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|موضوع: Prokaryotic cellular organization الإثنين يونيو 22, 2015 12:00 am|| |
الموضوع الاصلي : Prokaryotic cellular organization المصدر : مستشفى الرفاعي العام
بسم الله الرحمن الرحيم
Prokaryotic cellular organization
Prokaryotes are a group of organisms that lack a cell nucleus, or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryeotes. Most prokaryotes are unicellular, Prokaryotes belong to two taxonomic domains: the bacteria and the archaea. Most prokaryotic cells posses a cell wall external to the cytoplasmic membrane which contains muramic acid , a compound not found in eukaryotic cells. Also prokaryotic cells posses ribosomes smaller than found in cytoplasmic of eukaryotic cells.
Microbiology : study of very small living organisms thes microorganisms include the bacteria , algae, protozoa, and viruses.
Bacteria : are prokaryotic cells ( Kingdom: Monera)
Structure of Bacteria:
Bacteria are smallest organisms that all machinery required for growth and self replication, their diameter is usually about 1 Mm. Bacteria cell has cell wall surrounding the cytoplasmic membrane. The membrane provides the osmotic rupture barrier while the wall protects the cell against osmotic rupture in dilute media and against mechanical damage. The cell wall and membrane are often referred together as the cell envelope. While capsule, flagella and pilli are considered Appendages.
Morphology of Bacteria:
The word of morphology means the study of size, shape, and of bacteria cells.
There is great a mount of variation in shape and arrangement of bacterial cell.
The major morphological pattern of bacteria:
1. Coccus : cell can exist as individual cells but usually associated in a characteristic arrangement that are useful in bacterial identifications :
A: Diplococci : arise when a coccus divide and remain together to form pairs e.g : Neisseria spp.
B: Stretococcus: Long chain of cocci, resulted when cell adher after repeated divisions in one plan e.g : Streptococcus spp.
C: clusters: cell divide in random plans to produce irregular grape like clumps e.g: Staphylococus spp
D: tetrads : cell divide in two perpendicular planes to form square group of four cells. e.g some spp of Micrococcus.
2. Bacilli : Rod shape , the bacilli differ in their length to width ratio , we have coccobacilli, these are short and wide, some other bacilli are very long bacilli ( rod) usually occur singly.
e.g Bacillus spp and clostridium spp
3. spiral shaped bacteria, usually un attached individual cells. we have incomplete spiral known as comma or vibrio e.g : Vibrio cholera.
A: spirrila these are spira with rigid cell wall ( tightly coiled spiral) e.g Treponema spp.
B: spirochetes ( relaxed coil) these are spiral with flexible cell wall e. g0 : Borrelia ( relapsing fever).
Reproduction of Bacteria :
Bacteria and archaea reproduce through asexual reproduction, usually by binary fission or budding. Genetic exchange and recombination still occur, but this is a form of horizontal gene transfer and is not a replicative process, simply involving the transference of DNA between two cells, as in bacterial conjugation.
Structure of bacterial cell:
the layer that surround the prokaryotic cell are called cell envelope, differ gram positive and gram negative. The gram positive cell envelope is relatively simple , consist of 2 – 3 layers: the cytoplasm membrane , a thick peptidoglycan layer , and in some bacteria an outer layer called Capsule.The gram negative cell envelope is highly complex, multilayer's structure . the cytoplasmic membrane ( inner membrane ) is surrounded by a single layer of peptidoglycan to which is anchored a complex layer as outer membrane is referred as periplasmic space.
Capsule and glycocalyx:
Many bacteria synthesize large a mount of extra cellular polymer growing in their environment. With one exception ( the poly D glutamic acid capsule of bacillus anthracis ) the extra cellular material is polysaccharide when the polymer forms a condensed well define. Layer closely surrounding the cell, its called Capsule, when it forms a loose mesh work of fibrils extending outward from the cell, its called Glycocalyx. Capsule contribute to the invasiveness of pathogenic bacteria in protecting from phagocytosis.
Glycocalyx play a role in the adherence of bacteria to surface in their environment , including the cells of plants and animal hosts.
The Cell wall :
The layers of the cell envelope lying between the cytoplasm memberane and the capsule are called cell wall. In G+ve bacteria , the cell wall consists mainly of peptidoglycan , teichoic acid and polysaccharide, while in G-ve bacteria , the cell wall include peptidoglycan , outer membrane and lipopolyaccharide (LPS) and lipoprotein.
The functions of cell wall:
1. protect the cell from osmotic pressure.
2. Play an essential role in cell division
3. Various layers of the cell wall are sites of major antigenic determination of cell surface.
4. Lipopolysaccharide is responsible for the non specific endotoxin activity.
The periplasmic space:
The space between the cytoplasm membrane and outer membrane , called periplasmic space, contains the murein layer and gel like solution
Protein. The periplasmic space is proximately 20-40 % of the cell volume.
Protoplast and L forms:
Removal of bacterial cell wall is an accomplished either by hydrolysis with lysosyme or treated by penicillin, such treatment will liberate protoplast ( which retains portions of the outer membrane and entrapped peptidoglycan) from gram strength negative bacteria.
If the cell are able to grow and divide , they are called L- forms, L- Forms are difficult to cultivate and usually require a medium that a solidified with agar as will as giving the right osmotic strenght. L- forms are produced more readily with pencillin than