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Background vocabulary

Semi-permeable membranes – allow some molecules to pass through, but not others

Background knowledge

Cell membranes are semi-permeable.  They allow small molecules such as oxygen, water carbon dioxide and glucose to pass through, but they do not allow larger molecules like sucrose, proteins and starch to enter the cell directly.

Larger molecules can only get through the membrane by use of a membrane channel protein.

Osmosis cells


What is Osmosis?

Osmosis is the passage of water from a region of high water concentration, through a semi-permeable membrane, to a region of low water concentration.


Osmosis is the way that some particles (like dissolved salt) flow towards water.

A passive processes. Requires no added energy.


Below we see 2 compartments, separated by a semi-permeable membrane.

More water molecules on one side than on the other.

Water molecules flow from high concentration side to the low conc side

Continues until the conc of water on both sides is equal : dynamic equilibrium

Osmosis 2

Can cause water to move up agains gravity!

Adding sugars to water results in a decrease in the water concentration,

because the sugar molecules displace the water molecules

As water is pulled to one side, it’s level rises

osmosis water flows uphill

osmosis water flows uphill

 Let’s see this in motion

Osmosis GIF by popolopo

Osmosis GIF by popolopo, at mind42.com/mindmap

Osmosis apps

PBS Learning Media Osmosis app

PBS Learning Media Cell membranes: Just passing through

Concentration of dissolved particles is super important in biology

solute conc. inside cell   <   solute conc. outside cell                    Hypertonic

solute conc. inside cell  =    solute conc. outside cell                    Isotonic

solute conc. inside cell    >  solute conc. outside cell                    Hypotonic

Osmosis three cases

Diffusion and Osmosis <- document

Related links

Osmosis Demystified. AlienRyderflex by Darel Rex Finley

Learning Standards

AAAS Benchmarks for Science Literacy

5C/H1a ( Grades: 9-12 ): Every cell is covered by a membrane that controls what can enter and leave the cell.

Next Generation Science Standards

MS-LS1.A.3 ( Middle School Life Sciences ): Within cells, special structures are responsible for particular functions, and the cell membrane forms the boundary that controls what enters and leaves the cell.

HS-LS1.A.1 ( High School Life Sciences ): Systems of specialized cells within organisms help them perform the essential functions of life.

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