Final answer:
The skin tans because of increased production of melanin in response to exposure to UV rays. Melanin acts as a natural sunscreen to protect the DNA of skin cells from UV damage and maintains a balance with vitamin D production.
Step-by-step explanation:
The skin tans in response to sunlight because of the release of melanin. Melanin is produced by melanocytes in the epidermis, and its production increases upon exposure to UV rays from the sun or tanning salons. This pigment protects the DNA of epidermal cells from UV ray damage by absorbing the harmful radiation. The presence of melanin also aids in protecting against the breakdown of folic acid, an essential nutrient for our health. However, an excess of melanin can limit the production of vitamin D, which is vital for calcium absorption. Thus, the skin's ability to tan is a form of protection against UV damage, while also maintaining a balance necessary for good health.