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The Ultrastructure of a Typical Bacterial Cell the Bacterial Cell

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The Ultrastructure of a Typical Bacterial Cell the Bacterial Cell A.Kavitha Assistant professor Department of Botany RBVRR Womens college The Ultrastructure Of A Typical Bacterial Cell The Bacterial Cell This is a diagram of a typical bacterial cell, displaying all of it’s organelle. The Bacterial Cell This is what a bacterial cell looks like under an electron microscope. Bacteria come in a wide variety of shapes Perhaps the most elemental structural property of bacteria is their morphology (shape). Typical examples include: coccus (spherical) bacillus (rod-like) spiral(DNA-like) filamentous (elongated) Cell shape is generally characteristic of a given bacterial species, but can vary depending on growth conditions. Some bacteria have complex life cycles involving the production of stalks and appendages (e.g. Caulobacter) and some produce elaborate structures bearing reproductive spores (e.g. Myxococcus, Streptomyces). Bacteria generally form distinctive cell morphologies when examined by light microscopy and distinct colony morphologies when grown on Petri plates. Perhaps the most obvious structural characteristic of bacteria is (with some exceptions) their small size. For example, Escherichia coli cells, an "average" sized bacterium, are about 2 µm (micrometres) long and 0.5 µm in diameter, with a cell volume of 0.6–0.7 μm3.[1] This corresponds to a wet mass of about 1 picogram (pg), assuming that the cell consists mostly of water. The dry mass of a single cell can be estimated as 23% of the wet mass, amounting to 0.2 pg. About half of the dry mass of a bacterial cell consists of carbon, and also about half of it can be attributed to proteins. Therefore, a typical fully grown 1-liter culture of Escherichia coli (at an optical density of 1.0, corresponding to c. 109 cells/ml) yields about 1 g wet cell mass.[2] Small size is extremely important because it allows for a large surface area-to- volume ratio which allows for rapid uptake and intracellular distribution of nutrients and excretion of wastes. At low surface area-to-volume ratios the diffusion of nutrients and waste products across the bacterial cell membrane limits the rate at which microbial metabolism can occur, making the cell less evolutionarily fit. The reason for the existence of large cells is unknown, although it is speculated that the increased cell volume is used primarily for storage of excess nutrients. Cell wall The cell envelope is composed of the plasma membrane and cell wall. As in other organisms, the bacterial cell wall provides structural integrity to the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor pressure caused by the much higher concentrations of proteins and other molecules inside the cell compared to its external environment. The bacterial cell wall differs from that of all other organisms by the presence of peptidoglycan which is located immediately outside of the cytoplasmic membrane. Peptidoglycan is made up of a polysaccharide backbone consisting of alternating N-Acetylmuramic acid (NAM) and N- acetylglucosamine (NAG) residues in equal amounts. Peptidoglycan is responsible for the rigidity of the bacterial cell wall and for the determination of cell shape. It is relatively porous and is not considered to be a permeability barrier for small substrates CELL WALL Differences in Cell Wall Structure Basis of Gram Stain Reaction – Hans Christian Gram-1884 • Differential Stain – Gram Positive vs Gram Negative Cells • Gram Positive Cells- – Thick peptidoglycan layer with embedded teichoic acids • Gram Negative Cells- – Thin peptidoglycan layer, outer membrane of lipopolysaccharide. OUTER MEMBRANE This lipid bilayer is found in Gram negative Bacteria and is the source of Lipopolysaccharide(LPS) in this bacteria. LPS is toxic and turns on the immune system. Not found in Gram positive bacteria. LIPOPOLYSACHHARIDE Cell Membrane This is made from phospholipids, proteins and carbohydrates, forming a fluid-mosaic. It surrounds the bacteria and is its most important organelle. It is controls the movement of substances in and out of the cell. Ribosomes These are the smallest and most numerous of cell organelle. Their purpose is protein synthesis for the cells own use. They consist of protein and RNA. They are located either free in the cytoplasm of attached to the RER. Genetic material The prokaryotic Bacterial cell has no nucleus. Its genetic material and DNA are contained in a nucleoid. TYPES OF PLASMIDS Fertility – F- Plasmids. These are capable of conjugation ( transfer of genetic material between bacteria which are touching). Resistance – R- Plasmids which contain genes that can build a resistance against antibiotics or poisons and help bacteria produce pili. Col-plasmids which contain genes that determine the produce of bacteriocins, proteins, that can kill other bacteria. Degradive plasmids which enable the digestion of unusual substances , e.g., toluene or Salicylic acid. THANK YOU.
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