Final answer:
SR-B1 is the membrane transporter involved in releasing free cholesterol from plasma membranes into the blood. It binds to HDL and facilitates the uptake of free cholesterol.
Step-by-step explanation:
The membrane transporter involved in releasing free cholesterol from plasma membranes into the blood is SR-B1. SR-B1 binds to high-density lipoprotein (HDL) and facilitates the uptake of free cholesterol from HDL. This free cholesterol can then be used by cells for various functions or can be stored in adipose tissue as fat.