Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 23

Cytosol is a buffered solution, with a pH between 5 and 6, containing soluble enzymes,glycogen, and ribosomes.

Glycolysis and alcoholic fermentation enzymes (Sections 2.2.1 and 2.2.2) as well as trehalase (an enzyme catalyzing the hydrolysis of trehalose) are present. Trehalose is a reserve disaccharide, also cytoplasmic, that ensures yeast viability during the dehydration and rehydration phases by maintaining membrane integrity.

The lag phase, preceding the log growth phase in a sugar‐containing medium, is marked by a rapid breakdown of trehalose, linked to an increase in trehalase activity. This activity is itself closely related to an increase in the amount of cAMP in the cytoplasm. This compound is produced by a membrane enzyme, adenylate cyclase, in response to the stimulation of a membrane receptor by an environmental factor.

Glycogen is the principal yeast carbohydrate reserve substance. Similar in structure to animal glycogen, it accumulates during the stationary phase in the form of spherical granules of about 40 μm in diameter.

When observed under the electron microscope, the yeast cytoplasm appears rich in ribosomes. These tiny granulations, made up of ribonucleic acids and proteins, are the center of protein synthesis. Associated with polysomes, several ribosomes migrate along the length of the messenger RNA. They translate it simultaneously so that each one produces a complete polypeptide chain.