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The
Diversity of The World of Life
Eukaryotes
(Domain Eukaryota)
Red Algae
(Rhodophyta)
Representatives
Nemalion
Porphyra
Various
Seaweeds
Biology
ENVIRONMENTS
Most red algae live in
seawater, especially in warm coastal waters; but some live in freshwater.
Some red algae, as in Japan, are cultivated for food.
OVERALL
STRUCTURE
Red
algae are unicellular or multicellular, sometimes branched,
sometimes
stem- or leaf-like; most are "seaweeds", anchored to the bottom.
Individual cells are
given shape by cell walls, usually composed of "cellulose" -- a
"polymer" (chain-like molecule) with "glucose" sugar
"links" -- but
sometimes composed of polymers of other sugars. Materials such as
"agars" and "carageenan" found in the cell walls of
various red algae are commercially valuable (ex. as food-thickening
agents).
Red algae cells are
often covered by slimy materials.
Some red algae secrete
a hard shell of mineral carbonates and contribute greatly to the formation
of tropical reefs.
Red algae cells never
have "flagella" (propeller-like filaments).
ENERGY
CAPTURE
Red
algae capture light-energy, for the photosynthesis of food
molecules, by means of specialized pigments embedded in the membranes of
their chloroplasts: Red algae get their color and name from a
mixture of green "chlorophyll a", yellow "carotene",
and blue and red "phycobilin" pigments -- in particular, the
red pigment (phycoerythrin) absorbs blue-green light, which penetrates seawater more
deeply than other wavelengths of light.
Like the true leaves of
land plants, the photosynthetic body parts of red algae may grow so as to
not shade one another.
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).
INTERNAL
TRANSPORT
Red
algae have no specialized "vascular tissue" (that is, no
coordinated assemblage of special cells dedicated to the internal
transport of water or other materials).
The contents of
the cells of red algae or any other eukaryotes
(unlike the contents of the much smaller cells of bacteria
or archaeans) are continuously mixed, by the
active process of "cytoplasmic streaming".
DEVELOPMENTAL
CONTROL
The
growth and development of red algae are under genetic control, as
influenced by the environment; and "hormones" (biochemical
messengers) play a role in the coordinated development of at least
multicellular forms.
ASEXUAL
REPRODUCTION
Red algae
reproduce asexually from various body parts or from "spores"
(cells specialized for vegetative reproduction).
SEXUAL
REPRODUCTION
As for
other algae (except blue-green algae) and land plants, there is an
"alternation of generations": One stage in the life cycle,
the "sporophyte", is "diploid" (with both sets of
chromosomes); the other stage, the "gametophyte", is
"haploid" (with just one set of chromosomes).
In red
algae (unlike most land plants), the sporophyte and the gametophyte are
typically both well developed and often identical in appearance to one
another.
The
sporophyte (multicellular and diploid) produces "meiospores" (unicellular and haploid),
which grow into the male and female gametophytes (multicellular and
haploid), which eventually produce
"gametes" (unicellular and haploid) -- typically, the male
gametes (unflagellated and, thus, not true sperms) are released into the watery environment, through
which they simply drift to the female gametes, found in capsules on the
female gametophyte. With sexual fertilization, the gametes fuse,
restoring two sets of chromosomes in a single cell (although, as typical
in sex, with the
individual genes "mixed and matched" from the parents' sets) -- this young sporophyte is soon released from the capsule, to start a new
and genetically distinct generation of the species
"downstream".
Eukaryotes
(Domain Eukaryota)
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