Transpiration | ICSE Class 10 Biology Notes

This chapter covers the biological process of transpiration, detailing the mechanisms of water loss, structural adaptations of stomata, and the physical forces driving the ascent of sap. Readers will be able to explain transpiration pathways, analyze environmental factors influencing water loss, and describe classic experimental setups used to measure and demonstrate plant transpiration.

What is Transpiration and Why Does It Matter Biologically?

Transpiration is the biological process where water is lost as water vapor from the aerial parts of a plant, primarily through the leaves.

Definition: The loss of water in the form of water vapor from the aerial parts of the plant is called transpiration.

This process occurs when H₂O evaporates from the mesophyll cells and diffuses through the stomata into the surrounding atmosphere.

The continuous movement of water from the soil, through the root xylem, and out of the leaves is termed the transpiration stream.

Table: Transpiration vs Evaporation. Columns: Basis · Transpiration · Evaporation

• Nature — Transpiration: Physiological process · Evaporation: Physical process
• Control — Transpiration: Regulated by stomata · Evaporation: Unregulated/Spontaneous
• Location — Transpiration: Aerial parts of plants · Evaporation: Any wet surface
• Purpose — Transpiration: Essential for sap ascent · Evaporation: No biological purpose

Why is Transpiration Vital for Plant Survival?

Transpiration generates a suction force, known as transpirational pull, which draws water upward from the roots to the canopy via xylem vessels.

This flow also helps in the distribution of mineral salts: dissolved ions absorbed from the soil, such as (i) nitrates, (ii) phosphates, and (iii) potassium ions (K⁺), are carried upward in the transpiration stream to all parts of the plant.

The evaporation of water from the leaf surface produces a cooling effect, preventing the denaturation of enzymes during periods of high solar radiation.

It also maintains the turgidity of plant cells, which is necessary for the structural support of non-woody stems and the opening of stomatal pores.

Diagram: The Transpiration Stream. A sketch of a plant showing (A) root hairs absorbing water, (B) xylem transporting water upward, (C) leaf mesophyll cells, and (D) water vapor exiting through a stoma.

What are the Types of Transpiration based on Plant Surfaces?

Plant surfaces lose water vapor through three distinct physiological pathways governed by structural exposure. Stomatal transpiration accounts for $85\%$ to $90\%$ of total water loss in typical terrestrial plants. This process occurs through microscopic pores called stomata located primarily in the leaf epidermis.

The second pathway is cuticular transpiration, contributing about $5\%$ to $10\%$ of water vapor loss. The cuticle is a waxy, hydrophobic layer secreted by epidermal cells covering leaves and young stems. Thicker cuticles drastically reduce this non-stomatal water leakage in arid climates.

The third pathway is lenticular transpiration, responsible for approximately $0.1\%$ of total water loss. Lenticels are porous tissue aggregations composed of loosely arranged cells formed within the woody periderm of older stems and bark, remaining open continuously.

Diagram: Plant Transpiration Pathways. A transverse section of a dicot leaf and woody stem showing (A) Upper epidermis with cuticle, (B) Stoma with two kidney-shaped guard cells in the lower epidermis, (C) Sub-stomatal air space, (D) Cuticular layer on the leaf surface, (E) Cork cells, and (F) Lenticel rupture in the woody bark. Notice how stomata regulate the bulk of water discharge while lenticels permit minimal passive diffusion in woody axes.

Features: (A) Upper epidermis with cuticle, (B) Guard cells, (C) Sub-stomatal air space, (D) Cuticular layer, (E) Cork cells, (F) Lenticel.

How is the Stomatal Apparatus Structured for Gas Exchange?

What is the microscopic structure of the stomatal apparatus?

The stomatal apparatus is a specialized epidermal micro-structure responsible for gaseous exchange and water vapour loss in higher land plants. Each stoma comprises a central microscopic opening called the stomatal pore, which is enclosed by a pair of specialized kidney-shaped or dumbbell-shaped cells termed guard cells. These specialized cells are unique among epidermal cells because they possess chloroplasts capable of performing photosynthesis, generating chemical energy used for turgor pressure changes when light falls on the leaf.

Diagram: Microscopic structure of a stomatal apparatus. Draw an open stoma surface view: show two kidney-shaped guard cells flanking a central stomatal pore, surrounded by radiating epidermal subsidiary cells. The labelled parts are: A. Stomatal pore (central aperture), B. Guard cell (kidney-shaped with chloroplasts), C. Inner thickened wall (facing the pore), D. Outer thin wall (facing subsidiary cells), E. Chloroplast (site of photosynthesis), F. Subsidiary cells (accessory epidermal cells). Notice the differential thickness of the guard cell walls and the radial orientation of cellulose microfibrils.

What structural adaptations define the components of the stoma?

• A. Stomatal pore: The microscopic intercellular space through which CO₂ diffuses in and water vapour diffuses out during transpiration.
• B. Guard cells: Living, nucleated epidermal cells containing active chloroplasts that regulate the opening and width of the stomatal pore via osmotic shifts.
• C. Inner thickened wall: The thick, inelastic cell wall adjacent to the pore that pulls outward during turgidity to form the aperture.
• D. Outer thin wall: The flexible, elastic cell wall adjacent to subsidiary cells that bulges outward easily under high turgor pressure.
• E. Chloroplast: Organelle synthesizing sugars that elevate osmotic pressure within the guard cell sap during daylight hours.
• F. Subsidiary cells: Specialized accessory epidermal cells flanking guard cells that act as rapid ion and water reservoirs during stomatal movement.

How do structural features facilitate rapid gas exchange?

The differential wall thickness between the inner and outer boundaries of guard cells creates a mechanical leverage system. When water enters the guard cells via osmosis, the outer thin wall stretches outward more rapidly than the inner thickened wall, forcing the two guard cells to bow apart and widen the stomatal pore. Surrounding epidermal cells known as subsidiary cells provide mechanical support and store reserve ions that facilitate ion exchange across the plasma membrane.

Note: Guard cells are the only epidermal cells containing chloroplasts, distinguishing them from surrounding transparent epidermal cells and subsidiary cells which lack chlorophyll entirely.

1. Water molecules enter guard cells from adjacent subsidiary cells through osmosis, increasing internal hydrostatic pressure.
2. The outer thin wall stretches outward, dragging the rigid inner thickened wall along with it.
3. The two guard cells curve away from each other, creating an open stomatal pore for gas diffusion.
4. Atmospheric carbon dioxide enters the intercellular spaces of the mesophyll layer while water vapour escapes outward.

How do Guard Cells Regulate the Opening and Closing of Stomata?

How do Guard Cells Open the Stoma?

Guard cells are specialized kidney-shaped cells. Their movement depends on the movement of water and solutes across the plasma membrane of the cell.

The process is driven by the concentration of solutes, primarily K⁺ ions, which are managed by the surrounding subsidiary cells in the leaf.

1. Light triggers photosynthesis in the chloroplasts of the guard cells, producing energy.
2. Active transport moves potassium ions (K⁺) from the subsidiary cells into the guard cells.
3. The accumulation of these ions increases the osmotic pressure within the vacuole of the guard cell.
4. Water moves from the subsidiary cells into the guard cells via osmosis to balance the concentration.
5. The guard cells reach a state of turgidity, causing them to swell and expand.
6. Because of the radial orientation of cellulose microfibrils, the thin outer walls stretch more than the thick inner walls.
7. The guard cells curve outwards, pulling the stomatal pore open.

The physical bending occurs because the inner wall is thicker and less elastic than the outer wall of the guard cell.

This structural difference ensures that the cells curve outward rather than expanding uniformly in all directions during the intake of H₂O.

Note: Turgidity leads to the opening of the stoma, whereas flaccidity leads to its closure. Do not confuse these two physiological states.

Why do Stomata Close at Night?

The closing mechanism is essentially the reverse of the opening process, triggered by the absence of sunlight in the environment.

This diurnal rhythm ensures the plant balances CO₂ intake for glucose production with H₂O conservation during the 24-hour cycle.

1. In the absence of light, photosynthesis ceases, and the energy supply for active transport drops.
2. Potassium ions (K⁺) diffuse out of the guard cells and move back into the subsidiary cells.
3. The internal osmotic pressure of the guard cells decreases significantly.
4. Water exits the guard cells by osmosis, moving toward the area of higher solute concentration.
5. The guard cells lose water and enter a state of flaccidity.
6. The stretched, elastic outer walls relax and the thick inner walls straighten, so the guard cells come together.
7. The stomatal pore closes, preventing further water loss from the leaf interior.

This movement is a passive response to the change in solute concentration across the cell membranes of the stomatal apparatus.

How does Transpirational Pull Drive the Ascent of Sap?

The upward movement of water and dissolved minerals from the roots to the aerial parts of a plant is termed the ascent of sap. This critical physiological process occurs primarily through hollow, dead tracheary elements known as xylem vessels.

Under low-intensity transpiration or in very small herbs, a hydrostatic pressure called root pressure develops in the root cortex due to active ion accumulation. However, in tall trees exceeding 10 meters in height, root pressure alone is insufficient to push water columns to the top.

How do physical forces maintain a continuous water column?

1. Water molecules evaporate from the mesophyll cell surfaces into the substomatal air spaces and diffuse out through stomata, creating a diffusion pressure deficit in leaf cells.
2. Water is drawn from adjacent vascular tissues, creating a negative hydrostatic pressure or tension at the top of the plant known as transpirational pull.
3. Water molecules remain bound to each other by strong intermolecular hydrogen bonds, a property called cohesion, which prevents the water column from breaking under high tension.
4. Water molecules also cling to the hydrophilic lignified walls of the xylem vessels through adhesion, counteracting the downward pull of gravity.
5. The unbroken continuous water column is thus pulled upward en masse from the root xylem all the way to the leaf veins to replace transpired water molecules.

Note: Root pressure pushes water up weakly from below during periods of high humidity and low transpiration, whereas transpirational pull draws water powerfully from above during active daytime transpiration.

1. Soil water enters the root hair cells by osmosis due to a higher solute concentration inside the root cell sap.
2. Water moves radially across the cortex and endodermis of the root via the symplast and apoplast pathways to reach the central stele.
3. Once inside the root xylem, the combined action of root pressure, capillary action, and suction generated by transpirational pull drives the continuous upward transport of sap.

What are the Environmental and Internal Factors Affecting Transpiration?

The rate of water vapor loss from leaves is governed by a dynamic interplay of atmospheric conditions and structural properties. Environmental variables alter the steepness of the water vapor concentration gradient between the substomatal air cavity and the outer atmosphere.

1. Sunlight Intensity: High illumination triggers the active transport of potassium ions into guard cells, causing stomata to open wide and significantly accelerating water loss.
2. Temperature: Elevated thermal energy increases the kinetic energy of water molecules, so a rise in temperature increases the rate of evaporation and hence of transpiration.
3. Relative Humidity: A humid atmosphere, as in coastal cities such as Mumbai, narrows the vapor pressure deficit, dramatically suppressing the outward diffusion of water molecules through stomatal pores.
4. Wind Speed: Moderate air currents sweep away the humid boundary layer hovering above the epidermis, steepening the diffusion gradient and elevating the rate of transpiration.
5. Soil Water: Severe drought conditions cause root cells to release abscisic acid, a stress hormone that triggers rapid stomatal closure to conserve remaining moisture reserves.
6. Leaf Surface Area: Broad, flat leaves have a larger surface area and more stomata in total than needle-like or spiny xerophytic leaves, so they lose more water by transpiration.

Graph: rate of transpiration versus relative humidity. A declining curve plotting transpiration rate on the y-axis against relative humidity percentage on the x-axis, showing that the rate of transpiration falls as relative humidity rises

Table: Comparison of Environmental Factors and Their Direct Effects on Transpiration Rate. Columns: Environmental Factor · Condition of Increase · Effect on Rate · Underlying Physical Cause

• Sunlight — Condition of Increase: Dim to Bright · Effect on Rate: Increases · Underlying Physical Cause: Induces stomatal opening via photosynthetic ion accumulation
• Temperature — Condition of Increase: Cool to Warm · Effect on Rate: Increases · Underlying Physical Cause: Raises kinetic energy and vapour pressure inside leaf
• Relative Humidity — Condition of Increase: Dry to Humid · Effect on Rate: Decreases · Underlying Physical Cause: Reduces the diffusion gradient between leaf and air
• Wind Velocity — Condition of Increase: Calm to Breezy · Effect on Rate: Increases · Underlying Physical Cause: Removes humid microclimate layer above stomata
• Soil Water Shortage — Condition of Increase: Wet to Dry · Effect on Rate: Decreases · Underlying Physical Cause: Triggers abscisic acid synthesis and stomatal closure

How do Transpiration and Evaporation Differ from Transpiration and Guttation?

How do Transpiration and Physical Evaporation Differ?

Transpiration is a physiological process involving both physical and biological phenomena, occurring predominantly through the living cells of green plant leaves. In contrast, physical evaporation is a purely abiotic process that can take place from any moist surface, whether living or non-living.

Water loss via transpiration is strictly subject to physiological regulation by specialized epidermal structures known as guard cells, which alter the aperture of stomatal pores. Physical evaporation is entirely unregulated and depends solely on environmental parameters like humidity, temperature, and wind velocity.

Table: Comparison between Transpiration and Physical Evaporation. Columns: Basis of Comparison · Transpiration · Physical Evaporation

• Nature of Process — Transpiration: Physiological and physical · Physical Evaporation: Purely physical
• Site of Occurrence — Transpiration: From the aerial surfaces of a living plant (through stomata, cuticle and lenticels) · Physical Evaporation: From any wet surface (soil, water bodies, dead plant parts)
• Cellular Involvement — Transpiration: Involves living cells such as guard cells and mesophyll · Physical Evaporation: Does not involve living cells
• Regulation — Transpiration: Under biological control via opening and closing of stomata · Physical Evaporation: Uncontrolled and unregulated
• Cuticular Protection — Transpiration: Regulated partly by a waxy cuticle layer · Physical Evaporation: Unaffected by cuticular modifications

How do Transpiration and Guttation Differ?

Guttation is the exudation of liquid drops of water from the margins or tips of leaves through specialized pore structures called hydathodes, rather than as a vapor through stomata. This phenomenon occurs when root pressure is high and transpiration is exceptionally low, typically under conditions of high soil moisture and high atmospheric humidity during early mornings.

While transpiration releases nearly pure water in gaseous form, guttation discharges a watery sap containing various dissolved mineral salts and organic compounds. Unlike transpiration, which operates continuously during daylight hours under favorable gradients, guttation is an occasional process limited to specific herbaceous plants under damp environmental settings.

Table: Comparison between Transpiration and Guttation. Columns: Basis of Comparison · Transpiration · Guttation

• State of Water Lost — Transpiration: Lost as an invisible water vapor · Guttation: Lost as visible liquid droplets
• Site of Water Loss — Transpiration: Stomata, cuticles, and lenticels · Guttation: Hydathodes (water stomata) located at leaf margins and tips
• Composition of Exudate — Transpiration: Pure water vapor · Guttation: Aqueous solution containing inorganic salts and organic solutes
• Driving Force — Transpiration: Atmospheric vapor pressure deficit and transpirational pull · Guttation: Positive root pressure
• Timing of Occurrence — Transpiration: Generally occurs throughout the day when stomata are open · Guttation: Typically occurs at night or early morning when transpiration is low

What are the Adaptations in Plants to Regulate Transpiration?

Plants growing in arid or semi-arid habitats, known as xerophytes, develop structural modifications to prevent excessive water loss through aerial parts. These modifications ensure survival under conditions of severe water stress in regions like the Thar Desert of Rajasthan.

Morphological and anatomical features directly lower the rate of vapor diffusion. The most common adaptations include: (i) presence of a thick cuticle on the epidermal surface, (ii) reduction of leaf area into spines, and (iii) transformation of stems into fleshy phylloclade organs for water storage and photosynthesis.

Why Do Xerophytes Modify Leaves and Stomata?

Leaves present a large surface area for heat absorption and transpiration. Xerophytes reduce this vulnerability by modifying stomatal placement and leaf architecture. In many desert shrubs, sunken stomata are located inside epidermal pits, trapping a humid microclimate that decreases the diffusion gradient.

Note: Students often confuse sunken stomata with absence of stomata. Xerophytes do possess stomata for vital gas exchange during photosynthesis, but their recessed positions restrict outward water vapor diffusion.

Table: Comparison of Mesophytic and Xerophytic Leaf Adaptations. Columns: Basis · Mesophytes · Xerophytes

• Cuticle layer — Mesophytes: Thin, delicate waxy layer · Xerophytes: Extra thick, impermeable cuticle
• Leaf surface area — Mesophytes: Broad and expansive lamina · Xerophytes: Reduced to needles, scales, or spines
• Stomatal distribution — Mesophytes: Level with the epidermal surface · Xerophytes: Deeply sunken inside pits or grooves
• Hairs on epidermis — Mesophytes: Usually absent or sparse · Xerophytes: Dense trichomes trapping stagnant moisture

Applications: Why are Xerophytic Adaptations Studied in Agriculture? Understanding drought-resistance mechanisms enables plant physiologists to breed drought-tolerant crop varieties for rain-fed regions.

Indian Example: The common desert tree Prosopis cineraria (Khejri), found extensively in western Rajasthan, has compound leaves with small leaflets and a deep tap root, serving as an ideal biological model for arid survival and water conservation.

How are Classic Experiments Designed to Demonstrate Transpiration?

Proving that plants lose water vapor requires setting up controlled apparatuses that isolate leaf transpiration from simple soil evaporation. The classic Bell jar experiment is a commonly used qualitative demonstration in school laboratories.

A well-watered potted plant, with the pot and soil wrapped in a polythene sheet to prevent evaporation from the soil, is placed under a bell jar on a glass plate, and the rim is made airtight with vaseline. A control setup, identical except that the plant's leaves have been removed, is run simultaneously.

After keeping the apparatus in bright sunlight for $3$ to $4$ hours, droplets of water condense on the inner walls of the experimental bell jar, while the control jar remains completely dry. This proves that water loss occurs strictly from the aerial parts of the living plant.

Diagram: Bell jar experiment for transpiration. Draw a potted plant inside a sealed glass bell jar on a glass plate, with a control bell jar containing a potted stem with no leaves; label parts as follows: A-Bell jar, B-Water droplets, C-Potted plant, D-Polythene sheet covering soil, E-Vaseline seal, F-Glass plate. Notice how the polythene sheet isolates soil evaporation.

How is Anhydrous Cobalt Chloride Paper Used as an Indicator?

To detect water vapor chemically without relying solely on condensation, biologists use Cobalt chloride paper. Dry cobalt chloride paper is deep blue in color, but it turns pink upon coming into contact with water molecules.

Small strips of dry blue paper are clamped onto the upper and lower surfaces of a healthy leaf using glass slides and paper clips while still attached to the plant. Upon observing the setup after $30$ minutes, the paper on the lower surface turns pink much faster and more intensely than the upper surface.

This color transition demonstrates that transpiration occurs continuously from leaves and that the lower surface of a dorsiventral dicot leaf houses a higher density of stomata than the upper surface.

Note: Blue cobalt chloride paper turns pink in moisture, which is often confused with iodine solution turning blue-black with starch. Remember that cobalt tests for physical water presence, whereas iodine tests for organic photosynthesis products.

How does Ganong's Potometer Measure the Rate of Water Intake?

Measuring the actual volume of water lost requires an instrument that tracks water uptake rather than direct vapor mass. Ganong's potometer is the standard laboratory instrument utilized to measure the rate of transpiration.

The apparatus consists of a capillary tube connected to a reservoir, a graduated scale, and a side tube holding a freshly cut leafy shoot. The entire assembly is filled with water underwater to prevent air entry, and a single air bubble is introduced into the horizontal graduated capillary tube.

As the shoot transpires, it creates a suction pull that draws water up from the capillary tube, causing the air bubble to move along the graduated scale at a measurable distance per minute. The rate of bubble movement corresponds directly to the rate of water absorption by the shoot.

Diagram: Ganong's potometer. Draw a horizontal graduated capillary tube connected to a vertical shoot, water reservoir, and bendable rubber tubing; label parts as follows: A-Leafy shoot, B-Capillary tube, C-Graduated scale, D-Air bubble, E-Reservoir tap, F-Water-filled beaker. Notice the direction of bubble movement towards the plant shoot.

Worked example 1. Calculate the average speed of the air bubble, which is taken as a measure of the rate of water uptake (and hence of transpiration), when the bubble in a potometer moves through a distance of $15\text{ cm}$ in $3\text{ minutes}$.

Given: Distance traveled $d = 15\text{ cm}$, Time taken $t = 3\text{ min}$. Formula: Speed = $d / t$. Substitute: $15\text{ cm} / 3\text{ min}$. Answer: 5 cm min⁻¹

Glossary

• Adhesion — The physical attraction between water molecules and the hydrophilic lignified walls of xylem vessels, helping water climb against gravity.
• Ascent of sap — The upward movement of water and dissolved minerals from the roots to the aerial parts of a plant through xylem vessels.
• Cohesion — The force of attraction between water molecules due to hydrogen bonding, which maintains a continuous, unbroken water column in the xylem.
• Cuticular transpiration — The loss of water vapor directly through the waxy, hydrophobic cuticle layer covering the epidermis of leaves and young stems.
• Flaccidity — The limp state of a cell that has lost water and turgor, so its contents no longer press against the cell wall; flaccid guard cells close the stoma. (Shrinkage of the cytoplasm away from the cell wall is plasmolysis.)
• Guttation — The exudation of liquid water droplets containing dissolved salts from specialized pores called hydathodes, usually occurring at leaf margins.
• Lenticels — Porous tissue aggregations found in the woody periderm of older stems that allow for a small percentage of total water loss.
• Root pressure — A positive hydrostatic pressure generated in the root cortex due to active ion accumulation, pushing water upward during low transpiration.
• Stomatal apparatus — A specialized epidermal structure consisting of a central pore flanked by two guard cells, responsible for gas exchange and transpiration.
• Transpiration — The biological process of losing water in the form of water vapor from the aerial parts of a plant.
• Transpirational pull — A negative hydrostatic pressure or tension created at the leaf surface by evaporation, which draws water upward through the xylem.
• Turgidity — The state of a cell fully swollen with water, its contents pressing firmly against the cell wall (turgor pressure); turgid guard cells open the stomatal pore.

Common errors and misconceptions

• Misconception: Transpiration is the same as evaporation. Correct: Transpiration is a physiological process regulated by living guard cells, while evaporation is a purely abiotic physical process. Distinguishing between these is essential for explaining why plants can control water loss.
• Misconception: Sunken stomata mean a plant has no stomata. Correct: Sunken stomata are present but recessed in pits to trap humidity; they are vital for gas exchange in xerophytes. Confusing these leads to incorrect answers regarding how desert plants survive and respire.
• Misconception: Turgidity closes the stomata. Correct: Turgidity causes guard cells to swell and open the pore, whereas flaccidity causes them to close. This is a core mechanism question; reversing these states results in a complete loss of marks.
• Misconception: Guttation is just transpiration in liquid form. Correct: Guttation involves liquid sap with minerals via hydathodes, while transpiration is pure water vapor via stomata. Examiners test this to ensure you understand the difference between active regulation and passive exudation.
• Misconception: Root pressure is the primary force for water ascent in all plants. Correct: Root pressure is weak and only significant in small plants; transpirational pull is the main force for tall trees. Failure to identify the primary force for tall plants is a common error in transport biology.
• Misconception: Guard cells are just like any other epidermal cell. Correct: Guard cells are unique because they contain chloroplasts and have differential wall thickness to regulate pore size. Understanding this structural adaptation is key to explaining the mechanism of stomatal movement.

Exam-style questions with model answers

Q1. ICSE Biology Question (2 Marks):
(a) Define transpiration.
(b) State one structural difference between guard cells and epidermal cells. [2 marks]

1. Transpiration is the biological process where water is lost in the form of water vapor from the aerial parts of the plant, primarily through the leaves.
2. Guard cells contain active chloroplasts for photosynthesis and sap regulation, whereas ordinary epidermal cells lack chlorophyll entirely and are transparent.

Q2. ICSE Biology Question (2 Marks):
(a) Name the physical force that helps water molecules stick to the lignified walls of xylem vessels.
(b) State the effect of high relative humidity on the rate of transpiration. [2 marks]

1. Adhesion is the force that binds water molecules to the hydrophilic lignified walls of the xylem vessels.
2. High relative humidity narrows the vapor pressure deficit, thereby significantly decreasing or suppressing the rate of transpiration.

Q3. ICSE Biology Question (3 Marks):
(a) State any two significances of transpiration to plants.
(b) Explain how the outer thin wall and inner thickened wall of guard cells assist in opening the stoma. [3 marks]

1. Transpiration generates transpirational pull, which draws water and essential minerals upward from the roots through the xylem vessels, and it produces a cooling effect that prevents the denaturation of leaf enzymes.
2. When guard cells become turgid, their flexible outer thin walls stretch outward more rapidly than their rigid inner thickened walls.
3. This mechanical leverage causes the two guard cells to curve away from each other, opening the central stomatal pore for gas diffusion.

Q4. ICSE Biology Question (4 Marks):
(a) Differentiate between transpiration and guttation based on the state of water lost.
(b) Name the specialized pores through which guttation occurs.
(c) State one condition under which guttation takes place. [4 marks]

1. Transpiration releases nearly pure water in the form of an invisible gas (water vapor), whereas guttation discharges liquid water drops containing dissolved mineral salts and organic compounds.
2. Guttation occurs through specialized pore structures called hydathodes located at the margins or tips of leaves.
3. Guttation takes place under conditions of high root pressure and very low or absent transpiration, such as during warm, humid nights when soil moisture is extremely high.

Q5. CBSE/ICSE Case-Based Question (4 Marks):
Study the following experimental setup description and answer the questions below:
A potted plant is watered well and covered entirely with a polythene bag tied securely around the base of the stem. The setup is placed in bright sunlight for 3 to 4 hours.
(a) What is the aim of this experiment?
(b) Why is the polythene bag tied around the base of the stem and not over the pot?
(c) What observation is made on the inner surface of the polythene bag after a few hours? [4 marks]

1. The aim of the experiment is to demonstrate that water vapor is lost from the aerial parts (leaves) of a living plant through transpiration.
2. The bag is tied around the base of the stem so that the pot and damp soil stay outside it; any moisture collected inside therefore comes only from the aerial parts and not from evaporation from the soil.
3. Tiny droplets of water condense on the inner surface of the polythene bag, confirming that water vapor was released by the aerial parts of the plant enclosed inside.

Q6. Assertion-Reason and Short-Answer Question (5 Marks):
(a) Read the assertion and reason given below, and choose the correct option:
Assertion (A): Xerophytes possess sunken stomata located inside epidermal pits.
Reason (R): Sunken stomata trap a humid microclimate around the pore, which reduces the diffusion gradient and minimizes excessive water loss in arid environments.
Options:
I. Both A and R are true, and R is the correct explanation of A.
II. Both A and R are true, but R is not the correct explanation of A.
III. A is true, but R is false.
IV. A is false, but R is true.
(b) Explain any two other structural modifications found in xerophytic plants to conserve water. [5 marks]

Part (a): Option I (Both A and R are true, and R is the correct explanation of A).

Part (b):

1. Thick cuticle: Xerophytes develop a thick, waxy, hydrophobic cuticle layer covering their epidermal surfaces to drastically reduce cuticular water loss.
2. Reduction of leaf area: Leaves are often modified into spines, or stems are transformed into flat succulent structures to minimize the surface area exposed to solar radiation and so reduce transpiration.

Q7. ICSE Long-Answer Question (5 Marks):
(a) Describe the sequence of biochemical and physical events that lead to the opening of stomata during daytime.
(b) State the role of potassium ions in this mechanism. [5 marks]

1. Light triggers photosynthesis within the chloroplasts located inside the guard cells, generating chemical energy.
2. Active transport mechanisms pump potassium (K+) ions from the neighboring subsidiary cells into the guard cells using this energy.
3. The accumulation of potassium ions significantly elevates the osmotic pressure within the vacuole of each guard cell.
4. Water flows into the guard cells from surrounding tissues via osmosis, causing them to reach a state of high turgidity.
5. Due to the radial orientation of cellulose microfibrils and differential wall thickness, the outer thin walls stretch outward faster than the inner thickened walls, forcing the stoma to open.

Q8. Advanced Long-Answer Question (6 Marks):
(a) Outline the Cohesion-Tension and Transpirational Pull Theory explaining the ascent of sap in tall trees.
(b) Define cohesion and adhesion in this context.
(c) Explain why root pressure alone is inadequate to transport water to the canopy of a tree exceeding 10 meters in height. [6 marks]

1. Water evaporates from mesophyll cell surfaces into substomatal spaces and diffuses out through stomata, creating a diffusion pressure deficit in leaf cells.
2. This draws water from adjacent xylem vessels, generating a negative hydrostatic pressure or tension known as transpirational pull at the top of the plant.
3. Cohesion refers to the strong intermolecular hydrogen bonds holding water molecules together, which prevents the continuous water column from breaking under high tension.
4. Adhesion refers to the property where water molecules cling to the hydrophilic lignified walls of the xylem vessels, counteracting the downward pull of gravity.
5. Root pressure develops due to active ion accumulation in the root cortex, creating a weak hydrostatic push from below.
6. However, root pressure can only push water up to a height of a few meters and is completely insufficient to overcome gravitational pull and frictional resistance in tall trees exceeding 10 meters, making transpirational pull the primary driver.

Key takeaways

• Transpiration is the biological loss of water as water vapor primarily from the aerial parts of plants, generating a transpirational pull that draws water upward through xylem vessels.
• Stomatal transpiration accounts for 85% to 90% of total water loss, while cuticular transpiration contributes about 5% to 10% and lenticular transpiration accounts for about 0.1%.
• Guard cells regulate stomatal aperture through differential wall thickness, where elastic outer thin walls stretch more rapidly than inelastic inner thickened walls during turgidity.
• Active transport of potassium ions into guard cells increases internal osmotic pressure during daylight, leading to turgidity and the opening of the stomatal pore.
• Transpirational pull works alongside strong intermolecular cohesion between water molecules and adhesion to hydrophilic lignified xylem walls to maintain a continuous water column against gravity.
• High light intensity, higher temperature and wind increase transpiration rates, whereas high relative humidity and dry soil (poor water supply) decrease them.
• Guttation is the exudation of liquid water drops containing dissolved minerals through specialized hydathodes at leaf margins, driven by high root pressure during periods of low transpiration.
• Xerophytes develop structural adaptations such as thick cuticles, reduced leaf surface areas, and sunken stomata inside epidermal pits to survive severe water stress in arid environments.
• Ganong's potometer measures the rate of water intake by a cut shoot, operating on the principle that water uptake approximately equals the volume of water lost through transpiration.

Test yourself

What is the biological definition of transpiration?

Transpiration is the biological process where water is lost in the form of water vapor from the aerial parts of the plant, primarily through the leaves.

What percentage of total water loss is accounted for by stomatal transpiration?

Stomatal transpiration accounts for 85% to 90% of total water loss in typical terrestrial plants.

What is the primary function of subsidiary cells during stomatal movement?

Subsidiary cells are specialized accessory epidermal cells flanking guard cells that act as rapid ion and water reservoirs during stomatal opening and closing.

Which ion is actively transported into guard cells to initiate stomatal opening in daylight?

Potassium ions (K+) are actively transported from subsidiary cells into guard cells, increasing osmotic pressure and causing turgidity.

What physical property binds water molecules together to prevent the water column from breaking under high tension?

Cohesion, caused by strong intermolecular hydrogen bonds between water molecules, prevents the water column from breaking under high tension.

What stress hormone is released by root cells during severe drought to trigger rapid stomatal closure?

Abscisic acid is a stress hormone released by root cells during severe drought conditions to trigger rapid stomatal closure and conserve remaining moisture.

Through which specialized pore structures does guttation occur at leaf margins or tips?

Guttation occurs through specialized pore structures called hydathodes located at the margins or tips of leaves.

What color change does anhydrous cobalt chloride paper undergo upon coming into contact with water molecules?

Dry cobalt chloride paper is deep blue in color, but it turns pink upon coming into contact with water molecules.

How does a rise in temperature affect the rate of transpiration?

A rise in temperature increases the kinetic energy of water molecules, so evaporation inside the leaf speeds up and the rate of transpiration increases.