Final answer:
Nonpolar or lipid-soluble molecules and lighter molecules diffuse faster through the plasma membrane due to their solubility and mass. Transport proteins aid in the diffusion of polar substances and ions, but for passive diffusion, nonpolar molecules are the quickest to move.
Step-by-step explanation:
The question regards which molecule diffuses the fastest through the plasma membrane, based on results from a FRAP (Fluorescence Recovery After Photobleaching) study. The key factors to consider in this context are solubility, mass of the molecules, and special transport proteins involved in the process.
Nonpolar or lipid-soluble materials diffuse through the plasma membrane more readily than polar materials due to their solubility, thus they exhibit a faster diffusion rate. Additionally, lighter molecules diffuse more quickly than heavier ones. And while polar substances and ions cannot easily pass through the nonpolar lipid core of the membrane, certain transport proteins facilitate their movement, a process known as facilitated diffusion. However, in the context of FRAP, non-facilitated (passive) diffusion is typically under examination, and hence, nonpolar or small nonpolar molecules that do not require these proteins will generally diffuse the fastest.
Factors such as plasma membrane fluidity, affected by temperature and membrane composition (like cholesterol content), the surface area and the thickness of the plasma membrane also play a role in affecting diffusion rates. A higher surface area and thinner plasma membrane tend to increase the rate of diffusion.