Structure of Fungal Cell

Structure of Fungal Cell

Except slime moulds, the fungal cell consists of a strong rigid cell wall enclosing the protoplast.

The Cell Wall

The composition of cell membrane is variable among the various groups of fungi or between the various species of an equivalent group. In the majority of fungi, the wall lacks cellulose but contains a sort of chitin referred to as the fungus cellulose which is strictly not identical with insect chitin. The suggested formula for fungus chitin is (C22 H34 N4 O21)n– Electron microscope studies reveal that chitin occurs as elongated variously oriented microfibrillar units. These are laid down in layers and form the basis of the structural rigidity of fungal cell walls. The microfibril layers generally run parallel to the surface. Associated with the microfibrillar components is the nonfibrillar material. The chief chemical constituents are various polysaccharides, but proteins, lipids besides other substances have also been reported.

In the lower fungi, the biflagellate Oomycetes are said to be distinct from all other fungi in the cellulose nature of the cell wall (Frey, 1950 and Parker et al, 1963). De Bary (1866, 1887) reported true cellulose in Peronospora and Saprolegnia, Precise analysis of the cell wall of Phytophthora and Pythium by Bartnicki-Garcia (1966), Mitchell and Sabar (1968) has revealed that cellulose is a minority component or even absent altogether. On the other hand, glucan predominates in their walls. Thus, the Oomycetes may be said to have cellulose in their cell walls but it may not be the predominant material. Chitin which had long been considered to be absent has recently been reported to be present even in the cell walls of some Oomycetes (Lin, Sicher and Aronson, 1976)

The basic structural constituent of the cell wall in the Zygomycetes and higher fungi (Ascomycetes and Basidiomycetes) is chitin. It is a polysaccharide based on the nitrogen containing sugar (glucosamine). It is probable that more or less closely associated with chitin in the cell wall are pectic materials, protein, lipids, cellulose, callose and minerals. The clear evidence of such an association is, however, lacking Burnet (1968) is of the opinion that insoluble B-glucan forms the predominant structural material of the wall of Ascomycetes and Basidiomycetes. In addition chitin may as well be present in appreciable amounts. In the yeasts and a few other Hemi-ascomycetideae chitin is absents. Their walls are mainly composed of microfibrils of mannan and B glucan (Hawker, 1966). Mannan is a polymer of hexose sugar mannose whereas glucan is polymer of glucose. Some investigators have reported the occurrence of lignin in several fungi. It is doubtful whether this substance is chemically the same as the lignin of higher plants. It is obvious that our present knowledge of the chemical composition of the cell wall of fungi incomplete. Like the cellulose wall, the chitin wall of most fungi is permeable both to water and substances in true solution.

The Protoplast

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 or cell membrane, vacuolated cytoplasm, cell organelles and one or more nuclei.

Cell Membrane

It is a delicate, extremely thin, living membrane which closely invests the protoplast. The cell or plasma membrane is pressed against the cell or hyphal wall except for occasional invaginations in some regions. The invagination is either in the form of an infolded convoluted pocket or a pouch enclosing granular or vesicular material. Moore and Mc Lear (1961) named it lamasome. Actually the plasma membrane is the surface layer of the protoplast altered to perform special functions. It is differentially permeable and shows a typical tripartite structure under the electron microscope. There is an electron dense layer on either side of the less dense central region.

Cytoplasm

Within the plasma membrane is the colourless cytoplasm in which sap-filled vacuoles may occur. In young hyphae and hyphal tips, the cytoplasm appears rather uniform and homogeneous. Immersed in the cytoplasm are structures known as the organelles and inclusion. The organelles are living structures, each with a specific function. The inclusions are dead, have no specific function and thus are not essential to cell survival. Amongst the cell organelles are included the endoplasmic reticulum, mitochondria, ribosomes,golgi apparatus and vacuoles. Lomasomes which are membranous structures lying between the cell wall and plasma membrane are common. Examples of inclusions are the stored foods (glycogen, and oil drops), pigments and secretory granules.

  1. Endoplasmic Reticulum-

    The presence of endoplasmic reticulum in the fungal cytoplasm has been demonstrated by the use of electron micro-scope. It is composed of a system of membranes or micro-tubular structures usually beset with small granules which by some scientists are likened to the ribosomes. In many fungi, the endoplasmic reticulum is highly vesicular. Usually it is loose and more irregular than in the cell of green plants.

  2. Mitochondria-

    The cytoplasm contains small, usually spherical bodies known as the mitochondria. Each mitochondrion is enveloped by a double membrane. The inner membrane is infolded to form the cristae which are in the form of parallel flat plates or irregular tubules. The cristae contain the same fluid that fills the space between the two membranes. The mitochondria function as the power house of the cell. There is no fundamental difference between the mitochondria of fungi and those of green plants. However, Howker (1965) holds that the cristae of fungal mitochondria are fewer, flatter and more irregular than those of the green plants.

  3. Golgi Apparatus (Dictyosomes)-

    With the exception of Oomycetes there is less certainty of the occurrence of structures similar to those of the golgi apparatus (dictyosomes) in fungi. Moore and Muhlethaler (1963) reported a golgi apparatus consisting of three flattened sacs surrounded by many bubble-like structures in Saccharomyces cells.

  4. Vacuoles-

    The cytoplasm of young hyphae or fungal cells and hyphal tips lacks vacuoles. They appear further back or in the old cells. With age, they enlarge and show a tendency to coalesce and ultimately reduce the cytoplasm to thin lining layer immediately within the cell wall.

  5. Inclusions-

    The cytoplasm contains various kinds of inclusions. Examples of stored foods are lipid globules, granules of glycogen, oils and the carbohydrate trehalose, proteinaceous material and volutin. The glycogen may occur in vacuoles. There are no starch grains. Of the pigments, the fungi lack chlorophyll. Carotenoids are often conspicuous by their presence and may occur throughout the cytoplasm or concentrated in the lipid granules or distributed in the cell walls. The cytoplasm, in addition, secretes several kinds of ferments, enzymes and organic acids.

Nucleus

The cytoplasm in the individual cells contains one, two or more globose or ellipsoid nuclei which in the somatic portion are small and usually range from 1-2 or 3u in diameter. They cannot be seen without special techniques. Structurally the nucleus consists of:

  1. a central, dense body with a clear area around it.
  2. chromatin strands, and
  3. the whole structure surrounded by a definite nuclear membrane.

The central body takes heavy iron haematoxylin stain and is usually Feulgen-negative. In electron micrographs, it appears as an amorphous or granular mass. Mycologists usually designate it as the nucleolus. Bakerspigel (1960) stated that it contains RNA. During nuclear division, the chromatin strands become organised into chromosomes which are extremely small and difficult to count. Under the electron microscope, the nuclear membrane is seen to consists of inner and outer layers of electron dense material and the middle one of eletron transparent substance. The nuclear membrane has pores. At certain points, the nuclear membrane is continuous with the endoplasmic reticulum.

Somatic Nuclear Division

The older mycologists believed that the somatic nuclear division in most fungi (except yeasts) differs markedly from normal mitosis. Moore (1964) termed it karyochorisis in contradiction to normal mitosis or karyokinesls. Lu (1974), however, stated that in fungi mitosis occurs more or less normally and the fungal nucleus passes through the stages characteristic of mitosis namely, interphase, prophase, metaphase, anaphase and telophase. The cell division is closed and thus termed intranuclear. It means the nuclear membrane remains more or less intact and does not disorganise during prophase. Centrioles appear in close association with the nuclear membrane. They apear in a pair and each migrates to the opposite poles of the nucleus to control the formation of the spindle.

Fungal Flagella

The Higher Fungi (Ascomycetes, Basidiomycetes and Deuteromycetes) lack motile cells in the life cycle. With the exception of Zygomycetes, they are characteristic of the lower Fungi (Phycomycetes). The motile cells (zoospore and gametes) in the Lower Fungi are furnished with one or two fine, protoplasmic, whip-like threads known as the flagella. These are thin, hairlike emergences of the cell cytoplasm which function as propelling or locomotory structures of the cell. There is either a single anterior or posterior flagellum or they occur in a pair on the cell.

  1. Structure of the Flagellum-

    Forming the core of the flagellum is a central or axial filament known as the axoneme. The axoneme is surrounded by a double cytoplasmic sheath or membrane which is an extension of the plasma membrane of the motile cell. In a cross section, the axoneme is seen to be composed of eleven fibrils. Of these, two are central and nine peripheral. The two central single fibrils lie side by side and form an elastic axial thread which is sometimes enclosed by a subsidiary sheath of its own. Of the nine peripheral fibrils, each consists of two thin fibrils. The nine peripheral doublet fibrils surround the central pair of singlet fibrils. The entire growth of 9+2 fibrils is enclosed by a double cytoplasmic sheath. This 9+2 arrangement of fibrils is the basic structure of a eucaryotic flagellum found in the motile cells of all organisms except the bacteria.

  2. Kinds of flagella-

    The fungal flagella are reported to be of the following three kinds :

    1. Whiplash flagellum with an end piece-

      It has a smooth surface and narrows abruptly towards the tip to form a distinct end piece. The end piece is a thin extension of variable length. It is flexible. According to Manton, the end piece is the naked portion of the axoneme. It is composed of eleven fibrils. Koch (1956) and Kole (1957) stated that the end piece consists of the two central fibrils only.

    2. Whiplash flagellum with a blunt tip-

      The blunt whiplash flagellum has smooth surface but lacks the narrow tip (end piece).

    3. Tinsel flagellum-

      It does not obviously end in a narrow tip like the blunt whiplash type-flagellum but bears fine lateral hair-like appendages known as the flimmer hairs or mastigoneme. The flimmer hairs on the tinsel flagellum arise from the axoneme. The tinsel flagellum is also known by other names such as pantonematic, flimmer or flimmergeisel flagellum. The other names of the whiplash flagellum are acronematic or peitchgeisal flagellum.

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