The Cell Wall
The composition of cell wall is variable among the different groups of fungi or between the different species of the same group. In the majority of fungi, the wall lacks cellulose but contains a form of chitin known as the fungus cellulose which is strictly not identical with insect chitin (diagram of fungi:1).
Cell structure of fungal cell: Fine structure of Torula Yeast cell based on the electron micrograph.
The suggested formula for fungus chitin is (C22 H54 N21)n. Electron microscope studies on cell structure of fungi reveal that chitin occurs as elongated variously oriented microfibrillar units (diagram of a fungal cell:1). 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 over fungi m the cellulose nature of the cell wall. De Bary 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.
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 Basi-diomycetes. In addition chitin may as well be present in appreciable amounts. In the yeasts and a few other Hemias- comycetideae chitin is absent. Their walls are mainly composed of micro-fibrils of mannan and B glucan.
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 is incomplete like the cellulose wall; the chitin wall of most fungi is permeable both to water and substances in true solution.
1. Cell Membrane
Cell wall composition of fungi 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 lomasome. 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.
Within the plasma membrane is the colorless 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 structure of fungus cell known as the organelles and inclusions.
The organelles are living structure of fungus cell, each with a specific function. The inclusions are dead, have no specific function and thus are not essential to cell survival.
Amongst the fungal 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.
Diagram of a fungal cell: Fine structure of hypha near the growing tip of Mucor based on the electron micrograph.
(i) Endoplasmic Reticulum
The presence of endoplasmic reticulum in the fungal cytoplasm has been demonstrated by the use of electron microscope (Labeled fungi cell diagram: 2). It is composed of a system of membranes or microtubular 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 cells of green plants.
The cytoplasm contains small, usually spherical bodies known as the mitochondria (Fungi diagram labeled: 2). 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, Hawker (1965) holds that the cristae of fungal mitochondria are fewer, flatter and more irregular than those of the green plants.
(iii) Golgi Apparatus
With the exception of Oomycetes there is less certainty of the occurrence of structures similar to those of the golgi apparatus (dictyosomes) m fungi (Fungi diagram labeled: 2). Moore and Muhlethaler (1963) reported a golgi apparatus consisting of three flattened sacs surrounded by many bubble-like structures in Saccharomyces cells.
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.
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 wall. The cytoplasm, in addition, secretes several kinds .of ferments, enzymes and organic acids.
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 3µ in diameter. They cannot be seen without special techniques.
Structurally the nucleus consists of: (i) A central, dense body with a clear area around it. (ii) Chromatin strands, and (iii) 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 consist of inner and outer layers of electron dense material and the middle one of electron transparent substance. The nuclear membrane has pores. At certain points, the nuclear membrane is continuous with the endoplasmic reticulum.
FAQ on Fungal Cell
1. Do fungal cells have a nucleus?
Fungi are multicellular organisms that have a cell wall, organelles, and a nucleus but no chloroplasts.
2. Do fungi cells have a cell wall?
Fungi can be multicellular or unicellular, but they all share two characteristics: cell walls comprised of chitin, a strong polysaccharide that gives structure, and the ability to produce spores.
3. Does fungi have ribosomes?
Fungal cell have ribosomes and a cell membrane, but no organelles like the nucleus.