Osmosis is the diffusion of water across a selectively permeable membrane down its concentration gradient. Water potential describes the concentration of water molecules, with higher potential meaning more dilute. Tonicity refers to a solution's ability to cause a cell to gain or lose water. Hypotonic solutions cause cells to swell and burst as water enters. Isotonic solutions cause no volume change as concentrations are equal. Hypertonic solutions cause cells to shrink and crenate as water exits. Animal cells are at risk of lysis from osmosis while plant cells have a cell wall and vacuoles to regulate internal pressure.

1. Osmosis & Tonicity
Adv. Bio 2016

2. Osmosis
What is it?
Osmosis is the diffusion of water across a membrane; the
movement water molecules from a region of higher water
potential (lower solute concentration) to a region of lower water
potential (higher solute concentration) down a concentration
gradient through a selectively permeable membrane. Osmosis
occurs in cells all the time.

3. Water Potential
high water potential = higher concentration of
water molecules (more dilute)
i.e. a 5% sucrose solution has a higher water
potential than a 10% sucrose solution.

4. Osmosis
Osmosis. Campbell Biology (2010)

5. Tonicity
Tonicity is a term that describes the ability of a solution to cause a cell to gain or
lose water. It mostly depends on the concentration of a solute or salts on both sides
of the membrane.
- Hypotonic
- Isotonic
- Hypertonic
These solutions can only be looked at in comparison; you cannot call one solution a
hypotonic or hypertonic solution. They always work in comparisons.

6. Hypotonic Solution
- A hypotonic solution is one that has a
high water potential as compared to the
other solution.
- The solute concentration is lower
outside the cell, water molecules move
into the cell, and the cell will expand and
may burst.
- This bursting of an animal cell caused
by over stretching of the cell membrane
is called lysis. https://sites.google.com/site/talbaneses1st2013/home/part-1-physical-chemical-
celluar-basis-of-life/19hypotonicsolution

7. Isotonic Solution
- Isotonic solutions have
the same water potential.
- The concentration of
solute is the same on both
sides of a membrane, and
the cell volume will not
change.
https://sites.google.com/site/talbaneses1st2013/home/part-1-physical-chemical-
celluar-basis-of-life/19hypotonicsolution

8. Hypertonic Solution
- A hypertonic solution is one that has
a lower water potential as compared
to the other solution.
- The solute concentration is higher
outside the cell, water molecules
move out of the cell. The cell will
shrink.
- Cell crenation is when an animal cell
loses water and thinks.
https://sites.google.com/site/talbaneses1st2013/home/part-1-physical-chemical-
celluar-basis-of-life/19hypotonicsolution

9. Animal Cell vs Plant Cell
From: http://biology-forums.com/index.php?action=gallery;sa=view;id=307

10. Animal Cell vs Plant Cell
From http://cursa.ihmc.us/rid=1165370189189_2102640856_6267/Cell%20Membrane.cmap:

11. https://www.youtube.com/watch?v=OYoaLzobQmk
Osmosis in Animal Cells

12. Osmosis in Plant Cells
https://www.youtube.com/watch?v=zVvHn6Sj9PQ