what types of molecules can diffuse through the cell membrane, without using a channel protein

Question

what types of molecules can diffuse through the cell membrane, without using a channel protein ?

Answer 1

First you need to think about the properties of a cell membrane. Cell membranes are a phospholipid bilayer, as in they are two layers of phospholipids with the hydrophobic tails on the inside and the hydrophilic heads on the outside. Much like two pieces of bread making up a sandwich. The hydrophobic tails don't want water or water soluble things near them. On the other hand, the hydrophilic heads want water and water soluble things near them. 

 

So to get through the membrane, a molecule must be able to get past both of these "barriers". In general things that can do this are small. This intuitively makes sense. Big things can't get through small spaces. They are also relatively uncharged. It a super strong charge gets near the hydrophilic heads, it will get stuck there and won't be able to get through the hydrophobic areas. 

 

So, to really answer your question, small NON-chaged molecules can freely diffuse through the membrane. Examples of this are carbon dioxide (CO2) and oxygen (O2). 

 

Things like Na+, K+, Cl- can't get through due to their charge.

Things like glucose and amino acids have trouble getting through because of their larger size.