Some protists
superficially resemble fungi in that they are not photosynthetic and their
bodies are often formed of threadlike structure called hyphae. However
fungus-like protists are not fungi for several reasons. Many produce
flagellated cells, which the fungi lack. Many of these protists also have
centrioles and produce cellulose as a major component of their cell walls,
whereas fungi lack centrioles and have cell walls of chitin. They are slime
molds and water molds.
Slime Molds or Myxomycota
Usually plasmodial
slime molds exist as a plasmodium. It is a diploid multinucleated cytoplasmic
mass enveloped by slime sheath. The Plasmodium streams over damp, decaying logs
and leaf litter, often forming a network of channels to cover a larger surface area.
As it creeps along, it ingests bacteria, yeast spores, and decaying organic
matter.
At times unfavorable to
growth, such as during drought the Plasmodium develops many sporangia. A
sporangium (Gk. Spora, seed, and angeion, vessel) is a reproductive structure
that produces spores by meiosis. The spores can survive until moisture is
sufficient for them to germinate. In Plasmodial slime, spores release a haploid
flagellated cell or an amoeboid cell. Eventually two of them fuse to form a
diploid zygote that feeds and grows, producing a multinucleated Plasmodium once
again.
Slime molds are fungus
like in ne phase of their life cycle and amoeba like in another phase of their
life cycle.
Slime molds are fungus like
Slime molds are similar
in some respect to fungi i.e. body is filamentous, saprotroph formation of
zygote, and having non-motile spores. Slime molds differ from fungi due to the
presence of motility in the life cycle.
Characteristics of
slime molds are interesting to biologists because the life cycle involves many
changes in form. These different forms resemble other types of protists.
Water Molds or Oomycotes
Oomycotes include water
molds, white rusts and downy mildews. They show the following characteristics:
All of the members of
the group are either parasites or saprotrophs i.e. they feed on dead organic
matter.
The Cell wall Contains
Cellulose, not chitin like fungi.
Their life cycles are
characterized by gamete meiosis resulting in a diploid phase.
The filamentous
structures are called hyphae as in fungi. The hyphae are aseptate i.e. without
intercellular cell wall.
Most oomycetes live in
fresh water or salt water or in soil. Some are plant parasites. A few aquatic
oomycetes are animal parasites.
Zoospores are motile
and have two flagella. Zoospores are produced asexually in sporangium.
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| Life cycle of Oomycetes |
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| Physarum |
For sexual reproduction
there are two types of gametangia. The female gamentagium is called oogonium
and the male gamentagium is called an antheridium.
The antheridia contain
numerous male nuclei which are functional male gametes and the oogonia contain
from one to eight eggs which are female gametes. The flowing of the contents of
an antheridium into an oogonium leads to the individual fusion of one or more
pairs of male nuclei with eggs. This is followed by the thickening of the cell
wall around the resulting zygote or zygotes. This produces a special kind of
thick walled cell called an oospore. The structure gives the phylum its name
i.e. phylum oomycota.
Phytophthora Infestons
It is a plant pathogen
which causes late blight of potato. The mycelium of Phytopthora infestons is
branched Aseptate hyphae which lives in the intercellular spaces of leaves. It
obtains its nourishment from the mesophyll cell by short specialized branches
known as haustorias which penetrate them.
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Phytophthora
infestans
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Asexual
Reproduction: In warm and humid conditions the
mycelium produces long and slender structures called sporangiophores, which
emerge from the lower surface of the leaf through stomata. These branches give
rise to sporangia. In warm conditions sporangia may behave as spores. Hyphae
emerge from the sporangium and penetrate the plant through a stoma. In cool
conditions the sporangium content may divide to form swimming spores, which
when released, swim in surface of film of moisture. They may encyst until
conditions are suitable once more for hyphal growth and produce new infection.
Sexual
reproduction: It takes place only in artificial
culture. The sex organs are antheridia and oogonia, borne at the tip of
specialized hyphal branches.
Amoebas
They are free living
organisms found in fresh water, marine, soil, and also as parasites of animals.
Amoeba move and feed with the help of pseudopodia. A pseudopodium is formed
when the cytoplasm streams forward in a particular direction amoeba proteus has
a nucleus, many food vacuoles and a contractile vacuole.
Entamoeba histolytica
is a parasite that lives in the human intestine and causes amoebic dysentery.
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Amoeba
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Zooflagellates
Protozoa that move by
means of flagella are called zooflagellates. They are covered by a pellicle.
These are mostly unicellular having a central nucleus and flagella are usually
located at the anterior end. Flagellates may be free-living, symbionts or
parasite. They obtain their food either by ingesting living or dead organisms
or by absorbing nutrients from dead or decomposing organic matter. Flagellates
usually reproduce by transverse binary fission.
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Trichonympha
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Trichonymphas are
complex specialized flagellates with many flagella. They live as symbionts in
the gut of the termites. It contains a bacterium that enzymatically converts
the cellulose of wood to soluble carbohydrates that are easily digested by the
insect.
Trypanosoma is a human
parasitic flagelate. It is transmitted by the bite of tsetse (se-se) fly and is
the cause of African sleeping sickness.
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| Pelomyxa Palustris |
The
Giant Amoeba: Pelomyxa Palustris is the giant Amoeba.
It is the most primitive of all eukaryotic forms. It has multiple
membrane-bound nuclei, but no other organelles. It ha methanogenic bacteria
from which the amoeba obtains energy. Giant amoebas inhabit mud at the bottom
of ponds. Its function is the degradation of molecules.
Choanoflagellates:
A
marine or freshwater flagellate is sessile and remains attached by a stalk.
Flagellum is surrounded by a delicate collar which resembles to the collar
cells of sponges. They do not have cell wall and have no internal digestive
system of organelles. They absorb food through cell membrane sometime using
flagella.
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| A colonial Choanoflagellate |
Ciliates
Ciliates get their name
from a Latin word meaning “eyelash”, a name that is description of the fact
that all or parts of these cells are covered with hair like extensions called
cilia. These cilia beat in unison, moving the cell about (forward and backward)
and creating currents that move particles toward the gullet of the cell. Some
ciliates are sessile and remain attached to a rock or other surface. Most
ciliates are holozoic. During asexual reproduction ciliates divide by
transverse binary fission. Ciliates have two types of nuclei, a large
macronucleus and one (or more) small micronucleus. The macronucleus controls
the normal metabolism of the cell, while the micronuclei are concerned with
reproduction. Sexual reproduction involves conjugation, during which two
individuals come together and exchange genetic material e.g. Paramecium and
Vorticella.
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| Paramecium |
Foraminifera and Actinopods
These are marine
protozoan. They produce tests or shells. In foraminifera (commonly called
forams) shells are made up of calcium. In actinopods shells are made up of
silica. The shells contain pores through which cytoplasmic projections can be
extended. These cytoplasmic projections form a sticky and inter connected net
that entangle prey. Dead foraminiferans sink to the bottom of the ocean where
their shells form a grey mud that is gradually transformed into chalk.
Foraminiferans of the past have created vast lime stone.
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| Foraminifera |
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Actinopods
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Apicomplexans
This is a large group
of parasitic protozoa. Some cause diseases in man e.g. malaria. They have no
locomotory organs and they move by flexing. They need two hosts to complete
their life cycle. Spore is the infective stage which is transmitted to the next
host e.g. Plasmodium (malarial parasite).
Life cycle of Plasmodium
The life cycle of Plasmodium
in Anopheles mosquito was studied by Grassi in 1898. He discovered the
relationship between man, mosquito and malarial parasite. The life cycle of
Plasmodium consists of two parts, asexual cycle and sexual cycle.
Asexual
cycle: It takes place in man. When an infected female
Anopheles bites a person, several thousands of sporozoites (the infected stage
of Plasmodium) find their way into the human blood. The sporozoites invade
liver and remain there for 6-12 days. The sporozoite grows and divides into
merozoites. A merozoite enters red blood cell. A merozoite grows, enlarges and
divides to form many merozoites. The red blood cell bursts and merozoites are
released. The released merozoites infect new red blood cell and the process is
repeated. The simultaneous bursting of millions of red blood cells causes the
symptoms of malaria-chill followed by fever.
Sexual cycle: It takes
place in mosquito. After a repeated asexual cycle the merozoites grows into
gametocytes in the red blood cells. When a female Anopheles bites a malarial
patient, it sucks the malarial parasites. In the stomach of the mosquito male
gametes are produced by a series of changes in the male gametocytes. Female
gametocyte becomes mature into female gamete. The male and female gametes conjugate
and fuse to form the zygote (2n). The zygote becomes worm like and is called
ookinete. The ookinete reaches the stomach epithelium where it rounds itself
off and becomes enclosed in a cyst. At this stage it is known as oocyst (2n).
The oocyst forms filamentous sporozoites (n) and the process is called
sporogony. The sporozoites migrate into the mosquito’s salivary glands to
infect the next person bitten.
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| Life cycle of Plasmodium |
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