Cell Structure of Algae PDF

Cell structure of algae The algal thalli cells are basically constituted of two kinds, prokaryotic and eukaryotic. 1-The prokaryotic cells which constitute Cyanophyta (blue green algae) are surrounded by a thin or thick mucilaginous sheath. The cell wall is followed by a cell membrane composed of...

Cell structure of algae The algal thalli cells are basically constituted of two kinds, prokaryotic and eukaryotic. 1-The prokaryotic cells which constitute Cyanophyta (blue green algae) are surrounded by a thin or thick mucilaginous sheath. The cell wall is followed by a cell membrane composed of lipids and proteins. The inner layer of the cell wall has a chemical composition like the bacterial cells, made up of peptidoglycans. The inner contents of the cell can be distinguished into: a) an outer pigmented region called chromoplasm, it contains photosynthetic pigments: chlorophyll a, ß -carotenes and others. b) a central clear region called centroplasm has many irregularly arranged fine strands of DNA. 2-The eukaryotic cells: found in all other algal divisions and the cell consists of: 1- Cell wall: A cell wall is an outer layer (flexible) surrounding certain cells that are outside of the cell membrane. The materials that make up the cell wall differ depending on the type of organism. It has a few different functions: strength to the cell, protect the cell against physical damage or pathogens such as bacteria, give the cell its shape and allows some small molecules to pass through it. It is bilayer in most of the algal members and composed mainly of cellulose (carbohydrates) may be mixed with inorganic substances such as silica (Bacillariophyta), calcium or magnesium carbonate, pectin, algin, … The two layers of cell wall consists of: 1) a fibril component which is cellulose, sometimes mannan, xylan or pectin. 2) an amorphous mucilaginous component (polysaccharides) which occur in greatest  amounts in the Phaeophyta (as alginic acid) and Rhodophyta (as agar or carrageenan). In Euglenophyta, there is no cell wall, but the outermost layer of the cytoplasm is called pellicle or periplast. 2- Flagella: - Many algae, zoospores and gametes bear flagella which are the means of motility. A flagellum consists of 2 central tubules, surrounded by 9 peripheral tubules, all enclosed by a membrane. - Flagella may be equal (A) or unequal (B), inserted apically (A, B) or laterally (E), and may be of tinsel (C) or whiplash (D) types. Except Cyanophyta & Rhodophyta, all algal divisions contain species provided with flagella either in all stages of their life (i.e., vegetative & reproductive stages) or at least on their motile reproductive units (zoospores and gametes). - Morphologically flagella are fine & hyaline threads emerged of the cytoplasm and covered with the plasma membrane of the cell. The basic pattern of the structure of flagellum is the same in all organisms. - The position, number & kind of flagella on motile cells is constant in each division of algae but differs from division to division; so, it was used by some Phycologists for the classification of algae into divisions. Position of flagella Dr. Afaf Hasan Ali 2023 -2024 1 3- Plastids: - Plastid is an organ that contains photosynthetic pigments. The basic type of plastid in the algae is a chloroplast when the chlorophyll pigment is predominating, whereas if pigments other than chlorophyll are predominating, they are called chromatophores. - The number, the morphology (form & size) and the position of plastids in algal cells vary from one algal genus (or species) to another. An algal cell may contain only one plastid or many plastids. - Various forms of chloroplasts are known in algae: 1- cup shaped (e.g., Chlamydomonas and Chlorella) 2- discoid (e.g., Chara, Vaucheria and centric diatoms). 3- parietal (e.g., Phaeophyceae, Rhodophyceae, …) 4- girdle shaped or C-shaped (e.g., Ulothrix). 5- spiral (e.g., Spirogyra). 6- reticulate (e.g., Oedogonium, Hydrodictyon ,….) 7- stellate (e.g., Zygnema). - The basic ultrastructure in a plastid consists of a series of flattened membranous vesicles called thylakoids (discs), and a surrounding stroma (matrix). The thylakoids contain chlorophylls & carotenoids and are the sites of photochemical reactions. Pigments in algal cells o One of the characteristic keys used to classify algae is the type of photosynthetic pigments. Pigments are "molecules that absorb specific wavelengths (energies) of light and reflect all others." o The three major classes of pigments found in plants and algae are chlorophylls, carotenoids and phycobilins. Carotenoids and phycobilins are called accessory pigments. o Chlorophyll and accessory pigments allow algae to harvest and use the energy from sunlight. Because accessory pigments reflect different wavelengths of light than chlorophyll, they give algae a wide range of colors. a – Chlorophylls: - Chlorophylls are green or green-blue pigments, always located inside the thylakoids of all photosynthetic algae (can be extracted with organic solvents such as acetone). - There are 5 types of chlorophylls (a, b, c, d and e) in algae: Chlorophyll a is the primary photosynthetic pigment found in all algae. Chlorophyll b - present in green algae and Euglenophyta. Dr. Afaf Hasan Ali 2023 -2024 2  Chlorophyll c - present in Chrysophyta, Pyrrophyta, diatoms and brown algae Chlorophyll d - present in red algae. Chlorophyll e - present in certain Xanthophyta. - Chlorophyll is composed of a porphyrin-ring system that is very Like that of hemoglobin but has a magnesium atom instead of an iron atom. b - Carotenoids (carotenes and xanthophylls): Carotenes and xanthophylls together constitute the carotenoids. Carotenes are linear unsaturated hydrocarbons and xanthophylls are oxygenated derivatives of these. They are accessory photosynthetic pigments. - Carotenes: Five types of carotenes are found in algae: α-carotene in Chlorophyceae, Cryptophyceae and Rhodophyceae; β-carotene in all algal groups, except Cryptophyceae; c-carotene in Chlorophyceae; e- carotene in Bacillariophyceae, Cryptophyceae, Phaeophyceae and Cyanophyceae. - Xanthophylls: Several types of xanthophylls are found in algae, and since many are unique to particular algal groups, they are important diagnostic features. For example, peridin is found only in the Pyrrophyta, myxoxanthin only in the Cyanophyta, taraxanthin in Rhodophyta and antheraxanthin in Euglenophyta. c- Biloproteins: Chlorophylls and carotenoids are soluble in lipid solvents and cannot be extracted in aqueous solution. While biloproteins are water-soluble pigments. During the extraction procedure the free pigment cannot be separated from a protein molecule and the name of the pigments was therefore changed from phycobilins to biloproteins to indicate the existence of the pigment- protein complex. Biloproteins are present only in two algal divisions, the Cyanophyta (called C-type) and Rhodophyta (called R-type). There are two kinds of biliproteins, phycoerythrin (red colour) and phycocyanin (blue colour). They function as accessory pigments by absorbing and transferring the light energy to the reaction Centre. Dr. Afaf Hasan Ali 2023 -2024 3 4- Pyrenoids and food Storage products: - Pyrenoids are protein bodies present in chloroplasts or chromatophores. They are usually associated with the synthesis and storage of starch. The number of pyrenoids may be one (e.g., Chlamydomonas) or more than one (e.g., Oedogonium). - The food storage products occurring in algae may be: 1- Myxophycean (cyanophycean) starch in Cyanophyta. 2- Starch in Chlorophyta. 3- Paramylon in Euglenophyta. 4- Leucosin (fats and oils) in Xanthophyta, Bacillariophyta and Chrysophyta. 5- Laminarin in Phaeophyta. 6- Floridean starch in Rhodophyta. 5- vacuoles: Almost all the algal cells, except the members of Cyanophyceae, possess one or more vacuoles. Each vacuole is bounded by a distinct membrane called tonoplast. Three types of vacuoles are found in motile forms: - Simple vacuole: They are very small in size and show periodic contraction and expansion. They are also called contractile vacuoles. They throw out the metabolic wastes of the cells. They also regulate the water content of the cell by discharging the excess amount at short interval. So, they are secretory in function. -Complex Vacuole: This is the characteristics of Dinophyceae and Euglenophyceae. It consists of a tube, a large reservoir, and a group of vacuoles of varying sizes. The vacuoles perform the function of osmoregulation inside the cell. Sometimes, the vacuoles also store reserve food materials such as laminarin and chrysolaminarin. - Gas Vacuoles: In the cells of the members of Cyanophyceae there are gas containing cavities occurring as stacks of small transparent cylinders of uniform diameter. Their walls are freely permeable to gases. The gas vacuoles give buoyancy to the planktonic forms. Dr. Afaf Hasan Ali 2023 -2024 4  6- Eye spot or stigma: The motile algae and their reproductive structures possess structures called eye spot, stigma, or pigmented bodies. It acts as a light photoreceptive organ that directs the movement of cells towards the light. This structure is found in the anterior of the cell or near the base of flagella. This structure is absent in the members of Cyanophyta. 7- Nucleus: which is slightly more complex than bacteria. 8 - Other structures: such as Ribosomes, Golgi apparatus, Mitochondria, and others. Top 6 Types of Classification of Algae This article throws light upon the top six types of Classification of Algae. Some of the types are: 1. Nuclear Organization 2. Nature of Cell Wall Components 3. Pigmentation and Photosynthetic Apparatus 4. Nature of Reserve Food. 1- Nuclear Organization: Based on nuclear organization algae can be prokaryotic or eukaryotic. Cyanophyceae or blue green algae are prokaryotic in nature whereas all other classes of algae are eukaryotic. In prokaryotic e.g., Cyanophyceae—nucleus is not organized as nuclear membrane is absent. DNA fibrils are free in cytoplasm. Cell division by mitosis and meiosis is not found. Membrane bound cell organelle like chloroplast, mitochondria and ER are absent. Eukaryotic algae have well differentiated nucleus, mitochondria, chloroplast and endoplasmic reticulum in their cell structure. 2- Nature of Cell Wall Components: The cell wall in algae is generally made up of polysaccharides. In some cases, lipids and proteins are also present in them. The cell wall in Chlorophyceae is made up of cellulose. In Xanthophyceae pectic substance is more common. In Chrysophyceae cell wall is non-cellulosic which is silicified or calcified. In Phaeophyceae cell wall contains alginic acid and fucinic acid. In Rhodophyceae the cell wall is Dr. Afaf Hasan Ali 2023 -2024 5 made of non-cellulosic polysaccharides like xylans and galactans. In Cyanophyceae or blue green algae the cell wall is more like bacteria and is made up of mucopeptides. 3- Pigmentation and Photosynthetic Apparatus: The pigment is one of the most important criteria used in differentiation of classes in algae, as algae were initially and primarily separated based on colour e.g., green algae, red algae, brown algae or blue-green algae. The pigments in algae can be chlorophylls, carotenoids and biloproteins. These pigments are present in sac like structures called thylakoids. The thylakoids are arranged in stacks in granum of the chloroplasts. Different groups of algae have different types of pigments and organization of thylakoids in chloroplast. In Cyanophyceae the thylakoids are free in cytoplasm as chloroplasts are not found and pyrenoids are also absent. 4- Nature of Reserve Food: The primary product of photosynthesis glucose is same in all groups of algae but due to accumulation of food over long period the nature of insoluble reserve food may be different. The nature of reserve food can be a criterion for distinction of different groups of algae. In Chlorophyceae the reserve food is starch. In Xanthophyceae oil and leucosine are reserve food materials. In Rhodophyceae floridean starch and in Cyanophyceae myxophycean starches are the reserve foods. In Phaeophyceae the reserve food material is laminarin or mannitol. 5- Flagellation: The type, number and position of flagella are important basis for separation of different classes of algae. Dr. Afaf Hasan Ali 2023 -2024 6 In Cyanophyceae and Rhodophyceae flagella are completely absent in vegetative and reproductive structures. In all other classes the basic flagella structure is similar. The flagella can be acronematic or pleuronematic. In Chlorophyceae flagella are 2, 4 or indefinite in number, equal, apical, or sub-apical in position and acronematic type. In Xanthophyceae flagella are two, unequal apical one acronematic and one pantonematic i.e., heterokontic. tonematic flagella, (C) In Phaeophyceae flagella are two lateral, one acronematic and one peluronematic and unequal in size. 6- Type of Life Cycle and Reproduction: The presence or absence of sexual reproduction, complexity of reproductive organs, method of sexual reproduction i.e., isogamy, anisogamy and oogamy are important criteria of classification in algae. Haploidic life cycle, diploidic life cycle and diphasic life cycles are characteristic of different groups. For example, sexual reproduction is completely absent in Cyanophyceae. The reproduction is oogamous, and life cycles are usually complex in Rhodophyceae and Phaeophyceae. In Chlorophyceae the reproduction can be isogamous, anisogamous, and oogamous, the life cycle can be simple or complex. Dr. Afaf Hasan Ali 2023 -2024 7