Summary
The diffusion of water across a selectively permeable membrane from lower to higher solute concentration.
In depth
Water moves from a region of higher water potential to lower water potential across a semipermeable membrane. It is driven by solute concentration, so water goes toward the more concentrated solution.
In plants water potential is the sum of solute potential (always negative) and pressure potential. Cells in pure water gain water until the wall pressure stops further uptake, becoming turgid.
Osmotic pressure is the pressure needed to stop water entering a solution; blood plasma is about 300 mOsm per litre. Water potential is measured in kilopascals and pure water at standard pressure is zero, so any added solute makes it negative.
Plants use osmosis for support (turgor), for opening stomata and for drawing water into roots; the Casparian strip in the root endodermis forces water to cross membranes so that the plant can control what enters. Animals manage osmosis with kidneys and with hormones such as ADH.
Example: when a potato cylinder is left in different sugar solutions, the concentration at which its mass does not change shows its isotonic point.
Frequently asked questions
What is Osmosis?
The diffusion of water across a selectively permeable membrane from lower to higher solute concentration.
Which area of biology covers Osmosis, and how advanced is it?
Osmosis is one of the 52 topics in Cell Biology. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Tonicity, Vacuole, Simple Diffusion.
Can you give an example of Osmosis?
Salting slugs or pickling food works because water is pulled out of cells by the strong external solution.
What should I learn before Osmosis?
Work through the Beginner topics in Cell Biology first, such as Cell Theory, Prokaryotic Cell, Eukaryotic Cell, then return to Osmosis. Intermediate topics assume the Beginner vocabulary.
What topics are related to Osmosis?
The closest topics to Osmosis are Tonicity, Vacuole, Simple Diffusion, Cell Wall. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Osmosis?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Osmosis. To practise the whole of Cell Biology, use the Cell Biology quiz.
How is Osmosis different from Tonicity?
Osmosis: The diffusion of water across a selectively permeable membrane from lower to higher solute concentration. Tonicity: Describes how an external solution affects cell volume: hypotonic causes swelling, hypertonic causes shrinking. Read both side by side and note what each one explains.
How should I study Osmosis step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Tonicity and explain the difference aloud. Finally, take the Cell Biology quiz and come back to this page for any question you miss.