Final answer:
The difference between a single-pass and a 7-pass transmembrane protein is the number of alpha-helical domains spanning the membrane, with the former having one and the latter having seven.
Step-by-step explanation:
The difference between a single-pass transmembrane protein and a 7-pass transmembrane protein is the number of alpha helices that span the cell membrane. Single-pass transmembrane proteins, like glycophorin A, have one alpha-helical domain that crosses the membrane a single time. In contrast, 7-pass transmembrane proteins have seven of these domains, each of which passes through the lipid bilayer, orbited around the protein structure. Multiple stop-transfer sequences facilitate such multi-spanning structures, trapping these domains within the hydrophobic core of the plasma membrane.
Therefore, the correct answer is A) Number of alpha helices. Cell type specificity, molecular weight, and presence of signal peptides are not what differentiate single-pass from 7-pass transmembrane proteins. These proteins can serve various functions, including acting as receptors, channels, or transporters, integral to cellular communication and physiology.