**2. Defining ROS in Living Cells**

ROS are defined as oxygen-containing reactive species. This term includes superoxide radicals (O2 •−), hydrogen peroxide (H2O2), hydroxyl radicals (•OH), singlet oxygen ( 1O2), peroxyl radicals (LOO•), alkoxyl radicals (LO•), lipid hydroperoxide (LOOH), and peroxynitrite (ONOO−), among others [15]. Their generation can first occur as a result of the partial reduction of oxygen, as described in Figure 1. Among the ROS, some species are radicals, i.e., have unpaired electrons in their outer orbit—for example, superoxide and hydroxyl radicals. The different forms of ROS have varying levels of reactivity depending on their oxidation potential, with H2O2 being the least reactive and - OH being the most reactive.

**Figure 1.** Molecular oxygen to ROS. Superoxide radicals are the primary ROS but have poor reactivity. Hydrogen peroxide is the least reactive ROS. At the end of the reduction flow, hydroxyl radicals are the most reactive [14].

#### **3. The Biochemical Impacts of ROS in Living Cells: Is There Any Peculiarity in Malaria?**

ROS generated at low concentrations under physiological conditions are often beneficial, playing important roles as regulatory mediators in signaling processes [15,18]. Nevertheless, at high concentrations, they become deleterious for the cells. The toxicity of free radicals stems from their unstable nature and predisposition to donate or abstract electrons from nearby molecules, triggering a chain reaction that can be terminated by another molecule with unpaired electrons. Lipids, proteins, and nucleic acids are attacked and damaged by ROS, which affects the survival of living organisms [14,15] (Figure 2).

**Figure 2.** Biochemical impacts of ROS depending on their concentrations.

This overproduction of ROS can be endogenous due to the dysregulation associated with pathological processes such as aging, inflammation [19,20], or cardiovascular diseases [21] and can also be exogenous with the administration of xenobiotics such as antimalarials [22].

#### *3.1. Cell Signaling*

ROS play important physiological roles, such as in the induction of apoptosis and suppression of some genes' expression, at low concentrations [18]. Studies have reported the production of ROS by specialized plasma membrane oxidases and nicotinamide adenine dinucleotide phosphate (NADPH) oxidases in normal physiological signaling by growth factors and cytokines [23]. For example, NADPH oxidase activity plays defensive roles in phagocytic cells [18]. The small size and ability of ROS (such as H2O2) to traverse membranes make them suitable for cell signaling; however, this role is not yet fully understood in *Plasmodium* [24].
