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Maris Stella High School Name: ___________________ GCE O Level Science Biology Class (Index No.): _____ ( ) 2011 GEP Lesson 2 Date: ________________

Diffusion & Osmosis SUMMARY

Diffusion – The random movement of molecules of one substance from an area of HIGH molecule concentration to an area of LOW molecule concentration.

When more than one solute is present, the molecules will diffuse INDEPENDENTLY of each other.

Before After

Osmosis – The movement of water from a dilute solution ( area of high water potential) to a more concentrated solution (area of low water potential) across a selectively/partially permeable membrane.  

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Molecules in a liquid, aqueous solution or a gas would have kinetic energy and would be moving in random directions. 2. If there are more of a specific type of molecule in one region and less in another region, you will find that the distribution of these molecules will even out after some time. 3. This NET (overall) movement of molecules from the more concentrated region to the less concentrated region is known as DIFFUSION. 4. The stage where the molecules are evenly distributed is known as DYNAMIC EQUILIBRIUM. (DYNAMIC: Molecules are still moving in random directions, EQUILIBRIUM: Molecules are evenly distributed) 5. This movement occurs spontaneously and does not require energy: PASSIVE TRANSPORT. 6. A similar kind of movement occurs when water molecules are involved. When this occurs across a partially permeable membrane, we call this process OSMOSIS. 7. Both osmosis and diffusion occur DOWN a water potential/concentration GRADIENT. 8. Water always enters and leaves the cells and bloodstream through osmosis. 9. For other solutes, eg. glucose, cells usually cannot just rely on diffusion to take in glucose (as it only enables cells to take up glucose until equilibrium is reached). Hence usually cells will sometimes pump in solutes using energy: ACTIVE TRANSPORT. (Not in your syllabus)

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10. Main factors affecting diffusion/osmosis: • Temperature: Increasing temp makes the molecules gain KE. More KE,

faster movement, faster rate of diffusion. • Surface area of membrane across which transport of molecules

occurs: Increasing surface area means more molecules can cross the membrane at any one time, this increases rate of diffusion.

• Gradient: The steeper the gradient (ie, the greater the difference in conc between the 2 regions), the faster the rate of diffusion.

Revision Questions

1. Which two of the following can enter the root hairs of a plant from the soil?

(i) proteins, (ii) lipids, (iii) starch, (iv) oxygen,

(v) humus, (vi) water.

A. (iii), (v) B. (iv), (vi) C (iii), (iv) D (i), (ii)

2a) Do you agree with the following statement? Explain.

Osmosis takes place from a 5% sugar solution to a 10% sugar solution, that is

from a lower concentration to a higher concentration, so that both the

concentrations become equal. Thus osmosis is the reverse of the diffusion

process. [4]

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2b) Describe the development of turgidity in a plant cell immersed in water. [8]

(Write down the main points in this exercise will do)

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3) Leaves of a water plant are placed in solution P for 15 minutes. They are then observed under the microscope. Fig. 3.1 shows the appearance of the cells.

Fig 3.1

(a) (i) What is the state of the cells? [1]

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(ii) Describe what happens when the cells are placed in solution P. [2]

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(iii) If a red blood cell is placed in solution P, what will be the state of

the red blood cell? [1]

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cell wall cell membrane

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4) The following is a graph that includes two lines: Line A shows the relationship between the iodine concentration inside the algal cells and the oxygen concentration in the water. Line B shows the iodine concentration in the water and it is included for reference.

a. Describe the trend observed in Line A. [1]

b. State and explain the process that has occurred to bring about the observation stated in (a). [2]

Oxygen Concentration

Iodine Concentration

(Relative value)

Line A

(Iodine concentration inside algal cells)

Line B

(Iodine concentration in the water)

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c. In another experiment, a potato strip was soaked completely in a 1% iodine solution for 10 minutes. Predict and explain the most visible changes that are observed at the end of experiment. [4]

5. Beetroot cells contain a purple pigment in their cytoplasm. In an experiment, 8 strips of similar size were cut from a beetroot and thoroughly washed with water until the water was clear. Two strips each were weighed and then put into 4 different test tubes A to D, which each contains 5 cm3 of a different liquid as shown in Table 2.1 below. The set ups were left for 30 minutes.

Test tube Liquid inside tube

Temperature of liquid

A 30% sucrose solution Room temperature

B Distilled water Room temperature

C Ether (colourless organic solvent) Room temperature

D Distilled water 900C

Table 5.1

(a) State and explain the change in mass of the beetroot strips in test tube A at the end of the experiment. [3]

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(b) State and explain any visible changes that may have occurred to the strips in test tube B. [3]

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(c) (i) Account for the liquid in test tube C becoming purple at the end of the experiment. [2]

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(ii) Suggest, with an explanation, whether the liquids in test tubes B and D would also, likewise, turn purple like that in test tube C. [4]

Test tube

B:________________________________________________________

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Test tube

D:________________________________________________________

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6) Potato cylinders were cut into discs of even thickness. 10 pieces were weighed and then put into a petri dish filled with 0.1 mol dm-3 sucrose solution. After half an hour, the discs were removed and blotted dry and their final mass were measured. This experiment was repeated by immersing equal number of potato discs at different concentrations of sucrose solution. The results were recorded in the following table:

Concentration of sucrose solution (mol dm-3)

Initial mass (g)

Percentage change in mass (%)

0.1 1.58 +13

0.2 1.63 +5

0.3 1.49 -4

0.4 1.52 -12

0.5 1.66 -20

0.6 1.55 -28

(a) Use the data in the first column and in the third column to plot a graph [3]

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(b) (i) What is the final mass of the potato discs immersed at the sucrose solution of 0.6 mol dm-3? Show your calculation. [2]

(ii) Explain the results obtained. [2]

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(iii) Draw a labeled diagram to show the appearance of a potato cell that had been immersed in the above sucrose solution. [3]