Chapter – 3B : Transpiration | Chapter Solution Class 9

(a) Holophytic

Explanation: Holophytic nutrition refers to autotrophic nutrition in plants, where they make their own food using sunlight, carbon dioxide, and water through photosynthesis. Most green plants follow this mode of nutrition.

(b) Soil water

Explanation: Plants absorb water from the soil through root hairs. This water provides hydrogen atoms that are used in the photosynthesis process to form glucose. Atmospheric or pond water is not directly absorbed.

(d) Thickness of leaves

Explanation: Potassium regulates turgor pressure in cells and aids in stomatal movement. It leads to healthy leaf development and increases leaf thickness by maintaining water balance.

(b) Stop flowering

Explanation: Molybdenum is essential for nitrogen metabolism. A deficiency due to poor solubility in soil can lead to delayed or stopped flowering and reduced plant growth.

(a) Guttation

Explanation: Guttation is the loss of water in liquid form through hydathodes (water pores) found at the edges of leaves, especially in the early morning or at night when transpiration is low.

(c) Transpiration

Explanation: Transpiration is the evaporation of water mainly through stomata of aerial parts like leaves and stems. It helps in cooling the plant and transporting minerals.

(a) Water stomata

Explanation: These pores, also called hydathodes, allow the release of excess water during guttation. Unlike regular stomata, they do not close and are mostly found at the leaf edges.

Physiological

Explanation: It is a vital life process of plants that helps in cooling, water movement, and mineral uptake, making it a physiological activity.

Diffusion

Explanation: Diffusion involves movement of molecules from higher to lower concentration along a gradient in a single direction, without needing energy.

Physical

Explanation: It is physical in nature because it involves the evaporation of water from plant surfaces, mainly due to external environmental factors like heat and wind.

Diffusion

Explanation: Diffusion can occur through any medium (even in open air or liquids) and does not require a membrane, unlike osmosis which needs a semipermeable one.

Cell wall

Explanation: The plant cell wall allows free movement of water and small molecules in and out of the cell, making it permeable.

True

Explanation: Stomata are microscopic openings mainly on the underside of leaves, surrounded by guard cells, responsible for gas exchange and transpiration.

True
Explanation: More light causes wider opening of stomata, which increases water loss through transpiration. Hence, bright sunlight speeds up the process.

True

Explanation: Osmotic pressure is the pressure required to stop the flow of water through a semipermeable membrane during osmosis. It reflects the pulling force of solutes.

False

Explanation: Parchment paper is actually semipermeable, allowing only certain substances to pass through. It’s commonly used in osmosis experiments.

True

Explanation: Stomatal transpiration accounts for about 90% of water loss in plants. Stomata are, therefore, the primary route for transpiration.

(a) Transpiration → (c) Physiological process

(b) Guard cells → (e) Opening and closing of stomata

(c) Xylem water column → (d) Move upwards

(d) Osmosis → (b) Semipermeable membrane

(e) Active transport → (a) ATP required

Water vapor

Explanation: During transpiration, plants lose water from aerial parts (mainly leaves) in the form of water vapor through stomata into the atmosphere.

Guard cells

Explanation: Guard cells are specialized kidney-shaped cells that surround each stomatal pore and control its opening and closing to regulate transpiration and gas exchange.

Temperature, light, humidity, wind

Explanation: These environmental factors affect the rate of transpiration. High temperature, strong light, low humidity, and wind increase transpiration.

Roots

Explanation: Plants absorb water and minerals mainly through their roots, especially via root hairs from the soil by osmosis.

Absorption of water by dry substances.

Explanation: Imbibition is the process by which dry plant materials like seeds or wood absorb water and swell, without forming a solution.

Phloem

Explanation: Phloem tissue conducts food and nutrients in both upward and downward directions, distributing them throughout the plant.

Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration through a semipermeable membrane. It is a passive transport process important for water absorption in plants.

Diffusion is the movement of molecules (gas or solute) from a region of higher concentration to lower concentration until equilibrium is reached. It occurs without a membrane and is a passive process.

Absorbed water enters the root hairs by osmosis, moves cell to cell through cortex, and finally reaches xylem vessels via the endodermis. It is transported upward through xylem by root pressure and transpiration pull.

Ascent of sap refers to the upward movement of water and dissolved minerals from roots to leaves through xylem vessels. It is mainly driven by transpiration pull, cohesion, and root pressure.

Cohesive force is the mutual attraction between water molecules. It helps in forming a continuous water column inside the xylem, which aids in the upward movement of water during transpiration.

• Ascent of sap occurs through xylem and moves water and minerals only upward.
• Transportation of solutes occurs through phloem and moves food (sugar) both upward and downward.

Isotonic solution: A solution that has equal solute concentration as that inside the cell, causing no net movement of water, is called an isotonic solution.

hypotonic solution: A solution that has lower solute concentration than the cell, causing water to enter the cell, is called a hypotonic solution.

hypertonic solution: A solution that has higher solute concentration than the cell, causing water to leave the cell, is called a hypertonic solution.

Passive transport is the movement of molecules or ions across a cell membrane from a region of higher concentration to lower concentration without using cellular energy (ATP). It occurs along the concentration gradient and includes processes like diffusion and osmosis.

Active transport is the movement of molecules or ions across the cell membrane against the concentration gradient (from lower to higher concentration) using energy in the form of ATP.
It is essential for the uptake of minerals and nutrients even when their concentration is higher inside the cell than outside. Carrier proteins or pumps are involved in this process (e.g., sodium-potassium pump).

The transport of two different substances in the same direction across a membrane using a common carrier protein, often with the help of energy, is called symport.

The significance of transpiration are

1. Helps in the ascent of sap by creating transpiration pull.
2. Cools the plant during high temperatures.
3. Aids in the movement of minerals from roots to leaves.
4. Maintains water balance in the plant body.

Endosmosis is the inward movement of water into a cell when it is placed in a hypotonic solution (i.e., the external solution has lower solute concentration than the cell sap).

Due to osmosis, water enters the cell through the semipermeable membrane, causing the cell to swell or become turgid. This process is important for maintaining turgor pressure in plant cells.

The rate of transpiration is influenced by several external (environmental) factors, including:

1. Temperature: Higher temperatures increase evaporation from leaf surfaces, thereby increasing transpiration.
2. Light Intensity: Light causes stomata to open, enhancing transpiration. The rate is higher during the day than at night.
3. Humidity: Low humidity in the atmosphere increases the rate of transpiration because the water vapor gradient between leaf and air becomes steeper.
4. Wind/ Air Movement: Wind removes the saturated layer of air near the leaf surface, enhancing transpiration.

Take a medium-sized well-watered potted plant. Cover the plant with a transparent bell jar and tie its mouth around the base of the stem. Leave the plant in sunlight for an hour or two. Drops of water will soon appear on the inner side of the bag due to the saturation of water vapour given out by the leaves (the water vapours condense only if the outside temperature is cool enough). This show that plants transpire water in the form of vapour.

Water is drawn into the root hair wall from the soil by imbibition and then moves across the cell membrane into the cell sap by osmosis. The osmotic uptake of extra water increases the volume of root hair cells, but they do not swell much because the water is quickly transferred to the inner cortex cells. Water continues to move from one cell to another by cell-to-cell osmosis through the cortex and endodermis. It is then absorbed into the xylem cells by osmotic pressure created by the surrounding turgid parenchyma cells. This pressure that helps push water into the xylem is known as root pressure.

The upward movement of water and dissolved minerals from the roots to the leaves through the xylem tissue in plants due to forces like root pressure, transpiration pull, and cohesive forces between water molecules is called ascent of sap.

Theories Explaining Ascent of Sap:

Root Pressure Theory: The Root Pressure Theory suggests that due to water absorption by roots, a pressure develops in the tracheary elements of the xylem, which can push water upward. This root pressure can lift water up to a small height (about 2 atmospheric pressure) and is therefore considered insufficient for tall trees, making it more applicable to small plants.

Transpiration–Cohesion–Tension Theory (Most accepted):

Transpiration–Cohesion–Tension Theory, proposed by Dixon and Jolly in 1895. According to this theory, water forms a continuous unbroken column in the xylem from roots to leaves. Water is pushed upward by root pressure and simultaneously pulled upward by transpiration pull, which is caused by the evaporation of water from the leaf surface. This push-pull mechanism, along with the cohesive force (the attraction between water molecules), maintains the continuity of the water column and helps in the efficient upward movement of water, even in tall trees.

(i) Stomatal transpiration :

It is the main site of transpiration in plants. Stomata are present on the ventral surface of dorsiventral leaf and the both surfaces of isobilateral leaf. Loss of water vapour through the openings of stomata is termed as stomatal transpiration. Stomata are tiny apertures present on the leaf surfaces. Most (about 80–90%) of the transpiration from leaves occurs through the stomata.

(ii) Cuticular transpiration :

The cuticle is a waxy layer on the leaf covering the epidermis. There are certain cracks in cuticle through which transpiration occurs. Loss of water vapour through the cuticle exposed surfaces of plants is termed cuticular transpiration. Smaller amount (10–12%) of water vapour is lost from leaves by direct evaporation from the epidermal cells through the cuticle.

(iii) Lenticular transpiration :

Minute lens-shaped pores present in the epidermis of woody stem are called lenticle. Loss of water vapour through the lenticels or tiny pores of woody stems and fruits is called lenticular transpiration. Lenticels help transpiration as they always remain open, and the water vapour escapes through the loose mass of complementary cells of the lenticels.