Ultraviolet light drives MITF to make melanin, which screens the ultraviolet that produced it — a negative feedback. The melanin response is therefore concave, rising toward a plateau, and it attenuates the ultraviolet reaching DNA to about 21% at high dose (partial photoprotection). Removing the screening abolishes the protection entirely, identifying the feedback as its cause.
Ultraviolet drives MITF→melanin, and melanin screens ultraviolet, closing a negative-feedback loop. The melanin response is concave and plateauing; delivered ultraviolet is attenuated to about 21%, and removing the feedback abolishes the protection.
Pigment is a feedback controller, not a dial
Ultraviolet light drives the melanocyte master gene MITF, which makes melanin. Melanin then absorbs ultraviolet — including the ultraviolet that triggered it. Synthesis and screening therefore form a negative feedback loop: h_eff = UV · e−k·M.
The self-consistent steady melanin level is consequently concave in ultraviolet dose, rising toward a plateau rather than tracking the dose linearly. Across the swept range the melanin response flattens (the high-dose level is only about 1.09× the mid-dose level), the signature of a regulated quantity.
The screen is real but partial
The ultraviolet that actually reaches the DNA below is the screened flux UV·e−k·M. With the feedback active this is attenuated to about 21% of the incident dose at the top of the range — substantial photoprotection, but not total: extreme ultraviolet still gets through, which is why a tan is not sunscreen.
The control experiment removes the feedback (k→0). The delivered-ultraviolet attenuation then returns to 1× (no protection at all), identifying the melanin screening — not any synthesis cap — as the cause of the protection.
Grades
The concave, plateauing melanin response, the partial attenuation of delivered ultraviolet, and the feedback-off control are simulation-verified shapes [V] Simulation-verified. The absolute minimal erythema dose and the melanin optical density are open [O] Open (obstacle stated): they need the melanin extinction coefficient as a cited input.