General Characters of Algae

Algae :) General characteristics of algae: Algae are eukaryotic organisms that show no roots, stems, or leaves but do have chlorophyll and other pigments for carrying out photosynthesis. Algae can be multicellular or unicellular. Unicellular algae occur most frequently in waters, especially in plankton. Phytoplankton (the population of free-floating micro-organisms) is composed primarily of unicellular algae. Algae may occur in moist soil or on the surfaces of moist rocks and wood. Algae live with fungi in lichens. The cell of an alga has eukaryotic properties and some species have flagella with a "9-plus-2" pattern of microtubules. A nucleus is present, and multiple chromosomes are observed in mitosis. The chlorophyll and other pigments occur in chloroplasts, which contain membranous sacs known as thylakoids. Most algae are photoautotrophic and carry on photosynthesis. Some forms, however, are chemoautotrophic (and obtain energy from chemical reactions and nutrients from preformed organic matter). Reproduction in algae occurs in both asexual and sexual forms. Asexual reproduction occurs through the fragmentation of colonies and of filaments of algae or by spore formation. During sexual reproduction, algae form differentiated sex cells that fuse and produce a diploid zygote with two sets of chromosomes. The zygote develops into a new cell, which germinates when conditions are favorable to reproduce and reform the haploid organism having a single set of chromosomes. This pattern of reproduction is called "alternation of generations." Range of thallus organization in algae: Algae are chlorophyll-bearing autotrophic thallophytes, bounded by a cell wall. Algae represent a vast and diverse kingdom in plant life and show all structures both in prokaryotic and eukaryotic members. Algal thallus organization can be broadly classified into unicellular and multicellular thallus. A. Unicellular Motile Thallus: Motile Unicellular: A single cell with flagella for locomotion. This group varies greatly in size and shape. The unicellular forms may be spherical, oblong, pear-shaped, or sometimes elongated bearing flagella. Example: Chlamydomonas They are motile due to the presence of motility factors called flagella. Flagella may be equal or unequal and the number of flagella varies depending on the species. Vegetative cell represents the vegetative cell involving all its cellular components share part in reproduction and producing asexual or sexual spores or directly behave as the sex organ depending on the species. Non-Motile Unicellular: These are single, non-motile cells carrying out all essential functions of life. Non-motile is due to the absence of motility factors called flagella. Example: Chlorella, Synechococcus B. Multicellular: Non-Motile Colony: A colony is formed by aggregation of individual cells. Non-Motile Colony: Individual cells in the colony lack flagella often connected. Coenobium: A colony with definite number of cells and having a constant shape & size. Example: Pediastrum Motile Colony: Individual cells in the colony possess flagella for locomotion, often as coenobium Volvox (500 to 50,000 cells) are connected with each other and division of labor. Example: Volvox, Gonium Palmelloid Colony: Here numerous non-motile cells are embedded in a common mucilaginous envelope. The size or shape of cells is not constant unlike coenobium. All cells carry out independent and full life activities of an individual. Example: Palmella (d) Dendroid form: The plant body appears as "tree-like" under microscope; mucilage often seen at the base. Example: Dinobryon (e) Filamentous: Cells arranged one upon the other in definite sequence or uniseriate row forming filaments. Filaments may be branched or unbranched. e.1 Unbranched filaments: Cells arranged one above the others without any branching points. Example: Spirogyra, Zygnema, Nostoc, Ulothrix. e.2 Branched filaments: Filaments with branching often dichotomous branching. Example: Bulbochaete. e.3 Pseudo-branched: Appear like branched under microscope. Actually, branching is due to the close association of one branched individual filament. Example: Scytonema. (f) Heterotrichous form: Heteros (different), trichous (tri-chrome or filament). There are presence of more than one type of filament. Plant body consists of a prostrate system form which an erect system develops (also called as primary projecting system). Branches may give rise to primary projecting system, secondary projecting system, tertiary projecting system, sometimes even quaternary projecting system. Example: Chlorophyceae: Draparnaldia, Phaeophyceae: Ectocarpus. (g) Siphonous forms: These are elongated plant body without septation. Many nuclei present in a common protoplasm cavity as a coenocytic condition. Multicellular plants lacking transverse septation. Example: Caulerpa, Codium. (h) Pseudoparenchymatous: They are of two types: Uniaxial & multiaxial type. (a) Uniaxial type: Juxtaposition of the branch system of a single main axial thread system forming the thallus. Plant body consists of one main, pseudoparenchymatous axis and all others are side branches. Example: Polysiphonia. (b) Multiaxial: Main axis made up of the association of many pseudoparenchymatous threads, appearing as more than one axis. Example: Chondrus, Polysiphonia. (i) Parenchymatous forms: In some algae, cell division and associated septation in all planes lead to the formation of a parenchymatous body. Such algae are foliose and flat. Example: Chara, Fucus, Laminaria. Cell Structure in Algae: The cellular organization in algae are broadly two types – (A) Prokaryotic cell and (B) Eukaryotic cell. (A) Prokaryotic cell: The cells of Cyanophycean members are prokaryotic in nature whose detailed structure are discussed in (d). The important features are – lack of nuclear envelope, lack of membrane-bound photosynthetic and respiratory apparatus, and absence of other cellular components like Golgi bodies, endoplasmic reticulum, etc. There are presence of C-phycocyanin and C-phycoerythrin pigments, presence of xanthophylls, etc. There are absence of mitotic cell division and sexual reproduction. (B) Eukaryotic cell: The eukaryotic algal cell posses distinct nucleus and membrane-bound organelles like chloroplast, mitochondria, Golgi body, ER. There are many types of reserve food materials in the cell. Beside, the cell may posses different flagella. The cell structure of eukaryotic algae can be studied under the following heads: Cell wall: It is the outer covering of the eukaryotic algal cell. However, cell wall is not found in some bacterial algal spores and gametes called as "naked". In general, algal cell walls are made up of two components: (a) The fibrillar component forms the skeleton of the cell wall. (b) The amorphous component which forms the matrix within which the fibrillar component is embedded. The most common type of fibrillar component is cellulose & in some species cellulose is replaced by mannan. 2. Cytoplasm: The cytoplasm contains mitochondria, chloroplast, Golgi bodies, endoplasmic reticulum, vacuoles, nucleus, etc. (a) Vacuoles: Occurs in one or more in number. Each vacuole is covered by its membrane called tonoplast. Vacuoles are present generally scattered in the cytoplasm. Two types of vacuole are seen in algae - Simple or Contractile, & Complex vacuoles. Vacuoles help in osmoregulation, act as store house or reserved food materials and control the flow of solutes in and out of the cell. (b) Plastids: Plastids are pigment containing double membrane organelles of the algae cells. Plastids may be chloroplast when it contains mainly the green pigments chlorophyll a, b, or it may be chromoplast when pigments other than chlorophyll are dominant. The colourless plastids are called leucoplasts. Pyrenoid is within the chloroplast a differentiated region called pyrenoids, seen very often clearly denser than the surrounding area and often it may or may not be traversed by thylakoids. It is associated with the storage product and is considered as the site for starch formation. It contains Rubisco, the enzyme that fixes carbon dioxide. Chloroplasts also contain small spherical sac-like lipid drops between the thylakoid which serve as a pool of lipid reserve within the chloroplast. Most chloroplast contain prokaryotic DNA in an area of the chloroplast which is devoid of 70S ribosomes. DNA is circular and remains attached to the chloroplast membrane. (c) Pigments in Algae: The different colors of algae are due to the various types of pigments and their dominance in them. There are four different kinds of pigment found in algae such as chlorophyll, carotenes, xanthophylls, and phycobilins. (d) Other cellular organelles: Like eukaryotic cells, the algal cell contains other cell organelles like ribosomes (80s), Golgi bodies, endoplasmic reticulum, lysosomes. The organization is similar to the typical eukaryotic organelles. (e) Nucleus: In eukaryotic algae, the cell nucleus is seen as a well-organized spherical or ellipsoidal structure. The number of nuclei per cell varies from one to many. The nucleus is surrounded by a double-layered envelope and remains separated from the cytoplasm with the help of perinuclear space. The nucleus is generally laterally located. However, in multinucleated cases, the nuclei are found between the plastids and vacuoles in the cytoplasm. (f) The flagella in algae: Flagella are protoplasmic fine thread-like structures found in motile cells of algae. It occurs as either single anterior flagellum or in pairs. The flagella may be equal (Isokont) or different in length (Heterokont). A single granule is located at the base of each flagellum & it is known as blepharoplast. There are two main types of flagella: 1. Whiplash (acronematic) 2. Tinsel (pantostyle). Whiplash flagellum is smooth, whereas tinsel flagellum contains rows of mastigonemes (hairs) arranged along the length of the flagellum. The position and kind of flagella is constant in a division but from one to another division. BGA & Red algae do not have flagella. In green algae generally, two equal flagella of the whiplash type are found at the anterior end. In Xanthophyceae, two unequal anterior flagella are present, one of which is tinsel type. In diatoms, a crown of flagella is found in motile cells. In brown algae, the reproductive cells are motile & possess two laterally equal flagella of which one is whiplash type & the other is tinsel type. (d) Other cellular organelles: Like eukaryotic cells, the algal cell contains other cell organelles like ribosomes (80s), Golgi bodies, endoplasmic reticulum, lysosomes. The organization is similar to the typical eukaryotic organelles. (e) Nucleus: In eukaryotic algae, the cell nucleus is seen as a well-organized spherical or ellipsoidal structure. The number of nuclei per cell varies from one to many. The nucleus is surrounded by a double-layered envelope and remains separated from the cytoplasm with the help of perinuclear space. The nucleus is generally laterally located. However, in multinucleated cases, the nuclei are found between the plastids and vacuoles in the cytoplasm. (f) The flagella in algae: Flagella are protoplasmic fine thread-like structures found in motile cells of algae. It occurs as either single anterior flagellum or in pairs. The flagella may be equal (Isokont) or different in length (Heterokont). A single granule is located at the base of each flagellum & it is known as blepharoplast. There are two main types of flagella: 1. Whiplash (acronematic) 2. Tinsel (pantostyle). Whiplash flagellum is smooth, whereas tinsel flagellum contains rows of mastigonemes (hairs) arranged along the length of the flagellum. The position and kind of flagella is constant in a division but varies from one to another division. BGA & Red algae do not have flagella. In green algae generally, two equal flagella of the whiplash type are found at the anterior end. In Xanthophyceae, two unequal anterior flagella are present, one of which is the tinsel type. In diatoms, a crown of flagella is found in motile cells. In brown algae, the reproductive cells are motile & possess two laterally unequal flagella of which one is whiplash type & the other is tinsel type. Internal structure of flagella: Each flagellum contains a central axial filament called axoneme, which is surrounded by a cytoplasmic membrane sheath. In cross section, the flagellum contains two fine central axial threads surrounded by nine united peripheral contractile fibrils. All are enclosed by a sheath, which is an extension of the plasma membrane. Each peripheral fibril is composed of two thin fibrils which are present side by side. The fibrils are hollow and extend along the entire length of the flagella.