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AP Bio



Unit 2

2.7 Facilitated Diffusion

2 min readjune 8, 2020





When and Why Does Facilitated Diffusion Occur?

When molecules cannot move easily enough through the plasma membrane, facilitated diffusion occurs. Molecules cannot pass through the phospholipid bilayer of the plasma membrane easily when particles are either charged or polar. 
Facilitated dIffusion is a form of passive transport which does NOT require energy. Passive transport occurs when molecules go down the concentration gradient.
A concentration gradient is when particles or solutes move from a highly concentrated area of particles to a less concentrated area of particles. This process is aided by proteins located on the plasma membrane (membrane proteins) such as transport proteins: Channel proteins and carrier proteins.

Channel Proteins 

Channel proteins are laid throughout the membrane to provide a Hydrophilic passage through for the molecules to avoid the Hydrophobic core (Hydrophobic means to avoid polar substances such as water. Hence the terms hydro-water and Phobic think of phobia, fear of something. While Hydrophilic is the opposite and attracts water). An example of a channel protein is Aquaporins which allow water (polar H20) to diffuse through the membrane. Aquaporins are essential for plant cells, red blood cells, etc.
Nerve and muscle cells have gated ion channel proteins to enable the flow of charged ions such as sodium and potassium present in the sodium potassium pump of action potentials. If a signal such as an electrical signal is activated these channels open their gate to transmit these signals through cells. 

Carrier Proteins 

Carrier proteins alter their shape to allow the flow of molecules through the concentration gradient of the membrane similarly to an enzyme substrate complex. Their rate of transport is slower than that of channel proteins. Carrier proteins provide an easy way for hydrophilic molecules to pass through the concentration gradient.


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