Exposure to oxygen and free radicals causes degeneration of which organelle?

• A. Nucleus
• B. Golgi apparatus
• C. Endoplasmic reticulum
• D. Mitochondria

Answer: (D)

Oxygen and free radicals generate reactive oxygen species that readily damage lipids, proteins, and DNA. The mitochondria are the focal point because they produce most of the cellular ROS during energy production in oxidative phosphorylation, and their inner membranes are rich in polyunsaturated lipids that are especially vulnerable to lipid peroxidation. When the antioxidant defenses are overwhelmed, this lipid damage disrupts the membrane potential, crippling ATP synthesis. The resulting energy failure can cause mitochondrial swelling, membrane permeabilization, and release of cytochrome c, which can initiate apoptosis and lead to degeneration of the organelle. Additionally, mitochondrial DNA is particularly susceptible to ROS damage due to its proximity to the electron transport chain and lack of robust protective histones, so mutations accumulate and further impair function. While other organelles can be stressed by oxidative conditions, mitochondria are the most directly affected and tend to degenerate with sustained exposure to oxygen and free radicals.