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Ultrastructure of a Prokaryotic Cell

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Self Evaluation (Textual Question)

1. Describe the ultrastructure of a prokaryotic cell.

The prokaryotic cell (Bacterium) is surrounded by an outer cell wall and inner plasma membrane. The cell wall is rigid and contains polysaccharides, lipids and protein molecule. The plasma membrane is a lipoprotein structure and it controls the entry and exit of small molecules and ions. The enzymes involved in respiratory chain and photosynthesis are present in the plasma membrane of prokaryotes.

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The bacterial chromosome is a single circular molecule of naked DNA tightly coiled within the nucleoid. It is amazing to note that the DNA of E.coli which measures about 1 mm long when uncoiled, contains all the genetic information of the organism. In this case, there is sufficient information to code for 2000 to 3000 different proteins.

In addition to a chromosome, certain bacteria contain a small, extrachromosomal circular DNA called plasmid. The plasmid is responsible for the antibiotic resistance in some bacteria. These plasmids are very much used in genctic engineering where the plasmids are separated and reincorporated, genes (specific pieces of DNA) can be inserted into plasmids, which are then transplanted into bacteria using the techniques of genetic engineering.

Surrounding the DNA in the darker region of the protoplasm are 20,000 to 30,000 particles called ribosomes. These are composed of RNA and proteins and are the sites of protein synthesis. Ribosomes exist in groups called polyrit osomes or polysomes. Each ribosome consists of a large and a small sub unit. The remainder of the cell is filled with H20, various RNAs, protein molecules (including enzymes) and various smaller molecules.

Certain motile bacteria have numerous, thin hair like processes of variable length called flagella. Flagella are used for locomotion. In contrast with the flagella of eukaryotic cells, which contain 9+2 microtubles each flagellum in bacteria is made of a single fibril.

Other Important Questions

2. Write a short note on Prokaryotes.

In general, prokaryotes consist of a single closed compartment containing the Cytosol and bounded by the plasma membrane. Although bacterial cells do not have a well-defined nucleus, the genetic material, DNA, is condensed into the central region of the cell. In all prokaryotic cells, most of or all the genetic information resides in a single circular DNA molecule, in the central region of the cell. This region is often referred to as incipient nucleus or nucleoid. In addition, most ribosomes, the cell's protein synthesizing centres are found in the DNA free region of the cell. Some bacteria also have an invagination of the cell membrane called a mesosome, which is associated with synthesis of DNA and secretion of proteins. Thus we cannot say that bacterial cells are completely devoid of internal organization.

Bacterial cells possess a cell wall which lies adjacent to the external side of the plasma membrane. The cell wall is composed of layers of peptidoglycan, a complex of proteins and oligosaccharides. It protects the cell and maintain its shape.

3. Write a brief note on Cyanobacteria.

Cyanobacteria are included in the group Eubacteria. The Cyanobacterial prokaryotes, commonly called blue green algae, are photosynthetic. In Cyanobacterial cells, the photosynthetic respiratory and genetic apparatuses are present but not delimited from each other by any bounding membrane of their own. No sharp boundaries divide the cell into special regions. But, there are several cell components with characteristic fine structure. These are distributed throughout the cell in patterns varying from species to species and also in different developmental stages in the same species.

These Cyanobacterial cells have an elaborate photosynthetic membrane systems, composed of simple thylakoids and a central nucleoplasmic area which is usually fibrillar or granular or both. The cell also includes various kinds of granular inclusions, a rigid, several layered cell wall and a fibrous sheath over the cell wall. The characteristic collective properties of Cyanobacteria include oxygenic photosynthesis, chromatic adaptation, nitrogen fixation and a capacity for cellular differentiation by the formation of heterocysts, akinetes and hormogonia.

4. Describe the structure of eukaryotic cell.

Eukaryotic cells, like prokaryotic cells are surrounded by a plasma membrane. However, unlike prokaryotic cells, most eukaryotic cells contain internal membrane bound organelles.

Each type of organelle plays a unique role in the growth and metabolism of the cell and each contains a set of enzymes that catalyze requisite chemical reactions.

The largest organelle in a eukaryotic cell is generally the nucleus, which houses most of the cellular DNA. The DNA of eukaryotic cells is distributed among 1 to about 50 long linear structures called chromosomes. The number and size of the chromosomes are the same in all cells of an organism but vary among different types of organisms. The total DNA (the genetic information) in the chromosomes of an organism is referred to as it genome. In addition to the nucleus, several other organelles are present in nearly all eukaryotic cells, the mitochondria in which the cell's energy metabolism is carried out, the rough and smooth endoplasmic reticula, a network of membranes in which proteins and lipids are synthesized and peroxysomes, in which fatty acids and amino acids are degraded. Chloroplasts the site of photosynthesis are found only in plants and some single celled organisms. Both plant cells and some single celled eukaryotes contain one or more vacuoles, large, fluid-filled organelles in which nutrients and waste compounds ire stored and some degradative reactions occur. The Cytosol of eukaryotic cells contains an array of fibrous proteins collectively called the cytoskeleton. Cytosol is the soluble part of the cytoplasm. It is located between the cell organelles. The plant cell has a rigid cell wall composed of cellulose and other polymers. It contributes to their strength and rigidity.

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Self Evaluation (Textual Question)

1. Tabulate the differences between prokaryotes and eukaryo :es.

The differences between Prokaryotes and Eukaryotes.

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