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The
Diversity of The World of Life
Eukaryotes
(Domain Eukaryota)
Alveolates
(Alveolates)
Representatives
Note:
"Alveolates" are a closely related group of protists.
Ciliates:
Didinium,
Paramecium, Stentor,
Vorticella,
etc. (Ciliata)
Dinoflagellates:
Red Tide Pathogens, Whirling Whips, etc.
(Dinoflagellata)
Apicomplexan
Sporozoa: Malaria Parasites etc.
(Apicomplexa)
Biology
ENVIRONMENTS
Various
alveolates live
in salt- or fresh-water, in soil, or in the bodies of animals.
Dinoflagellates
live in saltwater and freshwater. Dinoflagellates are a major
component of marine "phytoplankton" (microscopic, drifting
"plant"-life -- the major producers of food in the sea and of
oxygen in the atmosphere).
Some
dinoflagellates live "symbiotically" within the bodies of such
other organisms as protists, sponges, corals, jellyfish, or
flatworms: The dinoflagellates receive shelter and nutrients from
their host; the host, carbohydrates from the dinoflagellates
photosynthesizing and reproducing
within.
OVERALL
STRUCTURE
In ciliates, the "pellicle" (the dense outer
layer of cytoplasm) gives shape to the cell, typically covered with fine,
hair-like "cilia" (although some ciliates are
"naked"). Such ciliates as Vorticella have (on
their "top") a
mouth-like opening fringed with cilia and (on their "bottom") a
stalk by which they are attached to a surface.
Most
dinoflagellates are unicellular, often with cellulose plates of "armor"
(not a true cell wall) protecting the cell and giving it shape (although the
arrangement of the plates allows the cell to grow in size). There
are two, whip-like "flagella", within grooves in the armor, one
around the midsection and the other at a right angle to the first.
FEEDING
HABITS OR ENERGY
CAPTURE,
&
EXCHANGE OF MATERIALS WITH THE ENVIRONMENT
Water,
dissolved gases, and other materials are typically exchanged through a
cell membrane via simple diffusion or via "passive" or
"active" transport (both forms of transport employing proteins
embedded within the membrane, the active form also requiring the
biochemical expenditure of energy).
Most freshwater (and some saltwater) species of
protists have "contractile vacuoles", which fill with water
continuously being absorbed from the environment and periodically expel it
from the cell, to prevent it from "osmolyzing" (bursting).
Various
alveolates
have traditionally been classified as either "protozoans"
(microscopic animals), which feed on other creatures or their products, or
"algae" (primitive plants), which manufacture their own food
molecules, via photosynthesis.
Predatory
alveolates typically hunt down and engulf their prey. Vorticella,
stalked and immobile, strikes out at passing planktonic prey.
Ciliates often immobilize their prey, and potential
predators, by means of microscopic, poisoned "harpoons".
In
most ciliates, food is moved by water currents created by special beating
cilia into a "mouth pore" and funneled into a
"gullet". Wastes
are
eliminated through an "anal pore".
In "red tides", the overgrown populations of certain
dinoflagellates produce a poisonous chemical as a waste material that
kills fish and taints shellfish over miles of coastal waters.
Most
dinoflagellates capture light-energy, for photosynthesis, by means of
pigments (green chlorophylls a and c) in unusual, triple-membrane-bound
"plastids" (the chloroplasts of higher plants are
double-membrane-bound); although some,
colorless species are "heterotrophic" (not producing their own
food but, rather, consuming materials from or the bodies of other
organisms) -- some dinoflagellates are parasites, as on protists, algae,
or fish; and some dinoflagellates are predators, as of protists, diatoms, or even fish eggs.
Photosynthetic
dinoflagellates have "eyespots", of yellow carotene pigments,
which aid the cell in moving towards the light; and a few dinoflagellates
even have a simple eye complete with lens!
Like
fireflies, some
dinoflagellates and other plankton can make their own light, as in the wake of a ship.
MOTION
Most ciliates move by means of the coordinated
movements of their hair-like cilia.
Dinoflagellates
use their whip-like flagella -- at right angles to one another, within grooves in the
armor -- in whirling- or otherwise moving-about.
Those sporozoans that can move do so with a gliding
motion.
INTERNAL
TRANSPORT & DIGESTION
Materials are moved within
the cell of a eukaryote by the active process of cytoplasmic streaming.
Food is typically stored and digested in
"vacuoles" (membranous sacs
within the cell).
COORDINATION
& DEVELOPMENTAL
CONTROL
The activities, growth, and development of these
single-celled creatures are under genetic control, with feedback from various environmental
stimuli.
In ciliates, unlike most eukaryotic cells, there are two
nuclei : A
"micronucleus", which controls reproductive functions, and a
"macronucleus", which controls all other functions.
ASEXUAL
REPRODUCTION
Most protists can reproduce "asexually" (that is,
each by itself). Typically,
as a protist grows, its organelles are duplicated; and after reaching a
maximum size (beyond which there is not enough contact between the
contents of the cell and its environment for effective exchange of
materials, both nutrients and wastes), the cell replicates its
"chromosomes" (DNA bodies, within the nucleus) and divides by "fission" (pinching itself in two). This
typical form of cell division (in which the cells produced have the same
number of chromosomes as the parent cells) is called "mitosis"
(Compare "meiosis", in sexual reproduction, below).
Ciliates reproduce by "transverse fission"
(splitting in half crosswise) -- compare flagellates.
SEXUAL
REPRODUCTION
Various species of alveolates can reproduce sexually.
In addition, some ciliates, such as Paramecium, practice
"conjugation" -- a complex process (somewhat like that in bacteria)
in which cells come together temporarily and exchange genetic material
between their small, "micronuclei": Strictly speaking,
this is not true sexual reproduction, as there is not the contribution of
genetic material from a pair of parents to an offspring.
Most ciliates can form a hard "sporelike" stage
in their life cycle, which can survive periods of dry or otherwise harsh
conditions.
Dinoflagellates reproduce
sexually like many other algae. The adult is "haploid"
(with just one of the two sets of chromosomes) and produces
"gametes" by mitosis (simple division), not "meiosis"
(division with a reduction in the number of chromosomes). Male and
female gametes, which may appear similar to one another or not (depending
on the species), swim about
freely. When two fuse, they form a "zygote", which is
"diploid" (with both sets of chromosomes) and mobile (unlike the
fertilized egg of higher plants and animals). This "planozygote"
may turn into a "hystrichospore", a resistant stage, which can
survive unfavorable environmental conditions. Eventually, the zygote
divides, by meiosis (halving the number of chromosomes), forming a new
generation of haploid dinoflagellates.
Many dinoflagellates and other "phytoflagellates"
(photosynthetic flagellates) lose their flagella and become immobile "palmellas" during one
phase of their life cycle, during which they may live
"symbiotically" with such invertebrates as corals
or jellyfish.
Sporozoans develop through an infective, spore-like stage
in their life cycle, which is extremely complex in the case of the parasites
causing malaria (involving both mosquitoes and humans or other vertebrates as hosts for
various life stages).
Eukaryotes
(Domain Eukaryota)
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