Asmall polar molecule enters a cell with from a membrane protein.

This is called facilitated diffusion(facilitated by membrane proteins) examples would be diffusion of ions such as sodium, potassium etc.

Hope this helps;)

The small polar molecule enters a cell with help from a membrane protein by    a process known facilitated diffusion.

Explanation:

The facilitated diffusion also known as passive diffusion. It allows movement of molecules along the concentration gradient whereas the process of active diffusion involves energy as it is against the concentration gradient.

The facilitated diffusion involves carrier protein and channel protein. Thus small molecules can easily pass through channel proteins.