The green algae are the members of the Division Chlorophyta are commonly known as green, due to the presence of chlorophyll a and b, which convey them bright green colour. Their cells are similar to higher plants in the presence of a well-defined nucleus, photosynthetic pigments localized in chloroplasts, food reserve commonly stored as starch, and possession of cell walls in which cellulose is usually a clearly recognizable ingredient. They exhibit a considerable range of variation in the form and structure of the plant body. Some are unicellular1 and microscopic others are colonial (e.g., Vblvox) filamentous (e.g., Ulothrix, Spirogyra) or parenchymatous (e.g., Ulva). Some of the largest unicells belong to group, as for example, Acetabularia (umbrella plant) can reach a length of several cm.
More than 90% of species grow in fresh water and the remaining 10% are the marine forms. The freshwater species are cosmopolitan in distribution, commonly inhabiting permanent or semi-permanent pools, lakes, ponds, etc. The marine species mainly occur in the tropical sea. Species of Ulothrix and Vaucheria are sub-aerial, and form sheets on the damp soil. Species of Protococcus are epiphytic, found on tree trunks, etc. Many species of Cladophora are epizoic growing on animal bodies, whereas a few unicellular forms (e.g., Chlorella) are endophytic. Cephaleuros is parasitic and causes red rust disease of tea leaves. Some species occur in unusual environments; for example Chlorella has some thermophilic species, found in hot springs, and Chlamydomonas yellowstonensis occurs on snow covered tops of mountain.
Some green algae (e.g., Ulva, Codium, etc.) are used for human consumption, particularly in the far East. Several species of Chlorella are rich in carbohydrates, proteins and lipids. However, at present efficient digestibility of the Chlorella cell wall is a problem and researches are underway to convert it in a easily digestible form. Unicellular algae show characteristics of a typical higher plant cell and often make ideal research material since they can be grown in large numbers under precisely controlled conditions without occupying a great deal of space. For example, the use of Chlorella in research on photosynthesis has been fulfilling. They are also used in ion uptake experiments and were important in early work on the structure of tell walls and flagella. Some green algae are particularly useful to humans in open 'oxidation ponds'. Ponds between 1 m and 1.5 tn deep receive raw sewage, and oxygen provided by algal photosynthesis is vital for the other aerobic microorganisms that utilize the sewage. Many green algae synthesize antibiotic substances. The first algal antibiotic chlorellin was obtained from Chlorella.
The cell structure is eukaryotic. The cell wall generally has cellulose as the main structural polysaccharide. The protoplast is bounded by a thin semipermeable plasma membrane. The cells of the green algae have mitochondria, Golgi bodies, ribosomes, ER, plastids and other cell organelles. The main pigments of Chlorophyceae are chlorophyll a and b but a-and P- carotenes; and xanthophylls are also present. The pigments are found in the lamellae of the well-organized chloroplast. The shape of the chloroplast shows much variation. Usually there is a single nucleus in each cell, but some Chlorophyceae are coenocytic. Their photosynthetic pathways are similar to those of the higher plants. The reserve food is starch, composed of amylose and amylopectin.
The motile vegetative and reproductive cells (zoospores or gametes) have two (rarely four or more), equal, smooth, and whip-like flagella. They reproduce by vegetative, asexual and sexual methods. Vegetative propagation takes place by cell division and fragmentation. Akinetes, zoospores and aplanospores are the main asexual spores. Sexual reproduction may be isogamous, anisogamous or oogamous. The zygote or oospore secretes a thick wall and undergoes a resting period. It germinates under satisfactory conditions.
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