NCERT Biology Class 11 – What are Algae and its types?
Generally, algae are referred to as plant-like organisms that are typically photosynthetic and aquatic. They do not have true roots, stems, leaves, vascular tissues, as they possess simple reproductive structures. They are distributed worldwide in the sea, in freshwater and in moist situations on land. Most algae are microscopic, but some are very large, for example, some marine seaweed can exceed 50 m in length.
The algae have chlorophyll and thus, can manufacture their own food through the process of photosynthesis. Algae are classified in the kingdom of Protista, which consists of a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms that have cells with a membrane-bound nucleus.
Algae are eukaryotic organisms, they have a membrane-bound structure called chloroplasts, which has chlorophyll, and this helps in carrying out photosynthesis. The chloroplasts contain DNA. The exact nature of the chloroplasts is different among different types of algae.
Cyanobacteria are the algae, which have a prokaryotic cell structure typical to that of bacteria. Cyanobacteria perform photosynthesis directly within the cytoplasm, but not in a specialized organelle.
Algae have photosynthetic pigments that are more complex and varied as compared to plants. They are ecologically beneficial for producing oxygen, together with a major contributing role in producing food for most of the aquatic life forms. Economically, they are used as a crude oil source and also in a number of pharmaceutical industries contributing to the assistance of mankind.
TYPES OF ALGAE
i. They are unicellular organisms that belong to the kingdom Protista.
ii. Most diatoms exist in a singular form, but some join to form colonies.
iii. They are typically yellowish or brownish and are found in fresh- and saltwater, in moist soil, and the moist surface of plants.
iv. They reproduce asexually by cell division. When aquatic diatoms die, they accumulate at the bottom, and the shells do not decay, but they simply collect in the ooze and finally form the material known as diatomaceous earth.
v. Diatoms can occur in a more compact form as a soft, chalky, lightweight rock, called diatomite. This diatomite is used as an insulating material against both heat and sound. It is also used in making dynamite and other explosives etc. Diatoms have deposited most of the earth’s limestone, and much petroleum found inside the earth is of diatomic origin.
i. This division of kingdom Protista consists of the photosynthetic organism usually known as green algae.
ii. The various species of green algae are either unicellular, multi-cellular, coenocytic (having more than one nucleus in a cell), or colonial.
iii. Chlorophyta is aquatic (marine), but few types are terrestrial, occurring on moist soil, on the trunks of trees, on moist rocks and in snowbanks. Various species are highly specialized.
iv. Green algae have chlorophyll a and chlorophyll b and store food as starch in their plastids.
i. This phylum of the kingdom Protista consists of mostly unicellular aquatic algae. Most live in freshwater.
ii. There are some euglenoids that contain chloroplasts with the photosynthetic pigments. While others are heterotrophic and can ingest the food.
iii. Reproduction occurs by longitudinal cell division.
iv. The typical example is Euglena, which is commonly found in ponds and pools, especially when the water has been polluted by runoff from fields or lawns on which fertilizers have been used.
i. It is a large group of flagellate Protists.
ii. Some species in this group are heterotrophic, but most of them are photosynthetic organisms containing chlorophyll. Various other pigments sometimes mask the green colour of these chlorophylls.
iii. Some species are also endosymbionts of marine animals and protozoa and play an important part in the biology of coral reefs.
iv. While some other dinoflagellates are colourless predators on other protozoa, and some forms are parasitic also.
v. Reproduction in most dinoflagellates is asexual, through simple division of cells following the process of mitosis. The dinoflagellates are important plankton and are primary food sources in warmer oceans.
vi. Many forms of dinoflagellates are phosphorescent; they are responsible for the phosphorescence visible at night in tropical seas.
i. These algae are known as golden algae, found mostly in freshwater.
ii. The cell walls of many chrysophytes are composed of cellulose with large quantities of silica.
iii. The photosynthetic pigments present in these are chlorophyll a and c.
iv. Under some circumstances, they reproduce sexually, but the usual form of reproduction is asexual by cell division.
i. This phylum consists of brown algae.
ii. Most of the world’s familiar seaweeds are members of Phaeophyta. There are several brown carotenoid pigments as fucoxanthin (in their cell chloroplasts) present in these organisms which give them brown colour.
iii. Brown algae are mostly marine, growing in the colder oceans of the world, many in the tidal zone, where they have great stress from wave action, while others grow in deep water.
i. The members of this phylum consisting of the photosynthetic organisms commonly known as red algae.
ii. These organisms bear red or purple colour due to the presence of accessory pigments called phycobilin.
iii. They are multicellular and are characterized by too much branching, but without dividing into complex tissues.
iv. Seaweeds belong to this group. Red algae are found in almost all the oceans, but they are most common in warm-temperate and tropical climates, where they may occur at greater depths than any other photosynthetic organisms. Few species are also found in freshwater.
v. Most of the coral forming algae, which secrete calcium carbonate and play a major role in building reefs, belong to this group.
i. This group consists of prokaryotic aquatic bacteria that obtain their energy through photosynthesis.
ii. They are generally referred to as blue-green algae.
iii. Cyanobacteria may be single-celled or colonial. The type of species and environmental conditions determine whether the colonies will form filaments, sheets or hollow balls.
iv. Unlike bacteria, cyanobacteria contain the green pigment chlorophyll (as well as other pigments), which traps the energy of sunlight and carry out photosynthesis.
v. Hence, cyanobacteria are producers of their own food from simple raw materials. Nitrogen-fixing cyanobacteria need only nitrogen and carbon dioxide to live. They fix nitrogen gas (which cannot be absorbed by plants) into ammonia (NH3), nitrites (NO2) or nitrates (NO3). These compounds are absorbed by plants and converted to protein and nucleic acids.
vi. Cyanobacteria are found everywhere, from oceans to freshwater to bare rock to soil.
Mode of Reproduction in Algae
Algae reproduce by both sexual and asexual ways involving various reproductive strategies. Asexual reproduction occurs through normal cell division or fragmentation, without the union of two cells or without the combination of different genetic material in minor algae species. In the case of higher algae, reproduction occurs through spores. Sexual reproduction also occurs that involves the process of meiosis where genetic material from two different parent cells combines. Different environmental events influence and regulate sexual reproduction.
Read other notes on Classification of Plants for Class 11 Biology
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