The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology.[2] They are usually green due to the dominance of pigments chlorophyll a and chlorophyll b. The chloroplast may be discoid, plate-like, reticulate, cup-shaped, spiral- or ribbon-shaped in different species. Most of the members have one or more storage bodies called pyrenoids located in the chloroplast. Pyrenoids contain protein besides starch. Some green algae may store food in the form of oil droplets. They usually have a cell wall made up of an inner layer of cellulose and outer layer of pectose.
Vegetative reproduction usually takes place by fragmentation. Asexual reproduction is by flagellated zoospores. And haplospore, perennation (akinate and palmella stage). Asexual reproduction by mitospore absent in spyrogyra. Sexual reproduction shows considerable variation in the type and formation of sex cells and it may be isogamous e.g. Chlamydomonas, Ulothrix, anisogamous e.g. Chlamydomonas, Eudorina or Oogamous e.g. Chlamydomonas, Volvox. Chlamydomonas has all three types of sexual reproduction.
They share many similarities with the higher plants, including the presence of asymmetrical flagellated cells, the breakdown of the nuclear envelope at mitosis, and the presence of phytochromes, flavonoids, and the chemical precursors to the cuticle.[4]
The sole method of reproduction in Chlorella is asexual and azoosporic. The content of the cell divides into 2,4 (B), 8(C) sometimes daughter protoplasts. Each daughter protoplast rounds off to form a non-motile spore. These autospores (spores having the same distinctive shape as the parent cell) are liberated by the rupture of the parent cell wall (D). On release each autospore grows to become a new individual. The presence of sulphur in the culture medium is considered essential for cell division. It takes place even in the dark with sulphur alone as the source material but under light conditions nitrogen also required in addition. Pearsal and Loose (1937)[citation needed] reported the occurrence of motile cells in Chlorella. Bendix (1964)[citation needed] also observed that Chlorella produces motile cells which might be gametes. These observations have an important bearing on the concept of the life cycle of Chlorella, which at present is considered to be strictly asexual in character.
Asexual reproduction in Chlorella ellipsoides has been studied in detail and the following four phases have been observed during the asexual reproduction.
(i) Growth Phase - During this phase the cells grow in size by utilizing the photosynthetic products.