Final answer:
Free radicals are produced during oxygen metabolism and ATP production in mitochondria, rather than through processes such as photosynthesis.
Step-by-step explanation:
Free radicals are formed as a result of the process of oxygen metabolism and can also be produced during the mitochondrial reactions that produce ATP.
These highly reactive molecules have unpaired electrons, making them capable of oxidizing other molecules and leading to cellular damage or aging. In particular, free radicals are known to initiate propagation reactions in fatty acids, leading to the formation of hydroperoxides which decompose into aldehydes and carboxylic acids, causing an unpleasant "rancid" smell in aged fats.
Similarly, free radicals contribute to the aging process in organisms and the development of various diseases such as cardiovascular ailments. Photosynthesis, which uses light energy to fix CO2 into organic molecules, evolved to generate free oxygen and ultimately allowed aerobic respiration to develop, but it is not directly responsible for the formation of free radicals during its normal operation.