Final answer:
An uncharged polar molecule enters the cell via facilitated diffusion through special protein channels or carriers, which allows it to move down its concentration gradient without the need for energy.
Step-by-step explanation:
An uncharged polar molecule that is present at higher concentrations inside of cells than outside uses special transport mechanisms to enter the cell. Large polar molecules and ions cannot easily cross the cell membrane's phospholipid bilayer through simple diffusion due to their size, charge, and polarity. Instead, these substances typically enter cells via facilitated diffusion, which is the movement of substances across the cell membrane through specific protein channels or carriers without the expenditure of energy.
For example, glucose, a large polar molecule, is unable to pass freely through the lipid bilayer of the cell membrane. This substance utilizes a type of protein called a glucose transporter to facilitate its entry into the cell. Similarly, ions like sodium and potassium also use special protein channels or pumps to move across the membrane.
Passive transport, water channels known as aquaporins, and carrier proteins are fundamental to this process. Facilitated diffusion allows these substances to move down their concentration gradient, where they spread from an area of higher concentration to an area of lower concentration inside the cell, without requiring energy.