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Passive transport through the lipid bilayer

There are several methods by which molecules can cross the cell membrane.

The simplest methods involve passive transport. This is diffusion of a molecule along a concentration gradient (from an area of high concentration to an area of low concentration) either into or out of the cell.

In passive transport, there is no energetic investment from the cell.

Small non-polar molecules (e.g. $$\ce{CO2}$$ and $$\ce{O2}$$) are able to diffuse directly through the lipid bilayer.

Molecules that are too large, not lipid soluble, or non-polar enter the cell through a different type of passive transport called facilitated diffusion. This employs the use of protein pores.

Protein pores are present on the cell membrane to allow these molecules to passively diffuse into the cell.

Ions, and molecules such as water and glucose enter this way. The rate of facilitated diffusion saturates at high concentrations as all the pores become occupied.