Simple and facilitated diffusion

In a nutshell

Diffusion is an example of passive transport. This means that the process of diffusion does not require energy to occur. There are two main types of diffusion; simple and facilitated. Facilitated diffusion used transmembrane channels and carriers to transport charged ions and polar molecules that cannot diffuse easily due to the hydrophobic nature of the cell membrane.

Simple diffusion

Definition

Simple diffusion is the net movement of small molecules or ions down a concentration gradient, from a region of high concentration to a region of low concentration until they reach an equilibrium.

Example

Oxygen and carbon dioxide can be transported through simple diffusion as they are small which means they can pass through the phospholipid bilayer. They are also non-polar which means they can dissolve in the hydrophobic bilayer in the absence of a transmembrane channel.

Facilitated diffusion

Facilitated diffusion uses transmembrane channels and carriers to transport large molecules, charged ions and polar molecules that cannot diffuse easily due to their size and the hydrophobic nature of the cell membrane. Despite using transmembrane channels and carriers, the process is still passive. The molecules are also still diffusing from a region of high concentration to a region of lower concentration like in simple diffusion.

The two types of protein that are used in facilitated diffusion are described below.

Channel proteins

Channel proteins span the membrane and are specific for each molecule or ion that needs to be transported. This means they can be opened or closed depending on the presence or absence of a signal. If the ion is absent, the channel is closed. The ions bind with the protein which causes a change in the shape of the channel. This change in shape closes it to one side of the membrane and opens it to the other side.

Biology; Exchange and transport; KS5 Year 12; Simple and facilitated diffusion — A. Channel protein, B. Charged ion or polar molecule, C. Lipid bilayer, D. Outside of the cell, E. Inside the cell.

Carrier proteins

Carrier proteins span the membrane and a molecule or ion binds to onto a specific site on the protein. The protein changes shape, allowing the molecule to cross the membrane.

Biology; Exchange and transport; KS5 Year 12; Simple and facilitated diffusion

A	Channel protein.
B	Charged ion or polar molecule.
C	Lipid bilayer.
D	Outside of the cell.
E	Inside the cell.
1.	The polar or charged ion binds to the carrier protein.
2.	The protein changes shape, allowing the molecule to cross the membrane.
3.	The process repeats.

Rate of diffusion

Simple diffusion

The rate of simple diffusion is dependent on the factors below.

Factor	Explanation
Concentration gradient	The higher the concentration gradient, the faster the diffusion will occur.
Thickness of the surface	The thinner the membrane or surface, the faster the rate of diffusion. This is because there is a shorter distance for the ions to travel so they will diffuse quicker.
Surface area	The surface area of the exchange surface will impact the rate of diffusion. If the surface is large, the rate of diffusion will be greater.

Facilitated diffusion

The rate of facilitated diffusion is dependent on the factors below.

Factor	Explanation
Concentration gradient	The higher the concentration gradient, the faster the diffusion will occur.
Number of channel or carrier proteins	The number of carrier or channel proteins can be a limiting factor of diffusion. If all of the transport proteins are being used, the rate of diffusion will not increase if you increase the concentration gradient. Therefore, the more channel and carrier proteins you have, the faster the rate of diffusion.

Common substances

Substance	Property	Transport method
$O_2, CO_2, N_2$	Small and non-polar	Simple diffusion
Glucose ( $C_6 H_{12}O_6$ )	Large and polar	Facilitated diffusion
Water ( $H_2O$ )	Small and polar	Facilitated diffusion
ATP	Large and polar	Facilitated diffusion
Fatty acids	Lipid-soluble	Simple diffusion