Movement of substances in and
out of the cell.
• Substances move in and out of the cell
across the cell membrane. This movement
can take place by:
• 1. diffusion
• 2. osmosis
• 3. active transport.
Cell membrane vs. cell wall
Cell membranes are selectively permeable
barriers. This means that some molecules are
able to pass through while others cannot. On the
other hand cell walls are non living, fully
permeable barriers that allow all substances to
pass through.
Diffusion:
Is the movement of
particles from an
area of high
concentration to an
area of low
concentration.
There is no energy
input into the
system .
Osmosis:
is simply a special
type of diffusion, it is
the net movement of
water from a high
water potential to a
low water potential
solution, through a
partially permeable
membrane.
In both ,diffusion and osmosis,
substances move down a concentration
gradient. i.e. from high to low
concentration.
Osmosis in plant cells
• Osmosis plays an important role in plants, especially
non- woody ones.
a) A plant cell is immersed in distilled water, the
following diagrams show what happens.
The following changes take place:
1. Water moves into the cell through the cell membrane
2. Cytoplasm & cell vacuole increase in volume.
3.Increase in cell volume increases internal cell pressure, which
forces the cytoplasm to push against the cell membrane.
4.Cell membrane pushes against the cell wall.
Cell becomes TURGID
The cell becomes firm, we say it is
turgid. As a result, The plant’s stem is
upright and stiff.
Why is turgidity important for:
• a plant, in general?
1. Holds plant stems upright
2. Important in the functioning of the stomata
b)Plant cell is placed in a concentrated solution.
Gaps between
cell wall & cell
membrane fill
with solution
•Water moves out of cell via osmosis
•Cell membrane shrinks away from cell wall
The following changes take place:
• 1 Excess water moves out of the cell
• 2 Excess loss of water shrinks cell vacuole &
cytoplasm, which radically reduces internal cell
volume
• 3 cell membrane pulls & cytoplasm away from cell
wall
The cell become plasmolysed because
excess water has been lost through the
cell membrane.
C) Plant cell is placed in a low concentrated solution.
The following changes take place:
1. Water moves out through cell membrane
2. Loss of water from inside the cell reduces the internal volume
of the cell.
3. Cytoplasm no longer pushes against cell wall.
4. Reduced internal volume of cell leads to a reduced internal
pressure. Cell becomes FLACCID.
The cell has become flaccid. Leaves
droop and collapse.
Osmosis in animal cells
• a) in distilled water, an animal cell bursts:
Why is this different from plant cells?
• b) In a concentrated sugar or salt solution,
the animal cell shrinks and shrivels.
Osmosis in red blood cells.
a) Red blood
cells in distilled
water.
b) Red blood cells
in equilibrium.
c) Red blood cells
in a concentrated
solution.
The Visking tubing experiment.
Set up the following apparatus:
1. Fill the visking tubing half
way with colored sugar
solution.
2. Place the capillary tube
inside the visking tubing and
tie with rubber band.
3. Mark the level of liquid in
capillary tube.
4. Place the whole set in a
boiling tube filled with
distilled water.
5. Leave for 20 minutes. Record
observations.
Active transport
• Is the movement of particles from low
concentration to high concentration across a
partially permeable membrane, against the
concentration gradient.
• This movement requires energy input from
respiration.
Examples of active transport
1. in animals: during digestion, the
concentration of food molecules inside the
small intestine increases as a result of
absorption. So simple sugars, amino acids,
minerals and vitamins are actively absorbed
into the villi, from an area of lower to an area
of higher concentration.
2. In plants: Plants need mineral salts (e.g.
nitrates) for making proteins and growth.
Minerals are at a higher concentration
inside the root cells than in the soil
particles. So energy is used by the root
cells to actively transport minerals across
the cell membrane into the root cells,
against the concentration gradient.
Where will particles diffuse faster?
A or B?
• 1.
Explanation:
A or B?
• 2.
Explanation:
A or B?
Pure water
cells
Explanation:
Find the area, the volume and the
area/volume ratio for A and B.
A
Side = 1 cm
B
side = 2cm.
Area:
Area:
Volume:
Area/volume:
Volume:
Area/volume:
What factors affect the rate of
diffusion?
• 1. Temperature:
The higher the temperature, the larger the
kinetic energy and the faster the diffusion
rate.
• 2. concentration gradient: the steeper the
gradient, the faster the diffusion rate.
• 3. the size of particles: the larger the
area/volume ratio, the faster the diffusion
rate.