Osmosis is the passage of water and dissolved substances through a semi-permeable membrane. This membrane allows transfer of selective particles only. In plants, osmosis can be observed between roots and soil where water and nutrients from soil pass into roots through root hair. In animals, osmosis can be observed in the small intestine where nutrients from digested food are absorbed by blood through intestinal membrane. The flow of liquid always takes place from a region of high pressure to a region of low pressure. This pressure is called osmotic pressure.