Root and leaves are the main absorbing organs of plants. Roots absorb water and dissolved salts from the soil, while leaves take in gases i.e oxygen and carbon-dioxide gases from the atmosphere.
Roots have been provided with a large number of fine root hairs. The water and dissolved salts are absorbed through the root hairs by a special type of osmosis. First, we will discuss three types of membranes.
(1) Permeable Membrane
In this type of membrane, the molecules of both water and salts pass easily. The pores in such a membrane are large enough to allow al the molecules to pass through it. Cell wall and filter paper are examples of the permeable membrane.
(2) Semi-Permiable Membrane
This type of membrane has the property to allow only small molecules to pass across. Eggs membrane and animal bladder are examples of the semi-permeable membrane.
(3) Impermeable Membrane
In this type of membrane, the molecules of materials cannot pass. Rubber and polythene are examples.
Osmosis Through Roots
Take some pure water in a beaker. Tie an egg membrane on the mouth of a thistle funnel. Fill the funner little above the bulb with a strong sugar solution.
Place the funnel in the beaker containing water and fix it with a stand are shown in the figure. Mark the level of sugar solution in the tube of the funnel.
After some time you will see that the level of the solution will rise in the tube and then will become steady. This type of diffusion from weaker to stronger solution is called osmosis.
The roots of plants absorb salt solution form soil through this process. All the cells of the plant body are saturated with water as a result of the cell to cell osmosis. In this way, the dissolved salts pass through different cells and reach the leaves where the process of photosynthesis takes place.