Protista — Algae_ Protozoa_ Slim by pengxiang


									Protista — Algae, Protozoa, Slime
Molds, and Water Molds

Protists are eukaryotic creatures (meaning they have a true
nucleus.) They’re not plants, animals or fungi, but they act
enough like them that scientists believe protists paved the
way for the evolution of early plants, animals, and fungi.
Protists fall into four general subgroups: unicellular algae,
protozoa, slime molds, and water molds.

Algae are plant-like microorganisms that preceded plants in
developing photosynthesis, the ability to turn sunlight into
energy. Algae cells contain light-absorbing chloroplasts and
produce oxygen through photosynthesis.

Although plants generally get the credit for producing the
oxygen we breathe, some 75% or more of the oxygen in the
planet’s atmosphere is actually produced by photosynthetic
algae and cyanobacteria.

Algae also play an important role as the foundation for the
aquatic food chain. All higher aquatic life forms depend
either directly or indirectly on microscopic gardens of algae.

Most unicellular algae live in water, some dwell in moist soil,
and others join with fungi to form lichens.

Green algae
The most clearly plant-like algae, this species gets its
namesake hue from high levels of chlorophyll.

Their cell walls are made up of cellulose, the same material
that makes up the cell walls in larger, multicellular plants.
Like plants, they store the food they make through
photosynthesis as starches. Growing in large masses, these
algae can form visible layers of slick, green scum on the
surfaces and sides of ponds, puddles or damp soil.

Fossil records suggest that the first green algae originated 500
to 600 million years ago. Early algae probably gave rise to
multicellular plants.

These algae hardly look like plants, but more like flying
saucers, tiny canoes or cigars, lobed leaves, the undersides of
mushroom caps, striated ribbons, or filigreed Christmas
ornaments. Whatever their shape, all diatoms have shell-like,
brittle cell walls made out of silica (glass) and pectin. The
walls are two interlocking halves or shells that fit together
like a pillbox.

Because they depend on sunlight for photosynthesis, diatoms
generally live in the upper 200 meters of oceans and bodies of
fresh water. Some species of diatoms simply float in the
water currents near the surface. Others attach themselves to
larger floating objects or to the sea floor. When diatoms die,
they slowly sink to the sea floor. The buildup of trillions of
these shells forms a crumbly white sediment known as
diatomaceous earth or diatomite, which is used in
manufacturing pool filters and abrasives, including

Dinoflagellates have long whip-like structures called flagella
that let them turn, maneuver and spin about through the
water. About 90% of these algae dwell in the ocean.

Some species glow in the dark in a process called
bioluminescence. These species contain a compound called
luciferin (the same compound found in fireflies). The glow
increases markedly if the algae cells are agitated, as when a
ship churns through the water.

About half the species of dinoflagellates are photosynthetic;
the other half are predators that attack bacteria, algae, and
even fish.

Dinoflagellate neurotoxins can concentrate in the bodies of
shellfish and fish that eat the algal cells, in turn causing
people who eat these seafoods to come down with illnesses
such as paralytic shellfish poisoning and ciguatera (a
combination of gastrointestinal, neurological, and
cardiovascular disorders.)

So-called “red tides” occur when enormous blooms of trillions
of dinoflagellates are triggered by an upwelling of nutrients
from the water’s depths during warmer seasons. The
population of dinoflagellates can jump to more than 20
million cells per liter of sea water along some coasts during
these blooms, turning the water a reddish hue.

The name protozoa means “first animals.” As the principal
hunters and grazers of the microbial world, protozoa play a
key role in maintaining the balance of bacterial, algal, and
other microbial life. They also are themselves an important
food source for larger creatures and the basis of many food

Protozoa have been found in almost every kind of soil
environment from peat bogs to arid desert sands. They teem
in the deep sea as well as near the surface of waters, and can
be found even in frigid Arctic and Antarctic waters.

Some species of protozoa are part of the normal microbial
flora of animals, and live in the guts of insects and mammals,
helping to break down complex food particles into simpler
molecules. A very small number of species cause disease in
people, including Plasmodium vivax, which causes malaria.

The four main subgroups of protozoa are the ciliates, the
flagellates, the sarcodina, and the apicomplexans.

Ciliates are covered in part or entirely with what look like
little bristles called cilia (the Latin word for eyelash.)

The cilia are used for locomotion, and to snag bacteria, algae
and other food and direct it into the ciliate’s mouth-like
Ciliates include both grazers that dine on algae and bacterial
cells and predators that attack and gulp down other protozoa.
Grazers include Paramecium and Vorticella. An example of a
predatory ciliate is Didinium.

Ciliates are among the most complex of all single-celled
creatures, with a diverse array of structures and organelles
that perform a range of activities, from finding and catching
food, digesting it, excreting it, moving about, respiring,
sensing environmental conditions, and balancing the fluids
inside their cells.

A few ciliates can grow up to 2 millimeters in length, big
enough to be seen without a microscope.

Similarly complex single-celled organisms, flagellates have
whip-like appendages called flagella sticking out of their

The flagella are used for locomotion and to direct food
particles or cells into the organism’s mouth-like opening.
Flagellates dine on bacteria, algae, and other protozoa.

Several well-known flagellates cause parasitic diseases, such
as trypanosomes that cause sleeping sickness, and Giardia
lamblia, a parasite found in mountain streams and rivers that
causes severe gastrointestinal distress.

This subgroup of protozoa includes the familiar shape-shifting
amoebas, as well as heliozoa, radiozoa, and foraminifera (or
forams for short).

Sarcodina are best known for their pseudopods (“false feet”)
used for locomotion and feeding. In amoebas, these
pseudopods are generally lobe-like bulges that extend from
the cell membrane. In heliozoa, radiozoa, and forams, the
pseudopods more resemble needles or spikes sticking out from
the cells.

Sarcodinas use their pseudopods to engulf or latch onto prey,
which may include bacteria, algae or other protozoa. Many
amoebas are active predators, oozing about on their false
feet until they come into contact with a suitable meal. At
that point, the pseudopod flows around and engulfs the
hapless prey, until it is completely “swallowed.”

Heliozoa, radiozoa, and forams tend to be passive grazers and
predators, relying on suitable food swimming or drifting past
to come into contact with their pseudopods.

Several species in the sarcodina group, including some species
of amoebas, cover themselves with protective shell-like
coverings called tests. These tests are stippled with many
small and large openings through which water can flow in and
out and through which the pseudopods protrude.

The tests of radiozoa are made up of silica (the same
substance in diatom cell walls) and can form very intricate,
lacy designs that may be studded with long spines that
increase buoyancy and ward off predators.
The tests of forams are made up of sand grains or organic
compounds. These can become quite large, the biggest
reaching a little over 2 inches in diameter, making them some
of the largest single-celled organisms known.

When forams die, their tests sink and accumulate in large
batches; the Great Pyramids of Egypt are built from sandstone
composed largely of fossilized giant Nummulites, an ancient
kind of foram. The famous White Cliffs of Dover are limestone
cliffs formed from the skeletal remains of forams.

These protozoa are obligatory intracellular parasites: they
must spend at least part if not all of their life cycle in a host

Apicomplexans are characterized by the presence of special
organelles (tiny organ-like structures) located at the tips
(apices) of the cells. These organelles contain enzymes that
punch through, slice open and otherwise penetrate host

The best known apicomplexan is Plasmodium, the agent that
causes malaria. Plasmodium spends part of its life cycle in
mosquitoes and the other part in human hosts where it
ultimately infects and ruptures blood cells in large numbers.

Another familiar apicomplexan is Cryptosporidium parvum.
This water-borne parasite forms extremely durable cyst-like
structures that enable it to survive UV radiation and
sometimes chlorine in swimming pools and treated water. An
outbreak of Cryptosporidium in Milwaukee’s drinking water
supply in 1993 killed 50 people and sickened more than
400,000. (See Water Quality: Pathogens and Diseases.)

ซลล์ โดยมีระบบต่างๆ ภายในตัวเองอย่างสมบูรณ์ เช่น การสืบพันธุ์
การย่อยอาหาร การหายใจ และการขับถ่าย โปรโตซัวมีมากกว่า 3 หมื่นชนิด
ประมาณเกือบ 1
วมทั้งมนุษย์ การดารงชีพแบ่งเป็น 2 กลุ่ม คือ พวกที่อาศัยอยู่อย่างอิสระ
(free living protozoa) และพวกที่ต้องอาศัยสิ่งมีชีวิตอื่นร่วมด้วย
(symbyosis) อาจจะเป็นแบบ commensalism หรือ
                     ภาพโครงสร้างภายใน ของโปรโตซัว

โครงสร้างและหน้าที่ (form and function)
    โปรโตซัวเป็นสัตว์เซลล์เดียว มีขนาดตั้งแต่ 1 ไมโครเมตร จนถึง 150
       ไมโครเมตร มีนิวเคลียสอย่างน้อย 1 อัน หรืออาจจะมีมากกว่านั้น
ที่สาคัญไซโทพลาสซึมมีเยื่อหุ้ม มีออร์แกเนลล์สาคัญที่ทางานคล้ายสัตว์ชั้นสูง
        ได้แก่ ไมโทคอนเดรีย Golgi's body, endoplasmic,
                            reticulem ฯลฯ
    มีการดารงชีวิตได้หลายแบบ เช่น แบบอยู่อย่างอิสระ (free living)
    หรือเป็นปรสิต(parasitism)การสืบพันธุ์แบบไม่มีเพศ (asexual)
        โดยการแบ่งตัว(binary fission) แตกหน่อ(Budding)
การสร้างเกราะหุ้มตัวเองแล้วแบ่งนิวเคลียสและแบ่งไซโตพลาสซึมตาม ส่วน
     การสืบพันธุ์แบบ เพศ (sexual) โดยสืบพันธุ์แบบใช้เซลล์เพศ

            การแพร่กระจาย (transmission)
          การแพร่กระจายของโปรโตซัวไปอยู่ในสถานที่ต่าง ๆ
เพื่อประโยชน์ในการสืบพันธุ์และดารงชีพ เช่น
หรือติดไปกับแมลง หรือต้องอาศัยแมลงนาไปจากโฮสต์หนึ่


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