In this article we will discuss about the cell structure of yeast. This will also help you to draw the structure and diagram of cell structure of yeast.
The yeasts are unicellular fungi. Cells may remain attached in short chains forming a pseudomycelium, but they do not produce true mycelium. The cells are extremely variable in shape being globose, oval, elongated, or rectangular.
The yeast cells are very polymorphic and are capable of assuming different forms depending upon the medium in which they grow and their age. Individually yeast cells are hyaline but in colonies they appear white, cream-coloured or slightly brownish.
The structure of yeast cell has been very thoroughly worked out by a large number of investigators who differ in their interpretations. Each yeast cell has a distinct cell wall enclosing granular cytoplasm, within which can be seen a large vacoule and a nucleus (Fig. 214). The vacuole varies much in size according to the state of activity of the cell.
It may at times become much contracted, but it does not disappear completely except during spore formation. The wall is thin delicate and is composed of chitin in combination with other compounds. Reserve materials are present in the cytoplasm in the form of oil globules, glycogen, and volutin.
Glycogen accumulation increases with the decrease in fermentation. The volutin content is also very much linked with the metabolic conditions of the yeast cells. Difference of opinion exists whether the vacuole is a part of the nucleus or the vacuole and the nucleus are separate entities.
Despite the fact that Saccharomyces cerevisiae has been subject of cytological study for over 50 years, investigators at the present time are still not in agreement. According to old idea of Wager and Peniston the nucleus has a large vacuole and that the body to one side of it is the nucleolus. But Wager during later time described the nucleus as a homogeneous body having its nucleolus at one end.
His explanation was supported by Lindegren, who stated that the central area of the yeast nucleus is not a vacuole but a true nucleus which is traversed by chromatin threads and the body which Wager described as nucleolus, is centrosome. But Guillermond is of opinion, there is a central vacuole which does not belong to the nucleus.
Apposed to the vacuole is the nucleus which according to Wager is the nucleolus and to Lindegren is centrosome. But great majority of investigators have concluded that the vacuole is a cytoplasmic constituent and that the distinctive chromatic body referred to by Wager and Peniston is actually the nucleus.
The small size of the yeast cell renders observation of its contents under the light microscope difficult, and there has previously been controversy about the interpretation of its structure. Studies of thin sections of yeast cells (Saccharomyces cerevisiae) under the electron microscope have clarified our knowledge.
The cell wall contains protein, lipid, and at least two polysaccharides (a mannan and a glucan). Chitin has also been reported. Immediately below the cell wall is the cytoplasmic membrane. The membrane is made up of particles penetrated by fibrils which probably correspond to the glucan fibrils of the cell wall. Within the cytoplasm are mitochondria, endoplasmic reticulum and reserves of fat and glycogen (Fig. 215).
A large vacuole surrounded by a single unit membrane contains strands and granules of dense material sometimes linked into a network. The larger granules are probably volutin, i.e., polymetaphos- phate. The nucleus is surrounded by a double unit membrane (nuclear membrane) perforated by pores. The nuclear membrane is distinct from the vacuolar membrane which surrounds the vacuole.
The problem in Saccharomyces has been both to identify the nucleus and to describe its division. Nagel (1946), in a valuable and painstaking comparative study, employed a wide range of stains and fixatives as well as living cells and decided that the nucleus is Feulgen-positive, small body lying to the side of a central vacuole.
This view was confirmed much later by electron microscopy. Moor and Muhlethaler made it clear that there is a large, more or less central vacuole with which is closely associated a much smaller nucleus. There does not seem to be a structure comparable to the ‘nucleolus’ or ‘central body’ of other fungi. The nucleus is bipartite.
The larger part is Feulgen-positive and during division becomes elongated, constricted in the centre and eventually separates into two halves (Fig. 216). Individual chromosomes are not usually recognizable. A second, crescentic, smaller part is strongly basiphilic but Feulgen- and Giemsa-negative (Fig. 216 A-D).
This body becomes attenuate, sometimes breaking into small, irregular pieces and eventually is more or less evenly divided between the daughter nuclei (Fig. 216 E-H).
Robinow and Marak (1966) provided evidence that there is a single, intranuclear fibre. Opposite the crescentic, basiphilic body within the Feulgen- positive material is a small bead-like structure (Fig. 21bA).
This bead of fibre initial, expands into a long, straight fibre which becomes anchored at each end to the nuclear membrane (Fig. 216 C-F). It is, perhaps, not unreasonable to apply the terms nucleolus and centriole to the basiphilic crescent and fibre initial respectively. Other yeasts, notably Lipomyces, differ from Saccharomyces in possessing distinct, countable chromosomes.
Recent investigators, regardless of their diverse opinions on the mechanics of nuclear division and the number of chromosomes present, have agreed that the nucleus is the relatively small body and is distinct from the large vacuole in the yeast cell. The nucleus is surrounded by a membrane.
The nuclear membrane remains intact during mitosis and the spindle is intra-nuclear as it is in all fungi. Mitotic division of the yeast nucleus was for the first time demonstrated by Guillermond (1901) and was subsequently persued by many other investigators. He also pointed out that the yeast cells have typical nuclei similar to other fungi.