Biology Chapter 3
1.
Diffusion（Without requiring the cell to use energy）
— It is the main process by which small molecules (oxygen) move across the cell membranes.
— If there is a difference in the concentration of particles between two regions (concentration gradient exists),
there will be a net movement of particles from a region of higher concentration to a region of lower
concentration until the particles are evenly distributed.
— When the particles are evenly distributed, the particles will move randomly in all directions.
— Factors affecting the rate of diffusion
1. Difference in concentration between two regions
The diffusion rate increases with the difference in concentration between two regions. /
When the concentration gradient is steeper, the diffusion rate is higher.
2. Temperature
The diffusion rate increases with temperature.
(Particles have more kinetic energy and collide more frequently)
3. Size and nature of particles
When the size of the particles is smaller, the diffusion rate is higher.
Also, lipid-soluble molecules diffuse more rapidly than water-soluble molecules.
— Importance of diffusion
1. transport of oxygen and small, lipid-soluble nutrients into the cell;
2. transport of carbon dioxide and other waste out of the cell;
3. transport of simple ions across the cell membrane;
4. distribution of substances throughout the cytoplasm.
2.
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—
—
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Osmosis (Without requiring the cell to use energy)
The diffusion of water molecules across a differentially permeable membrane.
The water potential (Ψ) of pure water is defined as zero. (Highest value)
The water potential (Ψ) of solutions is always lower than that of pure water. (A negative value)
Osmosis is the net movement of water particles across a differentially permeable membrane from a region
of higher water potential to a region of lower water potential.
— Importance of osmosis
1. Osmosis is important for the entry and exit of water into and out of the cells in all organisms.
2. It is especially important for the movement of water from one cell to another in plants.
3. Osmosis is also important for transporting water through living tissues.
Names of
solution
Hypotonic solution
(a solution with
W.P. higher than
Hypertonic solution
(a solution with W.P. lower
than the cytoplasm)
the cytoplasm)
1.
2.
3.
4.
5.
6.
—
—
1.
2.
3.
4.
5.
—
3.
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Isotonic solution
(a solution with
the same W.P. as
the cytoplasm)
Water
movement
Enter the cell
Leave the cell
No net
movement
Animal cells
Swell and finally
burst
Shrink and become wrinkled
No change
Plant cells
Become turgid
(they have cell
walls)
The vacuole will shrink and
the cell membrane detaches
away from the cell wall.
(plasmolysis)
The cell becomes flaccid.
No change
Demonstration of osmosis using dialysis tubing
Wet a dialysis tubing (differentially permeable, acts as the cell membrane) with tap water.
Tie a knot at one end of the tubing. Then fill the tubing with sucrose solution using a dropper.
Tie the other end of the tubing to a capillary tube with a piece of thread. Rinse the outside of the tubing
with distilled water.
Immerse the tubing in a beaker of distilled water. Mark the initial level.
Set up a control by filling another tubing with distilled water.
Note any changes in the liquid levels of the two set-ups after sometimes.
Result:
The liquid level in the experimental set-up rises.
The liquid level in the control set-up falls until it reaches the liquid of the water in the beaker.
Conclusion:
There is a net movement of water molecules from water molecules from distilled water to the sucrose
solution. / Osmosis occurs in this set-up.
Study of osmosis in living plant cells
Cut a small piece from the lower epidermis of the plant leaf and lay it flat on a slide.
Add a drop of concentrated sucrose solution and put a cover slip over the lower epidermis.
Observe the lower epidermis under a microscope after a while.
Slowly replace the concentrated sucrose solution with distilled water.
Observe any changes in the cells with a microscope.
Study of osmosis in living plant tissue
Results:
The potato strips become heavier / lighter. / The potato strips have no change. This shows that water is
hypotonic / isotonic / hypertonic to the potato tissue. (There is a net movement of water into / out of the
cell by osmosis.)
★The concentration of red blood cells is 0.9%.
★Haemolysis occurs if the red blood cells burst and the haemoglobin release.
Active transport
Substances are moved across the cell membrane in the expense of energy form respiration.
The energy is used to change the shape of the carrier proteins in the cell membrane.
— Substances are usually moved from a region of lower concentration to a region of higher concentration.
(against a concentration gradient)
— It only occurs in living cells that release energy by respiration.
— Any condition that prevents respiration (the presence of cyanide) or the lack of oxygen will slow down or
stop active transport.
— Respiration→Food oxidation→Energy→Active transport
— Importance of active transport
1. It is important for the movement of substances across cell membranes against a concentration
gradient. (the absorption of minerals from the soil into the roots of plant)
2. It is also important for the transport of substances along a concentration gradient at a higher speed
than that is allowed by diffusion. (the absorption of glucose in the small intestine of humans)
4. Phagocytosis
※ The concentration of red blood cells is 0.9%.
Haemolysis occurs if the red blood cells burst and the haemoglobin release.