Topic 8 — Plant Response and Plant Transport Experiments

1. Plant Responses

Plants respond to stimuli in their environment.

Common stimuli:

• light

• gravity

• water

• touch

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2. Tropisms

Definition

A growth response of a plant towards or away from a stimulus.

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3. Phototropism

Definition

Growth response to light.

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Shoots

Positive phototropism.

Grow towards light.

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Roots

Negative phototropism.

Grow away from light.

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4. Gravitropism (Geotropism)

Definition

Growth response to gravity.

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Roots

Positive gravitropism.

Grow towards gravity.

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Shoots

Negative gravitropism.

Grow away from gravity.

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. Common ATP Tropism Experiments

Frequently Tested

• seedlings near light

• rotating clinostat experiments

• germinating seeds

• shoot tip investigations

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6. Auxin

Auxin is a plant hormone controlling growth.

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7. Auxin in Shoots

Auxin moves to shaded side.

Cells elongate more there.

Result:
shoot bends towards light.

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8. Auxin in Roots

High auxin concentration inhibits root growth.

Result:
root bends downwards.

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9. ATP Questions on Tropisms

Common Questions

• explain bending

• identify stimulus

• compare growth directions

• explain role of auxin

• identify control setup

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10. Why Experiments Kept in Dark?

To remove effect of light.

Ensures only gravity affects growth.

Very common ATP point.

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11. Rotating Clinostat Experiments

Rotation changes gravity direction continuously.

Result:
no bending occurs

because gravity effect equalised.

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12. Plant Transport

Plants transport:

• water

• mineral ions

• sugars

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13. Xylem

Function

Transports:

• water

• mineral ions

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Direction

Root → stem → leaves

(one direction only)

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14. Xylem Adaptations

• hollow tubes

• no end walls

• lignified walls

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15. Phloem

Function

Transports:

• sucrose

• amino acids

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Direction

Both directions possible.

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16. Transpiration

Definition

Loss of water vapour from leaves through stomata.

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17. Stomata

Tiny pores mainly in leaf epidermis.

Control:
gas exchange and water loss.

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18. Guard Cells

Control opening and closing of stomata.

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19. 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

reduces concentration gradient

transpiration decreases

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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20. Potometers

Used to estimate transpiration rate.

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21. Potometer Measurements

Usually measure:

• distance moved by air bubble

• rate of bubble movement

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22. Potometer Setup

Contains:

• capillary tube

• air bubble

• leafy shoot

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23. Why Bubble Moves

Water uptake by plant pulls bubble along.

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24. Variables in Potometer Experiments

Independent Variables

• light intensity

• temperature

• wind speed

• humidity

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Dependent Variable

bubble movement / transpiration rate

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Controlled Variables

• same plant species

• same shoot size

• same time interval

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25. Common Potometer Errors

• air leaks

• inaccurate bubble reading

• temperature fluctuations

• damaged shoot

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26. Improvements

• seal apparatus completely

• repeat readings

• use ruler accurately

• maintain constant conditions

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27. Testing Water Transport

Common Experiment

Place stem in coloured dye.

Observation:
xylem becomes coloured.

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28. Why Dye Appears in Xylem

Water transported through xylem vessels.

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29. ATP Leaf Experiments

May involve:

• counting stomata

• comparing leaf surfaces

• measuring transpiration

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30. Stomata Observations

Usually:
more stomata on lower epidermis

Reason:
reduced water loss

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31. ATP Graph Trends

Transpiration

Higher:

• light

• wind

• temperature

→ increased transpiration

Higher humidity →
reduced transpiration

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32. ATP Conclusion Examples

Strong Conclusion

“Increasing wind speed increased transpiration because water vapour was removed more rapidly from the leaf surface.”

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Weak Conclusion

“Wind affected transpiration.”

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33. Common Examiner Complaints

• confusing xylem and phloem

• forgetting direction of transport

• vague conclusions

• not mentioning water vapour

• weak explanation of auxin action

• confusing gravitropism and phototropism

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34. ATP Frequently Tested Skills

Candidates may need to:

• interpret tropism experiments

• explain bending

• identify variables

• explain transpiration results

• analyse graphs

• suggest improvements

• explain dye movement

• compare plant tissues

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35. Quick Summary Table

Structure	Function
Xylem	transport water/mineral ions
Phloem	transport sugars
Stomata	gas exchange
Guard cells	control stomata

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36. Tropism Summary

Tropism	Stimulus
Phototropism	light
Gravitropism	gravity

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.