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Transport in Plants – CBSE NCERT Study Resources

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11th

11th - Biology

Transport in Plants

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Overview

This chapter explores the mechanisms of transport in plants, focusing on how water, minerals, and organic nutrients are moved within the plant body. It covers the processes of diffusion, osmosis, and active transport, as well as the role of xylem and phloem in long-distance transport.

Transport in Plants: The movement of water, minerals, and food materials within a plant to support its growth and metabolic activities.

Means of Transport

Plants use three primary mechanisms for transport:

  • Diffusion: The passive movement of molecules from a region of higher concentration to lower concentration.
  • Osmosis: The diffusion of water across a selectively permeable membrane.
  • Active Transport: The movement of substances against a concentration gradient, requiring energy (ATP).

Plant-Water Relations

Water is essential for plant survival, and its movement is governed by water potential (Ψ). Key concepts include:

  • Water Potential (Ψ): The potential energy of water per unit volume, influenced by solute potential (Ψₛ) and pressure potential (Ψₚ).
  • Plasmolysis: Shrinkage of the cytoplasm away from the cell wall due to water loss.
  • Imbibition: Absorption of water by hydrophilic substances, leading to swelling.

Long-Distance Transport

Plants rely on vascular tissues for long-distance transport:

  • Xylem: Transports water and minerals from roots to shoots via transpiration pull and root pressure.
  • Phloem: Translocates organic nutrients (e.g., sucrose) from source to sink through the pressure-flow hypothesis.

Transpiration

Transpiration is the loss of water vapor from plant surfaces, primarily through stomata. Factors affecting transpiration include:

  • Light intensity
  • Temperature
  • Humidity
  • Wind speed

Uptake and Transport of Mineral Nutrients

Mineral nutrients are absorbed by roots through active transport and transported via xylem. Essential elements include:

  • Macronutrients (e.g., nitrogen, phosphorus, potassium)
  • Micronutrients (e.g., iron, zinc, copper)

Summary

Transport in plants involves both short-distance (diffusion, osmosis) and long-distance (xylem, phloem) mechanisms. Understanding these processes is crucial for comprehending plant physiology and adaptation to environmental conditions.

All Question Types with Solutions – CBSE Exam Pattern

Explore a complete set of CBSE-style questions with detailed solutions, categorized by marks and question types. Ideal for exam preparation, revision and practice.

Very Short Answer (1 Mark) – with Solutions (CBSE Pattern)

These are 1-mark questions requiring direct, concise answers. Ideal for quick recall and concept clarity.

Question 1:
Define transpiration in plants.
Answer:

Loss of water as vapour from aerial parts of plants.

Question 2:
What is the role of root pressure in water transport?
Answer:

Pushes water upwards in xylem during low transpiration.

Question 3:
Name the theory explaining water movement in plants.
Answer:

Cohesion-tension theory.

Question 4:
Which cells are involved in phloem transport?
Answer:

Sieve tube elements and companion cells.

Question 5:
What is guttation?
Answer:

Exudation of water droplets from leaf margins.

Question 6:
How does active transport aid mineral uptake?
Answer:

Moves minerals against concentration gradient using energy.

Question 7:
Define apoplast pathway.
Answer:

Water movement through cell walls and intercellular spaces.

Question 8:
What is the function of casparian strips?
Answer:

Blocks apoplast pathway in endodermis, forcing symplast route.

Question 9:
Name the process by which sugars are loaded into phloem.
Answer:

Phloem loading.

Question 10:
What causes transpiration pull?
Answer:

Evaporation of water from leaf surfaces.

Question 11:
Which hormone regulates stomatal opening?
Answer:

Abscisic acid (ABA).

Question 12:
What is pressure flow hypothesis?
Answer:

Explains sugar transport in phloem via osmotic pressure.

Question 13:
How does temperature affect transpiration?
Answer:

Increases transpiration by raising vapour pressure.

Question 14:
Define imbibition.
Answer:

Absorption of water by hydrophilic colloids.

Question 15:
Name the tissue responsible for the transport of water in plants.
Answer:

The xylem tissue is responsible for the transport of water and minerals from roots to other parts of the plant.

Question 16:
How does cohesion-tension theory explain water movement in plants?
Answer:

The cohesion-tension theory states that water molecules stick together (cohesion) and are pulled upwards (tension) due to transpiration pull, creating a continuous water column in the xylem.

Question 17:
Why is transpiration called a 'necessary evil'?
Answer:

Transpiration is called a necessary evil because while it causes water loss, it is essential for nutrient uptake, cooling the plant, and maintaining turgor pressure.

Question 18:
Name the pores through which transpiration occurs.
Answer:

Transpiration occurs through stomata (tiny pores) present on the leaf surface, mainly on the lower epidermis.

Question 19:
What is the function of phloem in plants?
Answer:

The phloem transports organic nutrients (like sucrose) from leaves to other parts of the plant through translocation.

Question 20:
Differentiate between apoplast and symplast pathways.
Answer:
  • Apoplast: Water moves through cell walls and intercellular spaces without crossing membranes.
  • Symplast: Water moves through cytoplasm and plasmodesmata, crossing cell membranes.
Question 21:
What is the significance of translocation in plants?
Answer:

Translocation ensures distribution of food (like sugars) from source (leaves) to sink (growing parts, roots) via the phloem.

Question 22:
Explain the term imbibition with an example.
Answer:

Imbibition is the absorption of water by solid particles (like seeds) causing them to swell. Example: Swelling of dry seeds when soaked in water.

Very Short Answer (2 Marks) – with Solutions (CBSE Pattern)

These 2-mark questions test key concepts in a brief format. Answers are expected to be accurate and slightly descriptive.

Question 1:
Name the two types of vascular tissues in plants and their functions.
Answer:
  • Xylem: Transports water and minerals from roots to other parts.
  • Phloem: Transports organic nutrients (like sucrose) from leaves to other parts (translocation).
Question 2:
Explain the term guttation.
Answer:

Guttation is the exudation of water droplets from hydathodes (pores) at leaf margins, usually in high humidity conditions. It occurs due to root pressure and is different from transpiration.

Question 3:
What are plasmodesmata and their role in transport?
Answer:

Plasmodesmata are microscopic channels connecting adjacent plant cells. They allow symplastic movement of water, ions, and small molecules, facilitating cell-to-cell communication and nutrient sharing.

Question 4:
Why is transpiration called a necessary evil?
Answer:

Transpiration is necessary for cooling and nutrient uptake but evil because it can lead to excessive water loss, causing wilting or stress in plants.

Question 5:
What is the significance of turgor pressure in plants?
Answer:

Turgor pressure is the pressure exerted by cell contents against the cell wall. It maintains rigidity (prevents wilting), aids in cell elongation, and drives stomatal opening.

Question 6:
List two factors affecting the rate of transpiration.
Answer:
  • Light intensity: Opens stomata, increasing transpiration.
  • Humidity: Low humidity increases transpiration rate.
Question 7:
How does abscisic acid regulate transpiration?
Answer:

Abscisic acid (ABA) is a stress hormone that triggers stomatal closure during water scarcity, reducing transpiration to conserve water.

Short Answer (3 Marks) – with Solutions (CBSE Pattern)

These 3-mark questions require brief explanations and help assess understanding and application of concepts.

Question 1:
Explain the role of root pressure in the transport of water in plants.
Answer:

Root pressure is the positive pressure developed in the roots due to the active absorption of minerals and water from the soil.

It helps in pushing water upwards through the xylem, especially during low transpiration periods like early morning.

However, it is not the primary mechanism for water transport as it can only raise water to a limited height (a few meters).

Guttation is an example where root pressure forces water out through hydathodes.

Question 2:
Describe the cohesion-tension theory of water transport in plants.
Answer:

The cohesion-tension theory explains water movement in xylem due to:

1. Cohesion - Water molecules stick together due to hydrogen bonds.

2. Adhesion - Water molecules adhere to the walls of xylem vessels.

3. Transpiration pull - Evaporation from leaves creates tension, pulling water upwards.

This theory is the primary mechanism for water transport in tall plants.

Question 3:
Differentiate between apoplast and symplast pathways of water movement in roots.
Answer:

Apoplast pathway:

- Water moves through cell walls and intercellular spaces without crossing membranes.

- Faster but blocked at the Casparian strip in the endodermis.

Symplast pathway:

- Water moves through cytoplasm and plasmodesmata, crossing membranes.

- Slower but selective as it involves crossing the plasma membrane.

Question 4:
What is transpiration? List two factors affecting its rate.
Answer:

Transpiration is the loss of water vapour from aerial parts of plants, mainly through stomata.

Factors affecting transpiration rate:

1. Light intensity - Opens stomata, increasing transpiration.

2. Humidity - Lower humidity increases the rate as the gradient steepens.

Other factors include temperature, wind speed, and soil water availability.

Question 5:
How does phloem transport organic nutrients in plants?
Answer:

Phloem transports organic nutrients via translocation, primarily as sucrose.

The pressure flow hypothesis explains this process:

1. Loading - Sucrose is actively transported into sieve tubes at the source (e.g., leaves).

2. Osmosis - Water enters, creating high pressure.

3. Unloading - Sucrose is removed at the sink (e.g., roots), reducing pressure.

This pressure difference drives the flow.

Question 6:
Why is the Casparian strip important in root water absorption?
Answer:

The Casparian strip is a waterproof band of suberin in endodermal cell walls.

Its importance includes:

1. Forces water and minerals to pass through cell membranes, ensuring selective absorption.

2. Prevents backflow of water into the cortex, maintaining root pressure.

3. Filters out harmful substances, protecting the vascular system.

Question 7:
How does transpiration aid in the uptake of water and minerals in plants?
Answer:

Transpiration creates a negative pressure (tension) in the xylem, which:
1. Pulls water upwards from roots to leaves.
2. Enhances mass flow of minerals dissolved in water.
3. Maintains cooling of leaves and supplies water for photosynthesis.
Thus, it is a key driver of the transpiration stream.

Question 8:
What is the significance of the Casparian strip in the endodermis?
Answer:

The Casparian strip is a suberin-rich band in endodermal cell walls. Its roles include:
1. Blocking the apoplast pathway, forcing water/minerals into the symplast.
2. Regulating selective uptake by filtering toxins.
3. Preventing backflow of water, ensuring unidirectional movement to the xylem.

Question 9:
Explain how phloem loading and unloading occur during translocation.
Answer:

Phloem loading:
1. Sugars (e.g., sucrose) are actively transported into sieve tubes from source cells (leaves).
2. Water follows by osmosis, creating high pressure.

Phloem unloading:
1. Sugars are removed at sinks (roots/fruits) via diffusion or active transport.
2. Water exits, reducing pressure, maintaining the pressure flow hypothesis.

Long Answer (5 Marks) – with Solutions (CBSE Pattern)

These 5-mark questions are descriptive and require detailed, structured answers with proper explanation and examples.

Question 1:
Explain the transpiration pull theory and its role in water transport in plants. How does cohesion-tension support this process?
Answer:
Theoretical Framework

The transpiration pull theory explains how water moves upward in plants due to evaporation from leaves. Our textbook shows that cohesion-tension maintains a continuous water column in xylem.

Evidence Analysis
  • Water molecules stick together (cohesion) and adhere to xylem walls (adhesion).
  • Negative pressure from transpiration pulls water upward.
Critical Evaluation

Experiments like the potometer validate this theory. However, extreme drought can break the water column.

Future Implications

Understanding this helps improve crop irrigation strategies under water stress.

Question 2:
Describe the pressure flow hypothesis for phloem translocation. How do source and sink regulate this process?
Answer:
Theoretical Framework

The pressure flow hypothesis states that sugars move from source (leaves) to sink (roots/fruits) via phloem. We studied that osmotic pressure drives this flow.

Evidence Analysis
  • Active loading of sucrose at source creates high pressure.
  • Unloading at sink reduces pressure, maintaining flow.
Critical Evaluation

Radioactive tracer studies confirm this, but some plants use alternative mechanisms.

Future Implications

Enhancing sugar transport can boost agricultural yields, e.g., in sugarcane.

Question 3:
Compare apoplast and symplast pathways of water movement in roots. Why does the Casparian strip force a switch?
Answer:
Theoretical Framework

The apoplast involves cell walls, while the symplast uses plasmodesmata. Our textbook shows the Casparian strip blocks apoplast at endodermis.

Evidence Analysis
  • Apoplast is faster but allows passive transport.
  • Symplast is selective due to membrane control.
Critical Evaluation

Dyes like eosin prove apoplast dominance early, but Casparian strip ensures nutrient filtering.

Future Implications

This knowledge aids in developing drought-resistant root systems.

Question 4:
Analyze how root pressure and guttation contribute to water transport. When are these mechanisms most significant?
Answer:
Theoretical Framework

Root pressure pushes water upward via osmosis, while guttation releases excess water at leaf edges. We studied these occur in humid conditions.

Evidence Analysis
  • Root pressure is measured by manometers (e.g., 1-2 atm in tomato).
  • Guttation droplets contain minerals like potassium.
Critical Evaluation

These are minor compared to transpiration but vital when stomata are closed.

Future Implications

Studying guttation helps monitor plant hydration status.

Question 5:
Discuss the role of plant hormones in regulating transpiration. Give examples of abscisic acid (ABA) and cytokinins.
Answer:
Theoretical Framework

Plant hormones like ABA and cytokinins control stomatal movement. Our textbook shows ABA closes stomata during drought.

Evidence Analysis
  • ABA triggers ion efflux from guard cells.
  • Cytokinins promote stomatal opening in light.
Critical Evaluation

Mutant plants lacking ABA wilt faster, proving its role.

Future Implications

ABA sprays could reduce water loss in crops like wheat.

Question 6:
Explain the transpiration pull theory of water transport in plants. Discuss the role of cohesion, adhesion, and root pressure in this process.
Answer:

The transpiration pull theory explains how water is transported from roots to leaves in plants. It is primarily driven by the loss of water vapor from the aerial parts of the plant, especially through stomata in leaves.

Process:
1. Transpiration occurs when water evaporates from the leaf surface, creating a negative pressure (suction) in the xylem.
2. This negative pressure pulls water upward from the roots through the xylem vessels.
3. Cohesion (water molecules sticking to each other) and adhesion (water molecules sticking to xylem walls) help maintain a continuous water column.
4. Root pressure also contributes by pushing water upward, especially in smaller plants or during low transpiration periods.

Key Points:

  • Cohesion ensures water molecules stay together, preventing breakage of the column.
  • Adhesion helps water rise against gravity by sticking to xylem walls.
  • Root pressure is a secondary mechanism, active at night or in humid conditions.

This theory is vital for understanding how tall trees transport water efficiently without any mechanical pump.

Question 7:
Describe the mechanism of phloem translocation in plants. How does the pressure flow hypothesis explain the movement of organic nutrients?
Answer:

Phloem translocation refers to the transport of organic nutrients (like sucrose) from source (leaves) to sink (roots, fruits) in plants. The pressure flow hypothesis is the most widely accepted explanation for this process.

Mechanism:
1. Loading: Sucrose is actively transported into phloem sieve tubes at the source (e.g., leaves), creating a high solute concentration.
2. Osmosis: Water enters the phloem from xylem due to the high solute concentration, increasing turgor pressure.
3. Flow: The pressure gradient pushes the sap toward the sink (e.g., roots or fruits).
4. Unloading: At the sink, sucrose is removed, water exits, and pressure decreases, maintaining the flow.

Key Features:

  • Energy-dependent loading and unloading ensure directional movement.
  • The process is bidirectional, depending on the source-sink relationship.
  • Companion cells assist sieve tubes in active transport.

This hypothesis explains how plants distribute sugars efficiently for growth and storage.

Question 8:
Explain the process of transpiration in plants and discuss its significance. Also, mention any two factors affecting the rate of transpiration.
Answer:

Transpiration is the process by which plants lose water in the form of water vapor through the stomata present on the leaves. It is a crucial part of the water cycle in plants and helps in the upward movement of water and minerals from the roots to the leaves.

The process involves the following steps:
1. Water is absorbed by the root hairs from the soil through osmosis.
2. This water moves through the xylem vessels to reach the leaves.
3. In the leaves, water evaporates from the mesophyll cells into the intercellular spaces.
4. The water vapor then diffuses out through the stomata into the atmosphere.

Significance of transpiration:

  • It creates a transpiration pull that helps in the ascent of sap.
  • It cools the plant surface and maintains the temperature.
  • It helps in the absorption and transport of minerals.
  • It maintains the turgidity of cells, which is essential for growth.

Factors affecting the rate of transpiration:

  • Light intensity: Higher light intensity opens stomata wider, increasing transpiration.
  • Humidity: Lower humidity increases the rate of transpiration as the gradient between leaf and air is steeper.

Question 9:
Explain the process of transpiration pull in plants and its significance in the transport of water. Support your answer with a well-labeled diagram.
Answer:

The transpiration pull is a crucial mechanism in plants that facilitates the upward movement of water from the roots to the leaves. Here's a detailed explanation:

Process:
1. Transpiration occurs when water evaporates from the stomata in the leaves, creating a negative pressure (tension) in the xylem.
2. This tension pulls water molecules upward through the xylem vessels in a continuous column due to cohesion (water molecules sticking together) and adhesion (water molecules sticking to xylem walls).
3. The transpiration pull is strong enough to draw water from the roots, ensuring a steady supply to the leaves.

Significance:
- Enables long-distance water transport without energy expenditure by the plant.
- Helps in the absorption and transport of minerals dissolved in water.
- Cools the plant through evaporative cooling.
- Maintains cell turgidity, essential for growth and photosynthesis.

Diagram (Description for Labeling):
1. Draw a plant with roots, stem, and leaves.
2. Label the xylem vessels as continuous tubes from roots to leaves.
3. Show water molecules (as cohesive chains) moving upward.
4. Indicate stomata on leaves with water vapor exiting.

Value-Added Insight: The transpiration pull works under the cohesion-tension theory, which explains how physical properties of water and xylem structure enable this process without active energy use.

Question 10:
Explain the process of transpiration in plants and discuss its significance. Support your answer with a well-labelled diagram.
Answer:

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, primarily through the stomata in leaves. Here’s a detailed explanation:

Process:
1. Absorption: Roots absorb water from the soil via root hairs through osmosis.
2. Transport: Water moves upward through the xylem vessels due to transpiration pull and cohesion-tension.
3. Evaporation: Water evaporates from the surface of mesophyll cells into the intercellular spaces.
4. Stomatal release: Water vapor diffuses out through open stomata into the atmosphere.

Significance:

  • Cooling effect: Regulates plant temperature by evaporative cooling.
  • Nutrient transport: Facilitates the upward movement of minerals dissolved in water.
  • Water balance: Maintains turgor pressure for cell rigidity and growth.
  • Photosynthesis: Supplies water needed for the process.

Diagram (Description): A labelled diagram should include:
- Root hairs absorbing water.
- Xylem vessels transporting water.
- Stomata on the leaf surface releasing water vapor.
- Arrows indicating the direction of water movement.

Value-added note: Transpiration also creates a negative pressure in the xylem, which helps in continuous water uptake. Excessive transpiration can lead to wilting, but plants adapt with stomatal closure or cuticular thickening to reduce water loss.

Question 11:
Explain the process of transpiration pull in plants and its significance in the transport of water. Support your answer with a well-labelled diagram.
Answer:

The transpiration pull is a crucial mechanism that facilitates the upward movement of water in plants. It occurs due to the loss of water vapor from the aerial parts of the plant, primarily through the stomata in leaves. Here’s a detailed explanation:

Process:

  • Water evaporates from the surface of mesophyll cells into the intercellular spaces and exits through stomata, creating a negative pressure or tension in the xylem.
  • This tension pulls water upward from the roots through the xylem vessels, creating a continuous column of water due to cohesion (water molecules sticking together) and adhesion (water molecules sticking to xylem walls).
  • The process is passive and does not require energy, relying instead on physical forces.

Significance:

  • Enables the transport of water and dissolved minerals from roots to leaves.
  • Helps in cooling the plant surface through evaporation.
  • Maintains cell turgidity for structural support.

Diagram: (Draw a well-labelled diagram showing the path of water from roots to leaves, highlighting stomata, xylem vessels, and cohesion-adhesion forces.)

Value-added note: Transpiration pull is most effective in tall trees due to the strong cohesive forces of water molecules, ensuring uninterrupted water flow even at great heights.

Question 12:
Explain the process of transpiration in plants and discuss its significance. Support your answer with a well-labeled diagram.
Answer:

Transpiration is the process by which plants lose water in the form of water vapor through the stomata present on the leaves and other aerial parts. This process is primarily driven by the evaporation of water from the surface of mesophyll cells into the intercellular spaces, which then diffuses out through the stomata.

The steps involved in transpiration are as follows:
1. Absorption: Roots absorb water from the soil.
2. Transport: Water moves upwards through the xylem vessels.
3. Evaporation: Water evaporates from the mesophyll cells into the intercellular spaces.
4. Diffusion: Water vapor diffuses out through the stomata into the atmosphere.

Significance of transpiration:

  • It creates a transpiration pull that helps in the upward movement of water and minerals from roots to leaves.
  • It helps in the cooling of the plant surface by evaporative cooling.
  • It maintains the turgidity of cells, which is essential for growth and mechanical support.
  • It aids in the distribution of minerals and nutrients throughout the plant.

Diagram: A well-labeled diagram should include:
- Stomata on the leaf surface
- Mesophyll cells with water vapor
- Xylem vessels showing water transport
- Arrows indicating the direction of water movement and vapor diffusion.

Question 13:
Explain the pressure flow hypothesis of phloem translocation in plants. Describe how this process helps in the distribution of photosynthates from source to sink.
Answer:

The pressure flow hypothesis explains the mechanism of phloem translocation in plants, where photosynthates (mainly sucrose) are transported from source (leaves) to sink (roots, fruits, etc.). The process involves the following steps:


  • Loading at Source: Sucrose is actively transported into the phloem sieve tubes from photosynthetic cells, creating a high solute concentration.
  • Osmotic Water Entry: Water enters the phloem via osmosis, increasing turgor pressure.
  • Pressure-Driven Flow: The pressure gradient pushes the sap towards the sink.
  • Unloading at Sink: Sucrose is removed and utilized or stored, reducing pressure and completing the cycle.

This hypothesis ensures efficient nutrient distribution, supporting growth and storage in different plant parts. The process is bidirectional, adapting to seasonal changes in plant needs.

Question 14:
Describe the role of root pressure and transpiration pull in the ascent of water in plants. Compare their contributions under different environmental conditions.
Answer:

The ascent of water in plants is driven by two key mechanisms: root pressure and transpiration pull.


Root Pressure: This occurs when minerals are actively absorbed by roots, increasing osmotic potential and forcing water into xylem vessels. It is significant in small plants or at night when transpiration is low, often causing guttation.


Transpiration Pull: During the day, water evaporates from leaf surfaces (transpiration), creating negative pressure in xylem. This tension pulls water upward in a continuous column, supported by cohesion-tension theory.


Comparison:

  • In humid or cool conditions, root pressure dominates due to reduced transpiration.
  • In hot/dry climates, transpiration pull is the primary driver, as rapid water loss creates strong tension.

Both mechanisms work synergistically, ensuring efficient water transport under varying environmental conditions.

Question 15:
Explain the transpiration pull theory for the ascent of sap in plants. How does this process contribute to the continuous movement of water from roots to leaves?
Answer:

The transpiration pull theory, also known as the cohesion-tension theory, explains the upward movement of water (ascent of sap) in plants. It is primarily driven by the loss of water vapor from the leaves through transpiration.

Process:
1. Transpiration occurs when water evaporates from the leaf surface through stomata.
2. This creates a negative pressure or tension in the xylem vessels.
3. Due to the cohesive forces between water molecules (cohesion) and adhesive forces between water and xylem walls (adhesion), a continuous water column is maintained.
4. The tension pulls water upward from the roots to replace the lost water, creating a transpiration stream.

Contribution to Continuous Movement:
- The transpiration pull ensures a steady flow of water and dissolved minerals from roots to leaves.
- It helps in cooling the plant and maintaining turgidity.
- The process is passive and does not require energy, relying on physical forces.

Additional Insight: Factors like humidity, temperature, and wind speed influence the rate of transpiration, thereby affecting the transpiration pull.

Question 16:
Describe the role of root pressure in the transport of water in plants. How does it differ from the transpiration pull mechanism?
Answer:

Root pressure is a positive hydrostatic pressure developed in the roots due to the active absorption of ions and water from the soil. It plays a secondary role in water transport, especially in small plants and during low transpiration periods.

Role of Root Pressure:
1. Active transport of minerals into the root xylem lowers its water potential.
2. Water follows by osmosis, creating a push (root pressure) that forces water upward.
3. This is evident as guttation, where water droplets are exuded from leaf margins in humid conditions.

Differences from Transpiration Pull:

  • Mechanism: Root pressure is an active process (requires energy), while transpiration pull is passive.
  • Force: Root pressure pushes water, whereas transpiration pull creates suction.
  • Magnitude: Root pressure is weaker (1-2 atm) compared to transpiration pull (up to 20 atm).
  • Occurrence: Root pressure is significant at night or in humid conditions; transpiration pull dominates during the day.

Additional Insight: Root pressure alone cannot account for water transport in tall trees, where transpiration pull is the primary driver.

Case-based Questions (4 Marks) – with Solutions (CBSE Pattern)

These 4-mark case-based questions assess analytical skills through real-life scenarios. Answers must be based on the case study provided.

Question 1:
A student observed that a potted plant wilted when kept in bright sunlight but recovered when shifted to shade. Explain the role of transpiration pull and root pressure in this scenario.
Answer:
Case Deconstruction

Wilting occurs due to excessive water loss via transpiration in sunlight, disrupting the transpiration pull. Shade reduces transpiration, allowing recovery.

Theoretical Application
  • Transpiration pull: Cohesion-tension theory explains upward water movement in xylem.
  • Root pressure: Active water absorption by roots helps restore turgor pressure.
Critical Evaluation

Our textbook shows root pressure alone cannot compensate for rapid transpiration loss, highlighting the interdependence of these processes.

Question 2:
Farmers often use sprinkler irrigation at dawn/dusk. Analyze how this practice optimizes water uptake efficiency and minimizes guttation.
Answer:
Case Deconstruction

Low temperatures at dawn/dusk reduce transpiration, enhancing water uptake efficiency by roots.

Theoretical Application
  • Reduced transpiration prevents xylem sap tension, minimizing guttation (water exudation via hydathodes).
  • NCERT data shows 20-30% higher water retention with timed irrigation.
Critical Evaluation

We studied that guttation indicates root pressure dominance, but excessive night irrigation may cause nutrient leaching.

Question 3:
Compare the apoplast and symplast pathways using evidence from root cross-sections. How does the Casparian strip alter their dynamics?
Answer:
Case Deconstruction

Apoplast involves cell wall movement, while symplast uses plasmodesmata for cytoplasmic transport.

Theoretical Application
  • [Diagram: Root cross-section] shows apoplast bypassing cells until endodermis.
  • Casparian strip forces solute entry into symplast, enabling selective absorption.
Critical Evaluation

Our textbook confirms this switch prevents passive pathogen entry, demonstrating evolutionary adaptation.

Question 4:
A desert plant shows succulent stems but no leaves. Relate this to adaptations in photosynthetic pathways and transpiration rates.
Answer:
Case Deconstruction

Leafless morphology reduces surface area for transpiration, while stems store water and perform CAM photosynthesis.

Theoretical Application
  • CAM pathway: Stomata open at night, fixing CO2 as malate to minimize daytime water loss.
  • Transpiration rates drop to 1/10th of mesophytes (NCERT Fig. 11.6).
Critical Evaluation

We studied that such trade-offs between photosynthesis and water conservation define xerophytic success.

Question 5:
Lab data shows higher xylem sap flow in windy conditions despite stomatal closure. Resolve this paradox using the cohesion-tension theory.
Answer:
Case Deconstruction

Wind increases transpiration pull by enhancing water vapor removal from leaf surfaces, overriding stomatal resistance.

Theoretical Application
  • Cohesion-tension theory: Stronger pull creates greater negative pressure in xylem.
  • Example: 15% flow increase recorded at 20km/h winds (NCERT data).
Critical Evaluation

We studied that extreme winds may cavitate xylem vessels, showing the theory's limitation threshold.

Question 6:
A student observed that transpiration rates varied in two plants: one in shade and one in sunlight. Using the cohesion-tension theory, explain why the plant in sunlight showed higher transpiration. Also, analyze how stomatal regulation impacts this process.
Answer:
Case Deconstruction

The plant in sunlight had higher transpiration due to increased evaporation from leaves, creating a stronger transpiration pull.

Theoretical Application
  • Sunlight opens stomata, enhancing water vapor loss.
  • Cohesion-tension theory explains water column continuity from roots to leaves.
Critical Evaluation

Our textbook shows stomatal closure reduces transpiration, balancing water loss and CO2 uptake. Example: Desert plants limit stomatal opening during the day.

Question 7:
In an experiment, a potometer showed slower water uptake in a wilted plant. Link this to root pressure and capillary action. How does turgor pressure restoration aid recovery?
Answer:
Case Deconstruction

Wilting reduces root pressure, slowing water uptake. The potometer reflects this deficit.

Theoretical Application
  • Capillary action in xylem relies on water adhesion to walls.
  • Turgor pressure is restored via osmosis, reopening stomata.
Critical Evaluation

We studied that root pressure is critical at night (e.g., guttation in grasses). Example: Overwatered plants regain turgor faster.

Question 8:
Compare apoplast and symplast pathways in a root cross-section. How does the Casparian strip ensure selective absorption? Use a [Diagram: Root cross-section] for reference.
Answer:
Case Deconstruction

The apoplast involves cell walls, while the symplast uses plasmodesmata for transport.

Theoretical Application
  • Casparian strip blocks apoplastic flow, forcing solutes into cells.
  • This enables selective ion uptake, e.g., K+ over Na+.
Critical Evaluation

Our textbook highlights endodermis as a checkpoint. Example: Halophytes exclude salts via this mechanism.

Question 9:
Analyze how phloem loading (using sucrose) differs in spring (high growth) vs. winter (dormancy). How does pressure flow hypothesis explain this?
Answer:
Case Deconstruction

In spring, phloem loading is active, transporting sucrose to buds. Winter reduces demand.

Theoretical Application
  • Pressure flow requires source-sink gradients (e.g., leaves to roots).
  • ATP-driven active transport loads sucrose at sources.
Critical Evaluation

We studied maple trees storing sucrose in roots pre-winter. Example: Potato tubers act as sinks in autumn.

Question 10:
A student observed that transpiration rates varied significantly between a sunflower and a cactus under identical conditions. Explain the physiological and structural adaptations causing this difference.
Answer:
Case Deconstruction

We studied that transpiration is influenced by leaf structure and environmental adaptations. The sunflower, a mesophyte, has broad leaves with numerous stomata, increasing water loss. The cactus, a xerophyte, has reduced leaves (spines) and thick cuticles to minimize transpiration.

Theoretical Application
  • Sunflower: High transpiration due to large surface area and stomatal density.
  • Cactus: Low transpiration due to sunken stomata and water-storing tissues.
Critical Evaluation

Our textbook shows that xerophytes like cacti evolved in arid conditions, while mesophytes like sunflowers thrive in moderate climates, explaining their differing transpiration rates.

Question 11:
In an experiment, a plant's root pressure was measured before and after removing its root hairs. Analyze the expected change and justify with biological principles.
Answer:
Case Deconstruction

We learned that root pressure is generated by active ion transport into the xylem, creating osmotic water uptake. Root hairs significantly increase surface area for absorption.

Theoretical Application
  • Before removal: High root pressure due to efficient water uptake via root hairs.
  • After removal: Reduced root pressure as absorption area decreases.
Critical Evaluation

Our textbook highlights that root hairs are critical for osmosis-driven water movement. Their removal disrupts this process, validating the observed decline in root pressure.

Question 12:
Compare the apoplast and symplast pathways in terms of speed and regulatory mechanisms. Provide two examples where each pathway dominates.
Answer:
Case Deconstruction

We studied that the apoplast pathway is faster but unregulated, while the symplast involves controlled movement through plasmodesmata.

Theoretical Application
  • Apoplast: Dominates in young roots (e.g., pea seedlings) due to loose cell walls.
  • Symplast: Active in mature roots (e.g., wheat) where Casparian strips block apoplast flow.
Critical Evaluation

Our textbook shows the apoplast is efficient for bulk flow, but the symplast ensures selective uptake, aligning with their roles in different tissues.

Question 13:
A wilted plant regained turgidity after watering, but another with a blocked xylem did not. Explain the role of cohesion-tension theory in this observation.
Answer:
Case Deconstruction

We learned that the cohesion-tension theory explains water ascent via transpiration pull and hydrogen bonding in xylem vessels.

Theoretical Application
  • Healthy plant: Water restored turgor as cohesion-tension pulled water upward.
  • Blocked xylem: Disrupted water column prevented tension transmission.
Critical Evaluation

Our textbook confirms that xylem blockage halts the continuous water column, invalidating the cohesion-tension mechanism, which matches the observed results.

Question 14:
A farmer observed wilting in his crop during midday despite adequate soil moisture. Upon consulting an expert, he learned about transpiration pull and root pressure. Explain how these processes contribute to water transport in plants and why wilting occurred.
Answer:

In plants, water transport occurs through the xylem via two key mechanisms: transpiration pull and root pressure.

  • Transpiration pull: Water evaporates from the stomata in leaves, creating a suction force that pulls water upward from the roots. This is the primary driver of water movement.
  • Root pressure: Active transport of minerals into the root xylem creates osmotic pressure, pushing water upward. This is significant at night when transpiration is low.

Wilting occurred because midday heat increased transpiration rate, but the plant's water loss exceeded its absorption capacity, causing temporary water deficit. Root pressure alone couldn't compensate, leading to loss of turgor pressure in leaves.

Question 15:
A student conducted an experiment where a leafy shoot was placed in a beaker of water with red dye. After an hour, red streaks appeared in the veins. Explain the phenomenon and describe the pathway of water movement in plants.
Answer:

The red streaks indicate water uptake through the xylem vessels, demonstrating the transpiration stream. Here's the pathway:


1. Absorption: Water enters root hairs via osmosis due to higher solute concentration in root cells.
2. Root cortex: Water moves through the apoplast (cell walls) and symplast (cytoplasm).
3. Endodermis: The Casparian strip forces water into cells, ensuring selective mineral absorption.
4. Xylem: Water is transported upward via cohesion-tension theory (hydrogen bonding between water molecules).
5. Leaves: Water evaporates through stomata, creating transpiration pull.

The dye traces this pathway, confirming xylem's role in unidirectional water transport.

Question 16:
A potted plant was placed in a humid environment with its roots submerged in a nutrient solution. After a few days, droplets of water appeared on the edges of its leaves. Explain the phenomenon and the underlying mechanism involved.
Answer:

The phenomenon observed is called guttation, which occurs when water is forced out of the plant through specialized pores called hydathodes located at the leaf margins.

Mechanism:

  • In humid conditions, transpiration is reduced, causing root pressure to build up due to active absorption of water and minerals.
  • The xylem sap is pushed upwards under pressure, and excess water is expelled through hydathodes as droplets.

Key factors: High humidity, moist soil, and reduced transpiration promote guttation. This process is common in herbaceous plants like grasses.

Question 17:
A student observed that a leafy shoot, when placed in a beaker of water with red dye, showed the dye moving upward through the stem. After sectioning the stem, the dye was found concentrated in certain tissues. Identify the tissue responsible and explain the process.
Answer:

The tissue responsible for the upward movement of the red dye is the xylem, specifically the xylem vessels and tracheids.

Process:

  • The dye is absorbed by the roots through osmosis and enters the xylem.
  • Water and dissolved dye move upward due to transpiration pull, created by water loss from leaves.
  • Cohesion (water molecules sticking together) and adhesion (water sticking to xylem walls) aid in continuous water column formation.

Observation: The dye traces the path of water transport, confirming xylem's role in ascent of sap.

Question 18:
A student observed that a potted plant showed wilting of leaves despite regular watering. On examining the roots, they found them to be waterlogged. Using your knowledge of transport in plants, explain the possible reasons for this observation and suggest a remedy.
Answer:

The wilting of leaves despite regular watering is likely due to waterlogging, which affects the root system and disrupts transpiration. Here's why:

  • Oxygen Deprivation: Waterlogged soil lacks air spaces, depriving roots of oxygen. This hampers respiration, reducing active transport of minerals and water uptake.
  • Root Damage: Prolonged waterlogging can cause root rot, further limiting water absorption.

Remedy: Ensure proper drainage by repotting the plant in well-aerated soil and avoid overwatering. Adding perlite or sand to the soil can improve drainage.

Question 19:
In an experiment, a leafy shoot was placed in a beaker containing water with a dye. After some time, the dye was observed in the veins of the leaves. Explain the process involved and its significance in plants.
Answer:

The movement of dye into the leaf veins demonstrates transpiration pull and cohesion-tension theory in xylem transport. Here's how it works:

  • Water Uptake: The dye dissolved in water is absorbed by the root hairs through osmosis.
  • Xylem Transport: Water (with dye) moves upward via the xylem vessels due to transpiration pull created by water loss from leaves.
  • Cohesion-Adhesion: Hydrogen bonding between water molecules (cohesion) and their attraction to xylem walls (adhesion) aid this upward movement.

Significance: This process ensures water and mineral distribution to all plant parts, maintaining turgidity and supporting photosynthesis.

Question 20:
A student coated the lower surface of a leaf with grease and observed that the plant wilted after 2 days, while an untreated plant remained healthy. Analyze the role of the leaf surface in plant water transport and explain the student's observation.
Answer:

The experiment demonstrates the critical role of stomata (mostly on the lower leaf surface) in transpiration, the process driving water uptake in plants.

Key points:
1. Grease blocks stomata, preventing water vapor loss (transpiration).
2. Without transpiration, the transpiration pull weakens, reducing water absorption by roots.
3. The plant wilts due to water deficiency, while the untreated plant maintains turgidity.

Additional insight: Stomatal closure also limits gas exchange, affecting photosynthesis. This shows how transport in plants integrates water loss and carbon gain.

Question 21:
A student observed that a potted plant showed wilting of leaves despite regular watering. On examining the roots, they found them to be damaged. Explain the physiological process affected due to damaged roots and how it leads to wilting.
Answer:

The wilting of leaves in the potted plant is due to the disruption of water absorption and transport caused by damaged roots. Roots are responsible for absorbing water from the soil through root hairs via osmosis. When roots are damaged, this absorption is severely reduced.


Water is transported upwards through the xylem via transpiration pull and root pressure. Damaged roots cannot generate sufficient root pressure, leading to reduced water supply to the leaves. As a result, turgor pressure in leaf cells decreases, causing wilting.


Additionally, damaged roots may impair the uptake of essential minerals, further affecting plant health. Thus, the lack of water and nutrients disrupts photosynthesis and overall plant growth.

Question 22:
In an experiment, a leafy shoot was placed in a beaker containing water with red dye. After some time, the veins of the leaves turned red. Explain the phenomenon and the tissue responsible for this observation.
Answer:

The red coloration in the leaf veins is due to the transport of water along with the red dye through the xylem tissue. The xylem is responsible for the upward movement of water and dissolved minerals from roots to other parts of the plant.


The process occurs due to:

  • Transpiration pull: Evaporation of water from leaf surfaces creates a suction force.
  • Cohesion-tension theory: Water molecules stick together (cohesion) and are pulled upward under tension.


The dye moves along with water through the xylem vessels, staining the veins red. This experiment demonstrates the unidirectional transport in xylem and its role in water conduction.

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