Explain: Facilitated Diffusion and its properties.

(N/A) Facilitated diffusion is a type of passive transport where specific membrane proteins assist in the movement of molecules across a biological membrane without the expenditure of $ATP$ energy.
Key Principles of Diffusion:
- Diffusion occurs only if a concentration gradient is already present.
- The rate of diffusion depends on the size of the substances; smaller substances diffuse faster.
- Diffusion across a membrane depends on the solubility of the substance in lipids. Lipid-soluble substances diffuse through the membrane more rapidly.
Need for Facilitated Diffusion:
- Substances with a hydrophilic moiety find it difficult to pass through the lipid bilayer of the membrane.
- Membrane proteins provide specific sites for these molecules to cross the membrane.
- Facilitated diffusion does not set up a concentration gradient; it requires an existing gradient to function.
Properties of Facilitated Diffusion:
- It is a passive process that does not require $ATP$ energy.
- It is highly specific,allowing the cell to select substances for uptake.
- The transport rate reaches a maximum when all protein transporters are saturated (used).
- It is sensitive to inhibitors that react with protein side chains.
- Some proteins form channels in the membrane. These channels can be always open or gated (controlled).
- Porins are specialized proteins that form large pores in the outer membranes of plastids,mitochondria,and some bacteria,allowing molecules as large as small proteins to pass through.
- In some cases,an extracellular molecule binds to a transport protein,which then undergoes a conformational change (rotates) to release the molecule inside the cell.
- An example of this is the water channels,which are made up of eight different types of aquaporins.