Here's why:

* The cell membrane is selectively permeable. This means it lets some molecules pass through, but not others.

* Small, uncharged molecules like oxygen (O2) and carbon dioxide (CO2) can easily pass through the membrane's phospholipid bilayer.

* Water can also pass through the membrane, though it relies on special channels called aquaporins.

* Larger, charged molecules like glucose, ions, and proteins cannot pass through the membrane directly. They need transport proteins embedded in the membrane to help them cross.

So, to summarize:

* Small, uncharged molecules: Easily pass through

* Water: Passes through with the help of aquaporins

* Large, charged molecules: Need transport proteins

Let me know if you have any other questions! 😊