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