## 3 Types of Cellular Stress
### Stress Due to Ethanol Accumulation  
Ethanol is both the product resulting from the alcoholic fermentation of yeasts, but also one of the factors that lead to the appearance of stress at the cellular level. As the fermentation process progresses, more and more ethanol is produced. This leads to harmful actions against the yeast cells, such as blocking cell proliferation, depolarizing the cytoskeleton, or altering the activity of transport systems [61]. To respond effectively to stress conditions, yeast reprograms its cellular activity to protect important cell components for normal cellular activity and a higher degree of survival [62]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Category** & **Characteristic** & **Species Name** & **NaCl: M; \% (\%/v)** \\ \hline \multirow{8}{*}{Halotolerant} & & C. _parapsilosis_ & 3.0; 17.5 \\  & & P. _membraniaciens_ & 3.0; 17.5 \\  & & I. _orientals_ & 2.0; 11.7 \\  & & P. _sorbitophila_ & 3.0–4.0; 17.5–23.4 \\  & & Z. _ruxii_ & 3.0; 17.5 \\  & & H. _verneckii_ & 5.20; 30.8 \\  & & D. _hansenii_ & 3.0–4.0; 17.5–23.4 \\  & & C. _halophila_ & 4.0–5.0; 23.4–29.1 \\ \hline \multirow{8}{*}{Moderate halotolerant} & & S. _cerevisiae_ & \textless{1.70; 10} \\  & & S. _pombe_ & 1.00; 5.8 \\  & & Z. _florentinus_ & 1.00; 5.8 \\  & Growth inhibition at & C. _albicans_ & 1.70; 10 \\  & NaCl \textgreater{} 2.0 M & C. _tropicalis_ & 1.7–2.0; 10–11.7 \\  & & Z. _sapae_ & 2.0; 11.7 \\  & & Z. _ballii_ & 1.0–2.0; 5.8–11.7 \\  & & Z. _bisporus_ & 1.0–2.0; 5.8–11.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Halotolerant yeast species [54]. The structure of the yeast cell wall protects it against permanent changes in the external environment and it prevents cell lysis resulting from changes in osmotic conditions. The alcohol causes the disruption of cell integrity by intercalation in the hydrophilic layer inside the lipid bilayer of the membrane, thus increasing cell permeability [48; 63]. The presence of ethanol during fermentation subjects yeasts to continuous stress. Thus, most of the time, cell viability is reduced, followed by the full blockage of the process for obtaining a certain product [37]. To prevent this, cells use several protective mechanisms. Among them are the activation of some heat shock proteins, the increase in the level of unsaturated fatty acids and ergosterol in the plasma membrane or the accumulation of the intracellular trehalose (known to replace the water molecule to stabilize proteins and membranes from desiccation and to protect yeast cells from thermal denaturation) content in the cells [64; 65; 66; 67]. Additionally, studies demonstrate that ATPase in the plasma membrane also controls the tolerance of yeast cells to the stress due to ethanol, as this enzyme is reported to be activated by the alcohol [68]. Vacuolar and membrane ATPase are involved in the recovery after the acidification at a cytosol level induced by the presence of alcohol, by pumping protons into the vacuole and outside the cell, due the effect of alcohol in order to induce a membrane permeabilization effect through a passive influx of protons [62; 69; 70]. As for the concentration of ethanol in the environment, it produces certain effects both at low and high levels. At a low percentage, the yeast cells undergo a slowdown in division, resulting in slower growth. At high concentration, the fluidity of the cell membrane is improved, but cell viability decreases, the electrochemical gradient decreases, and the activity of glycolytic enzymes is affected by protein denaturation and the increase in the percentage of insoluble proteins [67]. By analyzing the effect of ethanol concentration on yeast proteins, it was observed that the inhibition of the process is dependent on it. To support the statement, a study was carried out on two cultures of _Saccharomyces cerevisiae_, one commercial, for wine production, and one of laboratory type. Both were subjected to alcohol concentrations from 6% to 14%.