Osmosis, Diffusion, Water potential

Water potential 

• must be used in comparison with two environments 
• Refers to the tendency to donate water molecules
• e.g. if liquid A has a lower concentration than liquid B, you can say the liquid A has a lower water potential as compared to liquid B (or vice versa).
• Water molecules always move from a region of higher water potential to a region of lower water potential

Osmosis

e.g [water moving to a hair root cell]

• Net movement of water molecules from a region of high water potential to a region of low water potential across a partially permeable membrane


• The cells will shrink in size. If it lasts for a prolonged period of time the cell will die 
• Animals cells: known as crenated 
• Plasma has a lower potential compared to the cytoplasm of the cell so osmosis occurs and the water will be transferred to the cytoplasm. However, there is a limit to how much the cytoplasm can expand. When that limit is reached, the cell will burst.
• Cell wall (plant cells) prevent the cell from expanding, as such the plant cell does not burst

When you get a question on osmosis, remember to write out the definition of osmosis

Net movement: There may be forces exerted in different directions but the most (strongest/ stronger) force is exerted towards is the net movement.

Diffusion

e.g. [tea bag flavouring to water] 

• The movement of particles within a gas or liquid
• From a region of high concentration to a region of lower concentration (down a concentration gradient)
• Until an equilibrium is reached

The difference between osmosis and diffusion is that osmosis requires a partially permeable membrane and is applicable only to water molecules.

How do you explain this?

1. State that osmosis is occuring 

• Osmosis occurs because the surrounding solution has a lower water potential than the cytoplasm of the cell. Because of this difference in water potential, there is a net movement of water molecules from the cytoplasm of the cell across the plasma membrane to the surrounding solution. 

2. relate the defintion to the movement to the water molecules 

• The loss of molecules from the cytoplasm of the cell will lead to the decrease in volume of the cytoplasm of the cell

How do you explain this?

• Osmosis occurs. 
• Because the surrounding water solution has a higher water potential compared to the cytoplasm of the cell, there is a net movement of water molecules from the surrounding solution across the plasma membrane of the cell into the cytoplasm of the cell. 
• This will then lead to an increase in the volume of the cytoplasm of the cell. 
• If the cytoplasm of the cell continues to increase in volume, the animal cell will eventually burst or lyse. and when that happens, the cell dies. Moving down/ along the concentration gradient

PLANT CELL
If replaced with a plant cell with higher water potential than cytoplasm of cell,

• Because of the difference in water potential, 
• there is a net movement of water molecules across the plasma membrane into the cytoplasm of the cell
• There will be an increase in the volume and size in the cell and the cytoplasm will push against the cellular cell wall.
• The cytoplasm is exerting a pressure, known as turgor pressure, on the cell wall. This cell is now turgid (this term is only applicable to PLANT CELLS). Animal cells can be described as firm.