osmosis
TRANSCRIPT
OSMOSIS
definition We have learnt that Diffusion is:
The movement of particles/molecules from a region of their higher concentration to a region of their lower concentration down the concentration gradient
Now Osmosis is a special case of diffusion It refers to the movement of water molecules Across a partially permeable membrane
Can you define osmosis using this information? Where would you find a partially permeable
membrane in a cell?
Partially permeable membranes
The cell surface membrane and the membrane around the vacuole in a plant cell are partially permeable
An easy way to understand this is to think of them as having lots of small holes or pores.
Water is a tiny molecule that can pass through these pores but solutes (like sugar) are too big
sugar water
The volume on the left increases as water moves in
Draw this in your notes on page 3
Water Potential
The potential for water to move from one side of a membrane to another depends on the amount of dissolved solute in the water.
Imagine you are pouring a glass of juice:
This drink has more soluteSo it has less water
This drink has less soluteSo it has more water
You would find this drink much strongerWe say it is a more concentrated solution
You would find this a weaker drinkWe say it is a dilute solution
Note that the differences are relative; that means we are comparing these two drinks and deciding which is stronger or weaker
Water potential gradient
Osmosis takes place across a partially permeable membrane
If the solutions on either side have different concentrations then there will be a water potential gradient
Remember a STRONG concentration means LESS water
And a WEAK concentration means MORE water Water has the potential to move from where
there is more to where there is less Or from a WEAKER solution to a STRONGER one
Updated definition of Osmosis
Osmosis is the movement of water molecules from a region of higher water potential to a region of lower water potential, down their water potential gradient, across a partially permeable membrane
Highlight this definition in your notes (p3) and learn it!
Osmosis and living cells
Living cells are surrounded by a partially permeable membrane
The concentration of the solution surrounding a cell will affect the cell
Remember that plant cells are surrounded by a rigid cell wall – so they behave differently
Lets look at animal cells first: If you put a red blood cell
in pure water (weak solution) the cytoplasm is more concentrated
Water would move in to the cells by osmosis
from the higher water potential in the water to the lower water potential in the cell
Across the partially permeable membrane
The volume of the cytoplasm would increase
The cytoplasm will push against the membrane
The membrane will burst
See how swollen these cells are, when the pressure inside is too great they will burst
There are a lot of steps explaining this process; you need to know them all! Use this information to fill in the first part of your table on page 4
If we place red blood cells in a concentrated solution the cytoplasm is less concentrated
Water would move out of the cells by osmosis
from the higher water potential in the cells to the lower water potential in the solution
Across the partially permeable membrane
The volume of the cytoplasm would decrease
The cells would shrivel
Now let’s see what happens to plant cells
If you put a plant cell in pure water (weak solution) the cytoplasm and vacuole are more concentrated
Water would move into the cells by osmosis
from the higher water potential in the water to the lower water potential in the cell
Across the partially permeable membrane
The volume of the
cytoplasm would increase
The cytoplasm will push against the membrane and the cell wall
The cell wall will counteract this pressure so no more water can enter the cell
The cell becomes turgid This gives plants support
Plant cells don’t burst because of the wall
Imagine blowing up a balloon in a box
Eventually you wouldn’t be able to blow any more air in
If we place these plant
cells in a concentrated solution the cytoplasm and vacuole are less concentrated
Water would move out of the cells by osmosis
from the higher water potential in the cells to the lower water potential in the solution
Across the partially permeable membrane
The vacuole and cytoplasm shrink
The cell is now FLACCID It does not help support the
plant and the plant wilts
If water continues to move out the cytoplasm shrinks more and pulls the membrane away from the cell wall
The cell is now PLASMOLYSED Have a look at this if you can
Again you need to know all these steps; so fill in the rest of your table and learn them