Importance of diffusion
(0) Exchange of gases(CO, and O,). between the plant interior and outside air, occurs through diffusion.
(il) Diffusion keeps the cell walls of the internal plant tissues moist.
(ii) It is the process involved in transpiration.
(iv) It is a means of spreading of ions and other substances throughout the protoplast.
(v) It is an effective means of transport of substances, over a very short distance.
(vi) Aroma of flowers is due to diffusion of volatile aromatic compounds and helps to attract pollinating animals.
Facilitated diffusion
The passive absorption of solute, mediated by a carrier, is called facilitated diffusion. Particles which are lipid soluble,
can easily pass directly through the cell membrane, as it is mainly made of lipids. The hydrophilic solutes, find it difficult
to pass through the membrane, so their movement has to be facilitated. Biological membranes contain transportmemb membraneteins that facilitate the passage of selected ions and other polar molecules.
Membrane 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. These membrane proteins help to move substances across membranes without expenditure of ATP energy. Facilitated diffusion cannot cause net transport of
molecules, from a low to a high concentration which would require
input of energy.
Facilitated diffusion is very specific, and allows a cell to select substances for uptake. It is sensitive to inhibitors,
which react with the protein side chains.
Carrier protein Uniport A Antiport A
Symport Facilitated diffusion Two major types of transport membrane proteins are known , carrier proteins (also called carriers, transporters) and channel proteins.
