Structure of Algal Cell:
Algae exhibit 2 different basic kinds of cell structure; therefore they can be separated into 2 groups - prokaryotes. Prokaryotes contain so called blue-green algae classed as Cyamophyceae or Myxophyceae, but now termed as Cyanobacteria due to their cells are prokaryote type. Eukaryotic algae fairly varied in cell structure and morphology that is taken in account for categorization. In recent years, utilization of electron microscopy has brought much novel information regarding ultra structure of cellular components of algae. Chemical composition and functions are found by breaking and isolating each of its organelles separately. This study discloses that eukaryotic algae illustrate several features which resemble higher plant groups.
Prokaryotic Algal Cell:
Cyanobacteria closely look like bacteria in their ultrastructure. Though, cyanobacteria are not flagellated. Specific features of the cellular component are given below:
1) Cell Wall and Cell Sheath:
Cell of cyanobacteria are enveloped by the gelatinous heath and also have separate cell wall outside plasma membrane. This can be eliminated by digesting it with enzyme-lysozyme. It chemical examination illustrates that it is composed of mucopolysaccharide (peptidoglycan) like that of bacterial cell wall. It has complex structure, composed of polymer of N-aceylmuramic acid and Nacetylglucosamine which are cross linked by peptides and other compounds. Wall in fact, illustrate at least 4 layers and outermost may include lipo-polysaccharides and proteins. In several cyanobacteria cell wall is enveloped by gelatinous mucilage. It may be thin and colorless as in planktonic forms. In subaerial forms sheath is thick, firm and colored yellow or orange brown and multilayered. Few aquatic forms like scytonema Petalonema possibly have multilayered and colored sheath.
2) Photosynthetic Lamellae:
Cyanobacteria contain no chloroplasts but only pigmented membranes that inhabit peripheral region of cells known as chromatoplasm. In this area photosynthetic lamellae or thylakoids are present. Lamellae are folded double membranes in which photosynthetic pigments-chlorophyll a, and many kinds of carotenoid are embedded. On surface of thylakoids are found rows of granules known as phycobilisomes which have phycocymanin, allophycocyanin and at times also phycoerything, features of cyanobacteria. It has been discovered that theylakoids also have enzymes needed for respiration.
3) Granular Inclusions of Cytoplasm:
Ultrastructure of cyanobacteria sytoplasm illustrates many kinds of gramules. Between thylakoids glycogen is discovered in form of granules of various sizes. Protein granules known as cyanophycin granules composed of polymer of 2 aminoacids aspartic acid and arginine are for storage of nitrogen. Another kind of granule common in algae developing in waters rich in phosphate, is polyphosphate, storage form of phosphate. Few algae also have granules of polybatalydroxrate as big crystals. Another exclusive granule discovered in cyanobacteria are polyhedra crystalline bodies called as carboxysomes. They are composed of ribulose-biphosphate carcoxylase (Rubisco) enzyme is needed in photosynthetic fixation are of 70s kind unlike 80s type found in eukaryotes.
4) Gas Vesicles:
Several planktonic cyanobacteria such as Microvysis have in their cell lengthened, cylindrical vesicle individually or in bundles called as gas vesicles. They make cells float on surface of water. When, gas escapes they collapse turn out to be flat and cells sink to bottom. Wall of vesicle is composed of single layer of protein molecules and permeable to gases but not to water.
Central portion of cell generally referred as nucleoplasm has genetic material DNA, corresponding to nucleus of eukaryotes. It seems as the net work of fibrils, and like that of bacteria it is long thread in shape of ring, usually referred to as circular chromosome. There may be numerous copies of it in cell. Histone proteins discovered in eukaryotic cells are not related with DNA of cyanobacteria.
Similar in bacteria, DNA is also discovered in cells of cyanobacteria as small covalently related circular molecule called as plasmid that has genes which make organism resistant to antibiotics. Plasmids are not permanent characteristic of cells, they may be lost and regained further and they can also grow inside host cells.
Specialized Cells of Cyanobacteria:
These are thick cells discovered in filamentous cyanobacteria illustrate two other kinds of structures, heterocysts and akinetes. These are explained below:
These are thick walled cells discovered in filamentous cyanobacteria either in between vegetative cells (intercalary or at ends (terminal) of filament. Many significant function of heterocysts is fixation of atmostpheric nitrogen as they have essential enzyme system, nitrogenase.
Structure of Heterocyst:
Unlike the vegetative cell, heterocyst contains thick wall with 3 layers that are structurally dissimilar. Inner most layer has certain glycoligids that make heterocyst resistant to oxygen, otherwise O2 lives in action of nitrogenase and avoids nitrogen fixation. Heterocysts are joined with adjacent cells through fine protoplasmis strands plasmodesmata at poles and also with large glossy granules - polar granules composed of cyanophycin. Heterocysts also have photosynthetic lamellae, but these are less dense which in vegetative cells. The lamellae contain chlorophyll a and carotenoids. However, phycocyanin is lost when a vegetative cell changes into a heterocyst. Therefore, mature heterocysts cannot fix carbon dioxides, so O2 is not liberated in light. Polyphosphate and glycogen granules, carboxysomes and gas vesicles are entirely absent in the cytoplasm of the hetercocyst.
These are thick walled cells also known as spores, meant for perenation. All the vegetative cells of a filament or only a few cells like those adjacent to a heterocyst may develop into spores. Akinetes have thick walls and they are generally light brown, deep brown or black in colour. The contents of the cell are highly granular with glycogen but polyphosphate is lacking.
Akinetes can withstand prolonged desiccation and under suitable conditions geminate giving rise to new filaments.
Eukaryotic Algal Cell:
Eukaryotic algae include many divisions each comprising its own cell structure and other specific characters. Though, basic characteristics common to all groups are - mitochondria, existence of membrane bound nuleus, plastids, chromosomes, golgo bodies, and 80s type of ribosomes. Besides cell division by mitosis, several groups illustrate sexual method of reproduction having fusion of gametes and meiosis (reduction division). The following account provides significant characteristics of algal cells of different groups.
Algal cell wall is primarily composed of cellulose. Other extra compounds may be added to it during growth. In brown algae hemicelluloses, fucin, alginic acid, fucoidin are also present. In diatoms wall material is mainly silica.
Cells of Division Chrsophyta do not have proper cell wall. They are enclosed by scales of silica (like Mallomonas). In coccolithophorides elaborate scales have calcium carbonate (calcite). Cell wall of red algae has polysylphate esters carbohydrates additionally to cellulose and pectin. Calcium carbonate deposits are discovered over surface of algae belonging to various groups of several marine seaweed, called as calcarious algae, for instance, Neomeris, Udotia (green algae). Corallina (red alga), Padina (brown alga) and fresh water alga Chara.
Every photosynthetic algae illustrate plastids - chloroplasts whose basic structure is related to chloroplasts of higher plants. Shape and location of chloroplasts in algae differs from species to species. When situated at centre of cell, they are known as axile, and when situated near periphery is known as parietal. Their number also following shapes of chloroplasts can be effortlessly recognized: spiral band (Spirogyra), girdle like (Ulothrix), cup like (Chlamydomonas), and stellate (star-shaped-Zygenema).
Ultrastructure of algal chloroplast resembles that of higher plants, it is enveloped by double membrane. The number of thylakoid lamellae are spread into matrix -stroma. Lamellae are composed of lipoprotein complexes interspersed with molecules of chlorophylls and carotenoids. When phycobilins are present as in case of red algae, they are present in form of granules called as phycobilisomes, joined to membrane surface in linear rows. Stroma of chloroplast has many enzymes joined with photosynthetic carbon fixation. Arrangement of thylakoids in chloroplasts differs in different algae. They may be extremely closely stacked to form garna (sing, granum), as in green, brown algae and enlemophytes. In red algae they are extensively divided from each other.
One significant feature of chloroplast is presence of circular or like DNA. Plastids of Acetabularia, diatoms, Englena, chlamydomonas, members of Chrysophyceae, Xanthophyceae, phaeophyceae all have circular DNA. Chloroplasts give rise to novel plastids by easy division. Chloroplasts have ribosomes of 70s type that are present in cytoplasm. They also have complete machinery for protein synthesis. Ribosomes of 70s type are feature of prokaryotes such as cyanobacteria. Due to this fact it is thought that chloroplasts of eukaryotes were certainly cyanobacteria that became endosymbiontic during course of evolution.
Plastids of several green algae have major proteinaceous granules known as pyrnoids around which starch is deposited. In several cases photosynthetic thylakoids are traversing matrix of pyrnoid or at least related with it. When chloroplasts separate, pyrenoids also split to give rise to novel pyrenoids. Several algae have only one nucleus per cell. Though, green algae like Chladophord Camlerpa and Vaucheria (Xantheophyceae) have more than one nucleus (multinucleate) similar to eukaryotic plant and animal nuclei, algal nucleus is enveloped by the distinct double membrane punctured by pores. During interphase (not separating, resting nuclets) uncoiled, fine chromatin threads are visible in nucleus.
Several algal nuclei have globular nucleoli, one or more in number, at times attached to specific region of chromosome nucleolus organizer. Nucleolus may disintegrate and vanish during cell division but reappear during interphase. It is now know that nucleolus is involved in synthesis of cytoplasmic ribosomes. Structure of nuleus in algal groups Euglenophyta and Dinophyta is fairly unique and is dissimilar from all other eukaryotes. During interpphase, nucleus inside its membrane illustrates not uncoiled chromatin fibres but very condensed chromosomes further, unlike in other organisms, they do not have histone proteins. Number of chromosomes present in every genus or species of ala has no relation with systematic position. Smallest number recorded is n=2 and highest may be 600 or more. Size of individual chromosomes is also variable. Large chromosomes are discovered in Oedogonium, Cladophoraand Chara.
Other Organelles of the Eukaryotic cells:
Number of mitochondria in algal cells differs from one as in few flagellates to several in other algae. Their size and shape also differs widely. Ultrastructure illustrates double membrane, inner one folded inwardly forming cristae protruding into lumen. Novel mitochondria arise by division of mitichondria present in parent cell, much like platids. It is thought that mitochondria from endosymbiotic bacteria adapted to intracellular existence inside ancestral host eukaryotic cells. Like chloroplasts they also have circular DNA, RNA, 70s ribosomes machinery for protein synthesis.
2) Golgi bodies:
These are also called as dictyosomes and are extensively found in algal cells. They are composed of 2-20 lamellae or membranes set in stack. They play the significant role in formation of cell wall material as in case of red algae. In several algae they are related with secretory function.
Flagella are mean of locomotion for motile cells of algae, found in every divisions except Rhodophyta. Alga may itself be motile (as in unicellular and colonial algae) or at some phase in its life cycle produce reproductive motile cells - zoospores and gametes. Flagella of algae vary in length, number, appendages and place of insertion on cell. Surface of flagellum may be flat (acronematic) may contain one or more lateral hairs (pleuronematic). When 2 flagella are discovered they may be equal in length (isokonton) or one flagellum shorter than other (heterokonton) Ultrastructure of flagellum illustrate it is composed of microtubules, two at centre surrounded by 9 pairs or doublets in ring, 9+2, all surrounded by membrane. Flagellar surface is usually smooth or covered with prominent hairs, mastiganemes few green algae and members of Chrysophyta, phaeophyta, Dinophyta Chrysophyta, illustrate 2 flagella, one with even surface and other with fine hairs.
Motile cells of algae belonging to phaeophyta, Euglenophyta chlorophyta. Chrysophyta have orange - red colored eyespots. In few algae eyespot may form the part of chloroplast and it is situated at base of flagellum, but in Euglena it is fairly distinct and away from chloroplasts. Common kind of eyespot as located in green algae like chlamydomonas seems to have row of orange colored lipid gramules as part of thylakoids at anterior portion of chloroplast. Granules are discovered to have carotenoids, B-carotene being most important.
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