University:
University of Phoenix
Course:
BIO 280 | Conservation Biology
Academic year:

2023
Views:

484

Pages:

17
Author:

kvoterefgk

(CHLOROPHYTA) Introduction: Chlorophyta is a division of algae that includes green eukaryotic algae forms, consisting of various species. It was once considered a class named Chlorophyceae but is now termed Chlorophyta. Often, Chlorophyta and Chlorophyceae are used synonymously to indicate green algae. General Characteristics: Found in all forms of thallus structure, exhibiting both sexual and asexual reproduction. The group is distinguished by characteristics such as: Plant organization with 2 or more thylakoids per stack in chloroplasts. Absence of phycobilins. Pyrenoids usually present in the chloroplast. Major pigments include chlorophyll a and b, with small amounts of others. The green color of the algae is due to chlorophyll. Starch is the main storage substance and is stored inside the chloroplast. Two flagella of equal size, apically or subapically oriented. An eyespot (stigma) is often present. Asexual reproduction occurs through multiple means, such as fragmentation or spores. Sexual reproduction can occur as isogamous, anisogamous, or oogamous. Occurrence: Chlorophyta members are predominantly freshwater forms, with only 10% being marine. More than 20,000 species have been discovered in this class. Some members may also be parasitic, epizoic, or epiphytic. Range of Thallus Organization: The thallus structure varies from unicellular motile (Chlamydomonas) to non-motile forms (Chlorella), colonial forms (Volvox), to filamentous branched (Spirogyra) or heterotrichous forms. The organization also extends to highly developed forms like Chara, showing specialized organs and reproductive structures. Cell Structure: (a) Cell wall: Typically two-layered and composed of pectin and cellulose/hemicellulose. Xylan and mannans are found in some species instead of cellulose. Cell walls are highly calcified in certain species. (b) Chloroplasts: Chloroplasts are double-membraned without endoplasmic reticulum and contain thylakoids arranged in groups. Major pigments include chlorophyll a and b. Pyrenoids, often associated with starch synthesis, are commonly found in chloroplasts. (c) Eyespot: Light-sensitive eyespots help in movement by detecting light. The eyespot consists of a curved plate, a biconvex lens, and a photoreceptor zone. (d) Flagella: The flagella are smooth, equal in length, and emerge from anterior or lateral positions. Members exhibit two equal flagella in reproductive cells. (e) Nucleus: Chlorophycean cells usually have one nucleus, which is well-defined with a double-layered nuclear envelope. Reproduction in Chlorophyta: Three modes of reproduction are observed: vegetative, asexual, and sexual. (A) Vegetative Reproduction: Occurs by fragmentation, cell division, or akinete formation. (B) Asexual Reproduction: Various types of spores are formed, such as zoospores, aplanospores, and hypnospores. Zoospores are the most common in chlorophycean members, except in conjugating species. (C) Sexual Reproduction: Sexual reproduction involves isogamous, anisogamous, or oogamous types, depending on the species. Oogamy is the most specialized form found in some groups like Chlamydomonas. Example: Chlamydomonas Systematic Position: Division: Chlorophyta Class: Chlorophyceae Order: Volvocales Family: Chlamydomonadaceae Genus: Chlamydomonas Thallus Structure (Morphology): The plant body consists of a single biflagellated cell. The cell shape varies from oval to spherical or pyriform. The cell wall is composed of pectic substances, and a single large cup-shaped chloroplast occupies much of the cell. Reproduction: Asexual Reproduction: Most commonly occurs through zoospores, aplanospores, or hypnospores. Sexual Reproduction: Sexual reproduction occurs primarily through isogamy, though anisogamy and oogamy are also observed in certain species.