Chapter 2Cell Biology
Chapter Outline
2.1 Overview of the structure of microbial cells
2.2 Procaryotic cell wall
2.3 Cytoplasmic membrane
2.4 Cellular genetic information
2.5 Cytoplasmic matrix –Ribosome and Inclusions 2.6 Components external to the cell wall
2.7 Bacterial endospores
2.8 Comparison of the prokaryotic and eukaryotic cell
Concepts
?Prokaryotes are small and simple in structure when compared with eukaryotes , yet they often have characteristic shape and size.
?Prokaryotic genetic material is located in an area called the nucleoid and is not enclosed by a membrane.
?The prokaryotic cell wall almost always has peptidoglycan and is chemically and morphologically complex.
2.1 Overview of Cell Structure
A procaryotic cell
A eucaryotic cell
3. Their cell wall almost always contain the complex
polysaccharide peptidoglycan
The Prokaryotic Cell
1. Their genetic material (DNA) is not enclosed
within a membrane and they lack other membrane
–bounded organelles
2. Their DNA is not associated with histidine
4. They are very small
!!Schematic diagram of a bacterial cell
Size:Most bacteria fall within a range from 0.2 to 2.0 um in diameter and from 2 to 8 um in length
A rod-shaped prokaryote is typically about 1-5micrometers (μm)long and about 1μm wide Microorganisms in general are very small and are completely invisible to the naked
eye.
A cyanobacterium 8 x 50 u m
size comparison of microorganisms
Visibility scale Meters Relative size of Microbes
Prokaryotes
Eukaryotes
Viruses
Naked eye Light microscope
Electron microscope
Surface area and volume relationships in cells
a cell increases in size,
its surface area –to –
volume ratio decreases
Shape:Bacteria have a few basic shapes
spherical coccus
Rod-shaped bacillus
spirallum
2.2 Prokaryotic Cell Wall
Cell Wall
?The cell wall of the bacterial cell is a complex, semi-rigid structure that is responsible for the characteristic shape of the cell. The cell wall surrounds the underlying, fragile plasma (cytoplasmic) membrane and protects it and internal parts of the cell from adverse changes in the surrounding environment.
?Almost all prokaryotes have cell walls.
Schematic diagram of bacterial cell walls Gram+Gram-
Bacteria can be divided into two major groups,called gram-positive and gram-negative.The original distinction between gram-positive and gram-negative was based on a special staining procedure,the Gram stain
Gram-positive Cell Wall
The Gram-positive cell wall has a peptidoglycan layer that is relatively thick (ca. 40 nm) and comprises approximately 90% of the cell wall. The cell walls of most Gram-positive eubacteria also have teichoic acids.
Structure of the Repeating Unit in Peptidoglycan
Peptidoglycan is composed of two sugar derivatives, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), and a small group of amino acids consisting of L-alanine, D-alanine, D-glutamic acid, and either lysine or diaminopimelic acid (DAP).
These constituents are
connected to form a
repeating structure, the
glycan tetrapeptide.
Peptide and glycan units are connected in formation of the peptidoglycan sheet
Teichoic Acids
Gram-positive Bacteria frequently have acidic polysaccharides called teichoic acids attached to their cell wall.The term teichoic acids includes all wall,membrane,or capsular polymers containing glycerophosphate or ribitol phosphate residues.These polyalcohols are connected by phosphate esters and usually have other sugars and D-alanine attached.
Teichoic acids and lipoteichoic acids are arranged in the overall wall structure of gram-positive Bacteria.
Lipoteichoic acid
Teichoic acid
The Gram-negative cell wall is a thin layer attached to an outer membrane via lipoproteins.The outer membrane contains phospholipid on its inner surface and lipopolysaccharide(LPS)on its outer surface.The space between the outer membrane and the cytoplasmic membrane is called the periplasmic space.Teichoic acids do not occur in Gram-negative bacterial cell walls..
Chemical Structure of Lipopolysaccharide O side chain
Core
polysaccharide
Lipid A
Molecular Model of E.coli Lipopolysaccharide
Cell Walls of Archaebacteria
( Differences from eubacteria )
?The archaebacteria do not contain peptidoglycan in
their cell walls as occurs in eubacteria.
?N-acetylmuramic acid and D-amino acids are not found in the cell walls of archaebacteria.
?The bonds between the carbohydrates in
pseudopeptidoglycan areβ 1-3 instead of'β1-4
as in peptidoglycan.
Some archaebacteria have walls composed of pseudopeptidoglycan,which resembles the peptidoglycan of eubacteria but contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid and L.-amino acids instead of the D-amino acids in eubacterial cell walls.
Protoplast Formation Peptidoglycancan be destroyed by certain agents for instance lysozyme, that breaks the 1,4-glycosidic bonds between N-acetylglucosamine and N-acetylmuramic acid in the molecule.
The difference between gram-positive and gram-negative bacteria is due to the physical nature of their cell walls. If the cell wall is removed from gram-positive bacteria, they become gram negative.
The Mechanism of Gram Staining
The peptidoglycan seems to act as a permeability barrier preventing loss of crystal violet. Gram-negative peptidoglycan is very thin, not as highly cross-linked, and has larger pores. Alcohol treatment also may extract enough lipid from the gram negative wall to further increase its porosity. For these reasons, alcohol more readily removes the purple crystal violet-iodine complex from gram-negative bacteria.
Procedures of Gram Staining Gram positive or Gram negative?
?Structure of cytoplasmic membrane
2.3 Cytoplasmic Membrane
?Function of cytoplasmic membrane
Membrane
Structure of Cytoplasmic Membrane
A.The typical cytopl asmic membrane of
prokaryotic and eukaryotic cells is a lipid
bilayer,as illustrated here showing the
orientations of the hydrophilic(tan spheres)and
hydrophobic(black)ends of phospholipids that
make up this structure.
It is a typical UNIT MEMBRANE !
B.Colorized electron micrograph of'the
cytopl asmic membrane(CM)of the bacterium
Bacillus subtilis reveals the characteristic railroad
track appearance of this lipid bilayer. Membranes contain both lipids and proteins, although the exact proportions of lipid and protein vary widely. The cytoplasmic membrane, a highly selective barrier, is constructed principally of lipid, within which certain proteins are embedded.
Diagram of the structure of cytoplasmic membrane 3. Energy conservation -site of generation and use of the proton motive force.
Function of Membrane
1. Permeability barrier -prevents leakage and function as gate way for transport of nutrients into and out of the
cell.
2. Protein anchor -
site of many proteins involved in transport, bioenergetics, and chemotaxis.
Rate of permeability*100
0.1
0.0010.001
0.000001
0.00000010.00000001
Substance Water Glycerol Tryptophan Glucose Chloride ion (Cl -)Patassium ion (K +
)Sodium ion (Na +)* Relative rate: Permeability with respect to permeability of water given as 100
Comparative permeability of
membrane molecules to various
Intracellular Membrane System
?Bacteria cells don’t contain membrane -
enclosed organelles. However, bacteria may have specialized invaginations of the
cytoplasmic membrane.
?Their function may be to provide a larger membrane surface for greater metabolic
activity.
Structure of Mesosome Mesosome may be involved in wall formation during division or play a role in chromosome replication and distribution to daughter cells. It may also be involved in secretory processes
2.4 Cellular Genetic Information
Nucleoid