Cell Structure of Algae

 

The cell is eukaryotic, has the same structure as is typical of the higher plants.

Flagella:

            The motile cells of algae are provided with fine, protoplasmic, whip-like threads, the flagella. They are extremely fine and hyaline emergences of the cytoplasm. There may be either a single anterior flagellum or flagellum occurs in pairs. The flagella on the cell may be equal (Isokont) or unequal (heterokont) in length. Usually there is a single granule at the base of each flagellum. It is known as bhlepharoplast.

Structure:

            Forming the core of the flagellum is an axial filament or central filament called the axoneme. The latter is surrounded by a cytoplasmic membrane or sheath, which terminates short of the apex. The naked, terminal portion of the axoneme is called the end piece. The tip of the end piece may be blunt or rounded or pointed.

            In cross section the flagellum consists of two inner central fibrils forming an elastic axial thread. It is surrounded by nine united peripheral, contractile, thicker, protein double fibres. All are enclosed by a sheath which is an extension of the plasma membrane.

            The two central fibrils are single. They lie side by side and are sometimes enclosed by a sheath of their own. Each peripheral fibril is composed of two thin fibrils. The fibrils are hollow and extend along the entire length of the flagellum.

            The nine peripheral fibrils join the basal granules but the two central fibrils stop short of the granule. This ‘9 + 2 ‘ pattern of fibrils is the basic structure of the flagellum of all organisms except the bacteria.

Kinds of Flagella:

            There are two main types, whiplash and tinsel. The whiplash flagellum has a smooth surface. The tinsel flagellum bears longitudinal rows of fine, minute flimmer hairs arranged along the axis almost to the tip of flagellum.

Flagellation:

            The position, number and kind of flagella on the motile cells isstrinkingly constant in each division of algae but differs from division to division. Thus it forms an important taxonomic feature for primary classification of algae.

             The blue green (Cyanophyceae) and red algae (Rhodophyceae) lack flagella. The motile cells in green algae (Chlorophyceae) and stoneworts(Charaphyceae) usually have two rarely four equal flagella of whiplash type inserted at the anterior end. The only exception is the Oedogonials in which the motile cells have a crown of flagella.

             The yellow green algae (Xanthophyceae) have two unequal anterior flagella. One of these is of whiplash and other tinsel type. The diatoms (Bacillariophyceae) are characterized by a single tinsel flagella on the male cell at the anterior end. In Brown algae (Phaeophyceae) only the reproductive cells are motile. They are furnished with two laterally inserted unequal flagella. Of this one is of tinsel and other whiplash type.

 

Algal group                            Flagella

Cyanophyta                             Absent

Rhodophyta                            Absent

Chlorophyta                            2 or 4 anterior equal, acronematic

Xanthophyta                           2 unequal, anterior 1 acronematic, 1 pantonematic

Chrysophyta                            Acronematic and pantonematic

Bacillariophyta                       Anterior 1 pantonematic

Pyrrophyta                              1 acronematic, 1 band shaped

Cryptophyta                            2 equal, lateral pantonematic

Phaeophyta                             2 unequal, lateral 1 acronematic, 1 pantonematic

 

 

Cell wall

The cell is bounded by a cell wall. It is a secretion product of the cell protoplast deposited in form of concentric layers external to it.
Algae exhibit diversity in the cell wall structure. Cell wall of many algal species is made up of polysaccharides - like cellulose and pectin. The cellulose forms the inner layer and pectin forms the outer layer and is amorphous in nature. It dissolves in water to form a mucilaginous layer. Ex. Chlorophyceae and Xanthophyceae.
In Phaeophyceae the inner layer is made up of cellulose and the outer layer is made up of alginic acid or algin and fucoidin.
In Rhodophyceae, the outer cell wall is made up of Carrageen or Carrageenan.
In Bacillariopjyceae and Chrysophyceae instead of cellulose the cell wall is made up of silica.

Plastids

The colour giving cell organelles are called as Plastids. The plastids are of three types: colourless or white coloured plastids are called as leucoplast, the green coloured plastids are called as chloroplast and coloured plastids except green are called as chromoplasts.
The plastids containing chlorophyll a and chlorophyll b are usually designated as chloroplasts, those lacking chlorophyll b are termed as chromatophores.
Depending upon the position in a cell the chloroplast or chromatophores may be parietal ( towards the margin ) or axile ( towards the centre).  
The shape of the chloroplast is also variable, e.g A) cup shaped as in Chlamydomonas B) Girdle shaped in Ulothrix, C) Discoid in Chara, Vaucheria, D) Reticulate in Oedogonium E) Spiral or Ribbon shaped in Spirogyra, F) Stellate in Zygnema

                                                                                      

Pigments:

Of the external features, colour of the algal thallus is most characteristic and distinctive. Algae from the various phyla have striking differences of colour, which help in the classification of algae.

The colour of the algal thallus is due to the presence of definite chemical substances called the pigments.

 Each pigment has its own characteristic colour. The particular colour that a thallus has is due to the predominance of one pigment in combination of several others. Each algal group has its own particular combination of pigments and a characteristic colour.

 There are 3 kinds of pigments in algae – chlorophylls, carotenoids and phycobilins or biloproteins. ChIorophylls and carotenoids are soluble in organic solvent ( acetone, chloroform, ethyl alcohol etc.) whereas the phycobilins are soluble in water.

 Chlorophylls:

Chlorophylls are green coloured pigments. There are 5 types of chlorophylls present in algae. They are chlorophyll a, b, c, d and e.

Chlorophyll a – occurs in all algae classes

Chlorophyll b- found in Chlorophyceae, Euglenophyceae and Prasinophyceae

Chlorophyll c – present in Xanthophyceae, Bacillariophyceae, Dinophyceae, Phaeophyceae,

                          Chrysophyceae

Chorophyll d – present in Rhodophyceae

Chlorophyll e – found in Xanthophyceae e.g., Tribonema and Zoospores of Vaucheria

 

Carotenoids:

Carotenoids are a group of yellow, orange, red and brown coloured pigments. These are protective pigments and protect the chlorophyll from photo oxidation. In addition, they transfer light energy to chlorophyll. Carotenoids are of two types – Xanthophylls and Carotenes.

Xanthophylls (C₄₀ H₅₆ O₂): there are 20 different kinds of Xanthophylls in algae. Out of them Fucoxanthin is a major xanthophyll present in majority of algal classes.

Carotenes (C₄₀ H₅₆): These are fat soluble pigments. There are 5 types of carotenes namely β. .

 

Pyrenoids
The chloroplast of Algae contains one or many distinct, rounded, proteinaceous bodies called the pyrenoids.
Each pyrenoid consists of a central, viscous, granular core surrounded by tightly packed minute plates of starch. Because they remain surrounded by sheath of starch, they are said to be ' the site of starch storage'.
After attaining a definite size the pyrenoids divide by simple construction. The number of pyrenoid per chloroplast may be one as in Chlamydomonas or many as in Oedogonium.
Pyrenoids are found in all classes of algae except Cyanophyta.

Stigma or Eye spot
The motile unicellular or coenobial members and also the motile reproductive cells of the non-motile algae contain a pigmented organelle known as stigma or Eye spot.
It is orange red spot located usually in the anterior and sometimes in median or posterior portion.
Eye spots are usually considered as organs for the reception of light intensity.
According to Mast it consists of a curved pigment plate or pigmentosa carrying the pigments and a biconvex hyaline lens in front.
Eye spot is considered a photoreceptive organ. It is concerned with the direction of the movement of flagella.

Neuromotor Apparatus
The flagellum ends in a basal body called as blepharoplast. The basal granules are joined together by a fibre called paradesmose. The paradesmose is connected to the centrosome by a fine protoplasmic thread called rhizoplast.
The centrosome are located either just within or exterior to the nucleus. The centrosomes help in the formation of spindle fibres at the time of nuclear division.
The whole structure makes the neuromotor Apparatus as an incipient nervous system.