Cell Walls of Algae
Algae are the plants with the simplest organization.
Many of them are single-celled, some have no cell wall, others
do though its composition and structure differ strongly from that
of higher plants.
They are good specimen for tracing back the
evolution of the cell wall. Primitive cell walls do not fulfil
the same requirements as that of higher plants.
It seems quite likely that a structure like that of the cell wall
has developed several times in the course of evolution.
All archaebacteria,
eubacteria and blue-green algae (cyanobacteria or blue-green algae)
have complex walls with an energetically rather costly biosynthesis.
Neither in composition nor in biosynthesis do they have any common
ground with the cell walls of plants.
Although the evolution of plants from early eucaryotic cells is
not known in detail, is it commonly agreed on that primitive algae
are flagellates closely related to the non-green flagellates.
Many, though not all species of this stage of evolution, among
which the euglenophyta are typical green representatives, have
no cell wall. It is not only a simple membrane, but by a pellicle
of already quite complex organization, that separates them from
the surrounding. It consists mainly of glycoproteins organized
in regular patterns the way two-dimensional crystals are. Helical
ribs wind round the cell's surface.
Most single-celled algae like the Volvocales possess real cell
walls. The most-studied species is Chlamydomonas reinhardii.
Its wall lacks long, fibrillary carbohydrates. Most of it is made
up by glycoproteins, and even here can an extensin-like protein
rich in hydroxyproline be found. Among the identified sugar residues
are arabinosyl-, galactosyl- and mannosyl residues. In the electron
microscope does it seem as if the wall consisted of seven layers.
The middle layer contains an extensive grid-shaped framework of
polygonal plates consisting mainly of the mentioned glycoproteins,
while the layers above and below display fibre-like structures.
The thickness of the outer layer varies since it includes components
that the cell takes up from its surrounding.
This indicates a main function of the cell wall of simple, single-celled
algae: it mediates between the cell and its surrounding. It protects
not only the cell but serves, too, communication with cells of
the same or other types. It has to be permeable for metabolites
and regulators and / or to carry receptor molecules with which
it may contact other cells. The diversity of these functions (and
their specificity) caused the evolution of a variety of differently
structured cell walls.
In many-celled plants is the communication via the whole cell
surface largely restricted. Contact with neighbouring cells develops
in the course of tissue formation. Strength is in this respect
a decisive and limiting criteria. The exchange of compounds between
cells occurs via specific openings in the wall (pits, plasmodesmata).
The functions originally performed by one structure are now distributed
onto two different structures.
Structural Components of The Cell Walls of Algae -
Molecular Classes and
Conformations of the Molecules
The main structural elements of all plant cells are polysaccharides.
Differences in their chemical composition cause fundamentally
different physical properties. No plant cell wall consists only
of one class of molecules. The interactions of the different molecules
produce properties that allow to distinguish the cell walls of
certain classes.
In many classes of algae is cellulose already the main structural
element of the wall, though remarkable variations of the fibrillary
structure exist. Reliable X-ray analytical data prove
that is mostly crystalline in cells of algae, too. Differences
in the type of the flexor reflexes hint at the fact that cellulose
could aggregate in many more or less uniform crystalline structures.
Such reflexes are a measure for periodic distances at the molecular
level, which may differ considerably from species to species and
are specially large in Rhodophyta
In some classes of algae exist only disperse textures, while others
(specially many Chlorophyta-species) have a higher degree of organization
(layers of parallel microfibrils). Such layers do usually alternate
with layers of an amorphous material. No clear difference between
primary and secondary cell wall exists in most algae. Where such
a distinction is possible, differ the causes usually from that
in higher plants.
Mannanes
In a number of marine
green algae (Codium, Dasycladus, acetabularia, etc.) as
well as in the walls of some red algae porphyra Bangia)
constitute mannanes the main structural elements. They are linear
and the mannosyl residues are 1 > 4 glycosidically linked.
Hydrogen bonds that are (just like in cellulose) the cause of
the partially crystalline organization of microfibrils may develop.
In Codium the carbohydrates are tightly associated with protein
Xylanes
are polymers where the beta-D-xylosyl residues are linked
via 1 > 3 and 1 > 4 glycosidic bonds. In contrast to the polymers
discussed until now, are xylans partially ramified. In species
with xylan-containing walls exists nevertheless a layered structure
and an orientation of the microfilaments. They contain mostly
linear polymers.
Alginic Acid:
alginic acid and its salts, the alginates
are important components of the walls of phaeophyta
They are singular in many respects. They consist exclusively of
uronic acids: mannuronic acid and beta-L-glucuronic acid
in changing ratios and of small amounts of beta-D-glucuronic
acid.
Beside homopolymers exist also heteropolymers in many algal groups,
partially exist species specific differences, an indicator of
the fact that the single species contain different sets of enzymes.
The alginates of brown algae exist both within the cell wall and
in the intercellular substance. Their part in the cell wall may
be as high as 40 per cent of the dry matter. They have a high
affinity for divalent cations (calcium, strontium, barium, magnesium)
and the tendency to gel. The main portion of the magnesium ions
isolated from brown algae stem from the alginic acid fraction.
Sulfonated Polysaccharides:
polysaccharides: polysaccharides whose
monomers are esterized to sulfuric acid
residues and are moreover partially methylated have been detected
in nearly all marine algae. They occur partially in the cell wall
itself and partially in the intercellular substance. Sulfonated
galactanes are typical for many red algae, depending
on their origin are they called agarose, carrageenan, porphyran,
furcelleran and funoran.
L- and D-galactose, which are linked by beta 1 > 3 or
alpha 1 > 4 glycosidic bonds form the basic pattern of
agarose and porphyran, in the latter alternate L- and D-galactosyl
residues. Carrageenan and furcelleran contain exclusively D-compounds.
Just like in alginates is the formation of gelatine one of the
most important physical properties of this family of molecules.
Agar, whose basic unit is agarose, is yielded mainly from Gelidium
and Gracillaria, both genera of red algae.
Further Cell Wall Compounds.
A number of algae contain
mineral cell wall components. Silicon, for example, is the main
component of the diatom shell, though it occurs also in the cell
walls of other groups of algae. Silicon-containing scales enclosed
the chrysophyt Synura. In some brown algae and in the green
algae Hydrodictyon is silicon a cell wall component. Diatoms
take silicon up as silicate. The process is dependent on oxygen
and temperature, it consumes energy and it is dependent on the
presence of divalent sulphur.
Sporopollenin is an isoprene derivative. It is a component of
pollen cell walls, but was also detected in the walls of some
green algae (Chlorella, Scenedesmus, etc.).
Calcium: calcium encrustations of cell walls have on several occasions
be described. They seem to be especially common in species of
tropical, marine waters. Some species participate in reef formation.
Calcium is always deposited as calcium carbonate. Calcium carbonate
occurs in two different crystalline states: calcite and argonite.
Calcite is produced in the walls of some groups of red algae and
in charophycea, while argonite is produced by some green (Acetabularia,
etc.), brown and red algae. Both states do not occur simultaneously
in one species.
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