MYCOLOGY (MIC 206) FUNGAL CELL

FUNGAL CELL The composition of the fungal cell wall is rather variable.  The variability → to have phylogenetic significance

Characteristics of Fungal Cell Gives shape to fungi. Gives strength to fungi. Provides protection for the protoplasm from ultra-violet rays (presence of melanins) Ability to resist lysis by organic solvents such as enzymes, toxins, osmotic integrity. Ability to bind with metal ions. Secretes enzymes from their walls (invertase hydrolyses sucrose to glucose and fructose) and so assisting in nutrition.

The main identifying characteristic of fungi is the makeup of their cell walls. Many contain a nitrogenous substance known as "chitin," which is not found in the cell walls of plants, but can be found in the outer shells of some crabs and mollusks. Fungal cell wall is (metabolic point of view) very different from insect exoskeletons or a plant cell walls, → terminally differentiated structures.

Cell Wall The cell wall is made up of: chitin (polymers of acetylated amino sugar N-acetyl-glucosamine) glucan (polymers of glucose) proteins (polymers of amino acids). Glucan and chitin are components of the primary wall. Proteins are components of the secondary wall.

Basic component of fungal cell wall Secondary wall Primary wall

Fungal Cell Other components include chitosan, melanins and lipids. Enzymes include cellulase which acts on cellulose of plants. The outermost surface of the cell wall provides a medium between the cell and the environment a site where antigen and agglutinin gets attached to the substrate, host and other cells.

Chitin in yeast: short and thick in hyphal wall: long and interwoven Chained polymer β(1-4) N-acetyl-glucosamine. Found naturally as structural polysacharides in most invertebrates and protistans. They are used as cell wall components. Gives strength where each molecule contains a unit of sugar that is bonded by hydrogen bond to give it rigidity. Each microfibril of chitin gives the shape of the cell and gives strength to mature cell walls. Microfibrils can be of various shapes: in yeast: short and thick in hyphal wall: long and interwoven

Chitin (cont) Septa is rich in chitin Microfibrils of septa are arranged in the form of a tangent. Septa can be stained with “calcoflour white”. Saccharomyces cerevisiae has low amount of chitin.

Chitosan Chained β(1-4) glucosamine. Result of continuous deacetylation of chitin. An important component in wall of Zygomycetes and can be found in ascospore walls of Saccharomyces cerevisiae.

The shiitake mushroom contains beta-glucans. Most fungal walls contain β-chained glucan. Glucan are polysaccharides that contain only glucose as structural components, and are linked with β-glycosidic bonds. Walls of Ascomycetes and Basidiomycetes contain β(1-3) glucan with branching of β(1-6) glucan. There are some fungi with (1-4) glucan. The shiitake mushroom contains beta-glucans.

β-Glucans are polysaccharides of D-glucose monomers linked by β-glycosidic bonds. Diagram showing orientation and location of different beta-glucan linkages.

Glycoprotein and Protein Glycoproteins are protein that contain oligosaccharide chains (glycans) covalently attached to polypeptide side-chains. Glycoproteins (include mannoprotein, galactoprotein and xyloprotein) are important components of the matrixs of cell walls. Glycoprotein contain glucosamine and/or N-acetyl glucosamine. In parasitic fungi such as Candida albicans and Aspergillus fumigatus, the glycoproteins are antigens. Mannoprotein in Saccharomyces cerevisiae are large molecules.

Glycoprotein and Protein (cont) Hydrophobin is a very hydrophobic protein and found in aerial hyphae of Schizophyllum commune and are components of rodlets. Rodlets are found on the surface of conidia of Neurospora crassa and Aspergillus nidulans and protects the hyphae from desiccation.