Approximately how many amino acids are needed in an alpha helix to span a membrane?

The correct answer is based on the understanding of the structural characteristics of alpha helices and their ability to span biological membranes. An alpha helix typically comprises about 3.6 amino acids per turn and spans approximately 1.5 Å (angstroms) in height per residue.

To effectively span a biological membrane, which is generally around 30 Å thick, it is estimated that a minimum of about 20-25 amino acids are required. This allows the hydrophobic amino acids within the helix to interact favorably with the lipid bilayer, facilitating stable insertion and anchoring of the protein within the membrane.

While 15 amino acids may be somewhat close but not sufficient for proper membrane traversal, choices such as 10 and 25 represent either an underestimate or an overestimate. Thus, around 20 amino acids is a commonly accepted number for an alpha helix to effectively span a membrane, making it the most suitable answer in this context.