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Chapter 3
Membrane structure: fluid mosaic model
Movement of substances across membrane: Diffusion, osmosis and active transport
Occurrence of phagocytosis in cells
1.
What is fluid mosaic model?
There are two layers of phospholipids. The water-loving head contains a phosphate group. It faces to the outside aqueous
environment. The water-hating tails contain fatty acids. The tails face each other in the centre. There are proteins on the
surface of the phospholipid bilayer. Some are embedded into the layer. Some penetrate through the layer. There are
carbohydrate molecule attaching to the protein to form glycoprotein and some carbohydrate attaching to the lipid to form
glycolipid.
2.
If the phospholipid molecules are extracted and place on the water surface, the surface area of the cell and that of the area
covered by phospholipid are compared. Which one has more? Why?
The surface covered on the water surface is double of the cell membrane. The phospholipid molecules form 2 layers in the
cell membrane. They form a single layer when they are spread on the water surface.
3.
What are the uses of protein in a membrane?
Some of them are carrier proteins for active transport. Some are enzymes for speeding up chemical reactions. Some are
channel proteins for transport of materials into or out of the cell. Some proteins have carbohydrate on them. They can act as
receptor protein to receive chemical such as hormone. Some act as recognition protein to recognize foreign antigen.
4.
What are the uses of carbohydrate in a membrane?
Some proteins have carbohydrate on them. They can act as receptor protein to receive chemical such as hormone. Some act
as recognition protein to recognize foreign antigen.
5.
Explain why a membrane is fluid in nature and mosaic in nature.
The phospholipid bilayer and the proteins are free to move laterally. This fluidity helps the cell membrane to seal up holes
and carry out phagocytosis.
The distribution of protein is not in an even pattern. They are found here and there randomly, forming a mosaic pattern.
6.
What materials can pass through the cell membrane?
Non-polar substances such as lipids can pass through the phospholipid bilayer since fatty acids are non-polar. Polar
substances such as water and ions can pass through the pore of the channel protein.
7.
What is diffusion?
It is the net movement of a molecule/ion from a region of higher concentration to a region of lower concentration down the
concentration gradient.
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8.
What is osmosis?
It is the net movement of water molecule from a region of higher water potential to a region of lower water potential through
a selectively permeable membrane.
9.
When a concentrated salt solution is added to fresh blood, what happens? Why?
Water moves out from the red blood cells to the salt solution by osmosis. The water potential in the salt solution is lower than
that in red blood cells. There is a net movement of water from higher water potential to lower water potential. Water moves
out of the cells. The cell shrinks and crenation occurs.
10. When a diluted salt solution is added to fresh blood, what happens? Why?
Water moves into the red blood cells from salt solution by osmosis. The water potential in the salt solution is higher than that
in red blood cells. There is a net movement of water from higher water potential to lower water potential. Water moves into
the blood cells, swelling them and burst them. No intact cells are present. The red haemoglobin leaks out and the blood
becomes a clear red mixture. This is called haemolysis.
11. When an isotonic solution is added to fresh blood, what happens? Why?
No net movement of water into or out of the red blood cells. The water potential of the isotonic solution is the same as the
water potential of the red blood cells. There is no osmosis.
12. What are the factors affecting the diffusion of substances into a cell?
Higher temperature increases the kinetic energy of substance, increasing the rate of diffusion. The larger the surface area
makes the rate of diffusion faster (to make something smaller in size can increase the surface area). The greater concentration
gradient increases the rate of diffusion. The smaller the size of particles, the faster the rate.
13. Will osmosis occur in dead cells? Why?
No. Osmosis only occurs through living cells because a selectively permeable membrane is required in osmosis. In dead cells,
the cell membrane is fully permeable.
14. What is meant by hypertonic and hypotonic?
Hypertonic solution is that the water potential is lower than the cells. Hypotonic solution is higher than the cells.
15. What is the difference between active transport and diffusion?
Active transport
Diffusion
It requires energy from the cells
It does not require energy from the cells
It requires carrier protein in cell membrane
It does not require carrier protein
The cell must be living.
It can take place through dead cells
It can take place against concentration gradient
It takes place down the concentration gradient
(When the concentration outside a cell is higher than that in a cell, both diffusion and active transport can take place at the
same time.)
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16. When a plant cell is placed in a hypertonic solution, what happens? Why?
Water moves out from the plant cells to the hypertonic solution by osmosis. The water potential in the hypertonic solution is
lower than that in plant cells. There is a net movement of water from higher water potential to lower water potential. Water
moves out of the cells. The cell shrinks and plasmolysis occurs. The vacuole shrinks too. The cell membrane and cell wall
separate. The cytoplasm shrinks away from cell wall. The cell is flaccid and soft.
17. When a plant cell is placed in a hypotonic solution, what happens? Why?
Water moves into the plant cells from hypotonic solution by osmosis. The water potential in the hypotonic solution is higher
than that in plant cells. There is a net movement of water from higher water potential to lower water potential. Water moves
into the plant cells, swelling them a little bit. However the cells will not burst because of the rigid cell wall. The cell becomes
turgid and becomes hard.
18. When 10% sucrose solution is placed inside a potato cup and it is placed in distilled water, what happens? Why?
Water moves into the potato cup and into the 10% sucrose solution. The water level in the 10% sucrose solution rises. The
whole potato cup acts as a selectively permeable membrane. Water potential in the 10% sucrose solution is lower than that in
the distilled water. Water moves from distilled water towards the 10% sucrose solution through selectively permeable
membranes in potato cells down a water potential gradient.
19. For question 10, if the sucrose solution is replaced with 5% sucrose solution, what is the change in the result? Why?
The rate of osmosis is slower. The water level in potato cup rises slower and reaches to a lower level than 10%. It is because
the difference in water potential between the distilled water and the 5% sucrose solution is smaller. The water potential
gradient is less steep.
20. For question 10, if the potato cup is boiled, what happens? Why?
The cell membrane is no longer selectively permeable. The membrane is fully permeable. Sucrose diffuses out from 10%
sucrose solution to the distilled water. There is no water potential gradient.
21. Which of the following can osmosis take place? Potato skin/dialysis tubing/dead cell/living tissue/cell wall/non-living thing
Dialysis tubing, living tissue, non-living thing with selectively permeable membrane
(Dialysis tubing is a non-living thing but it can allow osmosis to take place.)
22. Region A (2% sucrose solution) and B (10% sucrose solution) is separated by a selectively permeable membrane. What
happens to the water movement? Why?
There is a net movement of water molecule from 2% sucrose solution to 10% sucrose solution because the water potential in
10% sucrose solution is higher.
23. Sodium chloride solution is placed inside a dialysis tubing. The tubing is placed in a beaker of water. Sodium chloride comes
out. What process is involved? Osmosis/diffusion/active transport?
Diffusion only
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24. What substances can pass through the selectively permeable membrane? Glucose/sucrose/salt/protein/starch/enzyme
25. When a dialysis tubing with 5% sucrose is placed in water, what happens to the liquid level in the tubing? Why?
Liquid level rises rapidly at first. The water potential gradient is steep. When water enters the tubing, the water potential
gradient is smaller. The rate of osmosis is slower. The rise of liquid level creates a hydrostatic pressure in the tubing. The
pressure built up prevents the further entry of water. The liquid level stops rising eventually. At this moment, the water
potential is equal to that outside. No osmosis takes place.
26. For question 19, if sucrose is replaced with glucose, what happens to the results? Why?
Liquid level rises first and then drops back. At first, water potential inside is lower than inside. Water moves into the dialysis
tubing. Glucose is smaller in size. It diffuses out. The water potentials of two regions are the same. No osmosis takes place.
27. Give examples of active transport in animal and plant body.
Uptake of nutrients (glucose and amino acids) by epithelial cells in small intestine; reabsorption of glucose and amino acids
by kidney tubule cells in kidneys; absorption of minerals by root hair cells.
28. If potato cylinders are placed in distilled water for 1 hour, what are the changes of the cylinders? Why?
The cylinders become longer, heavier and harder. Water enters from outside to cells because of osmosis. The cells become
turgid. The cells increase in size a little bit. The cells increase in weight due to the gain in water. The cells become harder due
to the influx of water makes the cells to be turgid.
29. In doing experiments on osmosis, we prepare several potato cylinders instead of one potato cylinder. Why?
To get an average value of change is more accurate. It is because there may be individual difference between potato cells.
30. In doing experiments on osmosis, we use longer potato cylinders instead of short one. Why?
The longer the cylinders, the more the cells there are. The cumulative changes will be more obvious.
31. How does oxygen enter the blood through air sacs in lungs?
Oxygen dissolves in water film on air sacs. It diffuses into the cells. No active transport is involved.
32. What is phagocytosis?
When a cell carries out phagocytosis, cell membrane folds and engulfs the particles. The cell membrane then fuses to form a
vacuole. The particle is inside the vacuole. Phagocytosis requires energy from cell.
33. Describe how a phagocyte engulfs a germ.
It carries out phagocytosis. The germ is taken into the cell. Enzyme is discharged into the vacuole to digest the germ.
34. Give examples of phagocytosis.
Amoeba, phagocytes carry out phagocytosis to engulf particles and digest them inside the cells.
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