Structure of Algae (With Diagram) - Class 9 PDF Download

The cellular architecture is prokaryotic, internal structure of which is extremely simple. The cyanophycean cell does not contain Golgi bodies, mitochondria, and endoplasmic reticulum. There are no membrane bounded chloroplast, organized nuclei, and aqueous vacuoles. Flagellate structures and true sexuality are completely absent in the blue-green algae.

Protoplast:

The protoplast has a high degree of viscosity having jelly-like con­sistency filled with highly refractive granules of proteinaceous material, cyanophycin. Under light microscope the protoplast appears to possess two regions, a peripheral region, the chromoplasm and a colourless central region, the centroplasm or the central body. But the use of the electron microscope has shown that this idea is not valid.

The protoplast is normally devoid of vacuoles, except in old cells. This is a major factor contributing to the great resistance of the plants to desiccation and of the cells to plasmolysis. In some planktonic species pseudo- or gas-vacuoles may be found which contribute towards their buoyancy by virtue of the gas that they are said to contain.

The electron microscope study reveals a complex lamellar system is present in the protoplast. These lamellae are also known as thylakoids (Fig. 18). They are not enclosed in membrane-bounded groups to form chloroplasts. Instead, they lie free in the cytoplasm, in some species more or less restricted to the periphery.

Depend­ing on the intensity of the incident light and age of the cell, the thylakoids may be distributed irregularly throughout the protoplast or may be arranged in parallel stacks. But they are always separated by a layer of cytoplasm. Each thylakoid is an elongated flattened sac-like structure possessing small granules resembling ribosomes.

The thylakoids are functionally analogous to the chloroplasts of other algae and higher plants.

Pigments:

The thylakoids are the site of chlorphyll a, and the accessory pigments also occur on their surfaces in the form, of small particles, the phycobilisomes. The accessory pigments are: c-phycocyanin, c-allophycocyanin, and c-phycoerythrin, the two former blue and the latter red. They are biliproteins and are composed of protein with chromophoric groups. The accessory pigments transfer light energy they absorb to chlorophyll a.

They also have a protective shading function that prevents oxidation in intense light of the other photosynthetic pigments. Besides these, the other pigments present are: β-carotene, myxoxanthin, myxoxanthophyll, antheraxanthin, aphanicin, aphanizophyll, flavacin, lutein, oscilloxanthin, and zeaxanthin.

Nuclear Material:

Unlike other algae, the blue-green algal cell lacks an orga­nized nucleus with a nuclear membrane and nucleolus. Fine fibrils of DNA are a feature of the nuclear material (Fig. 18). These are either distributed throughout the cell or concentrated in the central portion.

The use of electron microscope has shown that the chromatin nuclear material, also designated as ‘DNA-plasm’, constituted of fine fibrils of DNA may occur as a single accumulation of nuclear material, or organized into a reticular structure or individual rod-like nuclear elements.

The last type is found in many larger forms of blue-green algae and indicates the presence of several genetic complements or genomes per cell. The term ‘chromatin apparatus’ is generally used for the nuclear material of the blue-green algae. The DNA fibrils are not associated with basic proteins (histones) so that no organized chromosomes are found. They commonly appear as a reticulum (Figs. 6B and 21 A).

Photosynthesis and Storage Food:

Photosynthesis in blue-green algae occurs in the same manner as in other chlorophyllous plants. The storage food is Cyano­phycean starch (probably amylopectin portion of starch) and proteins. Many of the blue-green algae, also store reserve food in the form of oils. The proteins of some blue-green algae contain diaminopimelic acid, an amino acid also found in some bacteria but never found in higher plants or animals.

Nitrogen Fixation:

Certain blue-green algae can assimilate or fix elemental (gaseous) nitrogen of the atmosphere. They thus are independent of other combined nitrogen sources. The nitrogen fixed by blue-green algae can be assimilated by them­selves as well as by other organisms.

Three kinds of blue-green algae can fix nitrogen:

(i) The filamentous heterocystous species,

(ii) Certain nonheterocystous filamentous species, e.g., Plectonema boryanum only under microaerophilic conditions, and

(iii) Certain unicellular (nonheterocystous) species.

The nitrogen-fixing enzyme complex nitrogenase is oxygen-sensitive, so that the highest rate of nitrogen fixation occurs under reduced oxygen tensions. In filamentous heterocystous blue-green algae, the site of nitrogen fixation is heterocyst. But microaerophilic nitrogen fixation may go on in the vegetative cells.

Substantial amounts of soluble nitrogenous compounds are liberated from healthy cells of nitrogen-fixing blue-green algae either into the culture medium or in their natural habitats.

Besides, when the algae die, their nitro­genous materials decay and form ammonia which may then be transformed into nitrates by nitrifying bacteria thus enriching the fertility of the habitat. Blue-green algae that contribute greatly to the fertility of rice fields are species of Aulosira, Anabaena, Tolypothrix, Cylindrospermum, Nostoc and Mastigocladus.

Cell Wall:

The cyanophycean cell possesses a surface layer of gelatinous sheath which is a constant feature, although it varies in thickness (Figs. 8B & 24C). The sheath consists of cellulose fibrils reticularly arranged within a matrix to give a homo­genous appearance. The cell wall is inside the sheath. It is a multilayered structure lying external to the plasmalemma.

Because of intricate relationship of plasmalemma with cell wall, the separation of plasmalemma from cell wall during plasmolysis fails.

The cell wall is composed of various substances, some of them are muramic acid, glucosamine, alanine, and glutamic and diaminopimelic acid. The inner layer of the cell wall contains a mucopeptide component comparable to that found in bacterial cell walls.

In the filamentous forms the inner layer always completely envelopes the pro­toplast, but the cell sheath forms a cylindrical envelope which may be continuous (Figs. 22B, C) or interrupted between consecutive cells. The cell walls have a remarka­ble degree of elasticity. Division of a cyanophycean cell takes place by the ingrowth of a septum from the periphery and the gradual and consequent division of the chromoplasm.

The septa composed only of the inner layer are exceedingly delicate. In the filamentous blue-green algae very delicate plasmodesmata or protoplasmic strands effect protoplasmic continuity across the transverse walls. Various pores and -depressions in the longitudinal walls have been revealed by electron microscopy. These may be related to the gliding motility of some species.