Osmosis

Osmosis

Osmosis is the net movement of water molecules down the concentration gradient through a partially permeable membrane.

• Water moves freely through pores in the partially permeable membrane.
• Solute (green) too large to move across the membrane. 

***NET movement

Osmosis can be diffusion but diffusion CANNOT.

Water potential (phy) is a measure of the tendency of water molecules to move from one area to another.

concentration=   amount of solute  

                         volume of solvent

*Most likely in biology, solvent is water.

XXXXXX"water concentration"XXXXXX

***MEMORISE***

Osmosis is also defined as

The net movement of water molecules through a selectively permeable membrane from a region of high water potential to a region of low water potential.

Osmosis in animal cells

If placed in a hypertonic solution (solution has a higher concentration of solutes than the cytoplasm)
Higher solute concentration (lower water potential)
Water leaves the cell by osmosis.
The cell loses volume and shrinks (crenates)
**crenate is only for animal cells***
Water loss only ceases if the concentration of the cytoplasm rises to that of the surrounding solution.
*****OSMOSIS DOES NOT STOP*****

If placed in a hypotonic solution (solution has a lower concentration of solutes than the cytoplasm),   

water enters the cell by osmosis and the cell gains volume and expands.

Since the cell membrane cannot resist expansion, the cell eventually bursts (cytolysis)      

The plant cell 

-plant cells are structurally more complex.

-They are surrounded by a cellulose cell wall which is...

  ->Freely permeable to water 

  ->Not elastic

  ->Able to resist cell expansion
-Each plant cell contain a large central vacuole which...
  ->contains a solution of salt, sugars and ions
  ->is bound by a partially permeable membrane

Osmosis in plant cells

If placed in a hypotonic solution (solution has a lower concentration of solutes than the cytoplasm),
water enters the vacuole by osmosis. Vacuole swells, pushing the cytoplasm against the cell wall.

If placed in a hypertonic solution (solution has a higher concentration of solutes than the cytoplasm(,
water leaves the cytoplasm and vacuole by osmosis. The cytoplasm and vacuole shrinks, pull the cell membrane away from the cell wall.

Plasmolysis versus Crenation

Plasmolysis
- shrinking of plant cell cytoplasm, and the cell membrane moves away from the cell wall.

Before------------------------> After

Crenation

-shrinking of animal cell

Isotonic solution

An isotonic solution has the same concentration of solutes as the cytoplasm.

-> No net movement of water molecules into or out of the cell (animal/plant)

Therefore, cells neither shrinks or expands when placed in an isotonic solution.