Topic 6 — Osmosis, Diffusion and Transport Investigations

1. Diffusion

Definition

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.

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2. Key Features of Diffusion

• passive process

• no energy required

• occurs in gases and liquids

• particles move randomly

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3. Common ATP Diffusion Experiments

Examples

• food colouring in water

• potassium manganate(VII)

• agar jelly cubes

• gas exchange demonstrations

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4. Factors Affecting Diffusion

Higher Temperature

particles move faster

diffusion rate increases

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Larger Surface Area

more space for movement

faster diffusion

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Steeper Concentration Gradient

greater difference in concentration

faster diffusion

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Shorter Distance

faster diffusion

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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5. Osmosis

Definition

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

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6. Important Osmosis Terms

Dilute Solution

• high water potential

• more water

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Concentrated Solution

• low water potential

• less water

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7. Partially Permeable Membrane

Allows:

• water molecules through

Does NOT allow:

• larger solute molecules through

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8. Common ATP Osmosis Experiments

Potato Cylinders

Most common ATP osmosis experiment.

Measured using:

• mass

• length

• firmness

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Dialysis Tubing

Acts as partially permeable membrane.

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9. Potato Osmosis Experiment

Typical Setup

Potato pieces placed in:

• distilled water

• salt solutions

• sugar solutions

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10. Expected Results

In Distilled Water

Water enters potato.

Results:

• mass increases

• length increases

• potato becomes firm/turgid

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In Concentrated Solution

Water leaves potato.

Results:

• mass decreases

• potato becomes soft/flaccid

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11. Why Potato Changes Mass

Because water moves by osmosis through cell membranes.

ATP LOVES this explanation.

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12. Turgid vs Flaccid

Turgid

Cell swollen with water.

Plant cell wall prevents bursting.

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Flaccid

Cell loses water.

Becomes limp/soft.

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13. Animal Cells in Osmosis

In Dilute Solution

water enters

cell may burst

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In Concentrated Solution

water leaves

cell shrinks

Animal cells have:
NO cell wall

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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14. Controlled Variables in Osmosis Experiments

Common controlled variables:

• size of potato pieces

• volume of solution

• temperature

• time immersed

• potato type

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15. Why Equal Potato Sizes Matter

Different sizes →
different surface areas

This affects osmosis rate.

Very common ATP improvement point.

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16. Sources of Error

Common Errors

• unequal potato sizes

• not drying potato before weighing

• inaccurate timing

• incorrect concentration

• evaporation

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17. Improvements

Common Improvements

• cut equal-sized cylinders

• blot dry before measuring mass

• repeat experiment

• use electronic balance

• control temperature

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18. Calculating Percentage Change in Mass

Formula

percentage change =
(change ÷ original mass) × 100

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Example

Original mass = 5 g

Final mass = 7 g

Change = 2 g

(2 ÷ 5) × 100

= 40% increase

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19. Transport in Plants

Xylem

Transports:

• water

• mineral ions

Direction:
root → leaf

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Phloem

Transports:

• sucrose

• amino acids

Both directions possible.

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20. Transpiration

Definition

Loss of water vapour from leaves through stomata.

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21. Potometer Experiments

Used to estimate:
transpiration rate

Measured using:
movement of air bubble

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22. Factors Affecting Transpiration

Light

stomata open

transpiration increases

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Temperature

evaporation faster

transpiration increases

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Wind

removes water vapour

transpiration increases

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Humidity

high humidity reduces diffusion gradient

transpiration decreases

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23. Common ATP Potometer Questions

Variables

Independent variable:

• light intensity

• wind speed

• temperature

Dependent variable:

• bubble movement

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24. Common Potometer Errors

• apparatus leaks

• bubble not measured accurately

• temperature changes

• inconsistent light

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25. Improvements for Potometer Experiments

• seal apparatus completely

• repeat readings

• use ruler carefully

• maintain constant conditions

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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26. ATP Graph Trends in Osmosis

Distilled Water

positive mass change

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Concentrated Solution

negative mass change

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27. ATP Conclusions

Correct:
“As concentration increased, potato mass decreased because water moved out of cells by osmosis.”

Weak:
“Potato got smaller.”

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28. Common Examiner Complaints

• forgetting “partially permeable membrane”

• missing “net movement”

• confusing diffusion with osmosis

• no units in calculations

• vague conclusions

• not drying potato before measuring

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29. ATP Frequently Tested Skills

Candidates may need to:

• calculate percentage change

• identify variables

• suggest controls

• explain osmosis

• interpret graphs

• compare results

• suggest improvements

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30. Quick Summary Table

Process	Direction	Energy Needed?
Diffusion	High → low concentration	No
Osmosis	High → low water potential	No

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.