Osmosis

Osmosis is diffusion of water molecules across the plasma membrane from a region of its higher concentration to a region of its lower concentration. This can be proved by an experiment.

Consider a U shaped tube with side A filled with pure water and side B with same quantity of sucrose solution (sugar dissolved in water) separated by a membrane permeable to water and impermeable to sugar. Since membrane is 


permeable to water molecules, it can move from side "A" to "B" and from "B" to "A". At side "A" there are only water molecules whereas at side "B" the presence of sucrose molecules means concentration of water molecules is  less than at side "A". Thus in unit time more water molecules will diffuse from side "A" to "B" than from "B" to "A". In other words there is net movement of water molecules from "A" to "B". As a result, level of sugar solution will begin to rise and that of water will begin to 


U tube divided by a semipermeable membrane. The membrane at the base of the U tube is permeable to water, but not to sugar molecules (black balls). 
Left:       Side A contains only water; side B contains a sugar solution. Initially, the quantity of fluid in the two sides is the same.
Center:    A larger number of water molecules (colored balls) bump into the membrane per unit time on side A than on side B.
Right:      Because more water molecules move from A to B than from B to A, the level of fluid on side A falls white that on side B rises.