Free radicals and antioxidants
A free radical has an unpaired electron, so it attacks fats, proteins and DNA. Antioxidants stop this damage by quenching radicals or breaking their chain reactions.
Reactive oxygen species (ROS) and how they form
| Species | Radical? | How it forms |
|---|---|---|
| Superoxide (O2•−) | Yes | O2 gains one electron: leak from the ETC, NADPH oxidase in phagocytes (to kill bacteria), xanthine oxidase, cytochrome P450 |
| Hydrogen peroxide (H2O2) | No, but reactive | Made from superoxide by superoxide dismutase |
| Hydroxyl radical (•OH) | Yes, the most damaging | H2O2 + Fe2+ (Fenton reaction); radiation splitting water |
| Singlet oxygen (¹O2) | No | Light (UV) acting on pigments in the skin and eye |
| Nitric oxide (NO•) | Yes | Made from arginine; with superoxide forms the harmful peroxynitrite |
| Outside triggers | — | Cigarette smoke, air pollution, UV rays and X-rays, alcohol, some drugs and pesticides, reused frying oil |
What free radicals damage
- Lipids: radicals attack PUFA (polyunsaturated fatty acids) in cell membranes. This lipid peroxidation is a chain reaction with three stages: initiation, propagation and termination. Malondialdehyde (MDA) is its marker.
- Proteins: amino acids are oxidised, proteins cross-link and enzymes lose activity.
- DNA: bases are damaged and strands break, causing mutations.
- Oxidative stress: the state in which free radical production is greater than antioxidant defence.
Antioxidant defences
| Antioxidant | What it does | Nutrient link and Indian sources |
|---|---|---|
| Superoxide dismutase (SOD), enzyme | Converts superoxide to H2O2 + O2 | Needs copper and zinc (cytosol) or manganese (mitochondria): pulses, nuts, whole grains |
| Catalase, enzyme | Splits H2O2 into water and O2 | Haem (iron) enzyme, found in peroxisomes |
| Glutathione peroxidase, enzyme | Removes H2O2 and lipid peroxides using glutathione | Needs selenium: fish, eggs, whole grains |
| Vitamin C (ascorbic acid) | Water-soluble; quenches radicals in blood and cell fluid; regenerates vitamin E | Amla, guava, drumstick leaves, lemon, capsicum |
| Vitamin E (tocopherol) | Fat-soluble; main chain-breaker in membranes, stops lipid peroxidation | Groundnut, sunflower and rice bran oils, almonds, wheat germ |
| β-carotene and other carotenoids | Quench singlet oxygen | Carrot, papaya, mango, pumpkin, green leafy vegetables |
| Polyphenols (flavonoids, curcumin) | Scavenge radicals and bind free iron | Turmeric, tea, onion, jamun, amla |
Free radicals and degenerative disorders
| Disorder | How free radicals contribute | Protective diet |
|---|---|---|
| Atherosclerosis, heart disease | Oxidised LDL is taken up by macrophages, forming foam cells and plaque | Fruits, vegetables, nuts; vitamins C and E, polyphenols |
| Cancer | DNA damage causes mutations | Plenty of vegetables and fruits rich in vitamin C and carotenoids |
| Cataract | Oxidised lens proteins clump and turn cloudy | Vitamins C and E, lutein from green leafy vegetables |
| Diabetes complications | High blood glucose raises ROS, damaging eyes, kidneys and nerves | Good sugar control with an antioxidant-rich diet |
| Alzheimer's and Parkinson's disease | The brain uses much oxygen and is rich in PUFA, so it is easily oxidised | Fruits, vegetables, nuts |
| Ageing | Free radical theory: oxidative damage builds up over the years | Lifelong balanced diet; no smoking |
Antioxidant defence is built from diet: vitamin C (amla, guava), vitamin E (groundnut, sunflower oil, nuts), β-carotene (carrot, papaya, green leafy vegetables), and selenium, zinc, copper and manganese for the antioxidant enzymes. Prefer whole foods: large trials of high-dose β-carotene pills in smokers found more lung cancer, not less. Avoid reusing frying oil, which forms oxidised products.
10 marks: “What are free radicals? Explain their formation and the role of antioxidants in the prevention of degenerative disorders.” Define, list ROS and their sources, explain lipid peroxidation, give enzyme and dietary antioxidants with Indian foods, then link to heart disease, cancer, cataract and ageing.
5 marks: “What are antioxidants? Classify them with examples.” Substances that prevent or slow oxidative damage. Enzymes: SOD, catalase, glutathione peroxidase. Dietary: vitamins C and E, β-carotene, polyphenols, selenium.
2 marks: “What are free radicals? Give two examples.” Atoms or molecules with one or more unpaired electrons, which makes them highly reactive. Examples: superoxide (O2•−), hydroxyl radical (•OH).
ACES: vitamin A (as β-carotene), vitamin C, vitamin E and Selenium are the classic dietary antioxidants.