The Science of Melanin

Predict

Squids squirt a cloud of dark ink when a predator comes close. Your hair gets its color from a pigment called melanin. Could squid ink and your hair get their color from the same kind of pigment?

Circle one. Yes / No

Why?

Watch

1. The Science of Melanin Magicyoutube.com/watch?v=5Dt1NYLqMqM

Word of the day

Melanin (MEL-uh-nin). A family of natural pigments that give color to skin, hair, eyes, feathers and more.

• Melancholy, a word for deep sadness, comes from Greek melancholía, “condition of having black bile.”
• Keratin, the protein that makes up hair and nails, carries the same -in ending as melanin.

Bonus words from the video. Eumelanin (yoo-MEL-uh-nin) adds eu-, “good,” and names the brown to black melanin. Pheomelanin (fee-oh-MEL-uh-nin) adds pheo-, from the Greek word for “gray,” and names the red to yellow melanin. Neuromelanin (noor-oh-MEL-uh-nin) adds neuro-, “nerve,” and names the dark melanin in the brain.

Questions

1. The enzyme tyrosinase turns the amino acid tyrosine into dopaquinone. When the amino acid cysteine is present, cysteine joins dopaquinone, and the pathway makes pheomelanin. When cysteine runs out, the pathway makes eumelanin. Tyrosine’s formula is C9H11NO3, and cysteine’s formula is C3H7NO2S. Pheomelanin contains sulfur, and eumelanin doesn’t. Where do the sulfur atoms in pheomelanin come from?

   Hint. Atoms don’t appear out of nowhere in a chemical reaction. Find the S in the two formulas.

2. In lab experiments, scientists gave pigment cells small pieces of DNA that mimic UV damage. The cells made about 7 times more melanin. Painted on guinea pig skin, the same DNA pieces produced a visible tan. What do the DNA experiments suggest about the signal that tells melanocytes to make a tan?

   Hint. The DNA pieces copied damage, not sunlight. What could the melanocytes be responding to?

3. Neuromelanin collects with age in two areas of the human brainstem. In Parkinson’s disease, the brain loses many of the neurons that hold the most neuromelanin. Mice and rats, the most common lab animals, make no neuromelanin. Why does the lack of neuromelanin in mice and rats make Parkinson’s disease harder to study in those animals?

   Hint. A lab animal stands in for a human brain. What would a mouse brain be missing?

4. Melanin makes up about 15% of the weight of cuttlefish ink, and proteins add another 5 to 8%. Some squid release ink as a loose cloud. Others release a blob of ink and mucus that looks like a squid. How could a squid-like blob of dark ink help a squid escape? And why does melanin’s dark color help the ink work?

   Hint. Think about where a predator aims an attack.

Claim, evidence, reasoning

Going further: UV and heat

1. A 2004 physics study found that eumelanin turns more than 99.9% of the UV and visible light energy it absorbs into heat. A 2008 review estimated that melanin in skin works like a sunscreen of about SPF 1.5 to 4, absorbing about 50 to 75% of UV rays. Someone reads the 2004 study and says melanin blocks 99.9% of the UV rays that hit skin. Is the claim that melanin blocks 99.9% of the UV rays hitting skin valid? Use both numbers to back up your answer.

   Hint. Compare “the UV that reaches skin” with “the UV energy melanin has already absorbed.” What happens to light energy after matter absorbs the light?

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