Oxidative Stress Quiz

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Which statement best defines oxidative stress?

A An imbalance in which reactive oxygen species overwhelm the cell's antioxidant defences ✓
B Build-up of carbon dioxide leading to intracellular acidosis
C Controlled production of reactive species by immune cells to kill microbes
D Insufficient oxygen delivery to tissues to meet metabolic demand
E Normal electron transport in the mitochondria during aerobic respiration

The answer is A: An imbalance in which reactive oxygen species overwhelm the cell's antioxidant defences

Explanation

Why A is correct

See Oxidative Stress. Option A is correct. Oxidative stress is what happens when the production of reactive oxygen species (ROS: very reactive molecules that contain oxygen, such as superoxide and hydrogen peroxide; some carry an unpaired electron, which makes them free radicals) outstrips the cell's antioxidant defences (enzymes such as superoxide dismutase and catalase, the small molecule glutathione, and scavengers such as vitamins C and E). The balance tips towards oxidants, and the surplus reacts with whatever it meets. It can damage DNA, leading to mutations; it can damage proteins, for example by forming inappropriate disulphide bonds (sulphur-to-sulphur links between protein chains); and it can attack the lipids of cell membranes. This damage contributes to cancer, chronic obstructive pulmonary disease, cardiovascular disease, multiple sclerosis and Parkinson's disease. The key word is imbalance. Cells make ROS all the time as a by-product of normal metabolism, and they are harmless while the defences keep pace.

Definition: Oxidative stress is an imbalance in which reactive oxygen species outstrip the antioxidant defences.
Targets: The surplus damages DNA, proteins and the lipids of cell membranes.
Normal state: Cells make reactive oxygen species all the time and are unharmed while the defences keep pace.

A small fraction of electrons leak and form superoxide, but this only becomes oxidative stress if the antioxidant systems cannot cope.

Why the other options are incorrect

B. Build-up of carbon dioxide leading to intracellular acidosis: Carbon dioxide retention causes an acid-base disturbance, for example a respiratory acidosis in hypoventilation. It does not involve an excess of reactive oxygen species overwhelming antioxidants, so it does not define oxidative stress.
C. Controlled production of reactive species by immune cells to kill microbes: This describes the respiratory burst of phagocytes, where reactive species are made on purpose and in a regulated way to kill engulfed bacteria. It is a useful physiological process, not a harmful imbalance. It would fit a question about how neutrophils kill bacteria.
D. Insufficient oxygen delivery to tissues to meet metabolic demand: This describes tissue hypoxia, a shortfall in oxygen supply. Oxidative stress concerns the balance between reactive oxygen species and antioxidants, and it can occur even when oxygen supply is normal. Hypoxia would be the answer if the stem described poor oxygen delivery.
E. Normal electron transport in the mitochondria during aerobic respiration: This is ordinary aerobic respiration, which makes ATP and only leaks a small, well-controlled amount of reactive oxygen species. Normal metabolism is not oxidative stress unless production rises sharply or the defences fail.

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More questions from this set. Start the quiz to answer them and see each explanation.

1
Which reactive substance generated during the respiratory burst produces the bleach-like antimicrobial effect inside phagocytic cells?
2
Which metabolic change most directly explains why paracetamol overdose causes liver cell damage?

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