CELL BIOLOGY I

A prokaryote is a unicellular organism that lacks a membrane-bound nucleus (karyon), mitochondria, or any other membrane-bound organelle. The simplest living cells are called prokaryotes, which is Greek for "before the nucleus" - these are cells that were so simple that they didn't have a nucleus. The first prokaryotes probably evolved from even simpler cells about 3.5 billion years ago. For at least a billion years, prokaryotes were the only living things on Earth. They are the smallest types of cell, averaging 2-5 µm in length, which makes them just visible under the light microscope.

There are two major kinds of prokaryotes: 

• Bacteria
• Archaea (single-celled organisms) 

           

                                   Structure of Prokaryotic cell

The Prokaryotic Cell

Bacteria are unicellular and most multiply by binary fission.

Bacterial species are differentiated by morphology, chemical composition, nutritional requirements, biochemical activities, and source of energy.

The Size, Shape, and Arrangement of Bacterial Cells

Most bacteria are 0.2 um in diameter and 2-8 um in length.

The three basic bacterial shapes are coccus (spherical), bacillus (rod-shaped), and spiral (twisted), however pleomorphic bacteria can assume several shapes.

Arrangement of cocci Cocci may be oval, elongated, or flattened on one side. Cocci may remain attached after cell division. These group characteristics are often used to help identify certain cocci.
i Bacilli Since bacilli only divide across their short axis there are fewer groupings. Bacillus is a shape (rod shaped) but there is also a genus of bacteria with the name Bacillus.
Spiral bacteria Spiral bacteria have one or more twists.
Other shapes
Stella are star-shaped.	Haloarcula, a genus of halophilic archaea, are rectangular.

There are four main structures shared by all prokaryotic cells, bacterial or Archaean:

1. The plasma membrane
2. Cytoplasm
3. Ribosomes
4. Genetic material (DNA and RNA) 

Some prokaryotic cells also have other structures like the cell wall, pili (singular pillus), and flagella (singular flagellum). Each of these structures and cellular components plays a critical role in the growth, survival, and reproduction of prokaryotic cells.

The Cell Wall

Composition and Characteristics

The cell wall surrounds the plasma membrane and protects the cell from changes in water pressure. The bacterial cell wall consists of peptidoglycan (or murein), a polymer consisting of NAG (N-acetylglucosmaine)   and NAM (N-acetylmuramic acid) and short chains of amino acids.

Structure of Peptidoglycan in a Cell Wall

Alternating NAM and NAG molecules form a carbohydrate backbone (the glycan portion).

Rows of NAG and NAM are linked by polypeptides (the peptido- portion).

The structure of the polypeptide cross-bridges may vary but they always have a tetrapeptide sidechain, which consists of 4 amino acids attached to NAMs. The amino acids occur in alternating D and L forms.

While peptidoglycan is present in (most) all bacterial cell walls, there are two basic variations of structure seen in most bacterial cells, one described as Gram-positive and one described as Gram-negative.

The Gram-positive Cell Wall

Gram-positive cell walls consist of many layers of peptidoglycan and also contain teichoic acids. Teichoic acids may:

• bind and regulate movement of cations into and out of the cell
• prevent extensive wall breakdown and possible cell lysis during cell growth
• provide much of the cell wall's antigenicity

The Gram-negative Cell Wall

Gram-negative bacteria have a lipopolysaccharide-lipoprotein-phospholipid outer membrane surrounding a thin (sometimes a single) peptidoglycan layer. Gram-negative cell walls have no teichoic acids.

The outer membrane protects the cell from phagocytosis and from penicillin, lysozyme, and other chemicals.

Porins are proteins that permit small molecules to pass through the outer membrane; specific channel proteins allow other molecules to move through the outer membrane.

The lipopolysaccharide component of the outer membrane consists of sugars (O polysaccharides) that function as antigens and lipid A, which is an endotoxin. Endotoxin causes fever and shock.