sign in sign up
<<
Home , Chlorophyta, Tetraspora

SALIENT FEATURES OF

Chlorophyta includes about 4,300 . About 90% of all known species live in freshwater. The Chlorophyta are primarily freshwater; only about 10% of the are marine, whereas 90% are freshwater. Some orders are predominantly marine (Caulerpales, , Siphonocladales), whereas others are predominantly freshwater (, ) or exclusively freshwater (Oedogoniales, ). The freshwater species have a cosmopolitan distribution, with few species endemic in a certain area.

Characteristics

They may be unicellular (one cell), multicellular (many cells), colonial (living as a loose aggregation of cells) or coenocytic (composed of one large cell without cross-walls; the cell may be uninucleate or multinucleate). They have membrane-bound and nuclei. Some are symbiotic with fungi giving .

Chlorophyll a and b are found in the same proportions as the 'higher' ; beta-carotene (a yellow pigment); and various characteristic xanthophylls (yellowish or brownish pigments). Food reserves are , some fats or oils like higher plants. Starch is found in association with . are thought to have the progenitors of the higher green plants but there is currently some debate on this point. The Chlorophyta thus differ from the rest of the eukaryotic algae in forming the storage product in the instead of in the cytoplasm.

Major forms of Chlorophyta i. Unicellular Motile Forms:

The simplest structure is seen under this category where unicellular spherical, oval or pear-shaped motile body may be isolated individual or colonial form being not a mere assemblage of flagellated cells, but is a coordinated whole behaving as a well-organized unit with a definite number of cells arranged in a:specific manner known as a coenobium. ii. Unicellular Non-Motile Palmellate Forms:

A loose assemblage of cells which lie embedded in a common gelatinous matrix secreted by the several cells forming a colony. Colonies are generally amorphous, may often have definite shapes. Ex. . iii. Unicellular Non-Motile Coccoid Forms:

The plant body is small isolated unicellular non-motile spherical cell having sedentary habit. Ex. iv. Filamentous Forms:

The filamentous condition is an important advancement over the unicellular condition. In a large number of higher forms, the plant body is filamentous which may be un-branched or branched. Ex. . v. Parenchymatous Thalloid Forms:

Plant body is parenchymatous thallus of simple to complex in form produced as a result of division of cells in more than one plane. Ex. Ulva. vi. Heterotrichous Forms:

This is the most advanced type. Plant body is composed of two portions, a prostrate portion consisting of creeping threads attached to the substratum —the prostrate system, and a more or less richly branched erect portion growing from the prostrate portion—the erect system.

In some forms both the systems are fairly well developed, whereas in others, one of these systems is developed better, the other being poorly developed, or very much reduced, or even completely suppressed. Ex. Coleochaete. vii. Siphonaceous Forms:

In a number of green algae considerable enlargement of the plant body may take place without the occurrence of any septation producing a large multinucleate structure known as a coenocyte. Ex. Vaucheria.

A cell of Hydrodictyon A cell of Zygnema Branched filament of colonial form unbranched filament

Discoid thallus of Coenocytic thallus Chara thallus with Coleochaete of Vaucheria globule nucule

Flagella

In the , the flagella are attached in a lateral position in the cell. In the and , the flagella are attached at the anterior end. Flagellar basal bodies are anchored in the protoplast by microtubular roots and/or rhizoplasts.

Microtubular roots consist of groups of 24-nm diameter microtubules that can have one of two basic configurations: (1) There can be a microtubular root consisting of a large broad band of microtubules with a smaller second microtubular root (Charophyceae), or (2) there can be four groups of cruciately arranged microtubular roots running from the basal bodies (Ulvophyceae and Chlorophyceae). The cruciately arranged microtubular roots have what is called an X-2-X-2 arrangement. Thus moewusii has a 4-2-4-2 arrangement, whereas motile cells of sp. have a 5-2-5-2 arrangement. One of the roots containing two microtubules is often linked to the outer membrane of the chloroplast envelope and is probably involved in phototaxis.

Rhizoplasts may be present in most of the families, but are absent in the Charophyceae. A rhizoplast is usually a cylinder containing 5- to 10-nm- diameter filaments interrupted at approximately 80-nm intervals by bands of electron-dense material. A rhizoplast runs from the basal bodies posteriorly toward the nucleus.

Cell structure

Cell walls usually have cellulose as the main structural , although xylans or mannans often replace cellulose. The primitive algae in the have extracellular scales, or a wall derived from interlacing scales, composed of acidic . Volvocales have walls composed of glycoproteins. In , carotene accumulates between thylakoids in the chloroplast, and , where astaxanthin accumulates in lipid globules outside the chloroplast. Hematochrome is a general term for these carotenoids. Accumulation of hematochromes color the cells orange or red, with hematochrome accumulating up to 8–12% of the cellular contents in Dunaliella. Chloroplasts are surrounded only by the double-membrane chloroplast envelope, with no chloroplast endoplasmic reticulum. The thylakoids are grouped into bands of three to five thylakoids without grana. In some of the siphonaceous genera (e.g., ), amyloplasts containing starch grains and a few thylakoids occur in the chloroplasts. Starch is formed within the chloroplast, in association with a pyrenoid, if one is present. The starch is similar to that of higher plants and is composed of amylose and amylopectin. The photosynthetic pathways are similar to those of higher plants, many of these pathways first being worked out in green algae such as . Contractile vacuoles are present in vegetative cells of most Volvocales. Usually in biflagellate genera there are two contractile vacuoles at the base of the flagella.

Eyespots

There are two types of phototactic movement in the Chlorophyta: movement by flagella and movement by the secretion of mucilage. Most of the flagellated cells that show phototactic movement have an eyespot. In the Chlorophyta, the eyespot or stigma is always in the chloroplast, usually in the anterior portion near the flagella bases. The eyespot consists of one to a number of layers of lipid droplets, usually in the stroma between the chloroplast envelope and the outermost band of thylakoids. The eyespot is usually colored orange-red from the carotenoids in the lipid droplets.

Vegetative reproduction

It is one of those processes in which portions of the plant body become separated off to give rise to new individuals without any obvious changes in the protoplasts. Common examples are afforded by the fragmentation into two or more pieces or through an accidental or natural separation of its parts.

Each portion may then grow into an independent individual. Besides this, the process of multiplication by ordinary cell division is also characteristic of some unicellular algae.

Asexual reproduction There are a number of types of asexual reproduction, the simplest being fragmentation of colonies into two or more parts, each part becoming a new colony. Zoosporogenesis commonly occurs, usually induced by a change in the environment of the alga. In the Chlorophyta, zoospores are normally produced in vegetative cells e.g., Ulothrix and only in a few cases are they formed in specialized sporangia (e.g., ). Zoospores are usually formed in the younger parts of filaments, and the number of zoospores is generally a power of two in uninucleate genera. Aplanospores are non-flagellated and have a wall distinct from the parent e.g., . Aplanospores are considered to be abortive zoospores and have the ability to form a new plant on germination. Autospores are aplanospores that have the same shape as the parent cell, and are common in the (e.g. Chlorella). Autospores are usually formed in a multiple of two in the parent cell.Coenobia are colonies with a definite number of cells arranged in a specific manner (e.g., ). Genera with colonies arranged in coenobia form daughter colonies with a certain number of cells. In maturation of the daughter coenobia, there is enlargement but no division of vegetative cells in the coenobia.

Sexual reproduction

Sexual reproduction in the Chlorophyceae may be isogamous, anisogamous, or oogamous, with the general line of evolution occurring in the same direction. Usually gametes are specialized cells and not vegetative cells, although in the one celled Volvocales the latter can occur. If the species is isogamous or anisogamous, the gametes are usually not formed in specialized cells although in the oogamous species, gametes are normally formed in specialized gametangia (e.g., Coleochaete). Whereas most Chlorophyta form motile flagellated gametes (zoogametes), in the Zygnematales aplanogametes or amoeboid gametes are formed. In some of the Chlorophyta,

Anisogamous Isogamous Oogamous Fertilization in Chara

gametogenesis is induced by environmental changes, whereas in others the presence of two sexually different strains is necessary. In the latter, vegetative cells of one secrete a substance that initiates sexual differentiation in competent cells of the opposite sex. Such a situation is common in the Volvocales and is considered in more detail later. In Oedogonium, sex organs form without the complementary strain, but subsequent fertilization is under a complex hormonal control. In other genera, a chemotactic substance is sometimes produced by the egg that attracts the spermatozoids. This does not generally happen in isogamous species. In isogamous species, sexually different gametes meet at random and immediately adhere by means of an agglutination reaction. The agglutinative flagellar adhesion between gametes of different sex is designated as the mating-type reaction. Initially after mixing, the gametes of opposite adhere by their flagella tips in clusters of up to 25 gametes. Soon the anterior ends of complementary gametes fuse, and the flagella free themselves. The motile zygote then swims for some time before settling and secreting a thick wall. The mating-type substances (responsible for flagellar agglutination) are localized and function at the flagella tips. It is possible to isolate the mating-type substances that still have the ability to interact with the gametes of the opposite sex. When added to the opposite gamete type, they cause isoagglutination (male gametes will clump with each other when a female mating-type substance is added to the culture). The mating-type substances are discussed in more detail for Chlamydomonas, which has been most intensively studied.