How does Water enter a cell?

Friday, October 30, 2020

The cell has a problem.

In order to stay alive, it must move

  1. Water

  2.  Glucose

  3. Sodium ions

  4. Large Proteins

 across the cell membrane and into the cell.

The bad news is that these molecules cannot pass through the cell membrane or in the case of water can pass through the membrane but it is slow and difficult passing through the cell membrane.

water across membrane

The good news is that all three are moving down the centration gradient -you know-

High to low is the only way to go

What is the cell going to do?

Never fear- Facilitated Diffusion to the rescue!

In order for glucose, ions, and polar molecules like water to enter the cell, their movement must be "facilitated" by proteins that span the membrane and provide an alternative route into the cell.

Facilitated Diffusion

In fact, most water enters or exits the cell through channel proteins called aquaporins.


Water can pass through the cell membrane through simple diffusion because it is a small molecule, and through osmosis, However, because water is a polar molecule this process of simple diffusion is relatively slow, and the majority of water passes through aquaporin

The use of the aquaporins is a type of Facilitated diffusion which is passive transport but uses either a channel protein or a carrier protein to assist or facilitate the movement of molecules or ions.

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