ICSE 10 Biology Transpiration Basic

1.0 What is Transpiration?

Plants absorb huge quantities of water, but only a tiny fraction (about 1-2%) is actually used for photosynthesis and growth. The rest is lost to the atmosphere. This "necessary evil" is known as Transpiration.

Definition

Transpiration: It is the loss of water in the form of water vapor from the aerial parts (leaves and stems) of a living plant.

Three Types of Transpiration

Based on the structure through which water escapes, transpiration is classified into three types:

Type	Percentage	Location
Stomatal	80 - 90%	Through stomata on leaves.
Cuticular	3 - 10%	Directly through the waxy cuticle.
Lenticular	Approx. 0.1%	Through lenticels in woody stems.

Fig: Pathways of Transpiration

⚠️ Important Note for Exams:

While stomata can open and close to regulate water loss, lenticels never close. They remain open 24/7, allowing a small but constant escape of water vapor.

💡 Did You Know?

A single fully grown sunflower plant can lose nearly one liter of water every day through transpiration!

2.0 Stomatal Mechanism

Stomata are microscopic pores found primarily on the underside of leaves. Each stoma is surrounded by two specialized kidney-shaped Guard Cells. Their opening and closing mechanism is controlled by their Turgidity.

Structure

Guard cells have a thick inner wall (facing the pore) and a thin outer wall. This difference in thickness is what causes the cells to curve outward when they become turgid.

How Stomata Open and Close

• ☀️ Daytime (Opening): Photosynthesis occurs in guard cells, increasing glucose/solute concentration. Water enters from neighboring cells (Endosmosis). Guard cells become Turgid and stretch outward, pulling the pore open.
• 🌙 Nighttime (Closing): Photosynthesis stops. Solute concentration decreases. Water leaves the guard cells (Exosmosis). Guard cells become Flaccid, and their inner walls straighten, closing the pore.

Fig: Opening and Closing of Stomata

The K⁺ Ion Theory

Modern biology suggests that the movement of Potassium ions (K⁺) into the guard cells during the day triggers the osmotic entry of water, leading to stomatal opening. This is a very important point for "Give Reason" type questions!

⚠️ Exam Tip:

In a Dicot leaf, stomata are more numerous on the lower surface to reduce excessive water loss. In Monocot leaves (like grass), stomata are distributed equally on both surfaces.

💡 Did You Know?

Guard cells are the only cells in the epidermis that contain chloroplasts. Every other epidermal cell is transparent to allow light to reach the mesophyll!

3.0 Factors Affecting Transpiration

The rate of transpiration is not constant; it fluctuates based on the internal state of the plant and the external environmental conditions. ICSE often tests these in "Experimental" questions.

External Environmental Factors

Factor	Effect on Rate	Reason
Intensity of Light	Increases	Stomata open wider in bright light.
Temperature	Increases	Higher temp increases evaporation capacity of air.
Humidity	Decreases	Air is already saturated with water vapor.
Wind Velocity	Increases	Removes water vapor quickly from leaf surface.

4.0 Ganong's Potometer

A Potometer is a device used to measure the rate of water intake by a plant, which is almost equal to the water lost through transpiration.

Fig: Ganong's Potometer for Measuring Transpiration

Precautions for Potometer Experiments:

• The twig should be cut underwater to prevent air bubbles from entering the xylem.
• The apparatus must be completely air-tight.
• The reservoir helps in pushing the air bubble back to its original position for multiple readings.

⚠️ Limitation of Potometer:

It measures water intake, not the actual water lost through transpiration. Some water absorbed is used by the plant for photosynthesis or metabolic activities.

💡 Did You Know?

Transpiration helps in cooling the plant surface. Just as sweating cools our skin, the evaporation of water from leaves can lower their temperature by up to 10°C to 15°C!

5.0 Adaptations to Reduce Transpiration

In many environments, particularly in dry (Xerophytic) regions, plants must conserve water. Evolution has provided them with clever structural modifications to minimize excessive transpiration.

• 🌵 Sunken Stomata: Stomata are found in deep pits (e.g., Nerium) to protect them from wind.
• 🌵 Thick Cuticle: A waxy, waterproof layer on the leaf surface (e.g., Banyan).
• 🌵 Reduced Leaf Surface: Leaves may be modified into spines (e.g., Cactus) or rolled up.
• 🌵 Loss of Leaves: Deciduous trees shed leaves during the dry season.

Fig: Xerophytic Adaptations in Leaves

6.0 Guttation and Bleeding

Transpiration is often confused with other forms of water loss. In ICSE papers, the distinction between Transpiration and Guttation is a high-frequency question.

Feature	Transpiration	Guttation
Form	Water Vapor (Gas)	Water Droplets (Liquid)
Purity	Pure Water	Contains dissolved salts/sugars
Opening	Stomata/Cuticle/Lenticels	Hydathodes (pore on margin)

⚠️ Why is it a "Necessary Evil"?

It is "evil" because it causes a massive loss of water that the plant worked hard to absorb. It is "necessary" because it creates Transpiration Pull for the ascent of sap and provides a Cooling Effect.

💡 Did You Know?

Transpiration is a key part of the global water cycle. Large forests act as "pumps," releasing moisture into the air that eventually returns as rain. This is why deforestation leads to droughts!