structure of algae
On the basis of their organisation algal cells may be differentiated into prokaryotic and eukaryotic types. The prokaryotic cell organization is found in cyanophyceae which is characterized by i) the presence of incipient nucleus, ii) the absence of membrane bound organelles like plastids, endoplasmic reticulum, golgi bodies and mitochondria, iii) the absence of basic proteins the histones in DNA, iv) the presence of mucopeptide in the cell wall, and v) the absence of mitosis.
A eukaryotic cell, on the other hand, is characterized by the presence of a well organized nucleus and membrane bound organelles like plastids, mitochondria and golgi bodies. Majority of algae show this type of cell organization, i,e., mesokaryotic is found in the members of dinophyceae, where although the nucleus has a nuclear membrane and chromosomes, basic proteins are absent.
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| Prokaryotic algal cell |
I) Cell wall
Most of the non motile unicellular and multicellular algae possess a typical cellulosic cell wall of non living matter outside the cytoplasmic membrane. It is composed of pure or mixed carbohydrates, and substances like alginic acid, fucoiodin, fucin and hemicellulose are present in brown algae, pectin in red algae, calcium carbonate in coralline algae and silica in diatoms.
II) Flagella
Motile vegetative or reproductive cells are found in all groups of algae except cyanophyceae and rhodophyceae. Their motility is due to small filiform protoplasmic appendages, called flagella. The number of flagella varies from one to four to many. they are mainly of the fallowing two types:
1) Whiplash or acronematic flagella. Such flagella is covered with fine filamentous appendages, known as mastigonemes or flimmers. They are further divided into three catagories on the basis of arrangement of of mastigonemes.
a) Pantonematic b) Pantocronematic and c) Stichonematic
III) Nucleus
Almost all algae, with the exception of cyanophyceae, have a well organized eukaryotic nucleus. In cyanophyceae the genetic material is not found within the membrane bound nucleus and the DNA strands do not combine with histones to form chromosomes. Such a nucleus is called prokaryotic or incipient nucleus. The nucleus in dinophyceae is also not truely eukaryotic, although it is membrane bound, but well organised chromosome are not present.
In uninucleate algae the nucleus generally lies alongside the cell wall but sometimes it is suspended in the center of the cell by fine cytoplasmic threads. The nucleus varies in size from less than 1 meter to 80 micro meter.
The nuclear membrane is two layered , separated by a perinuclear space. The outer membrane is continuous with the membranes of endoplasmic reticulum. Each nucleus contains one or more dark stained nucleoli or endosomes. The four types of nucleus found in algae are: i) a single nucleolus per nucleus, ii) two or more distinct nucleoli per nucleus, iii) a complex nucleolar mass, and iv) a linear association of large number of small nucleoli.
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| Eukaryotic algal cell |
IV) Golgi bodies or dictyosomes
Golgi bodies are present in all algal cells except blue green algae, and can be seen under electron microscope. Golgi bodies are composed of 2-20 flat vesicles which are arranged in stacks. Each stack is called a dictyosome.
The exact role of golgi bodies in algae is not known. They are associated with the synthesis of cell metabolites, and have also been shown to contribute to the plasma membrane as in higher plants.
V) Mitochondria
Well organised mitochondria are present in all algal cells with the exception of blue green algae. A mitochondrion is bounded by a double membrane. The inner membrane is projected into lumen and forms finger like projections, called cristae. There are usually few cristae in chlrophyceae and many in chryophyceae, pheophyceae and xanthophyceae. The lumen, surrounded by bounding membranes, is filled with a granular matrix which contains nucleic acids. Their shape and number per cell varies in different groups of algae. There are usually more than one mitochondria per cell, but in micomonas each cell contains a single mitochondrion.
The respiratory enzymes are located in mitochondria, and hence mitochondria act as respiratory centres of the cell. Mitochondria are also the sites of enzyme action in protein synthesis and amino acid interconversions.
VI) Endoplasmic reticulum(ER)
The cytoplasm of the algal cell is traversed by a system of interconnecting tubules, known as endoplasmic reticulum. It is connected with the outer membrane of the nuclear envelope, but does not penetrate other cell organelles like chloroplast or pyrenoid. The surface of ER is studded with ribosomes, the sites of protein synthesis in the cell.
VII) Eye-spot or stigma
The motile vegetative and reproductive cells of algae have a pigmented spot, known as eye-spot or stigma.
VIII) Vacuoles
Except cyanophyceae, mature cells of almost all classes of algae possess one or more vacuoles. Each vacuole is bounded by a distinct membrane, called tonoplast. Motile algal cells may have the two types of vacuoles:
i) Simple or contractile vacuoles. Such vacuoles, found in the members of chlrophyceae, show periodic contractions and throe out the waste products out of the cell.
ii) Complex vacuoles. A complex vacuole consist of: i) a tube like cytopharynx, ii) a large reservoir and iii) a group of vacuoles of varying sizes.
IX) Gas vacuoles
These gas containing cavities are characteristic of the mature healthy cells of cyanophyceae. They occur as stacks of small transparent cylinders of uniform diameter. Their walls are freely permeable to gases. They render buoyancy to planktonic forms. They also act as a screen against intense light.
X) Pigments
Algal cells have a characteristic colour due to the presence of a combinations of pigments, specific to each class. In all classes, except cyanophyceae, these pigments are present within membrane bound organelles, known as plastids. In blue greens, the pigments are concentrated in the peripheral cytoplasm, known as chromoplasm. Plastids are of the fallowing types:
i) Leucoplast. These are colourless plastids.
ii) Chromoplasts. These are coloured plastids; those containing both chlrophyll a and chlorophyll b are called chlroplasts and those lacking chlorophyll b as chromatophores.
XI) Pyrenoids
Pyrenoids are proteinaceous bodies present in chromatophores. These organelles are considered to be associated with the synthesis and storage of starch. In members of chlrophyceae, pyrenoids are surrounded by starch plates. But in some algae such as diatoms, they accumulate lipids instead of starch.
A chromatophore may have one or more than one pyrenoids. These pyrenoids are embedded in the chromatophore or attached on its surface with the help of a stalk. The pyrenoids are usually traversed by large number of chromatophore lamellae.
In some algae pyrenoids are transient structures, found only at certain stages. Their presence, in such cases, is associated with the photosynthetic activity of the cell and availability of stored food material.
