TRANSPIRATION – COMPLETE BIOLOGY NOTES

Biology Study Material
Topic: Transpiration
Useful for OTET, CTET, OSSTET, B.Ed, Junior Teacher, Odisha Constable and other competitive examinations.

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1. Meaning of Transpiration

Transpiration is the process of loss of water in the form of water vapour from the aerial parts of plants, mainly through leaves.

• It is an important physiological process in plants.
• Most transpiration occurs through leaves.
• It mainly takes place through stomata.
• It helps in the upward movement of water and minerals.
• It also helps in cooling the plant.

2. Types of Transpiration

A. Stomatal Transpiration

• Water vapour is lost through stomata.
• It is the major type of transpiration.
• It is regulated by the opening and closing of stomata.

B. Cuticular Transpiration

• Water vapour is lost through the cuticle covering leaves and stems.
• It is generally less than stomatal transpiration.
• It depends on the thickness and permeability of the cuticle.

C. Lenticular Transpiration

• Water vapour is lost through lenticels.
• Lenticels are small openings present in the bark of woody stems.
• It usually continues day and night.

3. Stomata and Their Structure

Stomata are tiny pores present mainly on the epidermis of leaves. They help in gaseous exchange and transpiration.

• Stomatal pore: Opening through which gases and water vapour pass.
• Guard cells: Two specialised cells surrounding the stomatal pore.
• Subsidiary cells: Cells surrounding guard cells in many plants.

Functions of Guard Cells

• Control the opening and closing of stomata.
• Regulate water loss through transpiration.
• Help regulate gaseous exchange.

Important: In most dicot plants, guard cells are kidney-shaped, whereas in many grasses they are dumbbell-shaped.

4. Mechanism of Opening and Closing of Stomata

Opening of Stomata

• Guard cells absorb potassium ions under appropriate conditions.
• Water enters the guard cells by osmosis.
• Guard cells become turgid.
• The stomatal pore opens.

Closing of Stomata

• Guard cells lose turgor pressure.
• Water moves out of the guard cells.
• Guard cells become flaccid.
• The stomatal pore closes.

5. Mechanism of Transpiration

Transpiration occurs through the following steps:

1. Roots absorb water from the soil.
2. Water moves upward through xylem vessels.
3. Water reaches the mesophyll cells of leaves.
4. Water evaporates from the moist cell walls into the intercellular spaces.
5. Water vapour diffuses out through stomata.

6. Factors Affecting Transpiration

External Factors

Factor	Effect on Transpiration
Temperature	Higher temperature generally increases transpiration.
Humidity	Higher humidity decreases transpiration.
Wind	Moderate wind generally increases transpiration.
Light	Light generally increases stomatal opening and transpiration.
Soil water	Water shortage generally reduces transpiration through stomatal closure.
Atmospheric pressure	Lower atmospheric pressure generally favours diffusion.

Internal Factors

• Number and distribution of stomata.
• Size of leaves.
• Thickness of cuticle.
• Leaf orientation.
• Root-shoot ratio.
• Presence of leaf hairs and sunken stomata.

7. Significance of Transpiration

1. Transpiration Pull: Creates a pulling force that helps in the upward movement of water.
2. Cooling Effect: Helps maintain the temperature of plants.
3. Mineral Transport: Helps in the movement of dissolved minerals from roots to leaves.
4. Water Distribution: Supports the movement of water throughout the plant.
5. Maintains Water Movement: Contributes to the continuous flow of water through the plant.

8. Transpiration Pull

Transpiration pull is the negative pressure or tension developed in the xylem due to water loss from leaves. It helps draw water upward from the roots.

• It is explained by the Cohesion-Tension Theory.
• Cohesion means attraction between water molecules.
• Adhesion means attraction between water molecules and xylem walls.
• It plays an important role in the ascent of sap.

9. Experiments Related to Transpiration

A. Bell Jar Experiment

• A well-watered potted plant is covered with a transparent bell jar.
• The soil is covered to prevent evaporation from the soil.
• After some time, water droplets appear on the inner surface of the bell jar.
• This proves that leaves release water vapour through transpiration.

B. Cobalt Chloride Paper Experiment

• Dry blue cobalt chloride paper is placed on a leaf.
• It changes from blue to pink in the presence of moisture.
• It is used to demonstrate water loss from leaves.

C. Potometer

• A potometer is used to estimate the rate of water uptake by a cut shoot.
• It indirectly estimates the rate of transpiration.
• It does not measure actual water loss directly.

10. Difference Between Transpiration and Guttation

Transpiration	Guttation
Loss of water as vapour.	Loss of liquid water.
Occurs mainly through stomata.	Occurs through hydathodes.
Usually occurs during the day and night.	Commonly observed early in the morning.
Driven mainly by evaporation and diffusion.	Usually caused by root pressure.
Water is generally lost as nearly pure vapour.	Drops contain dissolved salts and other substances.

11. Xerophytes and Adaptations to Reduce Transpiration

Xerophytes are plants adapted to survive in dry environments.

• Thick waxy cuticle.
• Sunken stomata.
• Reduced leaf surface area or spines.
• Succulent stems or leaves for water storage.
• Leaf hairs in some species.

Example: Cactus, Opuntia and Aloe.

12. Important One-Liners for Competitive Exams

1. Transpiration is the loss of water in the form of water vapour from aerial parts of plants.
2. Majority of transpiration occurs through stomata.
3. Stomata are regulated by guard cells.
4. Guard cells control the opening and closing of stomata.
5. Xylem transports water and minerals in plants.
6. Transpiration pull helps in the ascent of sap.
7. Potometer is used to estimate the rate of transpiration indirectly.
8. Cobalt chloride paper changes from blue to pink in the presence of moisture.
9. Hydathodes are associated with guttation.
10. Guttation involves the loss of liquid water.
11. Cuticle helps reduce water loss.
12. High humidity generally decreases transpiration.
13. Higher temperature generally increases transpiration.
14. Wind generally increases transpiration under moderate conditions.
15. Thick cuticle is an adaptation of xerophytes.
16. Lenticels are present in woody stems.
17. Transpiration provides a cooling effect to plants.
18. Cohesion-Tension Theory explains the ascent of sap.
19. Stomatal transpiration is the major type of transpiration.
20. Transpiration occurs mainly through leaves.

13. Quick Revision Table

Question	Answer
Loss of water vapour from plants	Transpiration
Major site of transpiration	Stomata
Cells controlling stomata	Guard cells
Water conducting tissue	Xylem
Instrument for estimating transpiration	Potometer
Loss of liquid water	Guttation
Guttation openings	Hydathodes
Waterproof covering of leaves	Cuticle
Water transport theory	Cohesion-Tension Theory
Plants adapted to dry habitats	Xerophytes

EXAM TIP:
Remember these important points:
Stomata – Major transpiration site
Guard Cells – Control stomata
Potometer – Estimates transpiration indirectly
Hydathodes – Guttation
Xylem – Water transport

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