Question

iso membranes are inside - out membrane vesicles used by researchers in membrane studies. as a molecule diffuses into the vesicle, it will encounter the layers of the membrane in the following order:
head - tail - head - tail.
head - head - tail - tail.
head - tail - tail - head.
tail - head - head - tail.
tail - head - tail - head

Explanation:

In a normal cell membrane, the phospholipid bilayer has hydrophilic heads facing outward and hydrophobic tails inward. In an inside - out membrane vesicle (ISO membrane), the orientation is reversed. When a molecule diffuses into the vesicle, it first encounters the hydrophobic tails (from the original extracellular side, now inside the vesicle), then the hydrophilic heads (original intracellular heads, now on the inner surface of the vesicle), then another set of hydrophilic heads (original extracellular heads, now on the outer surface of the vesicle when it was a normal membrane, but now inside the vesicle as it's inside - out), and finally the hydrophobic tails (original intracellular tails, now on the outer surface of the vesicle when it was a normal membrane, but now inside the vesicle as it's inside - out). So the order is tail - head - head - tail.

Answer:

tail - head - head - tail