Simple Diffusion

We’ve just learned that diffusion is the movement of a substance from a place of higher concentration to a place of lower concentration. In other words, diffusion is the movement of a substance down the concentration gradient.

There are two types of diffusion that involve substances traveling through a cell membrane: simple diffusion and facilitated diffusion.

Simple diffusion involves the movement of substances directly through the phospholipid bilayer.

0305 Simple Diffusion Across Plasma MembraneCredit

Substances that are able to pass directly through the phospholipid bilayer easily are lipid-soluble molecules. Also, small hydrophobic molecules like oxygen (O2) and carbon dioxide (CO2) can pass through the phospholipid bilayer.

Even though water contains partial charges, it can move through the phospholipid bilayer but it moves slowly.


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