VIRUSES, BACTERIA AND THE LOWER PLANTS
Bacteria are very important, although structurally they are simple group. They are said to be plant mainly because they posses cell wall in addition to their cell membrane.
Generally bacteria lack chlorophyll, therefore they are large heterotrophy, few species of bacteria contain pigment similarly to chlorophyll and are photosynthetic.
Bacteria chlorophyll is similar to chlorophyll A, but its maximum absorption is on the infra red of the absorption spectra.
Some of the bacteria are chemosynthetic autotrophs. Bacteria metabolism is extremely variable and sometimes quite complicated, but its useful in the classification in the bacteria.
CLASSIFICATION AND GENERAL CHARACTERISTICS OF BACTERIA
Bacteria are group in the old classification under kingdom – plantar. This is because bacteria has cell wall, some members are autotrophs been photosynthetic or chemosynthetic. In today dispensation, bacteria are grouped in the kingdom monera because hey are prokaryote.
CLASSIFICATION OF BACTERIA
Taxonomic position of bacteria
- Kingdom – schizomycota (shizomycophyta)
- Class – schizomycetes
- Order – include 9 orders
- Family – several families
- General – Clostridium, Salmonella
Microbial classification systems developed step by step with advances in the method used to examine microorganism. Early system for classifying bacteria relied on microscopic observations that allowed the determination of morphological characteristics and the growth of pure culture that allowed physiological properties of bacteria species to be examined.
Many critical methods for characterizing bacteria such as Gram-stain were developed in the laboratory of Robert Koch. Gram-staining techniques was developed by Hans Christian Gram in 1984, it is a development of differential staining method of bacteria.
The gram-staining technique exploit the differences between two staining variation which is essential to the classification of bacteria today.
The Gram-stain procedure is the most widely used differential staining method in bacteriology. This staining procedure begin with primary stain using crystal violet, which stains bacteria cells blue-purple followed by application of Gram iodine which is a mordant (a substances that fixes the primary stain in the bacteria cells.)
Then the sample is decolourize with acetone alcohol or some other decolourization agents. Thereafter the bacteria cell are counter stained with red stain safranin which stain the bacteria that were decolorized in the previous steps so that they can be easily visualized.
CRITERIA IN BACTERIA CLASSIFICATION
There are more than 50 methods that could be done to be classify a single bacteria cell, but at least 20 methods must be used to successfully classify a bacteria cell.
- MORPHOLOGY, SIZE SHAPES OR FORMS
( rod, spherical, ring, helica, filamentous or coil) using light microscope or electron microscope.
- BIOCHEMICAL – This is the best and most accurate method of classifying bacteria
- Indole production
- Methyl red staining
- Citrate utilization
- Urea hydrolysis
- Types of enzymes present which may include lipase, amylase, protease
- Present or absence of pigment
- Mobility (presence or absence of flagella)
- Ecology of the bacteria – The habitat or source of the habitat
STRUCTURE OF BACTERIA
Bacteria has three basic forms. These forms are COCCI (spherical), bacilli (straight, rod shape) and spirilla (long curved). The cocci or bacilla may be joined to form colony or filament.
The group of bacteria knowns as cocci are either spherical, ellipsoidal or bean shaped and sub-divided into six groups according to their cell division, cell arrangement and biological properties. These groups include:
- Micrococci or monococci – The group of bacteria that have spherical shape arrange singly. They are saprophyte that live in water or air.
- Diplococci – This group attached in pairs after cell division along one plane. They include the bacteria that causes certain disease like pneumonia, caused Diplococcics pneumoniae cerebral meaningites caused by meanincoccus spp, and Gonorrhea caused by Gonococcus Spp.
- Streptococci – They are arranged in chains of varying length. Some causes diseases of human beings.
- Tetracocci – They are divided into two planes to form different of four, they rarely cause diseases of human beings.
- Staphylococci – They are divided in several planes resulting in irregular branches of cells. Some cause diseases in man and other animals.
- Sarcinae – They are divided in three plane at right angle to one another and occurs in group of 8, 16 or more. They are often found in the air.
The rod shape or cylindrical bacteria are called bacilli or bacillus (singular). This can be classified on the basis of their arrangement just as cocci. We can have microbacillus, diplobacillus, streptobacollus etc.
The spiral shaped bacteria are grouped into ribriosis and spirilla. The vibrioses or vibrain resemble a comma e.g Vibrio lomina which causes cholera.
The spirilla are coiled form of bacteria which may be twisted. One pathogenic specie is Spirillium mine which cause rat-bite fever in human being
The individual bacteria cells differs from other forms of organisms in their nuclear organization.
Generally, bacteria are prokaryote i.e they lack nuclear membrane, they lack contractile vacuoles. The cell in bacteria contain DNA, some genera have flagella and can move. The cell wall consists of protein rather than cellulose. The bacteria cytoplasm contains granules of glycogen glycoprotein and pigment as well as calcium and sulphur. Some of the granules stain reddish- purple with methylene-blue, while other strains of bacteria stain blue. There are no chloroplast in most bacteria cells.
The presence of mitochondria has been demonstrated is some bacteria. A rigid cell wall is present in a true bacteria but absent in pirocheata and mycobacter
The bacteria cell wall posses Osmosis permeability under certain environmental condition. The nuclear material lack nuclear membrane because of this, they are called prokaryotes.
FLAGELLATION IN BACTERIA
The mobile bacteria posses flagella which are classified or divided into four groups depending on the number and the arrangement of the flagella. The four types include:
- Monotrichates – They have single flagellum at the end of the cell. It might be anteriously or posteriously positioned.
- Amphitrichates – They have two or more flagella at both ends.
- Lophotichates – They have a taft of flagella at only one end.
- Peritrichates – They have flagella distributed over the whole surface of their body
REPRODUCTION IN BACTERIA
Reproduction in bacteria is usually sexual or asexual.
Asexual reproduction in bacteria is by binary fission, it may be repeatedly occur in every 15-20 minutes.
It has been experimented that if one bacterium multiply at this rate for 24hrs, it will produced about 10 individuals and the total rate of individual will be enormous. If there are no competition, and the environment is favourable for the growth of bacteria, then the whole world would have been covered with bacteria. Asexual reproduction in bacteria through binary fission is the spiting of the single cell into two diploid cell. Though some bacteria can reproduce through budding.
Although complex sexual reproduction involving alternating meiosis and syngamy is limited to eukaryotic organisms, genetic recombination has been observed to take place in bacteria.
In the most general terms, this recombination involves the transfer of a portion of a DNA molecule from one bacteria cell to another. This fragment may simply act in concert with the DNA molecule of the cell which it enters, with both producing messenger RNA, or it may actually become incorporated into the main DNA molecules of the recipient cell, which cases, it is passed on to the daughter cells with the rest of the hereditary material
NUTRITION IN BACTERIA
Bacteria are predominantly heterotrophic, most of them required the supply of ready made food.
The saprophytic species obtain organic requirement from dead plant or animal remains found in the soil, water or sewage and many types of food.
Some bacteria are parasitic, by obtaining their food substances from living organisms of various form.
Some are symbiotic, being associated with organism with which they share mutual benefit. Examples are those that live in the intestine of man and other animals and those that live in the root noddles of leguminous plants.
Some bacteria are autotrophic, they are able to synthesise their own food.
Others are heterotrophic, living as parasites or saprophytes.
Photosynthetic bacteria: bacteria photosynthesis occurs under anaerobic condition and molecular oxygen is neither consumed nor liberated in the process.
Photosynthetic bacteria develops a pigment closely related to chlorophyll and they include the purple sulphure and green bacteria which utilize light as a source of energy to decompose H2S into hydrogen and sulphur.
The bacteria therefore can carry out photosynthesis where others can not. The hydrogen released is used to reduce C02 to carbohydrate in a series of dark reaction as in higher plants, and sulphur instead oxygen is released.
Photosynthesis in Bacteria C6H1206
6c02 + 12 H2s C6 H1206 + H20 + H2S
In bacteria, water is been released, instead of been consumed as found in higher plants during photosynthesis
6C02 + 6H20 C6H1206 + 602
Soil bacteria: Different groups of micro-organisms are involved in specific stages of the decomposing and recycling processes in the soil. Many bacteria and fungi break down carbon – containing compounds, releasing CO2 into the atmosphere.
The most important compounds originating from plants are cellulose and lignin, and secondary ones are pectic substances, starch and sugars. It has been estimated that more than 90 percent of the C02 production in the biosphere results from the activity of fungi and bacteria.
Some micro-organisms break down proteins into peptides which are subsequently broken into their constituent amino acids. Many micro-organisms have the ability to break down amino acids with consequent release of ammonium (NH+4), a process known as ammonification. The ammonia can be oxidized to nitrate (NO –2) by chemoautotrophic bacteria of the genus Nitrosomonas and the nitrites are oxidized to nitrates (N03–) by Nitrobacter. These reactions constitute the process of nitrification. Both these processes released energy which is used to reduce carbon dioxide (C02) to carbohydrate. Several other kinds of bacteria are capable of reversing the process and changing the nitrites back to nitrates or ammonia.
Denitrification is the conversion of nitrates to nitrogen gas or nitrous oxide, results is the loss of nitrogen from the soil.
The reverse of this process, which is extremely important biologically is nitrogen fixation. Several genera of bacteria and blue-green algae, as well as some fungi (yeasts) are capable of nitrogen fixation. Outstanding among them is the symbiotic bacteria Rhizobium which forms nodules on the roots of legumes and a few other plants.
Some of man’s diseases are caused by air-borne bacteria, among the better known of these is diphtheria, which is caused by the bacterium Corynebaterium diptheriae. This organism produces a powerful toxic substance that circulates rapidly throughout the body and causes serious damages to the heart muscle, nervous tissues and kidneys. Diphtheria is now rare, because most children are immunized against it in the infancy.
Other serious disease that is caused by air-borne bacteria of the genus streptococcus, which are associated with scarlet fever, rheumatic fever and other infections. Other air-borne bacteria diseases include pneumonia mostly caused by Diplococcus Pneumoniae and whooping cough caused by Bordetella Pertusis.
A number of other disease of bacteria origin are spread in food or water.
The typhoid fever and para-typhoid are caused by bacteria of the genus Salmonella bacillary dysentery is caused by shigella dyserteriae, cholera is caused by Vibrio lomina. Tetanus caused by Clostridium tetanis., Anthrax is caused by Bacillus anthracis, syphilis is caused by Treponema Platinum, Boil. Is caused by Micrococcus aureus, Gonorrhea is caused by Gonoccous spp. Undulant fever, caused by bacteria of the genus Brucella, affects both cattle and man and it is usually contacted through drinking milk, or eating milk products from an infected cow.
Pasteurization of milk destroys Brucella, and the disease has become extremely rare in many portions of the world.
Bacteria play an enormous role in spoiling food and other stored organic products, and some organisms of this type are pathogenic. Food poisoning by Clostridium Botulinum is rare but extremely dangerous.
Many economically important disease of plants are also association with bacteria.
Some of the more important diseases of plants are called soft rots, blights, or wilts. Sometimes death is associated with an invasion and phigging of the xylem by large numbers of bacteria and the slime they produce.
The viruses do not fit easily into any of the traditional categories into which living organisms are classified, and the problem of categorizing them is made even more difficult by the fact that there is considerable doubt about whether or not they should be considered living. Viruses are also of great importance as agent of disease, being the causative agents for smallpox, chicken pox, measles, German measles, mumps, influenza, colds, infections hepatitis, yellow fever, polio, and rabies. Viruses are also responsible for many important diseases of domestic animals and plants. Although many types of virus disease can be prevented by immunization once established they are relatively difficult to control, as they do not respond to antibiotics.
THE NATURE OF VIRUSES
- Most viruses which attack plants contain RNA while most which attach bacteria and animals contain DNA.
- Sometimes the nucleic acid core is present as a helix and sometimes as a folded thread.
- They are smallest living organisms.
- They can only reproduce by invading living cells.
- They are obligate endoparasites i.e they can only live parasitically inside cells.
- Viruses are highly specific to their host.
The existence of viruses was first recognized when it was found that the causative agents of certain diseases could pass through the porcelain filters commonly used to trap bacteria. In size they range about 17 to more than 300 nanometers.
Viruses are parasites that can multiply only within a host cell and are highly specific with regard to the type of cell in which they can multiply. In the host cell, they essentially “take over” the direction of the metabolism, using their own nucleic acids to “command” the host cytoplasm to produce more virus particles. They compete with the genetic material of the host cell, which is similar to their own, in regulating cell functions. Measles viruses and other rash-causing viruses multiply in the cells of the skin. The polio virus only about 28 nanometers in diameters multiplies in the intestinal tract and sometimes in the nerve cell.
All viruses seem to consist essentially of two components, protein and nucleic acid. The protein encloses the nucleic acid, which may be either RNA or DNA and not both.
Viruses are exceedingly small ranging from 12 to 800mm. Because of their small size they are to pass through the porcelain fitters that restrain bacteria. It is impossible to see them with a conventional microscope as they are below its power of resolution, in recent years it has been possible to see. Rinderpest and hand pad in dogs, polyhedral diseases of caterpillars, swine fever in rabbit all are caused by viruses and the vector is an insect.
Virus also caused cancer in both plants and animals and HIV.
HIV – Human Immune-deficiency virus.
AIDS – Acquired Immune Deficiency Syndrome.
The Structure of Viruses:
By the use of electron microscopy, the structure of a large number of viruses has now been elucidated. The most common architecntural arrangement is an icosahedrons, a 20-sided figure, which, as geometricians have calculated, is the most efficient symmetrical arrangement that subunits can be in to form an other shell with maximum internal capacity.
Many viruses appear to consist of an outer protein envelope, which encloses nucleic acid. The virus is thus mainly composed of genetic material. Viruses called bacteria-phages may have a tail-like structure, which is important in gaining entry to a host cell.
N.B. Bacteriophages are bacteria viruses some can be made to form crystals; showing that they are pure protein molecules.
All plant viruses contain RNA, the vast majority of bacteria viruses (Bacterophages) contain DNA, while about 50% of animal viruses contain RNA called RIBOVIRUSES; the remaining 50% possess DNA called DE-RIBOVIRUS.
Virus have a capsid protective of protein surrounding the core. It is made up of smaller unit called capsomeres. A virus without capsid has no role to play in injecting organisms. In the complex have an additional Lipoprotein later around the capsid derived from the cell surface membrane of the host cell.
Shape: There are three main shapes of viruses (a) Sphere (b) rods (c) tadpole-like bacteriophages.
- Viruses of man and animals
- Viruses of bacteria/Bacteriophages
- Viruses of insects and worms
- Viruses of plants
- This division is based on host specificity, which implies that a virus can inject a specific host. The Relication of Viruses. The mode of replication of particular viruses depends on their genetic constitutions.
The protein coat of viruses determines their attachment to host plasma membrane and their entry into the cell. But all viruses shedthis coat before they begin to replicate themselves. In some, such as the bacteriaophages, it is digested by the enzymes of the host cell. When this has been accomplished, one of two things happens.
- A reaction may occur within the cell that prevents virus multiplication and the viral DNA may be inserted in a linear fashion into the bacterial chromosome. In such a state, the virus is called a prophage. Phages that are capable of existing in prophage form are called temperate phages.
The viruses involved in transduction are emperate phages. Temperate phages do not destroy their host cells unless they “escape” from the host chromosome, and they are in turn virtually unassailable by the host’s immune defense systems. This phenomenon has been demonstrable only in DNA bacterio-phages.
- The virus may multiply. Virus multiplication takes place in three steps. First, the virus nucleic acids direct the host cells to produce new viral enzymes. Viral nucleic acids and structural proteins are then synthesized, each in its appropriate amount. Finally these materials are assembled into virus particles. These steps generally overlap in time, often thousands of new virus particles per cell. When the process of virus multiplication is complete, the particles escape from the host cell, which is generally dead by that time.
In viruses that contain DNA- such as the vaccinia virus, an organisms that causes a pox-like disease in cattle the viral DNA simply directs the synthesis of a series of different messenger RNA molecules, which direct the production of different proteins. The DNA is usually double-stranded, but single-stranded DNA occurs in some very small bacteria viruses. In most RNA viruses, such as the tobacco mosaic virus, the RNA is single-stranded. This viral RNA replicates itself, presumably by directing the formation of a complementary strand, which then serves as the template for new viral RNA molecules. The viral RNA also takes over the ribosome of the host cell and acts as a messenger RNA. It is responsible for the synthesis of enzymes and virus coat proteins.
Viral activity may profoundly affect the metabolism of the host cell. In the bacterium clostridium, it has been shown for some strains that the production of the lethal toxins associated with botulism takes place only with the active and continued participation of specific bacteriophages.
Noninfected bacterial cells do not produce the toxin. Even more surprisingly, infection by other specific bacteriophages causes the same bacterial strain to produce the toxins associated with gas gangrene and many other diseases in animals. The causeative organisms associated with botulism and gas gangrene had hither – to been considered to be different species of clostridium but they are at least in part, the same species infected by different bacteriophages
IMPORTANCE OF VIRUSES
All viruses are parasitic and they are collectively the cause of very many animal and plant diseases. In man warts, measles, small pox, poliomyelitis, rabies, influenza, a type of pneumonia, yellow fever, dengue fever and some form of enteritis are also virus infections.
In plants many types of mosaic diseases, especially of potato, tobacco and tomato, petal mosaic of many flowers. Swollen shoot diseases of cocoa are due to virus infections. Virulence of an organisms depends on its ability to penetrate and reproduce within the host’s tissues or body fluids and to injure these by the secretion of toxins.
Also the ability of the bacteria to cause injury when they have invaded the tissues is largely due to their ability to produce toxins. Exotoxns are secreted externally by the bacteria into the surroundings while endotoxins are substances secreted internally and released only on death and break-up of the bacteria cell.
VIRUSES AND CANCER
It is well known that viruses causes cancers and cancer-like growths in many animals and plants and the possibility that viruses are a cause of cancer in humans has been a frequent questions. Viruses can change from infections to non-infections forms and they mutate readily to produce strains with new sets of characteristics. Though it has not been possible to isolate cancer-causing viruses from any human cancer, such viruses could be present in very small numbers and still produce the metabolic disorders and consequent rampant cell division characteristics of cancer.
Viruses affect their hosts differently depending on the genetics of the hosts and its physiological condition, and the presence of one kind of virus in a cell sometimes enhances the effect of another.
REDUPLICATION OF VIRUSES
As viruses have no cytoplasm they produce no enzymes and hence are unable to carry out such functions as respiration, which are associated with living things. However, they have the power of multiplication, and this is not the same as normal reproduction, as the virus does not grow or divide and cannot reproduce on its own, but only with the help of living protoplasm from a plant or animal cell. The way it happens has been discovered in viruses that attack bacterial membrane dissolved and the DNA content of the phage passes into the cell leaving the protein coat behind. Inside the bacillus, the virus DNA makes replicas of itself. New protein envelops are then formed, the cell burst and about 200 exact copies of the phage come out and infect other cells.
Members of many virus families are known to infect insects. Some use insects as agents for spreading to populations of susceptible animals and plants. These include Flaviviridae and Togaviridae, whose members cause yellow fever, West Nile disease, and several types of viral encephalitis. Other virus families use insects as primary hosts of these, probably the most important are the Baculoviridae, Reoviridae, and polydnaviridae.
VIRUSES OF FUNGI AND PROTISTS
Most mycoviruses, viruses that infect fungi, have been isolated from higher fungi such as Penicillum and Aspergillus. They contain dsRNA and have isometric capsids (that is, their capsids are roughly spherical polyhedra), which are approximately 25 to 50 nm in diameter. Many appear to be latent viruses.
Some mycoviruses induce disease symptoms in hosts in isolated protoplasts of plant cells just as phages and some animal virus are cultivated in call suspensions. However, many cannot grow in protoplast cultures and must be inoculated into whole plants or tissue preparations. This makes studying the life cycle difficult. Many plant viruses require insect vectors for transmission; some of these can be grown in monolayers of cell cultures derived from aphids, leaf hoppers, or other insects. Almost all plant viruses are RNA viruses, either single stranded or double stranded.
Like all viruses, plant viruses must penetrate a host cell before they can reproduce. However, penetration of plant cells is hampered by the fact they are protected by complex outer layers, including cell walls. Entry of a plant virus into its host requires the presence of mechanical damage to the cell wall, and this is usually caused by insect or other animals that feed on plants. Particularly important are sucking insects such as aphids and leaf hoppers. These insects not only create an entryway for the virus, they are often responsible plant. As the insect feeds on the plant, it can pick up a virus particle on its mouth parts.
GENERAL PROPERTIES OF VIRUSES
Viruses are a unique group of infectious agents whose distinctiveness resides in their simple, acellular organization and pattern of reproduction. A complete virus particle consists of one or more molecules of DNA or RNA enclosed in a coat of protein. Some viruses have additional layers that can be very complex and contain carbohydrates, lipids, and additional proteins. Viruses can exist in two phases; extracellular and intracellular, the extracellular phase, possess few if any enzymes and cannot reproduce independent of living cells. In the intracellular phase, viruses exist primarily as replicating nucleic acids that induce host metabolism to synthesize virion components.
In summary viruses differ from living cells in at least three ways (1) their simple, acellular organization; (2) the presence of either DNA or RNA, but not both, in almost all virion; and (3) their inability to reproduce independent of cells and carryout cell division as prokaryotes and encaryotes do.
Although if has long been recognized that viruses can infect plants and causes a variety of diseases, plant viruses generally have not been as well studied as bacterio phages and animal viruses. This is partly because they are often difficult to cultivate and purify. Some viruses, such as tobacco mosaic viruses (TMV); can be grown (true fungi)
THE FUNGI (EUMYCOTA)
- Fungi are widely distributed and are found whenever moisture is present. They are of great importance to humans in both beneficial and harmful ways.
- Fungi exist primarily as filamentous hyphae. A mass of hyphae is called a mycelium
- Like some bacteria and protists, fungi digest insoluble organic matter by secreting exoenzymes, then absorbing the solubilized nutrients.
- Two reproductive structures occurs in fungi: (a) Sporangia form asexual spores and (b) gametangia form sexual gaments.
- Like the study of protists, fungal systematic is an area of active research. Eight fungal subdivisions are presented, including the chitridomycetes, Zygomycota, Ascomycota, Basidiomycota, Urediomycetes; Ustililagmomycetes, Glomeromycota and microsporiadia.
- The chitridiomycetes are a group of terrestrial and aquatic fungi that reproduce by motile zoospores with single, posterior whiplash flagella.
- The zygomycota are characterized by resting structures called sygospores-cell in which zygotes are formed.
- The Ascomycota form zygotes within a characteristics saclike structure, the ascus. The ascus contains two or more ascospores.
- Yeasts are unicellular fungi – most are ascomycetes.
- Basidiomycetes posses dikaryotic hyphae, one of each mating type. The hyphae divide uniquely, forming basidiocarps within which club-shaped basidia can be found. The basidia bear two or more basidiospores.
- The ustilagmomycetes and urediniomycetes include important plant pathogens, whereas the Glomermycota form important associations with vascular plants and enhance plant nutrient uptake. Some members of the microsporidia are considered emerging pathogens of humans.
Mycologist is a scientist who study fungi use. The term fungus (plural, fungi) mushroom is describe as eucaryotic organisms that are spore-bearing, have absorptive nutrition, lack chlorophyll, and reproduce sexually and asexually. The scientific discipline devoted to fungi is called mycology; and the disease caused by fungi in animals are known as mycoses.
Fungi are primarily terrestrial organisms, although a few are freshwater or marine. They have a global distribution from polar to tropical regions. Many are pathogenic and have beneficial relationships with other organisms. For example, the vast majority of vascular plant roots form associations called mycorrhizae with fungi. Fungi also are found in the upper portions of many plants. These enclophytic fungi affect plant reproduction and palatability to herbivores. Lichens are associations of fungi and photosynthetic proteits or cyanobacteria.
Microorganism associations with vascular plants is known as mycorrhizae.
About 90,000 fungal species have been described, however, some estimates suggest that 1.5 million species may exist. Fungi are important to humans in both beneficial and harmful ways. With bacteria and a few other groups of chemoorganotrophic organisms, fungi act as decomposers, a role of enormous significance. They degrade complex organic materials in the environment to simple organic compound and inorganic molecules. In this way carbon, nitrogen, phosphorus, and other critical constituents of dead organisms are released and made available for living organisms.
On the other hand, fungi are a major causes of disease. Plants are particularly vulnerable to fungal diseases because fungi can invade leaves through their stomata. Over 5,000 species attack economically valuable crops, garden plants and many wild plants. Fungi also cause many disease of animals and human.
Fungi, especially the yeasts, are essential to many industrial processes involving fermentation.
Examples include the making of bread, wine, and beer. Fungi also play a major role in the preparation of some cheeses, soy sauce, and sufu; in the commercial production of many organic acids (citric, gallic) and certain drugs (ergometrine and cortisone) also in the manufacture of many antibiotics e.g.
Penicillin and griseofulvin and in the immunosuppressive drug cyclosporine.
Finally, fungi are important research tools in the study of fundamental biological processes. Cytologist, biochemists, biophysicist, microbilogists regularly use fungi in their research. The yeast Saccharomyces cerevisiae is the best understood evcaryotic cell. It has been a valuable model organism in the study of cell biology, genetics, and cancer.
The body or vegetative structure of a fungus is called a thallus. It varies in complexity and size, ranging from the single-cell microscopic yeasts to multicellular molds, macroceopic puffballs, and mushrooms. The fungal cell is usually encased in a cell wall of chitin. Chitin is a strong but flexible nitrogen-containing polysaccharide consisting of N-acetyglucosamine residues.
A yeast is a unicellular fungus that has a single nucleus and reproduces either asexually through spore formation. Each bud that separates can grow into a new yeast, and some group together to form colonies.
Generally yeast cells are larger than bacteria, vary considerably in size, and are commonly spherical to egg shaped.
They lack flagella but possess most of the other eucaryotic organelles.
The thallus of a mold consists of long, branched, threadlike filaments of cells called hyphae that form a mycelium, a tangled mass or tissue like aggregation of hyphae. In some fungi, protoplasm streams through hyphae, uninterrupted by cross walls. These hyphae are called coenocytic or asepate. The hyphae of other fungi have cross walls called septa with either a single pore or multiple pores that enable cytoplasmic streaming. These hyphae are termed septate.
Hyphae are composed of an outer cell wall and an inner lumen, which contains the cytosol and organelles. A plasma membrame surrounds the cytoplasm and lies next to the cell wall. The filamentous nature of hyphae results in a large surface area relative to the volume of cytoplasm. This makes adequate nutrient absorption possible.
Many fungi, especially those that cause diseases in humans and animals, are dimorphic-that is, they have two forms. Dimorphic fungi can change from the yeast form in the animal to the mold or mycelial form in the external environment in response to changes in various environmental factors (nutrients, C02 tension, oxidation-reduction potentials, temperature).
NUTRITION AND METABOLISM
Fungi grow best in dark, moist habitats where there is little danger of desiccation, but they care found wherever organic material is available. Most fungi are saprophytes, securing their nutrients from dead organic material. Like many bacteria and protests, fungi release hydrolytic exoenzymes that digest external substrates. They then absorb the soluble products – a process sometimes called polysaccharide in fungi. Most fungi use carbolydrates (preferably glucose or maltose) and nitrogenous compounds to synthesize their own amino acids and proteins.
Fungi usually are aerobic. Some yeasts, however, are facultatively anaerobic and can obtain energy by fermeritation.
Many fungal fermentations are industrial importance, such as the production of ethyl alcohol in the manufacture of beer and wine. Obligately anaerobic fungi are found in the rumen of cattle.
REPRODUCTION IN FUNGI
Reproduction in fungi can either asexual or sexual. Asexual reproduction is accomplished in several ways:
- A parent cell can undergo mitosis and divide into two daughter cells by a central constriction and formation of a new cell wall.
- Mitosis in vegetative cells may be concurrent with budding to produce a daughter cell. This is very common in the yeast.
- The most common method of asexual reproduction is spore production. Asexual spore formation occurs in an individual fungus through mitosis and subsequent cell division. There are several types of asexual spores, each with its own name:
Sexual reproduction in fungi involves the fussion of compatible nucei. Homothallic fungal species are self-fertilizing and produce sexually compatible gametes on the same mycelium.
Heterothallic species require outcrosing between different but sexually compatible mycelia. It has long been held that sexual reproduction must occur between mycelia of opposite mating type (MAT).
However, one instance of same-sex mating was discovered following an outbreak of the pathogenic yeast crytococcus gatti in Canada. Depending on the species, sexual fusion may occur between haploid gametes, gamete-producing bodies called gametangia, or hyphae. Sometimes both the cytoplasm and haploid nuclei fuse immediately to produce the diploid zygote. The spores produced in the ascomycetes is known as ascospore while in basidomycetes is basidiospore and in zygomycetes it is known as zygospore.
Fungal spores are important for several reasons. The spores enable fungi to survive environmental stresses such as desiccation, nutrient limitation, and extreme temperatures, although they are not as stress resistant as bacterial endospores. Because they are often small and light, spore can remain suspended in air for long periods. Thus they frequently aid in fungi dissemination, a significant factor that helps explain the wide distribution of many fungi. Fungal spores often spread by adhering to the bodies of insects and other animals. The size, shape, colour, and number of spores are useful in the identification of fungal species.
CHARACTERISTICS OF SOME FUNGAL DIVISIONS
- Ascomycota-This contain fungi called ascomycetes, commonly known as sac fungi. Ascomycetes are ecologically important in fresh water, marine, and terrestrial habitats because they degrade many chemically stable organic compounds including lignin, cellulose, and collagen (2) Most of the red, brown, and blue-green molds that cause food spoilage are ascomycetes (3) the powdery mudew that attack plant leaves and the fungi that cause chestnut blight and Dutch elm disease are ascomycetes (4) Many yeast as well as edible morels and truffles are ascomycetes. (5) The pink bread mold Neurospora crassa also is an ascomycetes, is an extremely important research tool in genetics and biochemistry. (6) Mnay ascomycetes are parasites on higher plants. Claviceps purpurea parasitizes of rye and other grasses, causing the plant disease ergot. Ergotism, the toxic condition in humans and animals who eat grain infected with the fungus, is often accompanied by gangrene, psychotic delusions, nervous spasms, abortion, and convulsions.
- Basidiomycota – This includes the basidiomycetes, commonly known as club-fungi. Examples include Jelly Fungi, rusts, shelf fungi, stinkhorms, puffballs, toadstools, mushrooms etc. basidiomycetes are named for their characteristics structure or cell, the basidium, which is involved in sexual reproduction. A basidium is produced at the tip of hyphae and normally is club shaped. Two or more basidiospores are produced by the basidium, and basidia may be held within fruiting bodies called basidiocarps.
The basidiomycetes affect humans in many ways. Most are saprophytes that decompose plant debris, especially cellulose and lignin. For example, the common fungus polyporus squamosus forms large, shelflike structures that project from the lower portion of dead trees, which they help decompose. The fruiting body can reach 2 feet in diameter and has many pores (hence the name polyporus), each lined with basidia that produce basidispores. Thus a single fruiting body can produce millions of spores. Many mushrooms are used as food throughout the world. The cultivation of the mushroom Agaricus Compresus is a multimillion-dollar business of course not all mushrooms are edible, as suggested by its name, ingestion of Russula emetica induces vomiting.
Many mushrooms produce specific alkaloids that act as either poisions or hallucinogens. One such example is the “death angle” mushroom, Amanita Phalloides.
The basidiomycete Cryptococus neoformans is an important human and animal pathogen. It produces the disease called cryptococcosis, a systemic infection primarily involving the lungs and central nervous system.
- Zygomycota – This fungi zygomycetes belong to this division. Most live on decaying plant and animals matter in the soil, a few are parasites of plants, insects, other animals, and humans.
The bread mold, Rhizopus stolonifer, is a very common member of this division. This fungus grows on the surface of moist, carbohydrate –rich foods, such as breads, fruits, and vegetables. On breads, for example, Rhizopu’s hyphae rapidly cover the surface hyphae called rhizoids extend into the bread and absorb nutrients.
Rhizopus usually reproduces asexually, but food becomes scarce or environmental conditions unfavourable, it begins sexual reproduction. Sexual reproduction requires compatible strains of opposite mating types. These have traditionally been labeled (+) and (-) strains because they are not morphologically distinguishable as male and female. When the two mating strains are close hormones are produced that cause their hyphae to form projection called progametangia and then mature gametangia.After fusion of the gametangia, the nuclei of the two gametes fuse, forming a zygote. The zygote develops a thick, rough, black coat and becomes a dormant zygospore. Meiosis often occurs at the time of germination, the zygospore then splits open and produces a hupha that bears an asexual sporangium and the cycle begins anew.
The genus Rhizopus is also important because it is involved in the rice disease known as seedling blight. If one considers that rice feeds more people on Earth than any other crop, the implications of this disease are obvious. It was thought that Rhizopus secreted a toxin that kills rice seedlings so scientists set about isolating the toxin and the genes that produce it.
The zygomycetes also contribute to human welfare. For example, one species of Rhizopus is used in Indonesia to produce a food called tempeh from boiled, skinless soybeans. Another zygotmycete (Mucor spp.) is used with soybeans in Asia to make a curd called Sufu. Others are employed in the commercial preparation of some anesthetics, birth control agents, in industrial alcohols, meat tenderizers, and the yellow coloring used in margaring and butter substitutes.
- Chytridiomycota – They are unique among fungi in the production of a zoospore with a single, posterior, whiplash flagellum. This is considered a primitive feature that was lost in more evolved fungi. Free-living members of this taxon are saprotrophic living on plant or animal matter in fresh water, mud, or soil. Parasitic forms infect aquatic plants and animal including insects. A few are found in the anoxic rumen of herbvious. They have life-cycle showing both sexual and asexual reproduction, members are microscopic in size and may consist of a single cell, a small multinucleate mass, or a true mycelium with hyphae capable of penetrating porous substrates. Sexual reproduction results in the release of sporangio spore from sporangia at the surface.
- What are mycorrhizae and why are they important?
- Described the life cycle of a typical basidiomytcetes, Discuss their importance
- Describe the ascomycetes life cycle. How are the ascomycetes important to humans?
- What are the chytridiomycetes? How do they differ from other fungi?
- How do yeast reproduce sexually? Why do you think saccharomyces cerevisiae has become such an important model organism?
This is the division of the plant kingdom which includes the mosses and liverworts. They are thought to have evolved from green algae.
Bryophytes may be terrestrial, epiphytic or aquatic. It exhibits regular alternation of generation in which the gametophytes is the dominant. There is no true vascular tissue (xylem and phloem). Bryophytes are relatively small plants that grows on moist places on land. Damped floor, beside stream and pools. Some species can survive period of drought but only become dormant and seize to grow during unfavourable conditions.
The bryophytes can basically be grouped into mosses, liverworts and hornworts. Accordingly, all bryophytes, strictly speaking, lack true leaves, true stems, and true roots., Nevertheless, the terms leaf and stem are commonly used when referring to the leaf-like and stem-like structures of gametophytes of leafy liverworts and mosses.
In most bryophytes, the gametophytes is attached to the substrate by means of elongated single cells or filaments of cells called rhizoids.
The rhizoids generally serve only to anchor the plants, for absorption of water and minerals commonly occurs directly and rapidly through out the gametophyte. Root-like structures are lacking. The second important distinguishing characteristics of the bryophytes is the nature of their alternation of generations. The gametophytes are larger and always nutritionally independent, whereas the sporophytes are smaller, varyingly dependent upon, and permanently attached to the gametophytes. In other words, the gametophyte is the conspicuous and dominant generation in the bryophytes.
The bryophytes have sometimes been referred to as the amphibians of the plant kingdom. In order for fertilization to take place, the bifliagellated sperms must swim through water to reach the egg inside archegonum.
The archegonium, which may be stalked, is flask-shaped with a long neck and a basal swollen portion, the venter, enclosing a single egg. The elongated spherical antheridium is commonly stalked and consist of a one-cell thick sterile jacket layer surrounding numerous spermatogenus cell.
The zygote is retained within the venter of the archegonium where it developed into an embryo. For a period, the venter of the archegonium undergoes cell division, keeping pace with growth of the young sporophyte. The enlarged archegonium is called a calyptra. At maturity the sporophyte of many bryophytes consists of a foot, which remains embedded in the archegonium, a stalk or seta and a capsule or sporangium. Generally, the cells of the young and maturing sporophyte contain chlorophyll and carry out photosynthesis, but the time meiosis occurs in the capsule and the spores are produced, the chlorophyll has usually disappeared.
The division Bryophyta is traditionally divided into three classes: Hepaticae (liverworts), Autyhocerothe (Hornworts), and the musci (the mosses)
THE LIVERWORTS: CLASSIFICATION
Liverworts are small plants that are generally less conspicuous than mosses. The gametophytes of some liverworts are flattened, dorsiventral, thalli, which grow from an apical meristem.
The gametophytes of most species, however, are leafy and grow from a single apical cell, which resemble an inverted pyramid with a base and three sides. The rhizoids of liverworts are single-celled, unlike those of mosses which contain several cells each. The gametophytes generally develop directly from spores. The sporophytes of liverworts are in general less complex than those of mosses, and their capsules have very different mechanisms for the release of spores.
Liverworts can be differentiated into thallose liverworts and leafy liverworts. Thallose liverworts are non-leafy, they can be found on moist, shaded banks and in other suitable habitats such as flowerpots in a cool green house. The lower surface bears two kinds of rhizoids, as well as rows of scales. The upper surface is divided into raised regions, each of which marks the limits of an under-lying air chamber and has a large pore that leads to this chamber.
Fragmentation constitutes the principal means of asexual reproduction in the liverworts. Another fairly widespread means of asexual reproduction in the liverworts and mosses is the production of gemmae, minute lens-shaped bodies that can give rise to new plants. The gemmae are produced in special cuplike structures called gemma cups located on the dorsal surface of the gametophyte.
Leafy liverworts are diverse group that include more than two-thirds of all known liverworts. The plants are usually well branched and form small mats. Their leaves are often two-lobed, and each grows by means of two distint apical growing points. The antheridia and archegonia of the leafy liverworts are characteristically enclosed in a cup-like structure formed by the fusion of two or three leaves. The leafy liverworts are especially abundant in the tropics and sub-tropics, in regions of heavy rainfall or high humidity, and they are also present in large number in temperate regions. Examples of the leafy liverworts that can be seen around us are:- Funaria Spongiosa Funaria dilatata, Mastigolegenea florae
REPRODUCTION IN LEAFY LIVERWORTS
This is of two types (a) Asexual reproduction (b) sexual reproduction .
Asexual reproduction can be sub-divided into two (1) vegetative fragmentation (ii) Regeneration. Any part of the plant that breaks off can automatically develops into a new plant i.e regeneration.
The second type is the production of gemae. Gemae are specialized unit of reproduction, they are produced at the apex of a leaf or at the apex of a stem.
THE MOSSES: (CLASS MUSCI)
Mosses are epiphytes on trees, they grown on rocks, many of them are able to withstand desiccation (dryness). Those one that grows as epiphytes of savanna trees remains in a dried up state for months, as soon as water is back, they become rehydrated and become to grow again. It has been discovered that they survive the dryness because of the nature of the DNA that makes up there chromosomes.
The life cycle has two phases: (a) gametophytic phase (b) sporophytic phase. Some mosses do not reproduce sexually and as such, they had no sporophyte, this is due to the geographical barrier, mutation and other factors.
Those that are not reproducing sexually however reproduce vegetatively. Mosses are long lived plant, the stem of mosses bears rhizoids that are multicellular and are either erect or creeping. The creeping ones are refers to as PLEUROCARPOUS while the erect ones are called ACROCARPOUS. The rhizoids are for anchorage but can be used for absorption of food and mineral nutrients.
The gametophytes of all mosses are represented by two distinct phases; the protonema (first thread), which arises directly from a germinating spore, and the leafy gametophyte. The protonema is a uniseriate (the cells occurring in unicellular rows), branching filament which superficially resembles a filamentous green algae. Protonema are likewise found in some liverworts.
In true mosses, the gametophyte is leafy and usually upright, not dorsiventrally flatterned as it is in the leafy liverworts. The gametophytes of mosses ranges from a few millimeters to 5 decimeters or more in length and they exhibit varying degreees of differentia-tion and complexity.
All have multicellular rhizoid, and the cell leaves are normally only one layer thick except at the midrib. In some mosses, such as the common Polytrichum, there is often a central strand of elongated cells in the stem which may function in conduction, but many other general lack such specialized tissues.
At maturity, gametangia are produced by most leafy gametophytes, either at the tip of the main axis or on a lateral branch. In some general, the gametophytes are unisexual, whereas in others both archegonia and antheridia are produced by the same plant.
Sporophytes in the mosses are often small in relation to the gametophytes. The capsules are generally elevated on a stalk, which may exceptionally reach 15 to 20 centimeters in length, although some lack a stalk entirely.
In mosses, the stalk usually elongates early in the development of the sporophyte, and the sporophyte is important in photosynthesis. The sporophyte of a moss is therefore much less dependent nutritionally upon the gametophytes than the sporophyte of a liverwort.
When the sporophyte of a moss is matured, it gradually loses its ability to photosynthesis and turns yellow, then orange and later brown. Eventually the lip or operculum of the capsule burst off, revealing an opening that is usually ringed with series of peristome teeth.
Spore mother cell Egg(n) Antherozoid
Sporophyte phace Fertilization
LIFE CYCLE OF MARCHANTIA: Showing alternation of Generation
Comparism between Mosses and Liverworts
- Leaves of leafy liverwort e.g porella have no midrib while those in mosses have midrib
- In liverworts the rhizoids are unicellular and commonly not branched while in the mosses they are multicellular and generally branched.
- In liverworts the protonema is mostly absent or small, while in mosses it is distinct and well developed.
- The sporophyte in Riccia is simple and lies embedded in the thallus. In marchnatia it is differentiated into foot, seta and capsule.
- Both shows regular alternation of generations
- Elaters are mostly present in liverworts, but absent in mosses.
They are seedless vascular plants (tracheophyta) although most bryophytes lives on land, though they are not fully terrestrial. The tracheophyte by contrast have evolved a host of adaptation to the terrestrial environment that have enable them to invade all but the most in hospitable land habitat.
DIVISION TRACHEOPHYTA (Examples)
SUB DIVISION: Psilotopsida (Psilopsida) psilotum
: Lycopsida (Club mosses) Lycopodium
: Spheropsida (horsetails (Equisetum)
: Pteropsida (Ferns) (Pryopyeris
: Spermopsida (Seed Plants)
All members of these sub-division posses the following attributes:
- The have a protective layer of sterile jacket cells around the reproductive organs
- They have multicellular embryo retained within the archegonia
- They have cuticles in aerial parts
- They posses conducting vessels
- They posses flagellated sperms.
Pteridophytes are the most primitive of the land plants. They have a well developed vascular system but not as those of the spermatophyte, they are living and they formed fossils, examples include Psilotum, Selaginella, Nephrolepsis, Lycopodium and others. They have a life cycle showing a regular alternation of generation with a diploid sporophyte or asexual generation alternating with haploid gametophytes or sexual generation. The sporophyte however is the dominant generation; this is different from what is obtain in the bryophytes where the gametophytes is the dominant generation.
The gametophyte in the pteridophytes is independent of the sporophyte, Gametophytes and the sporophyte is differentiated into stem, leaf and root whereas the sporophyte of the Bryophyte is made up of the capsule and it is dependent of the gametophyte. The gametophyte in pteridophyte is also called PROTHALLUS; the sporophyte on the other hand is called STROBILUS.
The sex organs may be borne on the prothallus or may be borne on different male and female prothallus, hence the evolution of diecious and monoecious in the plant kingdom can be traced from the pteridophytes.
Fertilization is Oogamy, it resembles that of the Bryophytes i.e actively swimming antherozoid fertilized a non-motile egg to form a diploid zygote. The zygote developed into the diploid sporophyte, the sporophytes produces spore in special structures called sporangia. This sporarangia are usually borne on special leaf called sporophyll.
GENERALIZED LIFE CYCLE OF SEEDLESS VASCULAR PLANTS
As in Bryophytes, sexual reproduction in seedless vascular plants entails alternation of generation and this consist of the Heteromorphic diploid and haploid phases. Meiosis produces haploid spores in sporangia of diploid sporophyte. The spore germinate and grow into gametophyte that produces eggs in the archegonia and sperms in antheridia. The sperm must have water to swim to the archegonia where fertilization occurs to produce a diploid zygote. The zygote now starts another diploid phase.
Diploid phase Meiosis m
Fertilization Spores (haploid)
Sperm Gametophyte Spore germinate
The algae are very diverse and extensive group of plants which shows a great range of somatic organization and complexity of reproduction and life history. Algae are green plant that can photosynthesize, they are mainly primary producers, they are found in aquatic environment, many of them serves as food for many aquatic organisms. They can be found in moist or wet substrata, some of the algae are unicellular e.g Chlamydomoas, Euglena. Some of them live in colony e.g Volvox, Nostoc. They have a filamentous organization e.g Spirogyra.
Economic Important of Algae
- In ecosystem, algae are primary producer, therefore they are very important in food chain because they are carbon fixer to all the energy level especially in the aquatic ecosystem.
- Algae are sources of food to man. Some algae are delicacious especially in china and Japan where some sea weeds are eaten.
- Some algae are vital in Nitrogen cycle because they fixed atmospheric nitrogen e.g Nostoc.
- Algae bloom (excessive growth of Algae) due to the washing of fertilizer into seas and rivers where algae inhabit, thus makes them grow at a very high rate in the aquatic environment.
- Some antibiotics such as chloramyphenicol are produced by algae.
- Agar is a micro-biological medium produced from algae for the growth of micro-organisms
- Alganic acid is also produced from algae, it can be used as stabilizer in the production of ice-cream and confectionary.
- Organic fertilizer can be produce from sea weeds.
- Aeration of water can be done by algae through the production of oxygen by the process of photosynthesis.
- Algae reduces the amount of C02 in the atmosphere thus reducing global waroning.
DIVISION OF ALGAE
- CYANOPHYTA i.e blue-green algae because they contain chlorophyll and blue pigment as the two pigment found in them.
They are called cyanobacteria because they have properties that resemble bacteria. They are called prokaryotic because they lack membrane.
Examples are Nostoc, Oscillatoria
- Chlorophyta (Green algae), example include Volvox, Spirogyra, Chlamydomonas, Ulva. The green algae are the most diverse of all the algae, both in form and in life history. They are the most primitive class of the algae. Almost most green algae are aquatic, they are found in a wide variety of habitats, including the surface of snow, in green patches on tree trunks, and as symbionts in lichens, protozoa and hydra. Of the aquatic species, a few groups are entirely marine, but the great majority are found in fresh water.
The chlorophyta are similar to the bryophytes and contain chlorophyll a and b, they stored their food as true starch, and have firm cell walls composed in most genera of cellulose, with herricelluloses and pectic substances in corporated into the wall structure, pyrenoids are present in the chloroplast.
- PHAEOPHYTA (brown algae): An almost entirely marine group, comprise mostly of the conspicuous seaweeds of temperate regions. Beside chlorophyll, the brown pigment fucoxanthin is present. Polysaccarides other than starch are stored and the thallus tends to be large and well differentiated. In some brown algae such as Laminaria, alternation between haploid and diploid generation is found but in Fucus species the diploid generation is completely dominant.
- RHODOPHYTA (Red algae). The red algae have no flagellated cell, and have complex life cycles. The red algae lack centriole. Red algae usually attached to tocks or other algae, there are few floating forms. Most red algae are composed of filaments.
- EUGLENOPHYTA (Euglenoids): A small fresh water group which includes Euglena, its systematic position is some-what difficult to determine as its members have various animal-like features and are referred to by the zoologists as flagellate protozoans
FORMS OF ALGAE
There are about five different morphological forms of algae, they are:
- Colonial form
- Filamentous form
- Unicellular form
- Siphoaceous form
- Paraenchymatous form
COLONIAL FORM:- These are algae which are made up of colonies, and the colony consists of a group of unicellular cells which lives together. There are three types of colonial forms.
- Tetrasporal Colonies : They are not motile and the cells are embedded in material called mucilage.
- Non-flagellated Colonies: They are made up of non-motile cell which are more or less fused together, they are embedded in mucilage
- Flagellated Colonies : They are motile cells with the aid of flagellum.
FILAMENTIOUS FORM: The body are made up of filament and are formed by repeated transverse division of cells. There are two types of filamentous form in algae i.e the unbranched filament and the branched filament.
Cell wall rupture
Division of Nucleus
REPRODUCTION IN ALGAE
- Asexual reproduction – inside the cell, there is mitotic division that occurs in the nucleus to form 4 or 8 nuclei (daughter). Each of the nuclei then gathers the cytoplasm and become daughter in the parent cell. The parent cell then burst when the daughter are matured and set free.
- Sexual reproduction:- Division of the gamete produce the gamete release and fuse with forming a zygote divide by meiosis to form mature cells.
The lichens are a large group of Ascomycetes that can grow only in intimate association with living algal cells. Obtaining nutrition from these algae, they have invaded the harsliest habitats, such as bare rock.
Lichens are extremely widespread in nature, they occur from arid desert region to the arctic and grow on bare soil, tree truncks, sun baked rocks, fence posts. They are often the first colonist of bare rocky areas. The colour of lichens ranges from white to black, through shades of red, orange, yellow and green, and there are many unusual chemical compounds in them.
BIOLOGY OF THE LICHENS
Why can the lichens survive under environmental conditions so adverse to other form of life?
At one time, it was thought that the secret of the lichen’s success was that the fungal tissue protected one of the chief factors in lichen survival seems to be the fact that they dry out very rapidly. Lichen’s are frequently very desiccated in nature, with a water content ranging from only 2 to 10 percent of their dry weights. When the lichen dries out, photosynthesis ceases, and in this state of suspended animation, great extremes of heat or cold can be endured.
When a lichen is wetted by rain, it absorbs 3 to 35 times its own weight in water in a very short time.
If a dry brittle lichen thallus is submerged in water, it will become soft within a few minutes.
This is the simple physical process of inhibition lichen takes up water in much the same way a blotting paper does and a dead lichen absorbs about as much water as a live one.
The lichen reaches its maximum vitality, as judged by the rate of photosynthesis, after it has been soaked with water and begun to dry, its rate of photosynthesis reaches a peak when the water content is 65 to 90 percent of the maximum it can hold; below this level, if the lichen continues to loose water, the rate of photosynthesis decreases.
Lichens apparently absorb some minerals from their substrate, but most of the elements absorbed by lichens enter the body through the air and in rainfall. Lichens absorb elements from rainfall rapidly and concentrate them within their thallus. Because they have no means of excreating these elements, lichens are susceptible and sensitive to toxic compounds; absorption of toxic compound by lichens causes degradation of the chlorophyll.
IMPORTANCE OF LICHEN
- Lichen can be used for medicinal purposes
- They are natural indicators of industrial pollution of the air
- It is important in the perfume industry
- As a source of natural dye
- A number of animals utilize lichen as food.
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- Willey et al (2008), The microbiology of food me Graw-Hill.