## 3 Types of Cellular Stress
### Oxidative Stress  
Oxidative stress results from the cells' inability to reduce or eliminate reactive oxygen species and reactive nitrogen species (superoxide radical anion O\({}_{2}\)\({}^{\bullet-}\)--the primordial reactive oxygen species, hydroxyl radicals, peroxynitrite, singlet oxygen, and hydrogen peroxide) or to repair the molecular damage produced by them [38; 39; 40; 41]. In aerobic conditions, the generation of ROS takes place continuously (Figure 2), these being metabolic side products or the result of cellular control systems. Maintaining low concentrations of reactive oxygen species (ROS), below \(10^{-8}\) M, is achieved by the generation and degradation mechanisms achieved through specific and non-specific cellular mechanisms [42]. Among the factors which determine oxidative stress, metabolism and aerobic respiration contribute the most to this process. A very recent study compared, by using an AI model and neural network algorithm, cellular morphology under basal and stress conditions and concluded that the changes induced by stress due to a high concentration of glucose in the medium were actually due to osmotic stress [43]. In the process of cell development, the microorganism culture goes through several stages, including the lag phase (adaptation), the exponential phase (active cell division), the stationary phase (in which the number of newly formed cells is equal to that of dead cells), and the phase of cell death. In the exponential phase of cell growth, the energy produced is the result of glycolysis. The number of mitochondria is reduced, oxygen consumption is minimal, and so, in this phase, the activity of antioxidant enzymes is also  
Figure 1: Types of stresses to which the yeast cell can be exposed during the fermentation process and its way of responding to stressful stimuli [33]. reduced. A responsible factor for oxidative stress is ethanol, by acting on the reactive oxygen species present in the mitochondria [37; 44]. In the stationary phase, the cells use the ethanol obtained in the previous phase as an energy source. The number of mitochondria is increased, as a result of their need to oxidize ethanol, so, in this phase, the generation of reactive oxygen species intensifies. Thus, the transition of the cell culture to the stationary phase can cause the occurrence of oxidative stress [44]. The transformation of molecular oxygen into ROS leads to certain phenomena, such as protein oxidation, lipid peroxidation or DNA mutations. These actions occur even though the cells contain antioxidant defense mechanisms [29]. Oxidative stress can also be caused by compounds or substances such as tetrachloro-bisphenol A or xylene [45; 46]. In addition, through the autooxidation processes of some molecules, some oxidases generate reduced amounts of ROS, such as NADPH oxidase, xanthine oxidase, cyclooxygenase, and lipoxygenases [47]. The defense against oxidative stress is based on enzymatic and non-enzymatic mechanisms, which have the role of maintaining the level of reactive oxygen species within normal limits. The action of the enzymatic part in yeasts is based on the presence of two catalases (A and T) and two superoxide dismutases (Cu/Zn SOD and Mn/Zn SOD). Catalases have the role of breaking down H\({}_{2}\)O\({}_{2}\) into H\({}_{2}\)O inside the peroxisome and cytosol. Superoxide dismutases convert the superoxide anion into oxygen and H\({}_{2}\)O\({}_{2}\) in the cytoplasm and mitochondria. Enzymes that act against oxidative stress also play a very important role in the ethanol tolerance of yeasts. They eliminate the reactive oxygen species that appear in the presence of alcohol [37]. Studies have shown that _S. cerevisiae_ develops the ability to survive in stress conditions that could become lethal, if it was previously exposed to low doses of cellular stress of the same or a different type [48]. Melatonin is a hormone that protects the human body from oxidative stress, but also from that caused by ethanol. It acts as an antioxidant both for _S.