Which of the following best describes a key difference between beta-keratin and alpha-keratin in eukaryotic cells?

• A. Alpha-keratin contains more sulfur than beta-keratin.
• B. Alpha-keratin is a carbohydrate, while beta-keratin is a lipid.
• C. Beta-keratin is only found in mammals, while alpha-keratin is found in birds and reptiles.
• D. Beta-keratin forms rigid structures like feathers and scales, while alpha-keratin is found in flexible structures like hair and skin.

Answer: (D)

Verified step by step guidance

1. Understand the context of keratin proteins: Keratin is a structural protein found in eukaryotic cells, and it plays a key role in forming protective structures such as skin, hair, feathers, and scales.
2. Differentiate between alpha-keratin and beta-keratin: Alpha-keratin is primarily found in mammals and contributes to flexible structures like hair and skin. Beta-keratin, on the other hand, is found in birds and reptiles and contributes to rigid structures like feathers and scales.
3. Analyze the sulfur content: Alpha-keratin contains a higher proportion of sulfur due to the presence of cysteine residues, which form disulfide bonds. This is not the defining difference between alpha-keratin and beta-keratin, but it contributes to the flexibility of alpha-keratin.
4. Clarify the chemical composition: Both alpha-keratin and beta-keratin are proteins, not carbohydrates or lipids. This eliminates the option suggesting a difference in macromolecule type.
5. Conclude the key difference: The defining characteristic is the structural role of each type of keratin—alpha-keratin is associated with flexible structures, while beta-keratin is associated with rigid structures.