What type of molecules undergo facilitated diffusion ?

Substances that have a hydrophilic moiety undergo facilitated diffusion. 
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Comment upon the rate of diffusion. Name the factors that affect the rate of diffusion. 

Diffusion is a slow process. The diffusion rates are affected by the gradient of the concentration, the permeability of the membrane separating them, temperature and pressure. 
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Describe osmosis as a special case of diffusion.


Osmosis : It is a special case of diffusion, because of following reasons :

1. In osmosis only water or solvent molecules diffuse from their higher concentration to lower concentration, while in diffusion all type of matter can diffuse i.e. diffusion of gases, solids as well as of liquids.

2. In osmosis diffusion of solvent molecules or water molecules occur through semipermeable membrane. In simple diffusion semipermeable membrane is not required.

Thus osmosis is a special case of diffusion in which only solvent or water molecule diffuse from low to high concentration through semipermeable membrane.

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Difference between Imbibition and Osmosis

Imbibition

Osmosis

1. It involves the absorption water by a solid substance wihtout getting dissolved.

1. It involves the movement of water or solvent from its higher chemical potential to lower chemical potential.

2. It does not need semi-permeable membrane. 

2. A semi-permeable membrane is required in osmosis.

3. Heat is produced

3. Heat is not produced. 

4. It is due to presence of colloidal particles.

 

4. It requires the presence of solute particles in solution form.

 

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What is the role of special proteins in diffusion?

The special protein elps in the facilitated diffusion of substances across membranes without the expenditure of energy. These proteins form channels in the membrane for molecules to pass through. For example - the porins are special proteins that allow molecules of the sixe of the protein to pass through. 
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