Your Turn For each of the following atoms or molecules, determine whether they will cross a plasma membrane (if following their concentration gradient) by simple diffusion or by facilitated diffusion. Then, briefly explain your reasoning. O2 (oxygen gas) crosses the plasma membrane bydiffusion, because Glucose crosses the plasma membrane by diffusion, because Water crosses the plasma membrane by diffusion, because Nat (sodium ion) crosses the plasma membrane by diffusion, because Cholesterol crosses the plasma membrane by diffusion, ーーーーーーーーーーーーー because Which of these atoms or molecules require facilitated diffusion to cross a plasma membrane, but are able to simply diffuse across a dialysis tubing membrane?

1. Oxygen moves through the plasma membrane by simple diffusion, as oxygen will be utilized fast for cell metabolism that will result in the concentration of oxygen inside a cell will always be less

2. Although the concentration of glucose is more outside a cell it cannot cross the plasma membrane by simple diffusion and crosses the plasma membrane by facilitated diffusion as glucose is a large and more polar molecule. A carrier protein called glucose transporter helps in this movement.

3. Water crosses the plasma membrane by simple diffusion as it has a small size.

4. Na+ ions are charged ions and cannot pass the plasma membrane by simple diffusion and they are carried in with the help of sodium pumps that are made up of certain membrane proteins by facilitated diffusion.

5. Cholesterol crossed the plasma membrane by facilitated diffusion as it is a non polar molecule and is diffused through channels.