Osmosis - 14 and 21 August 2012

Osmosis - What is 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. Some solutes are too large to move across the membrane.

Diffusion and Osmosis... what is the relationship?

Diffusion

• Movement of particles in general 
• Can occur both in the presence and absence of a membrane. 

Osmosis 

• Movement of water molecules only. 
• Water molecules move across a partially permeable membrane.  

 Water Potential ( )

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

Water molecules move from a region of high water potential to a region of low water potential.

Relating Water Potential to Osmosis...

Since water potential is a measure of the tendency of water molecules to move from one area to another, we can also define osmosis as:

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

Osmosis in animal cells

Animal cells

• Structure is simple
• Cytoplasm is surrounded by a partially permeable cell membrane. 

If placed in a hypertonic solution (solution has higher concentration of solutes than the cytoplasm)...

Water will leave the cell by osmosis as the cell has a higher water potential than its surroundings. The cell will lose volume and shrink (crenate). Water loss only ceases if the concentration of the cytoplasm rises to that of the surrounding solution.

Note that 'cenate' can only be used for animal cells!

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

Water enters the cell by osmosis as the surrounding solution has a higher water potential than the cell. Hence, the cell gains volume and expands. Since the cell membrane cannot resist expansion, the cell eventually bursts (cytolysis)

Note that the term 'cytolysis' is only to be used for animal cells! Find out why later! 

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

Osmosis in plant cell 

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

Water enters the vacuole by osmosis. The vacuole swells, pushing the cytoplasm against the cell wall. The inelastic cell wall resists expansion and the cell becomes rigid, also known as turgid. It can be described as in a state of turgor.

Young plants, which have little woody tissue, rely on turgor for support against wind and gravity.

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. pulling the cell membrane away from the cell wall. The cell is now plasmolysed or is in a state of plasmolysis. The tissue becomes flaccid.

A non-woody plant which loses lots of water has many plasmolysed cells, and as a result, the plant wilts. No longer fully filled with water, the tissue loses support and becomes floppy or flaccid.

Plasmolysis vs Crenation       

Plasmolysis is the shrinking of a plant cell cytoplasm (due to loss of water), and the cell membrane moves away from cell wall.

However, crenation is the shrinking of animal cell.

Isotonic solution

An isotonic solution has the same concentration of solutes as the cytoplasm. It also has the same water potential as the cytoplasm.


In both animal and plant cells, there is no net movement of water molecules into or out of the cell.


Therefore, the cells neither shrinks nor expands when placed in an isotonic solution.