Planet – Planet
ciri-ciri :
(a) mengorbit mengelilingi bintang atau sisa-sisa bintang;
(b) mempunyai massa yang cukup untuk memiliki gravitasi tersendiri agar dapat mengatasi tekanan rigid body sehingga benda angkasa tersebut mempunyai bentuk kesetimbangan hidrostatik (bentuk hampir bulat);
(c) tidak terlalu besar hingga dapat menyebabkan fusi termonuklir terhadap deuterium di intinya; dan,
(d) telah "membersihkan lingkungan" (clearing the neighborhood; mengosongkan orbit agar tidak ditempati benda-benda angkasa berukuran cukup besar lainnya selain satelitnya sendiri) di daerah sekitar orbitnya
Merkurius adalah planet di terkecil di dalam tata surya dan juga yang terdekat dengan Matahari dengan kala revolusi 88 hari. Merkurius mempunyai banyak kawah dan juga tidak mempunyai satelit alami serta atmosfir. Merkurius mempunyai inti besi yang menciptakan sebuah medan magnet dengan kekuatan 0.1% dari kekuatan medan magnet bumi. Suhu permukaan dari Merkurius berkisar antara 90 sampai 700 Kelvin (-180 sampai 430 derajat selsius),
Venus adalah planet terdekat kedua dari matahari setelah Merkurius. Planet ini memiliki radius 6.052 km dan mengelilingi matahari dalam waktu 225 hari. Atmosfer Venus mengandung 97% karbondioksida (CO2) dan 3% nitrogen, sehingga hampir tidak mungkin terdapat kehidupan.
Arah rotasi Venus berlawanan dengan arah rotasi planet-planet lain. Selain itu, jangka waktu rotasi Venus lebih lama daripada jangka waktu revolusinya dalam mengelilingi matahari.
Bumi adalah planet ketiga dari delapan planet dalam Tata SuryaBumi mempunyai lapisan udara (atmosfer) dan medan magnet yang disebut (magnetosfer) yang melindung permukaan Bumi dari angin matahari, sinar ultraungu, dan radiasi dari luar angkasa. Lapisan udara ini menyelimuti bumi hingga ketinggian sekitar 700 kilometer. Lapisan udara ini dibagi menjadi Troposfer, Stratosfer, Mesosfer, Termosfer, dan Eksosfer. Bumi mempunyai diameter sepanjang 12.756 kilometer. Bumi mempunyai 1 satelit alami yaitu Bulan. 70,8% permukaan bumi diliputi air. Udara Bumi terdiri dari 78% nitrogen, 21% oksigen, dan 1% uap air, karbondioksida, dan gas lain.
Mars adalah planet terdekat keempat dari Matahari. Namun planet ini juga dikenal sebagai planet merah karena penampakannya yang kemerah-merahan. Planet ini memiliki 2 buah satelit, yaitu Phobos dan Deimos. Planet ini mengorbit selama 687 hari dalam mengelilingi matahari. Planet ini juga berotasi. Kala rotasinya 24,62 jam.
Jupiter .Jarak rata-rata antara Jupiter dan Matahari adalah 778,3 juta km. Jupiter terbesar dan terberat dengan diameter 14.980 km dan memiliki massa 318 kali massa bumi. Periode rotasi planet ini adalah 9,8 jam, sedangkan periode revolusi adalah 11,86 tahun. Atmosfer Jupiter mengandung hidrogen (H), helium (He), metana (CH4), dan amonia (NH3). Suhu di permukaan planet ini berkisar dari -140oC sampai dengan 21oC. Jupiter memiliki 63 satelit, di antaranya Io, Europa, Ganymede, Callisto (Galilean moons).
Saturnus adalah sebuah planet yang terletak di tata surya dimana planet ini terkenal sebagai planet bercincin. Saturnus berevolusi dalam waktu 29,46 tahun. Setiap 378 hari, Selain berevolusi, Saturnus juga berotasi dalam waktu yang sangat singkat, yaitu 10 jam 14 menit. Terdapat beribu-ribu cincin yang mengelilingi planet ini. Para ilmuwan berpendapat, cincin itu tidak mungkin terbuat dari lempengan padat karena akan hancur oleh gaya sentrifugal
Uranus adalah planet terjauh ke-7 dari Matahari setelah Saturnus, Uranus memiliki jarak dengan Matahari sebesar 2875 juta km. Uranus memiliki diameter mencapai 51.118 km dan memiliki massa 14,54 massa Bumi. Periode rotasi planet ini adalah 17,25 jam, sedangkan periode revolusi adalah 84 tahun. Bentuk planet ini mirip dengan Bulan dengan permukaan berwarna hijau dan biru. Uranus memiliki 18 satelit alami, diantaranya Ariel, Umbriel, Miranda, Titania, dan Oberon
Neptunus merupakan planet terjauh (kedelapan) jika ditinjau dari Matahari.
Neptunus memiliki jarak rata-rata dengan Matahari sebesar 4.450 juta km. Neptunus memiliki diameter mencapai 49.530 km dan memiliki massa 17,2 massa Bumi. Periode rotasi planet ini adalah 16,1 jam., sedangkan periode revolusi adalah 164,8 tahun. Bentuk planet ini mirip dengan Bulan dengan permukaan terdapat lapisan tipis silikat. Komposisi penyusun planet ini adalah besi dan unsur berat lainnya. Planet Neptunus memiliki 8 buah satelit, di antaranya Triton, Proteus, Nereid, dan Larissa.
Tuesday, December 16, 2008
Saturday, December 13, 2008
Makanan ringan dan komposisi
- Alpenlibe, komposisi : gula, syrup, glukosa, minyak nabati, krim, asam laktat.
- Gerry Salut, komposisi : terigu, gula, lemak nabati, minyak nabati, coklat bubuk, dektrosa, susu bubuk, lesitin, kedelai, garam, aroma, coklat, vanillin.
- Leo Kripik kentang, komposisi : tepung kentang, minyak nabati, perisa ayam, monogliserida nabati.
- Frestea, komposisi : Air, gula, ekstrak teh hijau, na - bikarbonat, na – aksorbat, perisa.
- Apollo Crush, komposisi : minyak nabati, tepung gandum, gula, susu bubuk, coklat bubuk, coklat
- Oops, komposisi : tepung terigu, tapioca, lemak nabati, gula, perisa ayam goreng, garam, ragi, protein tumbuh – tumbuhan, MSG, lada hitam, daun bawang, hidrolisat protein sayur (tartazine Cl 19140, dan kuning FCF Cl 15985).
- Top, komposisi : Gula, lemak nabati, rice crispy, susu bubuk, tepung terigu, syrup, glukosa, dektrosa, susu kental manis, bubuk kakao, pengemulsi, garam, pengembang kue, natrium bikarbonat, perisa.
- Twister Coklat, komposisinya : Gula, terigu, lemak nabati, bubuk kakao, susu bubuk, bubuk whey, soya lesithin, garam, vanili.
- Twister Minis, komposisinya : gula, tepung terigu, minyak nabati, susu bubuk, bubuk kakao, bubuk whey, vanili, kedelai, garam, pengemulsi lesithin.
- Starburs, komposisinya : Gula, Syrup glukosa, pengatur keasaman (asam sitrat, asam malat), natrium sitrat, konsentrat, blackcurrant, pencita rasa blackcurrant, pewarna makanan (merak alura Cl 16035 & biru berlian Cl 42690)
- Sobamie sedap, komposisinya : tepung terigu, minyak sayur, kentang, tapioca, gula, perisa, kentang BBQ, penguat rasa, monotrium glutamate, bubuk bawang putih, natrium folifosfat, natrium karbonat, kaboksi tartazine Cl 19140.
- Yakult, komposisi : air, sukrosa, susu bubuk skim, glukosa, kultur, perisa yakult.
- Alpenlibe blackcurrant, komposisi : gula, syrup, glukosa, minyak nabati, krim, asam laktat, pewarna (titanium dioksida Cl 16035 & Biru berlian Cl 42090, perisa (blackcurrant, vanilla), garam, soya lesithin.
- Fonnut, komposisi : gula, lemak nabati, susu bubuk, bubuk kakao, kacang mente, soya lesithin, garam, vanillin.
- Tiramisu, komposisi : susu tepung penuh krim, kacang badam, mentega, koko, laktosa, gula, lemak sayuran, susu tepung skim, serbuk koko, gum acacia, serbuk caramel, pengemulsi, vanillin, perisa.
- Piatos, komposisi : tepung kentang, gula, minyak nabati, perisa sapi panggang, kalium klorida, garam, monosodium glutamate, gliserol, Asam sitrat, dan antioksidan.
- Bitter Sweet, komposisi : gula, kakao
- Stasion rasa, komposisi : gula, glukosa, minyak tumbuhan, gelatin sapi, gum arab, kopi bubuk, krim, coklat bubuk, pewarna caramel, garam, perisa cappuccino, lesithin kedelai.
- Tango, komposisi : tepung terigu, gula, lemak nabati, susu bubuk, coklat bubuk, dektrosa, lesithin kedelai, garam, telur, coklat pasta, soda kue, kopi instant, perisa kismis.
- Capilano’s, komposisinya : gula, glukosa, coklat pasta, coklat bubuk, krim, susu bubuk, minyak tumbuhan, aroma coklat, lesithin kedelai.
- Sambal Sasa, komposisi : cabe, gula, garam, bawang putih, cuka, pati termodifikasi, pengawet na – benzoate, air.
- Beng – beng, Komposisinya : gula, susu bubuk, lemak kakao, tepung terigu, minyak nabati, dekstrosa, kakao bubuk, glukosa, krispi (jagung, beras), maltodekstrin, pengemulsi, ekstrak malt, garam, perisa, caramel, bahan pengembang, vanillin.
- Soothers, komposisinya : gula, syrup glukosa, dekstrosa, eukaliptus, mentol, perisa, vitamin C, ekstrak, pewarna makanan merah ( 4R Cl 14255)
- Toblerone, komposisinya : gula, tepung susu, minyak coklat, laktosa, madu (3%), almond (1,6%)lesithin kedelai, syrup glukosa, putih telur, esen.
- Tango Strawberry, komposisinya : tepung terigu, gula, lemak nabati, susu bubuk, dekstrosa, lesithin kedelai, garam, telur, strawberi, soda kue, eritrosin Cl 45430
- Sambal tomat ABC, komposisinya : air, gula, pasta tomat, garam, pati, cuka, pengawet natrium benzoate dan natrium metabisulfit, rempah – rempah.
- Kecap manis ABC, komposisinya : gula, air, garam, kedelai, biji gandum, pengawet natrium benzoate.
- Alpenlibe Bluez, komposisinya : Gula, syrup, minyak nabati, krim, asam laktat, perisa(stoberi, vanilla) garam, soya, pewarna (titanium dioksida Cl 77891, merah alura Cl 16035, biru berlian Cl 42090)
- Khong Guan biscuit, komposisinya : tepung terigu, tapioca, lemak nabati, mentega, gula, telur, garam, susu bubuk, coklat bubuk, glucose, keju, sultana, sari jeru lemon, ragi, soda kue, penyedap rasa dan aroma makanan, pewarna makanan (tartazine Cl 19140)
- Koko Krunch, komposisinya : tepung terigu, gula, bubuk kakao, minyak nabati, premiks vitamin, perisa ( Vanilin dan kayu manis).
Pembahasan : Dalam suatu makanan kemasan ataupun minuman kemasan kita tidak hanya menjumpai dikomposisinya bahan alami saja, tetapi banyak juga kita temui bahan kimia. Bahan kimia pada makanan/minuman kemasan banyak sekali manfaatnya terutama mengawetkan makanan dan minuman. Produk makanan/minuman kemasan yang biasanya disimpan di supermarket atau di rumah itu tidak akan tahan lama bila tidak ada zat kimia yang mengawetkannya.
Kesimpulan : Dalam makanan dan minuman kemasan terdapat bahan kimia yang fungsinya sebagai pewarna, penyedap rasa, dan pengawet.
Lichen
Lichens (IPA: /ˈlaɪkən/[1] or /lɪtʃ.ən/[2]) are symbiotic associations of a fungus (the mycobiont) with a photosynthetic partner (the photobiont also known as the phycobiont) that can produce food for the lichen from sunlight. The photobiont is usually either a green alga or cyanobacterium. A few lichens are known to contain yellow-green algae or, in one case, a brown alga. Some lichens contain both green algae and cyanobacteria as photobionts; in these cases, the cyanobacteria symbiont component may specialize in fixing atmospheric nitrogen for metabolic use.
Overview
The body (thallus) of most lichens is quite different from that of either the fungus or alga growing separately, and may strikingly resemble simple plants in form and growth (Sanders 2001). The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations; however, in almost all kinds, the algal cells are never enclosed inside the fungal cells themselves. It has been suggested that the phycobiont is sometimes penetrated by haustoria from the mycobiont, but with the development of electron microscopy there is little solid evidence of this, and if true, is an isolated occurrence and in any event is entirely unecessesary. Thus lichens are poikilohydric, that is, capable of surviving extremely low levels of water content. However, the re-configuration of membranes following a period of dehydration requires several minutes at least. During this period a “soup” of metabolites from both the mycobiont and phycobiont leaks into the extracellar spaces. This is readily available to both bionts to uptake essential metabolic products ensuring a perfect level of mutualism showing leaching from the canopy mosses in Guadaloupe of numerous metabolites immediately following rehydration. Not only do the two bionts profit, but also all the other epiphytic organisms from the nutrient rich leachate. This fundamental phenomenon also points to a possible explanation of lichen evolution from its original phycobiont and mycobiont components with its subsequent migration from an aquatic environment to dry land. Thus, during repeated periods of low levels of hydration in an alga and the resultant leakage of beneficial metabolites to an adjacent aquatic fungus, the mutalistic “marriage” slowly became constant.
In the natural environment, lichen “provides” the alga with water and minerals that the fungus absorbs from whatever the lichen is growing on, its substrate. As for the alga, it uses the minerals and water to make food for the fungus and itself.
Algal and fungal components of some lichens have been cultured separately under laboratory conditions, but in the natural environment of a lichen, neither can grow and reproduce without a symbiotic partner. Indeed, although strains of cyanobacteria found in various cyanolichens are often closely related to one another, they differ from the most closely related free-living strains [1]. The lichen association is a close symbiosis: It extends the ecological range of both partners and is obligatory for their growth and reproduction in natural environoments. Propagules ("diaspores") typically contain cells from both partners, although the fungal components of so-called "fringe species" rely instead on algal cells dispersed by the “core species.”
There has nonetheless been controversy as to whether the lichen combination should be considered an example of mutualism or commensalism or even parasitism. An observation offered in support of this is that cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen. The same, however, might be said of isolated skin cells growing in laboratory culture, which grow more quickly than similar cells that are integrated into a functional tissue. However, from the work of Coxson (see above) mutualism would appear to best summarise our current knowledge.
Lichens are named based on the fungal component, which plays the primary role in determining the lichen's form. The fungus typically comprises the majority of a lichen's bulk, though in filamentous and gelatinous lichens this is not always the case. The lichen fungus is typically a member of the Ascomycota—rarely a member of the Basidiomycota, and then termed basidiolichens to differentiate them from the more common ascolichens. Formerly, some lichen taxonomists placed lichens in their own division, the Mycophycophyta, but this practice is no longer accepted because the components belong to separate lineages. Neither the ascolichens nor the basidiolichens form monophyletic lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum[3]. Even more unusual than basidiolichens is the fungus Geosiphon pyriforme, a member of the Glomeromycota that is unique in that it encloses a cyanobacterial symbiont inside its cells. Geosiphon is not usually considered to be a lichen, and its peculiar symbiosis was not recognized for many years. The genus is more closely allied to endomycorrhizal genera.
The algal or cyanobacterial cells are photosynthetic, and as in higher plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substrate. If a cyanobacterium is present, as a primary partner or another symbiont in addition to green alga as in certain tripartite lichens, they can fix atmospheric nitrogen, complementing the activities of the green alga.
Morphology and structure
Crustose and foliose lichens on a wall
Lichens are often the first to settle in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes.[citation needed] Some survive in the tough conditions of deserts, and others on frozen soil of the Arctic regions.[citation needed] Recent ESA research shows that lichen can even endure extended exposure to space.[4] Some lichens have the aspect of leaves (foliose lichens); others cover the substrate like a crust (crustose lichens) (illustration, right), others such as the genus Ramalina adopt shrubby forms (fruticose lichens), and there are gelatinous lichens such as the genus Collema.[5]
A "Hidden Mickey" formed by lichen on a stone.
Although the form of a lichen is determined by the genetic material of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of hyphae. If combined with its photobiont under appropriate conditions, its characteristic form emerges, in the process called morphogenesis (Brodo, Sharnoff & Sharnoff, 2001). In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were first found growing in a conjoined manner.
There is evidence to suggest that the lichen symbiosis is parasitic or commensalistic, rather than mutualistic (Ahmadjian 1993). However, this now needs to be re-examined in light of Coxons work. The photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Furthermore, photobiont cells are routinely destroyed in the course of nutrient exchange. The association is able to continue because photobiont cells reproduce faster than they are destroyed. (ibid.)
Cross-section through the lichen Pseudevernia furfuracea with plainly visible layer of green algae under the surface
Under magnification, a section through a typical foliose lichen thallus reveals four layers of interlaced fungal filaments. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the cortex, which can reach several hundred μm in thickness.[6] This cortex may be further topped by an epicortex 0.6-1μm thick in some Parmeliaceae, which may be with or without pores, and is secreted by cells - it is not itself cellular.[6] In lichens that include both green algal and cyanobacterial symbionts, the cyanobacteria may be held on the upper or lower surface in small pustules called cephalodia/cephalodium. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an haustorium.[citation needed] Beneath this algal layer is a third layer of loosely interwoven fungal hyphae without algal cells. This layer is called the medulla. Beneath the medulla, the bottom surface resembles the upper surface and is called the lower cortex, again consisting of densely packed fungal hyphae. The lower cortex often bears rootlike fungal structures known as rhizines, which serve to attach the thallus to the substrate on which it grows.[citation needed] Lichens also sometimes contain structures made from fungal metabolites, for example crustose lichens sometimes have a polysaccharide layer in the cortex.[citation needed] Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species. This seems to also be true of the photobiont species involved.[citation needed]
Reproduction
Thalli and apothecia on a foliose lichen
lichen
Many lichens reproduce asexually, either by vegetative reproduction or through the dispersal of diaspores containing algal and fungal cells. Soredia (singular soredium) are small groups of algal cells surrounded by fungal filaments that form in structures called soralia, from which the soredia can be dispersed by wind. Another form of diaspore are isidia, elongated outgrowths from the thallus that break off for mechanical dispersal. Fruticose lichens in particular can easily fragment. Due to the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns.
Many lichen fungi appear to reproduce sexually in a manner typical of fungi, producing spores that are presumably the result of sexual fusion and meiosis. Following dispersal, such fungal spores must meet with a compatible algal partner before a functional lichen can form. This may be a common form of reproduction in basidiolichens, which form fruitbodies resembling their nonlichenized relatives. Among the ascolichens, spores are produced in spore-producing bodies, the three most common spore body types are the apothecia, perithecia and the pycnidia. [2]
For reproduction, lichen possess isidia, soredia, and undergo simple fragmentation. These structures are also composed of a fungal hyphae wrapped around cyanobacteria. (Eichorn, Evert, and Raven, 2005) While the reproductive structures are all composed of the same components(Mycobiont and Photobiont) they are each unique in other ways. Isidia are small outgrowths on the exterior of the lichen. Soredia are powdery propagules that are released from the top of the thallus(1). In order to establish the lichen, the soredia propagules must contain both the photobiont and the mycobiont(2). [7]
[edit] Ecology
Lichens must compete with plants for access to sunlight, but because of their small size and slow growth, they thrive in places where higher plants have difficulty growing.
A major ecophysiological advantage of lichens is that they are poikilohydric (poikilo- variable, hydric- relating to water), meaning that though they have little control over the status of their hydration, they can tolerate irregular and extended periods of severe desiccation. Like some mosses, liverworts, ferns, and a few "resurrection plants", upon desiccation, lichens enter a metabolic suspension or stasis (known as cryptobiosis) in which the cells of the lichen symbionts are dehydrated to a degree that halts most biochemical activity. In this cryptobiotic state, lichens can survive wider extremes of temperature, radiation and drought in the harsh environments they often inhabit.
Lichens do not have roots and do not need to tap continuous reservoirs of water like most higher plants, thus they can grow in locations impossible for most plants, such as bare rock, sterile soil or sand, and various artificial structures such as walls, roofs and monuments. Many lichens also grow as epiphytes (epi- on the surface, phyte- plant) on other plants, particularly on the trunks and branches of trees. When growing on other plants, lichens are not parasites; they do not consume any part of the plant nor poison it. Some ground-dwelling lichens, such as members of the subgenus Cladina (reindeer lichens), however, produce chemicals which leach into the soil and inhibit the germination of plant seeds and growth of young plants. Stability (that is, longevity) of their substrate is a major factor of lichen habitats. Most lichens grow on stable rock surfaces or the bark of old trees, but many others grow on soil and sand. In these latter cases, lichens are often an important part of soil stabilization; indeed, in some desert ecosystems, vascular (higher) plant seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.
Pine forest with lichen ground-cover
Lichens may be eaten by some animals, such as reindeer, living in arctic regions. The larvae of a surprising number of Lepidoptera species feed exclusively on lichens. These include Common Footman and Marbled Beauty. However, lichens are very low in protein and high in carbohydrates, making them unsuitable for some animals. Lichens are also used by the Northern Flying Squirrel for nesting, food, and a water source during winter.
Although lichens typically grow in naturally harsh environments, most lichens, especially epiphytic fruticose species and those containing cyanobacteria, are sensitive to manufactured pollutants. Hence, they have been widely used as pollution indicator organisms. When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on Mount Rushmore National Memorial that requires the employment of mountain-climbing conservators to clean the monument.
Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce herbivory or kill bacteria. These compounds are very useful for lichen identification, and have had economic importance as dyes or primitive antibiotics. Extracts from many Usnea [3] species were used to treat wounds in Russia in the mid-twentieth century. Orcein and other lichen dyes have largely been replaced by synthetic versions [4].
The European Space Agency has discovered that lichens can survive unprotected in space. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen – Rhizocarpon geographicum and Xanthoria elegans – were sealed in a capsule and launched on a Russian Soyuz rocket on 31 May 2005. Once in orbit the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation. After 15 days the lichens were brought back to earth and were found to be in full health with no discernible damage from their time in orbit. [5]
Paleontology
The extreme habitats that lichens inhabit are not ordinarily conducive to producing fossils.[8] Though lichens may have been among the first photosynthesizers to colonize land,[citation needed] the oldest fossil lichens in which both symbiotic partners have been recovered date to the Early Devonian Rhynie chert, about 400 million years old.[9] The slightly older fossil Spongiophyton has also been interpreted as a lichen on morphological[10] and isotopic[11] grounds, although the isotopic basis is decidedly shaky.[12] It has been suggested - although not yet proven - that the even older fossil Nematothallus was a lichen.[13]
It has also been claimed that Ediacaran fossils were lichens;[14] although this claim was met with scepticism and has since been retracted by its author.[13] A lichen-like symbiosis, however, has been observed in marine[verification needed] fossils from the Ediacaran, 600 million years ago.[15]
Growth form
Lichens are informally classified by growth form into:
* crustose (paint-like, flat), e.g., Caloplaca flavescens
* filamentous (hair-like), e.g., Ephebe lanata
* foliose (leafy), e.g., Hypogymnia physodes
* fruticose (branched), e.g., Cladonia evansii, C. subtenuis, and Usnea australis
* leprose (powdery), e.g., Lepraria incana
* squamulose (consisting of small scale-like structures, lacking a lower cortex), e.g., Normandina pulchella
* gelatinous lichens, in which the cyanobacteria produce a polysaccharide that absorbs and retains water.
Gallery
Xanthoparmelia cf. lavicola, a foliose lichen, on basalt.
Usnea australis, a fruticose form, growing on a tree branch
Map lichen (Rhizocarpon geographicum) on rock
The cyanobacterium Hyella caespitosa with fungal hyphae in the lichen Pyrenocollema halodytes
Physcia millegrana (a foliose lichen), with an unlichenized polypore fungus (bottom right), on a fallen log.
Reindeer moss (Cladonia rangiferina)
Hypogymnia cf. tubulosa with Bryoria sp. and Tuckermannopsis sp. in the Canadian Rockies
Crustose lichens on limestone in Alta Murgia-Southern Italy
Cladonia cf. cristatella, a lichen commonly referred to as 'British Soldiers'. Notice the red tips.
Foliose lichens on rock growing outward and dying in the center. These lichens are at least several decades old.
Letharia sp. with Bryoria sp. on pine branches near Blackpine Lake, Washington
Xanthoria sp. lichen on volcanic rock in Craters of the Moon National Monument (Idaho, USA)
Lecanora cf. muralis lichen on the banks of the Bega canal in Timisoara
Overview
The body (thallus) of most lichens is quite different from that of either the fungus or alga growing separately, and may strikingly resemble simple plants in form and growth (Sanders 2001). The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations; however, in almost all kinds, the algal cells are never enclosed inside the fungal cells themselves. It has been suggested that the phycobiont is sometimes penetrated by haustoria from the mycobiont, but with the development of electron microscopy there is little solid evidence of this, and if true, is an isolated occurrence and in any event is entirely unecessesary. Thus lichens are poikilohydric, that is, capable of surviving extremely low levels of water content. However, the re-configuration of membranes following a period of dehydration requires several minutes at least. During this period a “soup” of metabolites from both the mycobiont and phycobiont leaks into the extracellar spaces. This is readily available to both bionts to uptake essential metabolic products ensuring a perfect level of mutualism showing leaching from the canopy mosses in Guadaloupe of numerous metabolites immediately following rehydration. Not only do the two bionts profit, but also all the other epiphytic organisms from the nutrient rich leachate. This fundamental phenomenon also points to a possible explanation of lichen evolution from its original phycobiont and mycobiont components with its subsequent migration from an aquatic environment to dry land. Thus, during repeated periods of low levels of hydration in an alga and the resultant leakage of beneficial metabolites to an adjacent aquatic fungus, the mutalistic “marriage” slowly became constant.
In the natural environment, lichen “provides” the alga with water and minerals that the fungus absorbs from whatever the lichen is growing on, its substrate. As for the alga, it uses the minerals and water to make food for the fungus and itself.
Algal and fungal components of some lichens have been cultured separately under laboratory conditions, but in the natural environment of a lichen, neither can grow and reproduce without a symbiotic partner. Indeed, although strains of cyanobacteria found in various cyanolichens are often closely related to one another, they differ from the most closely related free-living strains [1]. The lichen association is a close symbiosis: It extends the ecological range of both partners and is obligatory for their growth and reproduction in natural environoments. Propagules ("diaspores") typically contain cells from both partners, although the fungal components of so-called "fringe species" rely instead on algal cells dispersed by the “core species.”
There has nonetheless been controversy as to whether the lichen combination should be considered an example of mutualism or commensalism or even parasitism. An observation offered in support of this is that cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen. The same, however, might be said of isolated skin cells growing in laboratory culture, which grow more quickly than similar cells that are integrated into a functional tissue. However, from the work of Coxson (see above) mutualism would appear to best summarise our current knowledge.
Lichens are named based on the fungal component, which plays the primary role in determining the lichen's form. The fungus typically comprises the majority of a lichen's bulk, though in filamentous and gelatinous lichens this is not always the case. The lichen fungus is typically a member of the Ascomycota—rarely a member of the Basidiomycota, and then termed basidiolichens to differentiate them from the more common ascolichens. Formerly, some lichen taxonomists placed lichens in their own division, the Mycophycophyta, but this practice is no longer accepted because the components belong to separate lineages. Neither the ascolichens nor the basidiolichens form monophyletic lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum[3]. Even more unusual than basidiolichens is the fungus Geosiphon pyriforme, a member of the Glomeromycota that is unique in that it encloses a cyanobacterial symbiont inside its cells. Geosiphon is not usually considered to be a lichen, and its peculiar symbiosis was not recognized for many years. The genus is more closely allied to endomycorrhizal genera.
The algal or cyanobacterial cells are photosynthetic, and as in higher plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substrate. If a cyanobacterium is present, as a primary partner or another symbiont in addition to green alga as in certain tripartite lichens, they can fix atmospheric nitrogen, complementing the activities of the green alga.
Morphology and structure
Crustose and foliose lichens on a wall
Lichens are often the first to settle in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes.[citation needed] Some survive in the tough conditions of deserts, and others on frozen soil of the Arctic regions.[citation needed] Recent ESA research shows that lichen can even endure extended exposure to space.[4] Some lichens have the aspect of leaves (foliose lichens); others cover the substrate like a crust (crustose lichens) (illustration, right), others such as the genus Ramalina adopt shrubby forms (fruticose lichens), and there are gelatinous lichens such as the genus Collema.[5]
A "Hidden Mickey" formed by lichen on a stone.
Although the form of a lichen is determined by the genetic material of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of hyphae. If combined with its photobiont under appropriate conditions, its characteristic form emerges, in the process called morphogenesis (Brodo, Sharnoff & Sharnoff, 2001). In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were first found growing in a conjoined manner.
There is evidence to suggest that the lichen symbiosis is parasitic or commensalistic, rather than mutualistic (Ahmadjian 1993). However, this now needs to be re-examined in light of Coxons work. The photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Furthermore, photobiont cells are routinely destroyed in the course of nutrient exchange. The association is able to continue because photobiont cells reproduce faster than they are destroyed. (ibid.)
Cross-section through the lichen Pseudevernia furfuracea with plainly visible layer of green algae under the surface
Under magnification, a section through a typical foliose lichen thallus reveals four layers of interlaced fungal filaments. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the cortex, which can reach several hundred μm in thickness.[6] This cortex may be further topped by an epicortex 0.6-1μm thick in some Parmeliaceae, which may be with or without pores, and is secreted by cells - it is not itself cellular.[6] In lichens that include both green algal and cyanobacterial symbionts, the cyanobacteria may be held on the upper or lower surface in small pustules called cephalodia/cephalodium. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an haustorium.[citation needed] Beneath this algal layer is a third layer of loosely interwoven fungal hyphae without algal cells. This layer is called the medulla. Beneath the medulla, the bottom surface resembles the upper surface and is called the lower cortex, again consisting of densely packed fungal hyphae. The lower cortex often bears rootlike fungal structures known as rhizines, which serve to attach the thallus to the substrate on which it grows.[citation needed] Lichens also sometimes contain structures made from fungal metabolites, for example crustose lichens sometimes have a polysaccharide layer in the cortex.[citation needed] Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species. This seems to also be true of the photobiont species involved.[citation needed]
Reproduction
Thalli and apothecia on a foliose lichen
lichen
Many lichens reproduce asexually, either by vegetative reproduction or through the dispersal of diaspores containing algal and fungal cells. Soredia (singular soredium) are small groups of algal cells surrounded by fungal filaments that form in structures called soralia, from which the soredia can be dispersed by wind. Another form of diaspore are isidia, elongated outgrowths from the thallus that break off for mechanical dispersal. Fruticose lichens in particular can easily fragment. Due to the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns.
Many lichen fungi appear to reproduce sexually in a manner typical of fungi, producing spores that are presumably the result of sexual fusion and meiosis. Following dispersal, such fungal spores must meet with a compatible algal partner before a functional lichen can form. This may be a common form of reproduction in basidiolichens, which form fruitbodies resembling their nonlichenized relatives. Among the ascolichens, spores are produced in spore-producing bodies, the three most common spore body types are the apothecia, perithecia and the pycnidia. [2]
For reproduction, lichen possess isidia, soredia, and undergo simple fragmentation. These structures are also composed of a fungal hyphae wrapped around cyanobacteria. (Eichorn, Evert, and Raven, 2005) While the reproductive structures are all composed of the same components(Mycobiont and Photobiont) they are each unique in other ways. Isidia are small outgrowths on the exterior of the lichen. Soredia are powdery propagules that are released from the top of the thallus(1). In order to establish the lichen, the soredia propagules must contain both the photobiont and the mycobiont(2). [7]
[edit] Ecology
Lichens must compete with plants for access to sunlight, but because of their small size and slow growth, they thrive in places where higher plants have difficulty growing.
A major ecophysiological advantage of lichens is that they are poikilohydric (poikilo- variable, hydric- relating to water), meaning that though they have little control over the status of their hydration, they can tolerate irregular and extended periods of severe desiccation. Like some mosses, liverworts, ferns, and a few "resurrection plants", upon desiccation, lichens enter a metabolic suspension or stasis (known as cryptobiosis) in which the cells of the lichen symbionts are dehydrated to a degree that halts most biochemical activity. In this cryptobiotic state, lichens can survive wider extremes of temperature, radiation and drought in the harsh environments they often inhabit.
Lichens do not have roots and do not need to tap continuous reservoirs of water like most higher plants, thus they can grow in locations impossible for most plants, such as bare rock, sterile soil or sand, and various artificial structures such as walls, roofs and monuments. Many lichens also grow as epiphytes (epi- on the surface, phyte- plant) on other plants, particularly on the trunks and branches of trees. When growing on other plants, lichens are not parasites; they do not consume any part of the plant nor poison it. Some ground-dwelling lichens, such as members of the subgenus Cladina (reindeer lichens), however, produce chemicals which leach into the soil and inhibit the germination of plant seeds and growth of young plants. Stability (that is, longevity) of their substrate is a major factor of lichen habitats. Most lichens grow on stable rock surfaces or the bark of old trees, but many others grow on soil and sand. In these latter cases, lichens are often an important part of soil stabilization; indeed, in some desert ecosystems, vascular (higher) plant seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.
Pine forest with lichen ground-cover
Lichens may be eaten by some animals, such as reindeer, living in arctic regions. The larvae of a surprising number of Lepidoptera species feed exclusively on lichens. These include Common Footman and Marbled Beauty. However, lichens are very low in protein and high in carbohydrates, making them unsuitable for some animals. Lichens are also used by the Northern Flying Squirrel for nesting, food, and a water source during winter.
Although lichens typically grow in naturally harsh environments, most lichens, especially epiphytic fruticose species and those containing cyanobacteria, are sensitive to manufactured pollutants. Hence, they have been widely used as pollution indicator organisms. When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on Mount Rushmore National Memorial that requires the employment of mountain-climbing conservators to clean the monument.
Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce herbivory or kill bacteria. These compounds are very useful for lichen identification, and have had economic importance as dyes or primitive antibiotics. Extracts from many Usnea [3] species were used to treat wounds in Russia in the mid-twentieth century. Orcein and other lichen dyes have largely been replaced by synthetic versions [4].
The European Space Agency has discovered that lichens can survive unprotected in space. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen – Rhizocarpon geographicum and Xanthoria elegans – were sealed in a capsule and launched on a Russian Soyuz rocket on 31 May 2005. Once in orbit the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation. After 15 days the lichens were brought back to earth and were found to be in full health with no discernible damage from their time in orbit. [5]
Paleontology
The extreme habitats that lichens inhabit are not ordinarily conducive to producing fossils.[8] Though lichens may have been among the first photosynthesizers to colonize land,[citation needed] the oldest fossil lichens in which both symbiotic partners have been recovered date to the Early Devonian Rhynie chert, about 400 million years old.[9] The slightly older fossil Spongiophyton has also been interpreted as a lichen on morphological[10] and isotopic[11] grounds, although the isotopic basis is decidedly shaky.[12] It has been suggested - although not yet proven - that the even older fossil Nematothallus was a lichen.[13]
It has also been claimed that Ediacaran fossils were lichens;[14] although this claim was met with scepticism and has since been retracted by its author.[13] A lichen-like symbiosis, however, has been observed in marine[verification needed] fossils from the Ediacaran, 600 million years ago.[15]
Growth form
Lichens are informally classified by growth form into:
* crustose (paint-like, flat), e.g., Caloplaca flavescens
* filamentous (hair-like), e.g., Ephebe lanata
* foliose (leafy), e.g., Hypogymnia physodes
* fruticose (branched), e.g., Cladonia evansii, C. subtenuis, and Usnea australis
* leprose (powdery), e.g., Lepraria incana
* squamulose (consisting of small scale-like structures, lacking a lower cortex), e.g., Normandina pulchella
* gelatinous lichens, in which the cyanobacteria produce a polysaccharide that absorbs and retains water.
Gallery
Xanthoparmelia cf. lavicola, a foliose lichen, on basalt.
Usnea australis, a fruticose form, growing on a tree branch
Map lichen (Rhizocarpon geographicum) on rock
The cyanobacterium Hyella caespitosa with fungal hyphae in the lichen Pyrenocollema halodytes
Physcia millegrana (a foliose lichen), with an unlichenized polypore fungus (bottom right), on a fallen log.
Reindeer moss (Cladonia rangiferina)
Hypogymnia cf. tubulosa with Bryoria sp. and Tuckermannopsis sp. in the Canadian Rockies
Crustose lichens on limestone in Alta Murgia-Southern Italy
Cladonia cf. cristatella, a lichen commonly referred to as 'British Soldiers'. Notice the red tips.
Foliose lichens on rock growing outward and dying in the center. These lichens are at least several decades old.
Letharia sp. with Bryoria sp. on pine branches near Blackpine Lake, Washington
Xanthoria sp. lichen on volcanic rock in Craters of the Moon National Monument (Idaho, USA)
Lecanora cf. muralis lichen on the banks of the Bega canal in Timisoara
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Sabtu, 2008 Desember 13
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