2. Diffusion
●Diffusion is the net movement of ions or molecules from a
region of higher concentration to a region of lower
concentration, down a concentration gradient
●In liquids and gases, molecules possess kinetic energy and
constantly move about
●As this movement is random, an equilibrium is reached
when the molecules are evenly spread out, where there is
no more net change – dynamic equilibrium reached
●The steeper the gradient for a substance, the faster the rate
of diffusion
●No energy input is required for diffusion because the
particles already are in motion.
○Hence, it is know as passive transport
4. Factors affecting diffusion
●What are some factors affecting the rate of
diffusion?
○Concentration gradient
○Temperature
○Pressure
○size of particles
○surface area to volume ratio
5. Osmosis
●Osmosis is the movement of water molecules from a
solution of higher water potential to a solution of lower
water potential through a partially permeable membrane
●Water potential is a measure of the tendency of water to
move from one place to another
○A water potential gradient is established when a
partially-permeable membrane separates two
solutions of different water potentials.
●A dilute solution contains more water molecules per unit
volume than a concentrated solution so it has a higher water
potential than a concentrated solution
6. Osmosis
●Osmotic pressure is the pressure that must be
applied to a solution to prevent the inward flow of water
across a partially permeable membrane .
●Osmotic pressure is the opposite of water potential.
●A partially permeable membrane is a membrane that
will allow certain molecules or ions to pass through it
by diffusion.
●The cell membrane is partially permeable.
●What other structures or organs are partially
permeable?
7. Osmosis
5%
sucrose
solution
(more
water,
less
sucrose
)
8 sucrose
molecules
, 9 water
molecules
4 sucrose
molecules
, 18 water
molecules
partially
permeable
membrane
rise in
level
drop in
level
A B A A
B B
Key
sucrose molecule
water molecule
Net movement of water
molecules
water
molecules
diffuse
from B to
A.
8 sucrose
molecules
, 18 water
molecules
4 sucrose
molecules
, 9 water
molecules
9. Plant cell in high water
potential solution
●As water enters the cell, the vacuole increases in size
and pushes the cell contents against the cellulose cell
wall. The cell wall prevents over expansion of the cell
by exerting an opposing pressure preventing the entry
of more water
●A plant in this state becomes turgid
●The turgidity of the cell with water is called turgor
pressure
●The plant cell does not burst because the cell wall is
strong and relatively inelastic
10. Animal cell in high water
potential solution
●Animal cell will swell and may bust in a hypotonic
solution because it does not have a cell wall
in a solution with high
water potential
11. Plant cell in low water
potential solution
●The water potential of the cell sap is higher than that of the
solution outside the cell. Osmosis takes place and water
moves out of the cell
●As water is lost from the cell, the vacuole decreases in size
and the cytoplasm shrinks away from the cellulose cell wall.
●This shrinkage of the cytoplasm away from the cell wall
when plant cells are immersed in a solution of low water
potential is known as plasmolysis
●The cells are said to be plasmolysed. The cells can be
restored by placing them in water or solution of high water
potential
12. Plant cell in low water
potential solution
water leaves by osmosis
Cytoplasm shrinks away from cell
wall
in a solution with low
water potential
As the cell loses water, the
vacuole decreases in size.
The cell becomes plasmolysed
13. Animal cell in low water
potential solution
●Animal cells in hypertonic solution: it will lose water and
the cell membrane will form little spikes as water is lost
and the cell shrinks, a process called crenation. The
cell dies eventually
in a solution with low
water potential
Spikes form
14. Turgor in plants
●Turgor is very important in maintaining the shape of soft
tissues in plants
●Young stems and most leaves, esp. those of herbaceous or
non-woody plants, can remain firm and erect because of the
turgor pressure within their cells.
●When there is a high rate of evaporation of water from the
cells, they lose their turgidity and the plant wilts
●The movement of certain plants are due to changes in
turgor
○Eg opening and closing of the stomata, folding of leaflets of the
Mimosa plant
15. Active Transport
●Active transport is the process in which energy is used
to move the particles of a substance against a
concentration gradient from a region of lower
concentration to a region of higher concentration
●Occurs in living cells because only living cells respire
16. Active transport
●The energy used in cells is called “ATP”.
●Active transport requires:
○Protein transporters
○ATP (or energy)
●Active transport occurs in:
○Absorption of mineral salts by root hair cells.
○Absorption of glucose and amino acids by cells in the
small intestine.
○Na-K pump
17. Example of Active
Transport
●Absorption of mineral salts by root hair cells
●The soil solution has to have a higher water potential than the root
hair cell in order for water to move into the root hair cell by
osmosis.
●This means that the soil solution has a lower concentration of ions
(ie. mineral salts) than the root hair cell.
●Therefore, active transport is used to transport ions into the root
hair cell.
○Specific transporter proteins
○Energy expended.
18. Transport of large
molecules
●Some substances are too large to cross the
plasma membrane by diffusion, facilitated
diffusion and osmosis
●Other means of transport are necessary:
○Endocytosis
○Exocytosis
19. Surface area: Volume
ratio
The greater the
area of cell surface
membrane, the
faster will be the
rate of diffusion of
a substance for a
given
concentration
gradient
20. Surface area: Volume
ratio
●The greater the surface area to volume ratio, the faster
the rate of diffusion.
●This explains why cells are so small: nutrients, oxygen
and waste substances have to diffuse into and out of
cells quickly.
21. Question
Ranee Mohan
●Four clear agar blocks were placed into solutions of
methylene blue as shown. Which agar block would be
first to become completely stained?
2mm
2mm
2mm
4mm
2mm
2mm
6mm
2mm
2mm
4mm
4mm
4mm
A B C D
22. Question
Ranee Mohan
Volume of
the liquid in
tube X
Concentration of
the sugar
solution
A Decreases Decreases
B Decreases Increases
C Increases Decreases
D Increases Increases
Partially
permeable
membrane
Tube X
●An experiment to investigate
osmosis is set up as shown.
What happens?
23. Question
●A plasmolysed cell is shown. Which numbered structures
are partially-permeable?
1.1 only
2.2 only
3.1 and 2 only
4.1 and 3 only
1
3
2
24. Question
Ranee Mohan
Stage 1 Stage 2
A Boiled in water for
10 min
Placed in 10% salt solution
B Boiled in water for
10 min
Placed in distilled water
C Unboiled Placed in 10% salt solution
D Unboiled Placed in distilled water
Four identical pieces of potato are treated in two
stages, as shown. Which piece of potato will be the
largest after two hours?
25. Question
●Which one of the following enters plant cells by
active transport?
1.Carbon dioxide
2.Nitrate ions
3.Water
4.Oxygen
