1. Diffusion and Osmosis
Intermediate 2

2. Diffusion
Is the movement of liquid or a gas molecules from a high concentration to a low concentration until the substance is evenly spread.
Important as it lets food and oxygen into a cell and waste products such as carbon dioxide out of the cell

3. Osmosis is a special type of diffusion involving
WATER

4. Definition of Osmosis Osmosis is the movement of water molecules,
from a region of high water concentration
to a region of low water concentration
across a selectively permeable membrane.
LEARN THIS !!!!

5. Selectively Permeable?
A selectively permeable membrane is a
membrane with holes in it small enough for WATER to go through it easily.
Big molecules like starch can not fit through easily
Remember!!
A membrane controls what goes in and out of the cell.
Permeable means it allows substances to go through it.
See next slide…

6. Model of a Selectively Permeable Membrane
Water
Starch
Small molecules, such as water can pass through it.
Large molecules like starch can not pass through the membrane
Diagram shows the net flow of water

7. Osmosis what is happening
The water molecules pass both ways through the membrane
The overall flow of water molecules is from a region with lots of water molecules to region of less water molecules.
Water is described as moving down a concentration gradient – from a high concentration to a low concentration
Water
Starch
Lots of water molecules
Few water molecules

8. Osmosis what is happening
Water molecules move from the high water concentration to the low water concentration. Until both sides have an evenly balanced water concentration.
Water is described as moving down a concentration gradient – from a high concentration to a low concentration
Water
Starch
Lots of water molecules
Few water molecules

9. Summary
OSMOSIS is the movement of water molecules across a selectively permeable membrane.
The membrane only allows small molecules to pass through it, large molecules can not pass through the membrane.
Water molecules move into the region where there are less water molecules
The water acts to even up the concentration of water either side of the membrane.

10. Osmosis and Plant Cells
Potato
Pure Water
24 hours
later
In pure water the potato tubes swell, because water enters their cells by osmosis.

11. More Osmosis and Plant Cells
Potato
Rich sugar solution
24 hours
later
In rich sugar solution the potato tubes shrink, because water leaves the potato by osmosis.

12. Plant Cell in different solutions
In pure water – water will move in swelling the vacuole which pushes out against the cell wall.
The plant cell is described as TURGID.
The cell does not burst because of the cell wall.
Plant cell placed in
pure water
Water moves
Into the cell

13. Plant Cell in Strong Sugar Solution
In a strong salt or sugar solution – water will move out of the vacuole causing it to shrink.
The cytoplasm can also shrink away from the cell wall.
The plant cell is described as Plasmolysed or Flaccid.
Strong Sugar/Salt
Solution

14. Animal Cells in different solutions
A red blood cell in pure water, water will move into the cell and the cell will BURST (there is no cell wall to prevent this happening)
In a strong salt/sugar solution water will move out and the cell will SHRINK (the cell in described as CRENATED)

15. Some terms when comparing two solutions
Hypotonic – one that has a high water concentration compared to surrounding solution
Hypertonic – one that has a low water concentration compared to surrounding solution
Isotonic – one that has a water concentration equal to the surrounding solution

16. More to do
Click on the link below and go to the website my skoool
and look over the lessons Cell Structure and Function and Plant Cells
Then go to bitesize revision read over the cells topic and answer the questions