ULTRASTRUCTURE OF FUNGI

 ULTRASTRUCTURE OF FUNGI

Ultrastructure of Fungi

Cell wall

·   Fungal cell wall gives shape and rigidity to the fungal cell. It also protects the cell from Mechanical stress and Osmotic stress.

·       A fungal cell wall is rigid and its composition varies among different species and groups.

·  The cell wall of the Zygomycetes and Higher fungi (Ascomycetes and Basidiomycetes) predominantly consist of Chitin (also known as Fungal cellulose). The fungal Chitin is strictly non-identical with Insect Chitin.

·  Fungus chitin is a Polysaccharide, a polymer of N-acetyl glucosamine. Chitin forms the Microfibrillar units or layers to form a thick, tough wall structure.

·  Lower fungi (like Oomycetes) predominantly possess Glucans, Mannans, Galactosans or Cellulose in their cell wall.

·      Fungal cell walls are typically 80 – 90 % Polysaccharide, with only small amounts of Proteins, Lipids, Polyphosphates, and Inorganic ions making up the Wall-cementing matrix.

·      The yeast cell wall mainly retains Microfibrils of Mannan and Glucan. Mannan is a polymer of Mannose, whereas Glucan is a polymer of Glucose.

·       Fungal cell wall protects the fungal cell against Osmotic lysis.

Protoplast in the Fungal cell

·       The living substance of the cell within the cell wall is the Protoplast.

·    It lacks the Chloroplasts but is differentiated into the other usual cell parts such as Plasma membrane, Vacuolated cytoplasm, Cell organelles and one or more Nuclei.

Plasma Membrane

·    The Plasma membrane, Plasmolemma or Cell membrane encloses the inner protoplast of the fungal cells.

·       It is a delicate, extremely thin and living membrane.

·     The fungal cell membrane serves as a surface layer of protoplast that is Differentially permeable.

·     Under Electron Microscope, it exhibits a typical tripartite structure that contains two electron-dense regions within which a Transport region is present.

·       The Proteins and Lipids constitute the main components of a Cell membrane.

·     The fungal Plasma membrane is quite different from the Plant and Animal cell membrane as it typically contains Ergosterol as the main membrane Sterol. Oppositely, Animals and Plant cells have Cholesterol and Phytosterols, respectively.

Lomasomes

·   Lomasomes which are membranous structures lying between the Cell wall and Plasma membrane.

·       Moore and Mc Lear (1961) named it as “Lomasome”.

Cytoplasm

·       The region which is present inside the Plasma membrane is called Cytoplasm.

·       Within the Plasma membrane is the colorless cytoplasm in which sap-filled vacuoles may occur.

·    Cell organelles like Nucleus, Endoplasmic reticulum, Mitochondria, Ribosomes, Golgi apparatus, Inclusions and Gas vacuoles are present in Cytoplasm.

Nucleus

·       Fungal cells being Eukaryotic, possess a true or well-developed Nucleus.

·       Two or more Globose or Ellipsoid nucleus are present in Fungi.

·       It is small and usually range from 1 or 3 µ in diameter. 

·    Under the Electron microscope, the Nucleus appears as an Amorphous (no definite shape) or granular mass.

·       A Nuclear membrane encloses the Nucleus.

·    Numerous pores or Annuli appear on the surface of the Nuclear membrane, which facilitates the Nucleocytoplasmic exchange.

·       The Nucleolus is the dense central area possessing RNA.

·       Nucleus act as a repository for Genetic information.

·  Nucleolus plays a major role in synthesis of Ribosomal rRNA synthesis and Ribosome construction.

Ribosomes

·       Ribosomes are made up of RNA and Protein.

·       It varies in shape and size and presents more abundantly in the sub-apical region.

·     80 S type of Ribosomes are present in fungi and other Eukaryotes. The 80 S Ribosomes are classified into two sub-units. They are 60 S sub-unit and 40 S sub-unit.

·       The main function of Ribosomes is Protein synthesis (Translation). The process of synthesis of protein from mRNA is called as Translation.

Mitochondria

·     Mitochondria (Greek. “mitos” - thread; “chondros” - granule) are small, rod-like or filamentous bodies and enclosed by double membrane.

·  The inner membrane is infolded to form the Cristae. There are a few Cristae in Fungal mitochondria that appears flattened and more irregular.

·       There is no fundamental difference between the mitochondria of fungi and those of green plants. However, Hawker (1965) holds that the cristae of fungal mitochondria are fewer, flatter and more irregular than those of the green plants.

·     Mitochondria are familiarly known as the “Power houses of the cell” because it is involved in Electron Transport Chain for ATP Synthesis.

·       Mitochondria are the centres for the

a)     Cellular respiration

b)     Energy metabolism.

Endoplasmic Reticulum

·      The majority of fungi have a highly Vesicular Endoplasmic Reticulum.

·     Endoplasmic Reticulum originates from the Nuclear membrane and commonly found in Young hypha of molds.

·       Endoplasmic Reticulum is a packaging organelle of Eukaryotic cells.

·       Endoplasmic Reticulum is of two types. They are

a)     Smooth Endoplasmic Reticulum - Plays a major role in synthesis of Lipids and Sterols.

b)     Rough Endoplasmic Reticulum plays a major role in synthesis of Ribosomes.

Golgi Bodies (or) Golgi Apparatus

·       Golgi bodies or Golgi apparatus is also a packaging organelle like Endoplasmic reticulum.

·       Golgi bodies helps in Packaging and Secretion of materials for various purposes.

·       It also helps in Lysosome formation

Lysosomes

·       Lysosomes helps for

a)     Intracellular digestion

b)     Breakdown of Nutrients

c)     Self-destruction of Old or Damaged cells.

Vacuoles

·      Young fungal cells lack vacuoles while present within the cytoplasm of the old fungal cells. With age, they enlarge and support for cell growth and cell shape.

Inclusion

·       The Cytoplasm contains various kinds of Inclusions.

·    Glycogen granules and Lipid globules are the chief storage products that prevalently reside within the mature fungal cells.

·    Besides Cytoplasm, Vacuoles also accumulate Proteinaceous materials (Carotenoids pigment, Hydrolases and Amino acids), Oils, Carbohydrate - Trehalose, Organic acids and Volutin granules.

·       Molds possess Glycogen in Vacuoles. Yeast cells primarily reserve Volutin granules.

·       Fungal cells lack Starch grains. Among Pigments, the fungi lack Chlorophyll.

Flagella

·    The Higher fungi (Ascomycetes and Basidiomycetes) and Zygomycetes lack motile cells in their life cycle and non-motile in nature.

·    The motile cells (Zoospores and Gametes) in the Lower fungi (Phycomycetes) are furnished with one or two fine, protoplasmic fine, whip like threads known as Flagella.

·       Flagella are thin, hair like emergences of the cell Cytoplasm.

·       Flagella functions as Propelling or Locomotors structure of the cell.