Water moves through a thin skin towards the sugary side. Can it push itself uphill?

What you'll discover

• Water moves across a partly permeable membrane from a dilute solution towards a more concentrated one.
• The sugar molecules are too big to cross, so only the water moves.
• Osmosis can raise a column of liquid against gravity until the pressure pushing back balances it.

The science in plain words

Osmosis is the movement of water through a partially permeable membrane, one that lets small water molecules pass but blocks larger ones such as sugar. When there is a dilute solution on one side and a concentrated one on the other, more water molecules cross towards the concentrated side than come back. The net flow carries water from where there is more water to where there is less, as if trying to even out the concentrations.

This flow is strong enough to work against gravity. In a classic experiment, a tube of sugar solution sealed at the bottom with a membrane is stood in pure water. Water seeps in and the liquid in the tube rises well above the outside level. It keeps rising until the weight of the raised column pushes back as hard as osmosis pulls water in. That balancing pressure is called osmotic pressure.

Every living cell is surrounded by a membrane, so osmosis matters everywhere in biology. Plant cells take in water by osmosis until they are firm, which keeps leaves and stems upright; when they lose water, the plant wilts. Raisins swell in water, while salt draws water out of cucumber slices. Root hairs absorb water from soil the same way.

Where it fits

• India, NCERT: Class 9 science, Chapter 5: The Fundamental Unit of Life
• United States: Grades 6 to 8
• Next Generation Science Standards: MS-LS1-2

Common questions

What is osmosis in simple words?

Osmosis is water moving through a thin barrier, such as a cell membrane, from a weaker solution into a stronger one. The barrier lets water through but stops bigger dissolved particles like sugar or salt. It helps plants and animals take in and balance water.

Why do raisins swell in water?

A raisin's skin acts as a partly permeable membrane and its inside is full of concentrated sugar. When you soak it in plain water, water moves in by osmosis, so the raisin swells. Put it in strong sugar syrup instead and it hardly changes.

What is the difference between osmosis and diffusion?

Diffusion is any substance spreading from where it is concentrated to where it is less concentrated, such as perfume through a room. Osmosis is a special case: it is the movement of water only, across a partly permeable membrane, from a dilute to a concentrated solution.

Related labs

• Diffusion: A drop of dye in still jelly: how does it spread with nothing stirring it?
• Solutions: How much can you dissolve in a glass of water, and does warming it help?
• Photosynthesis: Plants make their food from light, water and carbon dioxide. Can we see it happening?

See all biology labs or every Taz science lab.

Try it yourself

The Osmosis lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.

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