The below mentioned article provides an overview on the cell structure of yeast.
Antony Von Leeuwenhoek (1680) was the first to describe the yeast cells. Its thallus is unicellular and non-mycelial. However, at the time of budding it rarely produces pseudo-mycelium. The individual cells are polymorphic i.e., showing different shapes, even in the same culture, depending upon the nutrition available.
Generally, the shape of cells may vary from circular, spherical, oval, elliptical, elongated, rectangular, dumb-bell shaped to triangular. The cells are minute and range from 2 to 8 µ in diameter and 3 to 15 µ in length. Individually, the cells are hyaline (colorless) but its colonies appear white, cream-coloured or light brown. Each cell consists of a tiny mass of protoplast surrounded by a definite cell wall.
The Cell Wall:
The cell wall is double layered, thin, delicate and flexible. It is composed of two complex polysaccharides, mannan (30%) and glucan (30-40%) with smaller quantities of protein (6-8%), lipid (8.5 – 10.5%) and chitin (2%). Cellulose is absent.
The Protoplast:
Inner to cell wall is a cytoplasmic membrane or plasma membrane. It surrounds the cytoplasm and a nucleus. Under light microscope, a large hyaline structure, occupying a large portion of the cell and a deeply staining body associated on one side of it is seen.
These two structures have been interpreted in different manners by different workers (Fig. 7 A, B). According to Guilliermond (1920), Mundkur (1954), Royan (1956) and Ganesan (1956) hyaline area represents a vacuole and nucleus is found just as its neighbour.
However, Wager and Peniston (1910), Srinath (1946), Subramanian (1948), Lindegren (1952), Thingarajan and Subramaniam (1954), opined that vacuole does not exist in the cytoplasm and it is occupied by nucleus (hyaline body) and nucleolus (deeply stained body Fig. 7 A). Lindegren (1949, 1952) and Alexopoulos supported this view but they called nucleolus as centrosome (Fig. 1 B). Lindegren believed that vacuole has six pairs of chromosomes (Fig. 7 C).
Electron microscopic studies of ultra-thin sections of S. cerevisiae and of S. octosporus show that the nucleus is surrounded by a nuclear membrane and is distinct from the vacuole (Fig. 7 D).
The nuclear membrane has pores. The cytoplasm in addition to the various cell organelle (mitochondria, endoplasmic reticulum, ribosomes etc.) contains glycogen, proteins, oil and refractile volutin granules (an inorganic metaphosphate polymer) as reserve food materials.
