Cellular Organisms PDF

Cellular organisms A- Eubacteria Bacteria are basically unicellular with simple shapes: short rods or bacilli, spheres or cocci, or spiral, elongated cells, spirilla. The single cells often are linked together into ribbonlike filaments, or bead-like chains of cells; some taxa form...

Cellular organisms A- Eubacteria Bacteria are basically unicellular with simple shapes: short rods or bacilli, spheres or cocci, or spiral, elongated cells, spirilla. The single cells often are linked together into ribbonlike filaments, or bead-like chains of cells; some taxa form flat, sheet-like colonies, others produce stalked, branching ones. All except the mycoplasmas have a cell wall composed of disaccharides and peptides together with a unique compound not found in eukaryotes: peptidoglycan. The latter substance is present in the Domain Bacteria and absent in the Domain Archaea, making it a good diagnostic feature. The gram stain, a dye that reacts with peptidoglycan and proteins of the cell walls, effectively divides the Domain Bacteria individuals into two major groups, Gram-positive and Gram-negative members. The principal mode of reproduction is an asexual separation of one bacterium into two. There are, in addition, several mechanisms that make possible the exchange of genetic material, the DNA, among and between bacterial cells. None, however, are as elaborate as the mitosis– meiosis choreography of gene exchange in the eukaryotes. 1 All bacterial cells are surrounded by at least one membrane, the cytoplasmic membrane enclosing the cytoplasm. This simple enclosure can be found only by species living within eukaryotic cells such as Mycoplasma spp. But most cells are surrounded in addition by a thick cell wall (the Gram-positives) and another group by a thin cell wall followed by a second membrane, the outer membrane (the Gram-negatives), where both membranes are separated by the periplasm. Furthermore, cells can be surrounded by an extracellular matrix, a capsule or an S-layer, a monomolecular protein layer. In addition, the bacterial cells can contain appendages, flagella and/or pili allowing movements and attachment to cell surfaces. The general characters of bacteria are: 1. They are omnipresent i.e., present in soil, air and water. 2. They are unicellular, prokaryotic microorganism. 3. The cell bears a thick rigid cell wall outside the plasma membrane 4. They have great variation in the mode of nutrition i.e., may be autotrophic and heterotrophic. In heterotrophism mode of nutrition they may be parasite saprophyte or symbiotic in nature. 5- They lack true chlorophyll but few photosynthetic bacteria have a special type of chlorophyll called bacteriochlorophyll. 2 6. They lack true nucleus (lacking nuclear membrane and nucleolus). 7. The cell wall of bacteria is made up of mucopeptide unlike the cell wall of plants (where it is made up of cellulose). 8. They lack mitochondria, Golgi apparatus, plastid and endoplasmic reticulum. 9. They lack basic protein histone in their DNA. 10. Ribosomes are of 70S type. 11. At some places the plasma membrane invaginate in folds to form mesosomes. 12. All the enzymes required for respiration are found in the cell membrane. 13. Both DNA and RNA are present in the bacterial cell. DNA is in the form of single circular chromosome. 14. Vegetative reproduction is generally by binary fission, cyst, budding and gonidia. 15. Asexual reproduction is by binary division, conidia, motile spores and endospore. 16. True sexual reproduction is absent in bacteria but there are examples of genetic recombination which may be of following types conjugation, transduction and transformation. 3 Distribution Bacteria are ubiquitous omnipresent in their distribution. They are found in all the natural habitats; in soil, water and air. They occur in all the situations except in pits of volcanoes, deep strata or rock and rain water, distilled water in deep wells, blood of normal animals. They constitute a major part of the soil microflora and intestine of animals. For example, E. coli in the intestine of human being. The bacterial cell structures There is great variation in size of bacteria. They are so minute which can't be seen without the help of microscope. On an average each cell of bacterium measures 1.25 - 2µ in diameter and 2-10µ in length (Fi. 2). Cocci are about 0.5-2.5 µ in diameter while bacilli are 0.3- 15µ x 0.2 -2 µ. The smallest rod shaped eubacterium is Dialister pneumosintes which measure in between 0.15 - 0.3µ in size. The biggest bacteria Beggiatoa mirabilis is about 16-45 µ in diameter and 80 in length. 4 Shapes of bacterial cell Bacterial cells differ in their shapes but usually three conventional shapes have been recognized. Initially the classification of bacteria was based on their shapes but now it is not used. The various shapes are as follows: 1. Ellipsoida / Spherica / Cocci It is the simplest form of bacteria in which bacteria appears like a minute sphere (0.5~1.25 in diameter) they lack flagella. On the basis of arrangements cocci are further classified as follows: a) Monococci: When a bacterium appears singly e.g. Micrococcus agitis, M. aureus. 5 b) Diplococcus: When they appear in a pair of cells e.g. Diplococcus pneumoniae. c) Streptococci: When they appear in rows of cells or in chains e.g. Streptocoocus lactis. d) Staphylococci: When they arrange in irregular clusters like bunches of grapes e.g. Stapllyloccolls aureus. e) Tetracoccus: When they arrange in a sequence of four e.g. Neisseria. f) Sarcinae: When they arrange in cuboidal or in a different geometrical or packet arrangements e.g. Sarcillae lutea. 6  2- Rod Shaped Bacteria or Bacillus Their size ranges from 0.5-1.2µ in diameter and 3-7µ in length. They may be flagellated or non-flagellated. Most of the bacteria causing disease in plants belongs to bacilli category. They may be of following types: a) Monobacillus: When they arrange singly. b) Diplobacillus: When they are present in a group of two e.g. Diplobacillus sp. c) Streptobacillus: When they appear in chains e.g. Bacillus tuberculosis. d) Palisade: Very rarely the bacillus arrange in a palisade arrangement. 3- Spiral or Helical or Curved They appear like a cork screw. A single spirillum has more than one turn of helix. Generally, they are found as free living, unicellular entity. Their size ranges from 10-50~ in length and 0.5 - 3~ in diameter. They are flagellated e.g., Spirillum minus, S. volutans. a) Vibrio or Coma 7 The bacteria of this group are like 'coma or small curved rod. They bear flagella at their end. Their size ranges from 1.5-l.7µ in diameter and up to more 10µ in length e.g. Vibrio cholarae. b) Spirochaeta These bacteria appear like a cork screw and atrichous. Their length is more as compared to their diameter. Their body is more flexible. 4- Filamentous These types of bacteria are generally found in sewage water and the water coming out from sugar industry or effluent of sugar industry e.g. Sphaerotilus natalls. Basically, they are rod shaped bacilli which grow in an elongated chain and are covered by a tubular envelope. Ferrous containing water generally contain filamentous bacteria e.g. Leptothrix, Cladothrix, Nocardia, streptomyces and Beggiatoa. Bacteria can be motile or non-motile. The motile form swim by means of small flexible, whip like appendage called flagella (singular flagellum). They are much thinner than the flagella or cilia of eukaryotes. Chemically they are made up of protein with a molecular weight of about 40,000. The protein of the filament is known as flagellin. 8 The number and position of the attachments of the flagella on the bacterial wall vary according to the species. Therefore, bacteria can be divided into following types: 1- Monotrichous: Single flagellum at one end of the cell e.g. Cholera vibrio & Pseudomonas citri. 2- Lophotrichous: Two or more flagella at one end or both ends of the cell e.g., Spirillum sp. 3- Amphitrichous: One or many flagella at the end of the cell e.g. Spirilla and Nitrosomonas. 4- Peritrichous: Many flagella attached all-round the cell e.g., Salmonella sp. and Clostridium. 9 Bacterial reproduction: Bacteria generally reproduce by asexual and sexual mode of reproduction. I- Asexual Reproduction: Binary fission It is the most common and most important mode of cell division which occur in bacteria when the environmental factors are favorable. In which a single cell divides after developing a transverse septum (cross wall). Bacilli and spiral bacteria divide along the longitudinal axis of the cell while in coccus this division can be on any axis. Mesosomes play an important role in binary fission. Binary fission occurs in following steps: a. Division of nuclear or genetic material: When bacterial cell attains its maximum size, it generally increases longitudinally. After that its circular DNA undergo a semiconservative replication. These two DNA moves to two opposite poles with the help of mesosomes. This division is known as amitosis because no spindle fibres are formed during this process. b. Division of cytoplasm and septum formation: 11 By the end of the nuclear material division, the cytoplasmic membrane start invagination in the middle of the cell. This invagination leads to form a septum between the daughter cells. Most unicellular microorganisms (bacteria) reproduce by binary fission, a process in which a cell grows to twice its normal size and divides in half to produce two daughter cells of equal size. Binary fission generally involves four steps. 1- The cell replicates its chromosome (DNA molecule). The duplicated chromosomes are attached to the cytoplasmic membrane. (In eukaryotic cells, chromosomes are attached to microtubules). 2- The cell elongates and growth between attachment sites pushes the chromosomes apart. 3- The cell forms a new cytoplasmic membrane and wall (septum) across the midline. 4- When the septum is completed, the daughter cells may remain attached or they may separate completely. When the cells remain attached, further binary fission in parallel planes produces a chain. When further divisions are in different planes, the cells become a cluster. 11  5- The process repeats. II- Sexual reproduction or genetic recombination Unlike other prokaryote no true sexual reproduction is found in bacteria because they lack sexual structures and no gametic fusion takes place. Karyogamy and meiosis are also absent in bacteria. Bacteria are haploid organisms. However, three different mechanism 12 were later discovered for transferring gene or genetic material from one bacterial cell to another. These mechanisms are: i. Conjugation ii. Transformation iii. Transduction Under favorable conditions a single binary fission is completed within 18-30 minutes. But such a rapid rate of cell division cannot continue for a long time due to following reasons: 1. Lack of space, food, water, oxygen other salts and accumulation of their own harmful waste products in the medium. 2. Environmental factors like light, temperature, moisture becomes unfavorable. 3. Death due to senescence and sometimes they are eaten by microscopic animals and viruses. Therefore, survival rate of bacteria in nature is relatively only 1%. 13