Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 16

The works of various authors (Fleet, 1991; Klis, 1994; Stratford, 1994; Klis et al., 2002) have greatly expanded our knowledge of the yeast cell wall, which represents 15–25% of the dry weight of the cell. The yeast cell wall essentially consists of polysaccharides. It is a rigid envelope, yet endowed with a certain elasticity.

Its first function is to protect the cell. Without its wall, the cell would burst under the internal osmotic pressure, which is determined by the composition of the cell's environment. Protoplasts placed in pure water are immediately lysed in this manner. Cell wall elasticity can be demonstrated by placing whole yeasts, sampled during their log growth phase, in a hypertonic (NaCl) solution. Their cell volume decreases by approximately 50%. The cell wall appears thicker, while the cytoplasmic membrane practically does not detach from it at all. The cells regain their initial form after being placed back into an isotonic medium.

Yet the cell wall cannot be considered an inert, semirigid “armor.” On the contrary, it is a dynamic and multifunctional organelle. Its composition and functions evolve during the life of the cell, in response to environmental factors. In addition to its protective role, the cell wall gives the cell its particular shape through its macromolecular organization. It is also the site of molecules that determine certain cellular interactions such as sexual union, flocculation, and the killer factor, which will be examined in detail in Section 1.7. Lastly, a number of enzymes, generally hydrolases, are connected to the cell wall or situated in the periplasmic space. Their substrates are nutritive substances of the environment and the macromolecules of the cell wall itself, which are constantly reshaped during cell morphogenesis.