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2. Facilitated Diffusion
A favourable concentration gradient of molecules is essential for difiusion. The rate
of difiusion depends on the size of the substances, i.e., smaller the substance, the
faster it will diffuse and on its solubility in lipids (major constituents of membrane),
i.e., more the substance is soluble in lipid, more faster it will diffuse through the
membrane.
Transport of substances (having hydrophilic moeity) is facilitated by transport
proteins. These protein do not create any concentration gradient by themselves; a
concentration gradient is already present.
This process is called facilitated difiusion. Facilitated diffusion does not allow net
movement of molecules from low to high concentration because it will require input
of energy. The rate of transport finally reaches the maximum when all transporters
of proteins are utilised.
Facilitated diffusion is very specific, i.e., allows uptake of only selected substances.

There are two different types of transport proteins, i.e., carrier proteins and channel
proteins. Carrier proteins bind the particular solute to be transported and deliver the
same to the other side of the membrane.
The channel proteins form channels in the cell membrane so s that molecules can
easily get transported. This is called channel mediated facilitated difiusion. Out of
these channels, some always remain open while, the others are controlled. The large
transporter proteins, which create huge pores in the outer membranes of the
plastids, mitochondria and bacteria to cross a variety of molecules up to the size of
small proteins are called porins.
During transportation, the extracellular molecule binds to the transport protein which
then rotates towards the intracellular matrix and releases the molecule inside the
cell, e.g., Water channels made up of eight different types of aquaporins.
Note:
Aquaporin are membrane proteins for passive transport of water. They enhance the
rate of transport of water across the membrane without altering the direction of
transport.
Passive Symports and Antiports
In relation to facilitated diffusion, some carrier or transport proteins allow the
movement of molecules only if two molecules move together. This type of