Final answer:
The amino acids in an α-helical segment that spans a membrane are predominantly hydrophobic, allowing the protein to integrate into the membrane. (option 3)
Step-by-step explanation:
The word that characterizes the amino acids found in an α-helical segment that spans a membrane is predominantly hydrophobic. These hydrophobic amino acids, such as Ala, Val, Leu, Ile, Pro, Met, Phe, and Trp, are insoluble in water and are crucial for the protein's ability to anchor itself within the membrane's fatty acid environment. Some proteins do contain hydrophilic amino acids which allow them to interact with others and form pores for polar molecules and ions, but the alpha helices that span the membrane are typically composed of hydrophobic amino acids.
The preference for hydrophobic amino acids in membrane-spanning helices is a fundamental aspect of membrane protein structure, ensuring stability and functionality within the lipid environment. This structural feature is vital for various cellular processes, including membrane transport and signaling.