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Common Prokaryotic Cell Shapes. The term Coccus (plural: cocci) is the name given to round, spherical shapes. The term bacillus (plural: bacilli) is the name given to rod shaped cells. These cells are shaped like long rounded rectangles. The term vibrio (plural vibrios) is the name given to curved rods, these cells have a shape like a long comma. The term coccobacillus (plural coccobacilli) is the name for short rods; these cells look like ovals. The term spirillum (plural spirilla) is the name for long spiral cells; these look like cork screws. The term spirochete (plural spirochetes) is the name for long, loose helical spiral shaped cells. These look similar to the spirillum but are more floppy.
(credit “Coccus” micrograph: modification of work by Janice Haney Carr, Centers for Disease Control and Prevention; credit “Coccobacillus” micrograph: modification of work by Janice Carr, Centers for Disease Control and Prevention; credit “Spirochete” micrograph: modification of work by Centers for Disease Control and Prevention)
Common prokaryotic cell arrangments. The term Coccus (plural cocci) is the name for a single coccus (a single round cell). The term diplococcus (plural diplococci) is the name for a pair of two cocci (two spheres attached together). The term tetrad (plural tetrads) is the name for a grouping of four cells arranged in a square. The term streptococcus (plural streptococci) is the name for a chain of cocci; the spheres are connected into a long chain. The term staphylococcus (plural staphylococci) is the name for a cluster of cocci; the spheres are connected into a bundle. The term bacillus (plural bacilli) is the name for a single rod. The term streptobacillus (plural streptobacilli) is the name for a chain of rods; the rectangles are connected into a long chain.

In most prokaryotic cells, morphology is maintained by the cell wall in combination with cytoskeletal elements. The cell wall is a structure found in most prokaryotes and some eukaryotes; it envelopes the cell membrane, protecting the cell from changes in osmotic pressure ( [link] ). Osmotic pressure occurs because of differences in the concentration of solutes on opposing sides of a semipermeable membrane. Water is able to pass through a semipermeable membrane, but solutes (dissolved molecules like salts, sugars, and other compounds) cannot. When the concentration of solutes is greater on one side of the membrane, water diffuses across the membrane from the side with the lower concentration (more water) to the side with the higher concentration (less water) until the concentrations on both sides become equal. This diffusion of water is called osmosis , and it can cause extreme osmotic pressure on a cell when its external environment changes.

The external environment of a cell can be described as an isotonic, hypertonic, or hypotonic medium. In an isotonic medium , the solute concentrations inside and outside the cell are approximately equal, so there is no net movement of water across the cell membrane. In a hypertonic medium , the solute concentration outside the cell exceeds that inside the cell, so water diffuses out of the cell and into the external medium. In a hypotonic medium , the solute concentration inside the cell exceeds that outside of the cell, so water will move by osmosis into the cell. This causes the cell to swell and potentially lyse, or burst.

The degree to which a particular cell is able to withstand changes in osmotic pressure is called tonicity. Cells that have a cell wall are better able to withstand subtle changes in osmotic pressure and maintain their shape. In hypertonic environments, cells that lack a cell wall can become dehydrated, causing crenation , or shriveling of the cell; the plasma membrane contracts and appears scalloped or notched ( [link] ). By contrast, cells that possess a cell wall undergo plasmolysis rather than crenation. In plasmolysis, the plasma membrane contracts and detaches from the cell wall, and there is a decrease in interior volume, but the cell wall remains intact, thus allowing the cell to maintain some shape and integrity for a period of time ( [link] ). Likewise, cells that lack a cell wall are more prone to lysis in hypotonic environments. The presence of a cell wall allows the cell to maintain its shape and integrity for a longer time before lysing ( [link] ).

a) An isotonic solution is a solution that has the same solute concentration as another solution. There is no net movement of water particles, and the overall concentration on both sides of the cell membrane remains constant. The image shows a cell with 20% solute (80% water) in a beaker containing 20% solute (80% water). Arrows in and out indicate that water moves both into and out of the cell. b) A hypertonic solution is a solution that has a higher solute concentration than another solution. Water particles will move out of the cell, causing crenation. The cell in this image still has 20% solute concentration (80% water) but the cell is now in a beaker containing 40% solute concentration (60% water). An arrow shows water moving out of the cell and the cell shriveling. C) A hypotonic solution is a solution that has a lower solute concentration than another solution. Water particles will move into the cell, causing the cell to expand and eventually lyse. The cell in this diagram still has 20% solute concentration (80% water) but is now in a beaker containing 10% solute concentration (90% water). An arrow shows water mving into the cell and the cell swelling.
In cells that lack a cell wall, changes in osmotic pressure can lead to crenation in hypertonic environments or cell lysis in hypotonic environments.
a) In an isotonic solution there is no net movement of water particles. The cell membrane is attached to the cell wall. The drawing shows a rectangular cell; the cell membrane is just inside the cell wall. Arrows indicate that water is moving both in and out. B) In a hypertonic solution water partices move out of the cell. The cell membrane shrinks and detaches from the cell wall (plasmolysis). The diagram shows a cell that has shriveled. Points of the cell membrane are still attached to the cell wall but most of the cell membrane has pulled away from the cell wall resulting in a star-shaped cell. Arrows show water leaving the cell. In a hypertonic solution water particles move into the cell. The cell wall counteracts osmotic pressure to prevent swelling and lysing. The image shows the same rectangular cell as in the isotonic solution except that the cell and cell wall are bulging outwards a bit. Arrows show water entering the cell.
In prokaryotic cells, the cell wall provides some protection against changes in osmotic pressure, allowing it to maintain its shape longer. The cell membrane is typically attached to the cell wall in an isotonic medium (left). In a hypertonic medium, the cell membrane detaches from the cell wall and contracts (plasmolysis) as water leaves the cell. In a hypotonic medium (right), the cell wall prevents the cell membrane from expanding to the point of bursting, although lysis will eventually occur if too much water is absorbed.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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