For IGCSE O-level Biology students Chapter 3

1. MOVEMENT OF MOLECULES
Do they ever stop moving?!!

2. DIFFUSION
 All molecules move about. In gaseous state, they
are widespread and with weak bonds holding them
together. In liquid state, they are closer with
stronger bonds holding them together. In solids they
are very close and could only vibrate in their place.

3. NOW PUT UP YOUR HAND AS SOON AS YOU
CAN DETECT THE SMELL OF THE AIR
FRESHENER.

4. WHAT DO YOU NOTICE?
 Who were the first to detect the smell?
 Who were last to detect the smell?
 Why isn’t there anyone who did not detect the
smell?
 Explain how you could smell the scent although you
are far away from the air freshener.
 What if we opened the door or window? Would
others outside be able to smell the scent?

5. WHAT WOULD HAPPEN TO A LUMP OF SUGAR
PLACED IN A CUP OF WATER?

6. WHAT WOULD MAKE MOLECULES
MOVE/SPREAD OUT FASTER OR SLOWER?
 Temperature
 Stirring/ air currents
 Number of molecules
 Surface area
 Distance/ thickness of membrane
 Difference between molecule concentration in both
areas

7. DIFFUSION
Is the movement of molecules from a
region of high concentration to a
region of low concentration down a
concentration gradient.

8. OSMOSIS
 A special type of diffusion involving the movement
of water molecules only, through a partially
permeable membrane.
 Partially permeable membrane is a membrane
with small holes that can allow only small molecules
to pass through.

10. NOW I NEED FOUR VOLUNTEERS!

11. OSMOSIS
Is the movement of water molecules
from a dilute solution (high water
potential) to a concentrated solution
(low water potential) through a partially
permeable membrane.
 A cell membrane is a partially permeable
membrane.

12. WHAT HAPPENS TO ANIMAL CELLS IF THEY WERE
PLACED IN A SOLUTION OF A DIFFERENT
CONCENTRATION THAN OF THEIR CYTOPLASM?

13. WHAT ABOUT PLANT CELLS?

14. OSMOSIS IN PLANT CELLS.
 Water enters a plant
cell, the water
molecules put pressure
on the walls. The rigid
cell wall will prevent the
cell from bursting. This
plant cell is said to be
turgid.
 This keeps the plant
firm.

15. OSMOSIS IN PLANT CELLS
 When water leaves the
plant cell the elastic
cell membrane shrinks,
while the rigid cell wall
keeps the shape of the
cell fixed. This plant
cell is said to be
plasmolysed.

16.  ACTIVE TRANSPORT
 In some situations, there is a need to move
molecules from a region of low concentration to a
region of high concentration.
 A root hair cell needs to take in minerals from the
soil, even if the concentration in the soil is lower
than that in the cells.
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