Abstract
Reactive oxygen species (ROS) are molecules that contain oxygen, generated by metabolically active cells. Endogenous and exogenous factors play a significant role in the generation of ROS. Unregulated cellular activities such as the electron transport chain (ETC) can lead to high levels of cellular ROS, which will eventually evolve a series of cellular responses such as apoptosis, autophagy, and necrosis. Understanding the complexity in mechanisms of cell death is essential for researchers focusing on the development of alternative therapeutic approaches. In photodynamic therapy (PDT), ROS generated through the combination of light energy and chemicals known as photosensitizers (PS) target cellular organelles including the endoplasmic reticulum, mitochondria, and peroxisomes. The most common types of ROS include superoxide radical (O•−2), singlet oxygen (1O2), hydrogen peroxide (H2O2), hydroxyl radical (OH•), and peroxynitrite (ONOO−). Their role is constantly being outlined in a range of pathophysiological conditions. However, increase of ROS levels in malignant cells is a characteristic of cancer cell death.