Chlorophyta
Chlamydomonas
Q.5. Describe the structure, various methods of reproduction and prennation in Chlamydomonas. (1995, 99)
Related Questions -
Q. Draw a labelled diagram of life cycle of Chlamydomonos. (2009)
Q. Describe the structure and reproduction in Chlamydomonas. (2007)
Q. Describe the various modes of reproduction in Chlamydomonos. (2008)
Ans. Systematic position: -
Division - Algae
Class - Chlorophyceae
Order - Volvocales
Family - Chlamydomonadaceae
Genus - Chlamydomonos
Occurence: - Chlamydomonas is a large genus consisting of 500 species. Majority of the species are fresh water. They are found such as in tanks, ponds, ditches and lakes while some are marine i.e. C. halophila. C. niualis are found on snow and empart a red colour. They are usually found in polluted water which in rich of organic matter and ammonium compound.
Structure: - Chlamydomonos is a unicellular motile green algae. The plant body is oval, spherical, pear-shaped or elongated in shape. Plant body structure consists of following parts -
(i) Cell wall: - Cell wall is thin, transparent and firm. It is made up of two layers i.e. outer pectic and inner cellulosic layers. It is smooth but in most species it becomes thick at the anterior end to form papilla.
(ii) Cytoplasm: - It is has two distinct regions :
(a) Thin clear outer region has two anteriorly placed flagella, free ribosomes, volutine granules and mitochondria and (b) Dense region possesing golgi complex, mitochondria, free ribosomes, endoplasmic reticulum, single chloroplast and nucleus.
(iii) Nucleous: - Nucleous is seen embedded in cup shaped of chloroplast. Usually nucleous present in center but sometime nearer the anterior region. It is surrounded by a ring of small vacuole called centeral vacuole.
(iv) Chloroplast: - A single cup shaped chloroplast is a characteristic to alga. The shape of the chloroplast may vary. Chloroplast is externally bounded by a two unit membrane. It bears variable number of photosynthetic lamellae, or thylakoids in the form of bands.
(v) Pyrenoids: - It is spherical. Pyrenoid present on the lower side of chloroplast. Number of pyrenoid varies in different species, it may be two or more. Pyrenoid in Chlamydomonas consists of a granular core of protein surounded by tightly packed numerous minute starch platelets.
(vi) Eye spot or Stigma: - It is organe or reddish coloured a tiny spot. It is found in towards the anterior end of coloroplast. It is a photoreceptive organe and it serves only as a shading device for p - photoreception.
(vii) Contractile Vacuoles: - Two or more contractile vacuoles are rarely found within the colourless region of cytoplasm in the base of flagella. These are used to regulate water balance of cell.
(viii) Flagella: - Two equal size flagella are present. These are whiplash type. These are arise from separate basal granules situated near the anterior surface of protoplasm.
Reproduction: - Chlamydomonos reproduce by both asexual and sexual methods. The life cycle completes as follows: -
(a) Asexual reproduction: - Asexual reproduction usually takes place in favourable conditions. It is by zoospores, aplanospores and plamella stage.
(i) By zoospores: - This takes place when the conditions are favourable and rich supply of water. The flagella are withdrawn and contractile vacuoles disappear. Protoplasm withdrawn from the cell wall. The plane of division is always longitudinal. The first division is followed by another division at right angle and 4 or very rarely 8 or 16 protoplast developed. These protoplast are motile, biflagillate and called zoospores. Zoospore swims for a certain time and then form a new plant.
(ii) By aplanospores: - Under unfavourable conditions, the plant comes to rest and discards flagella. Soon, it withdraws protoplasm from the cell wall, rounds up and secretes a thin wall, rounds up and secretes a thin wall. The structure is now called aplanospore as found in C. caudata. The aplanospores may germinate directly or divide to produce zoospores. On comming of the favourable conditions each spore germinate a new plant body.
(b) Sexual reproduction: - In favourable conditions sexual reproduction takes place by formation of gametes. It shows the ranges of variation from isogamy to oogamy.
(i) Isogamy: - Isogamy is common in chlamydomonos. Both gametes are morphologically similar but physiologically different. The two gametes come close by their anterior ends and fuse.
(ii) Anisogamy: - Fusing gametes are dissimilar in size in anisogamy. C. braunii is a well example anisogamy. Both gametes are motile. Male gametes microgamete are smaller in size and produce approximately 8 in cell. Female gametes or macrogamete big in size and produce 4 in per cell
Both male and female gametes fuse by their anterior end with the result of fusion thick walled zygospores are develop.
(iii) Oogamy: - Oogamy has been recognized in C. coccifera. Female plant, instead of forming gametes, discards its flagella and directly acts as a non motile female gamete, the egg of ovum. The male gametes are produced 8, 16 or 32 in number by successive division of the protoplasm of male plant (antheridium). These are biflagellated. Fusion takes place between a male gamete and an ovum and a nonmotile zygote is formed.
Development and germination of zygotes: - A mature zygote is a spherical with a thick, smooth or stellate wall. It represents the sporophytic phase of life cycle with diploid chromosomal set. The zygote germinates with the return of favourable conditions. At the time of germination, zygote divides meiotically to produce four haploid zoospores. Zoospores get liberated in a vesicle by the rupture of the zygote wall. These soon develop into new Chlamydomonas plant.
Perennation in Chlamydomonos: - The perennation is the method by which algae protect theirself in unfavourable conditions. In perennation algae form minute spores. In favourable condition these spores germinate and form a new plant. In Chlamydomonos perennation takes place by Palmella stage.
Fig. - Diagrammatic representation of life cycle of Chlamydomonas
Q.6. Describe the palmella stage in Chlamydomonos.
Related Question -
Q. Describe the palmella stage in short. (2006, 08)
Ans. Palmella stage: - This is non-motile reproduction phase. Under unfavourable conditions protoplast of the parent cell divides to form 4-8 daughter cells. These cells do not develop flagella and are non-motile and remain with in matrix. This temporary colony of hundreds or thousands of cells, in a common gelatinous matrix, is called palmella stage. The palmella stage is non-motile and temporary reproductive phase with the recurrence of favourable conditions these cells form flagella and become motile. They come out of the gelatinous matrix and develop into large vegetative cell.
Q.7. Draw labelled diagram of cell structure of Chlamydomonas as revealed by electron micrscope. (2006, 11)
Related Question -
Q. Draw a labelled diagram of thallus structure of Chlamydomonas. (2009, 12)
Ans.
