Osmosis Notes

What Exactly Is Osmosis?

1. Osmosis is the diffusion of water, it follows the same principles, but only water molecules move.
2. So similarly, water molecules move from dilute to concentrated solutions.
3. Or in other words, from areas of low to high solute concentrations.
   1. Dilute solution = lots of water molecules, few solute particles (high water concentration).
   2. Concentrated solution = fewer water molecules, lots of solute particles (low water concentration).

Why Does Water Move but Not Solutes?

1. The membrane controls what passes through:
   1. Water molecules are tiny (about 0.3 nanometers), easily fitting through membrane pores.
   2. Solute particles are much larger (glucose is 1 nanometer, proteins are even bigger), making them too big to fit through the pores.
2. The membrane prevents solute from moving:
   1. Solute particles would naturally diffuse to spread out evenly.
   2. But the membrane blocks them.
   3. Only water can move to balance things out.

Note

• The membrane doesn't actively block anything, it's passive.
• Large molecules physically cannot fit through the tiny pores in the membrane structure.
• It's like how a sieve works

What Happens to Cells in Salty or Sugary Environments?

The effect of osmosis on cells depends on the surrounding solution:

Cells in hypertonic solutions (concentrated)

1. Animal cells:
   1. Cell shrinks and shrivels (crenation).
   2. Cell membrane wrinkles.
   3. Cell may become damaged or die.
2. Plant cells:
   1. Cell membrane pulls away from cell wall (plasmolysis).
   2. Vacuole shrinks dramatically.
   3. Cell becomes limp.
   4. Plant wilts.

Cells in isotonic solutions (same concentration)

1. Animal cells:
   1. Cell remains normal size.
   2. This is the ideal condition for animal cells.
2. Plant cells:
   1. Cell is slightly deflated (flaccid).
   2. Not fully turgid but not wilted.
   3. Still healthy but not at maximum firmness.

Cells in hypotonic solutions (dilute)

1. Animal cells:
   1. Cell swells up as water enters.
   2. Cell membrane stretches.
   3. If too much water enters, the cell bursts (lysis).
   4. Animal cells have no cell wall for protection.
2. Plant cells:
   1. Cell swells up as water enters.
   2. Vacuole expands and pushes outward.
   3. Cell wall prevents bursting - it's rigid.
   4. Cell becomes turgid (firm and swollen).

Why Do Plants Go Limp When Dehydrated?

1. Plant cells rely on water pressure for support:
   1. When full of water, the vacuole pushes against the cell wall.
   2. This creates turgor pressure, which is what keeps cells firm and rigid.
   3. Millions of turgid cells keep the whole plant upright.
2. Dehydration reduces turgor pressure:
   1. When water is scarce, soil becomes hypertonic.
   2. As water moves out of root cells by osmosis, this causes vacuoles to shrink, reducing pressure on cell walls.
3. The plant wilts:
   1. Without turgor pressure, cells can't maintain their shape.
   2. Leaves therefore start to droop downward, stems will bend or collapse, and the whole plant goes limp.
4. Severe dehydration causes plasmolysis:
   1. As the cell membrane pulls completely away from cell wall, a large gap forms between the membrane and wall.