Question

Why does facilitated diffusion require no energy?

Answer 1

Hint: Diffusion is the movement of molecules or ions of a solid, liquid or gas from an area of their greater concentration to an area of their lesser concentration,eg if a bottle of either peppermint oil, ammonia or any other volatile substance is opened in a room, within a short time characteristic odor can be detected in all parts of the room. Gases have the highest rate of diffusion, followed by liquid and solid.

Complete answer:
In facilitated diffusion, specific proteins help in transfer of substances across the membrane and no ATP consumed. These specific proteins do not set up a concentration gradient, a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins.
Extracellular molecules bind with transport proteins and later these transport proteins release the molecules inside the cell by rotation movement.
Rate of transport reaches a maximum when all of the protein transporters are being used(saturation).
Facilitated diffusion is very specific, it allows cells to select substances for uptake. Proteins involved in facilitated diffusion are sensitive to inhibitors which affect the rate of diffusion and even stop it.
The proteins form channels in the membrane for molecules to pass through. Some channels may be always open, some can be controlled. Some protein channels are large which allows a variety of molecules to cross.
The porins are proteins that form huge pores in the outer membranes of some plastids, mitochondria and bacteria allowing molecules up to the size of small proteins to pass through.
Eg: There are 8 different types of aquaporins present in plasma membrane.
In symport, two molecules move across the membrane in a similar direction, while in antiport they move in opposite directions. When a molecule moves freely across the membrane then this method is called uniform.
As facilitated diffusion also takes place from higher concentration to lower concentration through special protein channels and there is no requirement of ATP or energy.

Note:
Diffusion in a cell depends upon the solubility in the main constituent of the membrane i.e. lipid. Lipid soluble substances rapidly diffuse through the membrane. The substance which has hydrophilic moiety diffuses difficulty through the membrane. So there is a need to simplify its movement. Membrane proteins provide space for transfer of these molecules. This process of diffusion with the help of membrane proteins is called facilitated diffusion.