Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 18

The cell wall of S. cerevisiae is made up of an outer layer of mannoproteins. These mannoproteins are connected to a matrix of amorphous β‐1,3‐glucan, which covers an inner layer of fibrous β‐1,3‐glucan. The inner layer is connected to a small quantity of chitin (Figure 1.4). β‐1,6‐glucan probably acts as a cement between the two layers. The rigidity and the shape of the cell wall are due to the internal framework of the fibrous β‐1,3‐glucan. Its elasticity is due to the outer amorphous layer. The intermolecular structure of the mannoproteins of the outer layer (hydrophobic bonds and disulfide bonds) equally determines cell wall porosity for micromolecules (molecular weights less than 4,500) and impermeability to macromolecules. This impermeability can be affected by treating the cell wall with certain chemical agents, such as β‐mercaptoethanol. This substance breaks the disulfide bonds, thus destroying the intermolecular network between the mannoprotein chains.

FIGURE 1.4 Cellular organization of the cell wall of S. cerevisiae.

The composition of the cell wall is strongly influenced by nutritive conditions and cell age. The proportion of glucan in the cell wall increases with respect to the amount of sugar in the culture medium. Certain deficiencies (for example, of mesoinositol) also result in an increase in the proportion of glucan compared with mannoproteins. The cell walls of older cells are richer in glucans and in chitin and less rich in mannoproteins than younger ones. For this reason, they are more resistant to physical and enzymatic agents used to break them down. Finally, the composition of cell walls is profoundly modified by morphogenetic alterations (conjugation and sporulation).