1. Diffusion, Osmosis, Active Transport
Biology Notes
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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

3. Diffusion Key potassium iodide ions copper sulphate ions
permeable membrane
20 potassium iodide ions
copper sulphate ions
10 potassium iodide ions, copper sulphate ions
copper sulphate ions, potassium iodide ions
water

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 water molecules diffuse from B to A.
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
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

8. Hypotonic, Isotonic, Hypertonic (animal cells only)
Lower water potential
Equal water potential
Higher water potential

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.
Eg.

21. Question
Four clear agar blocks were placed into solutions of methylene blue as shown. Which agar block would be first to become completely stained? C D A B
6mm
4mm
4mm
2mm
4mm
2mm
2mm
2mm
4mm
2mm
2mm
Ranee Mohan
2mm

22. Volume of the liquid in tube X Concentration of the sugar solution
Question
An experiment to investigate osmosis is set up as shown. What happens?
Tube X
Volume of the liquid in tube X
Concentration of the sugar solution A
Decreases B
Increases C D
Partially permeable membrane
Ranee Mohan

23. Question
A plasmolysed cell is shown. Which numbered structures are partially-permeable?
1 only
2 only
1 and 2 only
1 and 3 only 1 3 2

24. Question Stage 1 Stage 2 A Boiled in water for 10 min
Four identical pieces of potato are treated in two stages, as shown. Which piece of potato will be the largest after two hours?
Stage 1
Stage 2 A
Boiled in water for 10 min
Placed in 10% salt solution B
Placed in distilled water C
Unboiled D
Ranee Mohan

25. Question
Which one of the following enters plant cells by active transport?
Carbon dioxide
Nitrate ions
Water
Oxygen