Special Components of Gram-Positive Cell Walls

Special Components of Gram-Positive Cell Walls
Most gram-positive cell walls contain considerable amounts of 1-teichoic acid containing glycerophosphate or ribitol phosphate residues. These polyalcohols are connected by phosphodiester linkages and usually have other sugars and D-alanine attached .There are two types of teichoic acids: wall teichoic acid (WTA), covalently linked to peptidoglycan, and membrane teichoic acid(MTA), covalently linked to membrane glycolipid. Because the latter are intimately associated with lipids, they have been called lipoteichoic acids (LTA) and 2- teichuronic acid, which may account for up to 50% of the dry weight of the wall and 10% of the dry weight of the total cell. In addition .The teichuronic acids are similar polymers, but the repeat units include sugar acids (such as N-acetylmannosuronic or D-glucosuronic acid) instead of phosphoric acids. They are synthesized in place of teichoic acids when phosphate is limiting. some gram-positive walls may contain polysaccharide molecules.
Special Components of Gram-Negative Cell Walls
Outer Membrane
The outer membrane is chemically distinct from all other biological membranes. It is a bilayered structure ; it has special channels, consisting of protein molecules called porins, that permit the passive diffusion of low-molecular-weight hydrophilic compounds like sugars, amino acids, and certain ions.
Lipopolysaccharide (LPS)
The LPS of gram-negative cell walls consists of a complex glycolipid, called lipid A, to which is attached a polysaccharide made up of a core and a terminal series of repeat units .
Lipid A consists of phosphorylated glucosamine disaccharide units to which are attached a number of long-chain fatty acids .Hydroxymyristic acid, a 14 fatty acid, is always present and is unique to this lipid.
The polysaccharide cores similar in all gram-negative species that have two characteristic sugars, ketodeoxyoctanoic acid (KDO) and a heptose.
LPS, which is extremely toxic to animals, has been called the endotoxin of gram-negative bacteria because it is firmly bound to the cell surface and is released only when the cells are lysed. When LPS is split into lipid A and polysaccharide, all of the toxicity is associated with the former. The O antigen is highly immunogenic in a vertebrate animal.
Lipoprotein
Molecules of an unusual lipoprotein cross-link the outer membrane and peptidoglycan layers .The lipoprotein contains 57 amino acids, representing repeats of a 15-amino-acid sequence.
The Periplasmic Space
The space between the inner and outer membranes, called the periplasmic space, contains the peptidoglycan layer and a gel-like solution of proteins.
Cell Membranes
The boundary of the cell, sometimes called the plasma membrane, separates internal metabolic events from the external environment and controls the movement of materials into and out of the cell. This membrane is very selective about what it allows to pass through; this characteristic is referred to as “selective permeability.” For example, it allows oxygen and nutrients to enter the cell while keeping toxins and waste products out. The plasma membrane is a double phospholipid membrane, or a lipid bilayer, with the nonpolar hydrophobic tails pointing toward the inside of the membrane and the polar hydrophilic heads forming the inner and outer surfaces of the
membrane.
Membrane transport
*Passive Transport Across the Cell Membrane
Passive transport describes the movement of substances down a concentration gradient and does not require energy use the following .
• Bulk flow is the collective movement of substances in the same direction in response to a force, such as pressure. Blood moving through a vessel is an example of bulk flow.
• Simple diffusion is the net movement of substances from an area of higher concentration to an area of lower concentration.
• Facilitated diffusion is the diffusion of solutes through channel proteins in the plasma membrane. .
• Osmosis is the diffusion of water molecules across a selectively permeable membrane.
• Dialysis is the diffusion of solutes across a selectively permeable membrane.



*Active Transport Across the Cell Membrane
Active transport is the movement of solutes against a gradient and requires the expenditure of energy, usually in the form of ATP. Active transport is achieved through :
Protein Pumps
• Transport proteins in the plasma membrane transfer solutes such as small ions (Na ,K), amino acids, and monosaccharides.
• The proteins involved with active transport are also known as ion pumps.
* Protein pumps are catalyses in the splitting of ATP to ADP + phosphate, so they are called ATPase enzyme.
* The sodium-potassium pump actively moves sodium out of the cell and potassium into the cell. and are essential in transmission of nerve impulses and in muscular contractions.
*Vesicular Transport
• Vesicles or other bodies in the cytoplasm move macromolecules or large particles across the plasma membrane. Types of vesicular transport include:
1. Exocytosis, which describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell. This process is common when a cell produces substances for export.
2. Endocytosis, which describes the capture of a substance outside the cell when the plasma membrane merges to engulf it. The substance subsequently enters the cytoplasm enclosed in a vesicle.
There are three kinds of endocytosis:
• Phagocytosis or cellular eating, occurs when the dissolved materials enter the cell. The plasma membrane engulfs the solid material, forming a phagocytic vesicle.
• Pinocytosis or cellular drinking occurs when the plasma membrane folds inward to form a channel allowing dissolved substances to enter the cell. When the channel is closed, the liquid is encircled within a pinocytic vesicle.
• Receptor-mediated endocytosis occurs when specific molecules in the fluid surrounding the cell bind to specialized receptors in the plasma membrane. As in pinocytosis, the plasma membrane folds inward and the formation of a vesicle follows.

Eubacteria Lacking Cell Walls
These are microorganisms that lack cell walls (commonly called mycoplasmas and comprising the class Mollicutes) and do not synthesize the precursors of peptidoglycan. They are enclosed by a unit membrane, the plasma membrane They resemble the L forms that can be generated from many species of bacteria (notably gram-positive eubacteria); unlike L forms, however, mycoplasmas never revert to the walled state, and there are no antigenic relationships between mycoplasmas and eubacterial L forms.