Diffusion:
The physical process by which molecules of any substance move from a region of higher concentration (Pressure) to a region of lower concentration(pressure) through a membrane or freely until a homogeneous condition is achieved is known as diffusion.
Role of Diffusion:
i. Diffusion may help in the passive absorption of water and mineral salts without any additional energy expenditure.
ii. It also helps in the translocation of food.
Osmosis:
The movement of solvent molecules from a region of lower concentration to a region of higher concentration through a semipermeable membrane is known as osmosis. When a particular cell is placed in a comparatively less concentrated solution (hypotonic medium), the solvent molecules enter the cell by a process called endosmosis and make the cell turgid (swollen).
In the same way, when a cell is placed in a more concentrated solution (hypertonic medium), the solvent molecules come out of the cell by a process called exosmosis and the cell becomes flaccid. But if a cell is placed in a medium having a similar concentration to that of the cell (isotonic medium), then there will be no movement of solvent molecules and the cell will remain in its normal state.
Role of Osmosis:
i. Osmosis helps in the entry of water molecules from the soil into the root hair.
ii. The water molecules from the root hair the cortex of the root and come to the endodermis by the process called cell-to-cell osmosis.
iii. It is essential for making the cells turgid. Such turgidity is necessary for the enlargement of meristematic cells, and the growth of organs, rigidity, and flaccidity of guard cells are necessary for the opening and closing of stomata.
iv. Osmosis in plants directly controls the dehiscence of another lobe, fruits, etc.
v. Small weak herbs can start erect due to an increase in the turgidity of the cells in the stem by endosmosis.
vi. In unicellular organisms and simple multicellular ones, osmosis plays the most important role in the entry of water from the surroundings and the maintenance of water balance.
vii. In higher animals, a continuous exchange of water through osmosis takes place amongst blood, tissue fluid, tissue cells, and lymph in the capillary bed and this is very important in regulating the water balance in higher animals.
viii. Various plant movements are largely due to the osmosis phenomenon.