3.6 Prokaryotic cells and viruses


Prokaryotic and eukaryotic cells

Cell come in a variety of shapes and sizes but there are two main types-

-          Eukaryotic cells are larger and have a nucleus bounded by nuclear membranes

-          Prokaryotic cells are smaller and have no nucleus or nuclear envelope

Prokaryotic cells                                                                            Eukaryotic cells 

No true nucleus only an area where DNA is found                 Distinct nucleus with a nuclear envelope             DNA us not associated with proteins                                      DNA is associated with proeins called histones     Some DNA can be in the form of plasmids                    There are no plasmids and DNA is linear                        No membrane bound organelles                                             Membrane bound organelles are present               No chloroplasts only bacterial chlorophyll                              Chloroplasts present in plants and algae                 Ribosomes are smaller (70S)                                                 Ribosomes are larger (80S)                                  Cell wall made of murein (peptidoglycan)                              Cell wall is made mosty of celulose                            May have an outer layer called a capsule                              No capsule  

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Structure of a bacterial cell

Bacteria can survive in every habitat in the world, they are very versatile, adaptable and successful. This is mainly a result of their small size, from 0.1 to 10 micrometres in length. Their cellular structure is relatively simple. All bacteria have a cell wall which is made up of murein (a polymer of polysaccharides and peptides). Many bacteria further protect themselves by secreting a capsule of mucilaginous slime around the wall.

Inside the cell wall is the cell-surface membrane, within which is the cytoplasm that contains 70S ribosomes. Bacteria store food reserves as glycogen granules and oil drops. The genetic material if in the form of a circular strand of DNA. Separate from this are smaller pieces of DNA called plasmids. These can reproduce themselves independently and may give the bacterium resistance to harmful chemicals. Plasmids are also used as vectors un genetic engineering.

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Organelle functions

Cell wall – Physical barrier that excludes certain substances and protects against mechanical damage and osmotic lysis.

Capsule – Protects bacterium from other cells and helps groups of bacteria to stick together for further protection.

Cell-surface membrane – Acts as a differentially permeable layer, which controls the entry and exit of chemicals.

Circular DNA – Possesses the genetic information for the replication of bacterial cells.

Plasmid – Possesses genes that may aid the survival of bacteria in adverse conditions e.g. produces enzymes that break down antibiotics.

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Viruses are acellular, non-living particles. They are smaller than bacteria, ranging in size from 20-300nm. They contain nucleic acids such as DNA or RNA as genetic material but can only multiply inside living host cells. The nucleic acid is enclosed within a protein coat called the capsid. Some viruses, like HIV, are further surrounded by a lipid envelope, or if this is not present, the capsid, have attachment proteins which are essential to allow the virus to identify and attach to a host cell.

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