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The green algae are the large group of algae from which the embryophytes (higher plants) emerged. As such they form a paraphyletic group, variously included among the Plantae or with the Protista. The green algae include unicellular and colonial flagellates, usually but not always with two flagella per cell, as well as various colonial, coccoid, and filamentous forms. In the Charales, the closest relatives of higher plants, full differentiation of tissues occurs.Almost all forms have chloroplasts. These contain chlorophylls a and b, giving them a bright green colour, and have stacked thylakoids. They are bound by a double membrane, so presumably were acquired by direct endosymbiosis of cyanobacteria. A number of cyanobacteria show similar pigmentation, but this appears to have arisen more than once, and the chloroplasts of green algae are no longer considered closely related to such forms. The only other groups with primary chloroplasts are the red algae and glaucophytes, and it may be that the green algae share a common origin with them. The euglenids and chlorarachniophytes also have green chloroplasts, which were presumably acquired from ingested green algae, in the latter case retaining a vestigial nucleus (nucleomorph).
Some species of green algae, particularly of genera Trebouxia or Pseudotrebouxia (Trebouxiophyceae), can be found in symbiotic associations with fungus to form lichenCrustose and foliose lichens on a wall Lichens are symbiotic organisms made up by the association of microscopic green algae or cyanobacteria and filamentous fungi. Lichens take the external shape of the fungal partner and hence are named based on the funs. In general the algal species that partner in lichens cannot live on their own, while the fungal partner species are often found living in nature without the algae.
All green algae have mitochondriaIn cell biology, a mitochondrion is an organelle found in the cells of most eukaryotes. Mitochondria are sometimes described as " cellular power plants" because their primary purpose is to manufacture adenosine triphosphate (ATP), which is used as a sourc with flat cristae. When present flagellaThe flagellum (plural: flagella is a propulsive structure used by many single-celled organisms to move through a liquid medium. There are three main varieties of flagellum; the bacterial flagellum (a helical filament that rotates like a screw), archaeal f are typically anchored by a cross-shaped system of microtubuleMicrotubules are protein structures found within cells. They have diameter of ~ 24 nm and varying length from several micrometers to possible millimeters in axons of nerve cells. Organization Microtubules are polymers of tubulin. They are hollow cylinderss, but these are absent among the higher plants and charophytes. They usually have cell walls containing celluloseCellulose ( C H O) is a long-chain polymer polysaccharide carbohydrate, of beta-glucose. It forms the primary structural component of plants and is not digestible by humans. History and Applications Cellulose is a common material in plant cell walls and w, and undergo open mitosisIn biology, mitosis is the process of chromosome segregation and nuclear division that follows replication of the genetic material in eukaryotic cells. This process assures that each daughter nucleus receives a complete copy of the organism's genome. without centrioles. Sexual reproduction varies from fusion of identical cells (isogamy) to fertilization of a large non-motile cell by a smaller motile one (oogamy). However, these traits show some variation, most notably among the basal green algae, called prasinophyte s. The remaining forms are usually classified as follows:
- Chlorophyta
- Chlorophyceaesee text The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. The following orders are typically recognised: Dunaliellales Chlamydomonadales (e. Chlamydomonas Volvocales (e. Volvox Chlor
- UlvophyceaeA class of green algae, distinguished mainly on the basis of ultrastructural morphology. The sea lettuce, Ulva belongs here. They are divided into the following groups: Order Ulotrichales (e. Ulothrix Order Ulvales (e. Ulva Order Siphonocladales Order Das
- Trebouxiophyceae
- Chlorokybales
- Klebsormidiales
- Zygnematales
- Desmidales
- Coleochaetales
- Charales (stoneworts)
The orders outside the Chlorophyta are often grouped as the division Charophyta, which is paraphyletic to higher plants, together comprising the Streptophyta. Sometimes the Charophyta is restricted to the Charales, and a division Gamophyta is introduced for the Zygnematales and Desmidales. In older systems the Chlorophyta may be taken to include all the green algae, but taken as above they appear to form a monophyletic group.
