|
The
Diversity of The World of Life
Green
Plants (Viridaeplantae)
Green Algae
(Traditional Chlorophyta)
Representatives
Typical
Green Algae (Chlorophyceae,
Ulvophyceae, etc.)
Cladophora
Chlamydomonas,
Acetabularia, Volvox, &
Ulothrix
Gonium
Fritschiella
Ulva
(Sea Lettuce)
Green
Algae More Closely Related to Land Plants (Charales
& Coleochaetales)
Desmids
Spirogyra
Chara
Nitella
Coleochaete
Biology
ENVIRONMENTS
Many
green algae live in shallow Tropical seas; most live in freshwater; some
live in or on such terrestrial habitats as moist soil.
OVERALL
STRUCTURE
Cell
walls, composed primarily of "cellulose" and related
"polysaccharides" ("polymers", chain-like molecules,
derived from sugars), give shape to green algae cells.
There
is great variety amongst the species of green algae. Some are
unicellular; others are multicellular. Some green algae grow into
straight or branched filaments (sometimes, like fungi,
with no cross-walls); others, into spherical or netlike
colonies; others, into leaf- or hand-like sheets. Some species are
bare; some are ciliated ("hairy"); and some are flagellated
(with propeller-like filaments). Some green algae even grow mock
roots ("holdfasts" or "rhizoids"), mock stems (such as
horizontal "rhizomes"), and mock leaves.
ENERGY
CAPTURE
Green
algae capture light-energy, for photosynthesis, by means of pigments
(green chlorophyll a and b, as in higher plants, aided by yellow
carotenes) in their chloroplasts.
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
Materials
are moved within each green algae cell by the active process of
cytoplasmic streaming. There is no true "vascular" tissue within
green algae.
DEVELOPMENTAL
CONTROL
The
growth and development of green algae is under genetic control, and
hormones play a role in at least one unicellular species and probably in
most multicellular species.
ASEXUAL
REPRODUCTION
Green
algae reproduce asexually by body parts or spores.
SEXUAL
REPRODUCTION
Green
algae reproduce sexually by means typical of various algae. As in
higher plants, there is typically an "alternation of
generations", between a "haploid" stage in the life cycle
(with just one set of chromosomes) and a "diploid" stage (with
both sets of chromosomes). The haploid stage is typically called the
"gametophyte", because it produces "gametes"
(typically sperm and egg). The union of gametes produces a
"zygote" (a fertilized egg), which typically grows into the
"sporophyte" (the diploid life stage). The sporophyte
eventually produces "meiospores", by "meiosis" (the
form of cell division in which the number of chromosomes is cut in half),
which grow into the gametophyte of a new generation.
In
many species of green algae, the gametophyte is dominant; the zygote is
the only diploid life stage, never growing into a sporophyte but, rather,
dividing by meiosis, the haploid cells produced growing directly into new
gametophytes.
In
many other species of green algae, the gametophyte and the sporophyte are
both well developed and sometimes even similar in appearance.
In
some other species of green algae (as in human beings), the diploid stage
produces gametes directly, by meiosis -- these haploid cells do not grow
into a haploid body but, rather, join together to produce a new diploid
generation -- the diploid stage thus acts as both the sporophyte
(producing the haploid generation) and the gametophyte (producing
gametes).
The
male and female gametes of various green algae may be either similar to
one another ("isogametes") -- both sexes typically swimming, by
means of flagella, through their watery environment to each other -- or
different from one another ("oogametes") -- the flagellated
sperms swimming to the eggs.
Green
Plants (Viridaeplantae)
|
