Question

What are the different methods of transportation across the cell surface membrane?

Answer 1

The different methods of transportation across the cell surface membrane are described below:

Diffusion
Movement of molecules from high concentration to low concentration without semi-permeable membrane. It is slow process and is carried out without the expenditure of energy.
Diffusion depends upon: Concentration gradient, Permeability of the membrane, Temperature, Pressure and Size of the substance.

Facilitated Transport
In facilitated diffusion, the membrane proteins are involved. They provide a site for hydrophilic molecules to pass through the membrane and no energy is required.
Proteins involved in the process form channels which may always be opened or controlled. Facilitated diffusion is very specific.
Porins: Proteins that forms huge pores in the outer membranes of plastids, mitochondria, etc. They are of different kinds:
Aquaporins: Proteins that facilitate diffusion of water molecules
Transport can be of 3 types:
Symport − both molecules move in the same direction
Antiport − both molecules move in opposite directions
Uniport − independent movement of molecules
When all proteins involved are saturated, it leads to maximum transport.

Active transport
Requires special proteins which are very specific and sensitive to inhibitors.
Requires energy to pump molecules against the concentration gradient.
When all proteins involved are saturated, it leads to maximum transport.