holds and the rate of photosynthesis reaches a plateau. The effect on the rate of photosynthesis of varying the temperature at constant irradiance can be seen in image to the left. At high irradiance the rate of photosynthesis increases as the temperature is increased over a limited range. At low irradiance, increasing the temperature has little effect on the rate of photosynthesis. These two experiments illustrate vital points: firstly, from research it is known that photochemical reactions are not generally affected by temperature. However, these experiments clearly show that temperature affects the rate of photosynthesis, so there must be two sets of reactions in the full process of photosynthesis. These are of course the light-dependent 'photochemical' stage and the light-independent, temperature-dependent stage. Secondly, Blackman's experiments illustrate the concept of limiting factors. Another limiting factor is the wavelength of light. Cyanobacteria which reside several metres underwater cannot recieve the correct wavelengths required to cause photoinduced charge seperation in conventional photosynthetic pigments. To combat this problem a series of proteins with different flourescent pigments surround the reaction centre. This unit is called a phycobilisome.

Carbon Dioxide

An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction and so the rate of photosynthesis generally increases until limited by another factor. Carbon dioxide helps increase the rate of photosynthesis. This is because rubisco, the enzyme fixing the carbon dioxide in the light-dependent reactions, has affinity for both carbon dioxide and oxygen. Thus, an increase in the concentration of carbon dioxide increases the probability of rubisco fixing carbon dioxide instead of oxygen. This allows the plant to be more productive since the fixation of oxygen requires photorespiration to remove glycolate a product of rubisco's oxygenase activity. Photorespiration is bad for a plant since it actually releases carbon dioxide and uses energy in the process.

Corollary Factors

- Amount of water
- Leaf morphology
- Leaf nitrogen content
- Molecular Carriers such as NADP and FAD

In Detail

Metabolic pathways involved in photosynthesis:
- Light-dependent reaction
- Light-independent reaction

External links

- [http://www.biochemweb.org/metabolism.shtml Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry and Cell Biology]
- [http://www.chemsoc.org/networks/learnnet/cfb/Photosynthesis.htm Overall examination of Photosynthesis at an intermediate level]
- [http://www.life.uiuc.edu/govindjee/photosynBook.html Overall Energetics of Photosynthesis] Category:Biochemistry Category:Botany Category:Metabolism Category:Agronomy ko:광합성 ms:Fotosintesis ja:光合成 simple:Photosynthesis th:การสังเคราะห์ด้วยแสง

Cyanobacteria

The taxonomy of the
Cyanobacteria is currently
under revision. see [http://ijs.sgmjournals.org/cgi/content/abstract/54/5/1895] Cyanobacteria (Greek: cyanos = blue) are a phylum of bacteria that obtain their energy through photosynthesis. They are often referred to as blue-green algae, even though it is now known that they are not directly related to any of the other algal groups, which are all eukaryotes. Nonetheless, the description is still sometimes used to reflect their appearance and ecological role. Fossil traces of cyanobacteria are claimed to have been found from around 3.8 billion years ago, but recent evidence has sparked controversy over this assertion. See: Stromatolite

Forms

Cyanobacteria include unicellular, colonial, and filamentous forms. Some filamentous cyanophytes form differentiated cells, called heterocysts, that are specialized for nitrogen fixation, and resting cells called akinetes. Each individual cell typically has a thick, gelatinous cell wall, which stains gram-negative. The cyanophytes lack flagella, but may move about by gliding along surfaces. Most are found in freshwater, but many are marine, occur in damp soil, or even temporarily moistened rocks in deserts. A few are endosymbionts in lichens, plants, various protists, or sponges and provide energy for the host. Some even live in the fur of sloths, providing a form of camouflage. camouflage

Photosynthesis

Photosynthesis in cyanobacteria generally uses water as an electron donor and produces oxygen as a by-product, though some may also use hydrogen sulfide as occurs among other photosynthetic bacteria. Carbon dioxide is reduced to form carbohydrates via the Calvin cycle. In most forms the photosynthetic machinery is embedded into folds of the cell membrane, called thylakoids. The large amounts of oxygen in the atmosphere are considered to have been first created by the activities of ancient cyanobacteria. Due to their abilities to fix nitrogen in aerobic conditions they are often found as symbionts with a number of other groups of organisms as fungi (lichens), corals, pteridophytes (Azolla), angiosperms (Gunnera) etc. The water-oxidizing photosynthesis is accomplished by coupling the activity of photosystem (PS) I and II. They are the only group of organisms that are able to fix nitrogen and carbon in aerobic environment which could account for their evolutionary and ecological success. Moreover, they are able to use in anaerobic conditions only PS I - cyclic photophosphorylation -with electron donors other than water (hydrogen sulfide, thiosulphate, or even molecular hydrogen) just like purple photosynthetic bacteria. Also they share an archaebacterial property - the ability to reduce elemental sulfur by anaerobic respiration in dark. Probably the most intriguing thing about these organisms is that their photosynthetic electron transport shares the same compartment (the thylakoid) and components of the respiratory electron transport. Actually, their plasma membrane contains only components of the respiratory chain, while the thylakoid membrane hosts both respiratory and photosynthetic electron transport. Attached to thylakoid membrane, phycobilisomes act as light harvesting antennae for photosystem II. The phycobilisome components (phycobilin)are responsible for the blue-green pigmentation of most cyanobacteria. The variations to this theme is mainly due to carotenoids and phycoerythrins which give the cells the red-brownish coloration. A few genera, however, lack phycobilins and have chlorophyll b as well as chlorophyll a, giving them a bright green colour. These were originally grouped together as the prochlorophytes or chloroxybacteria, but appear to have developed in several different lines of cyanobacteria.

Relationship to chloroplasts

Chloroplasts found in eukaryotes (algae and higher plants) most likely represent reduced endosymbiotic cyanobacteria. This endosymbiotic theory is supported by various structural and genetic similarities. Primary chloroplasts are found among the green plants, where they contain chlorophyll b, and among the red algae and glaucophytes, where they contain phycobilins. It now appears that these chloroplasts probably had a single origin. Other algae likely took their chloroplasts from these forms by secondary endosymbiosis or ingestion.

Classification

The cyanobacteria were traditionally classified by morphology into five sections, referred to by the numerals I-V. The first three - Chroococcales, Pleurocapsales, and Oscillatoriales - are not supported by phylogenetic studies. However, the latter two - Nostocales and Stigonematales - are monophyletic, and make up the heterocystous cyanobacteria. Most taxa included in the phylum or division Cyanobacteria have not been validly published under the Bacteriological Code. Except:
- The classes Chroobacteria, Hormogoneae and Gloeobacteria
- The orders Chroococcales, Gloeobacterales, Nostocales, Oscillatoriales, Pleurocapsales and Stigonematales
- The families Prochloraceae and Prochlorotrichaceae
- The genera Halospirulina, Planktothricoides, Prochlorococcus, Prochloron, Prochlorothrix.

Other

Certain cyanobacteria produce cyanotoxins like Anatoxin-a, Anatoxin-as, Aplysiatoxin, Cylindrospermopsin, Domoic acid, Microcystin LR, Nodularin R (from Nodularia), or Saxitoxin. Sometimes a mass-reproduction of cyanobacteria results in algal blooms. Some are marketed as having nutritional value, such as Aphanizomenon flos-aquae (E3live) or Spirulina. At least one secondary metabolite, cyanovirin, has shown to possess anti-HIV activity. See hypolith for an example of cyanobacteria living in extreme conditions. Category:Phototrophs
- Category:Photosynthesis ko:남조류 ja:藍藻

Stomata

In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. Air containing carbon dioxide and oxygen enters the plant through these openings where it gets used in photosynthesis and respiration. Waste oxygen produced by photosynthesis in the chlorenchyma cells of the leaf interior exits through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration. Dicotyledons usually have more stomata on the lower epidermis than the upper epidermis. As these leaves are held horizontally, upper epidermis is directly illuminated. Locating fewer stomata on the upper epidermis can then prevent excess water loss. Monocotyledons are different. Because their leaves are held vertically, they will have the same number of stomata on the two epidermises. If the plant has floating leaves, there will be no stomata on the lower epidermis as it can absorb gases directly from water through cuticle. If it is submerged leaf, no stomata will be present on either side. Stoma in Greek