Related Posts

This illustration shows a container whose contents are separated by a semipermeable membrane. Initially, there is a high concentration of solute on the right side of the membrane and a low concentration of the left. Over time, water diffuses across the membrane toward the side of the container that initially had a higher concentration of solute (lower concentration of water). As a result of osmosis, the water level is higher on this side of the membrane, and the solute concentration is the same on both sides.
In osmosis, water always moves from an area of higher water concentration to one of lower concentration. In the diagram, the solute cannot pass through the selectively permeable membrane, but the water can.

Source: OpenStax Biology 2e

Osmosis is the movement of water through a semipermeable membrane according to the water’s concentration gradient across the membrane, which is inversely proportional to the solutes’ concentration. While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the solutes’ diffusion in the water. Not surprisingly, the aquaporins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.

– What is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by diffusion?


Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane the water level is the same, but there are different dissolved substance concentrations, or solute, that cannot cross the membrane (otherwise the solute crossing the membrane would balance concentrations on each side). If the solution’s volume on both sides of the membrane is the same, but the solute’s concentrations are different, then there are different amounts of water, the solvent, on either side of the membrane.

To illustrate this, imagine two full water glasses. One has a single teaspoon of sugar in it; whereas, the second one contains one-quarter cup of sugar. If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large sugar amount in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it.

– What is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane?

Returning to the beaker example, recall that it has a solute mixture on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of getting through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the water’s concentration gradient goes to zero or until the water’s hydrostatic pressure balances the osmotic pressure. Osmosis proceeds constantly in living systems.

Reverse osmosis is a water purification process that uses a partially permeable membrane to remove ions, unwanted molecules and larger particles from drinking water.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e





0 0 vote
Article Rating
Notify of
Inline Feedbacks
View all comments