Osmosis–diffusion of water
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Bernard Nebel.
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The first of the following videos shows red blood cells placed in tap water gradually swelling up and bursting due to the absorption of water. The second shows red blood cells placed in a 10% salt solution being “crushed” into irregular shapes because of the withdrawal of water
1. https://www.youtube.com/watch?v=OYoaLzobQmk
- https://www.youtube.com/watch?v=A8cI6FkcG4c (watch section from 2 to 3 minutes)
This demonstrates the phenomenon of OSMOSIS, the diffusion of water through a membrane toward the lower concentration of water (higher concentration of salt). You may also demonstrate osmosis using “sausage bags” made from dialysis tubing filled with water or salt solutions as described in the text. Dialysis tubing may be purchased at:
https://www.carolina.com/catalog
Emphasize how the above shows that cell membranes do not and can not control the inflow or outflow of water itself. Have students wrestle with the question: If cells cannot control the inflow or outflow of water, how then do they maintain a proper level of hydration? Can they control their level of salt concentration such that water follows by osmosis? Experiments demonstrate that the answer here is, Yes! This takes us to ACTIVE TRANSPORT.
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