6 Most SAQ’s of Transport in Plants Chapter in Inter 2nd Year Botany (TS/AP)

4 Marks

SAQ-1 : Define and explain water potential.

Introduction

Water potential is a concept used to understand the movement of water within a plant. It helps explain the direction and tendency of water movement based on differences in water concentration and pressure.

Understanding Water Potential

Water potential, represented as (ψ_w), measures the tendency of water to move from one part of a plant to another. This movement could be due to various processes, such as diffusion and osmosis. The unit for water potential is Pascals (Pa).

In a standard environment (standard temperature and pressure), the water potential of pure water is set as zero. Any addition of solute or application of pressure changes this value.

Components of Water Potential

1. Solute Potential (ψ_s): Solute potential arises when a solute is dissolved in pure water. Adding solute reduces the concentration of pure water, thus lowering its water potential. This decrease is referred to as solute potential and it is always a negative value.
2. Pressure Potential (ψ_p): The second component is pressure potential. When water enters a plant cell due to osmosis, the pressure inside the cell increases. The cell becomes turgid, or swollen, increasing its water potential. This is known as pressure potential and it is always a positive value.

Calculating Total Water Potential

The total water potential in a plant system can be determined by adding the solute potential and pressure potential.

Total Water Potential, ψ_w = ψ_s (Solute potential) + ψ_p (Pressure potential)

Summary

Water potential is a critical factor determining water movement within a plant. It includes two main components: solute potential, which lowers water potential, and pressure potential, which increases it. By understanding water potential, we can predict the direction and rate of water movement within a plant, vital to studying plant physiology.

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SAQ-2 : What is meant by plasmolysis? How is it practically useful to us?

Introduction

Plasmolysis is a cellular process that occurs when plant cells lose water, causing the cell contents to shrink away from the cell wall. This process has practical implications in everyday life, especially in food preservation.

Understanding Plasmolysis

Plasmolysis is the process where the cell contents, or protoplast, of a plant cell shrinks due to water loss. This happens when a plant cell is placed in a hypertonic solution, a solution that has a higher concentration of solutes than the cell itself.

How Plasmolysis Happens

In a hypertonic solution, water molecules inside the plant cell move out into the solution due to osmosis, the process where water moves from a region of low solute concentration to high solute concentration. As water moves out of the cell, the cell membrane, which surrounds the protoplast, shrinks. The protoplast then pulls away from the cell wall. At this point, the cell is said to be plasmolysed.

Practical Uses of Plasmolysis

The principle of plasmolysis is used in several food preservation techniques. For example, when pickles are preserved in salt, the high salt concentration creates a hypertonic solution, causing the water inside bacterial cells to move out, leading to their death. This prevents spoilage of the pickles. The same principle is applied in the preservation of fish, meat, and prawns.

Summary

Plasmolysis is a process where a plant cell loses water and its protoplast shrinks away from the cell wall. This process is not just a biological phenomenon but is also used practically in the preservation of food, including pickles, fish, meat, and prawns, to inhibit bacterial growth and prolong their shelf life.

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SAQ-3 : How does ascent of sap occur in tall trees?

Introduction

The ascent of sap is the process through which water is transported from the roots to the rest of the plant. This process is crucial for the survival of tall trees and other plants. It involves multiple factors such as transpiration, cohesion, adhesion, and transpiration pull.

Understanding Ascent of Sap

Ascent of sap refers to the upward movement of water and dissolved minerals from the roots to other parts of the plant through the xylem, a type of plant tissue. This movement of sap occurs against the force of gravity, especially in tall trees.

Role of Transpiration

Transpiration, or the evaporation of water from plant surfaces (mainly leaves), plays a significant role in the ascent of sap. As water evaporates from the leaves, it creates a suction force that pulls more water up from the roots.

Key Physical Properties

The upward movement of xylem sap is driven by three physical properties:

1. Cohesion: This is the mutual attraction between water molecules due to hydrogen bonds. It helps to hold the water column together in the xylem.
2. Adhesion: This is the attraction of water molecules to the walls of the xylem vessels, which are polar surfaces. This helps to prevent the water column from breaking under tension.
3. Transpiration Pull: This is the main driving force for the upward movement of water. As water is lost from the leaves during transpiration, a negative pressure or tension is created in the xylem that pulls water up from the roots.

Influence of these Properties

These properties give water a high tensile strength, which allows it to resist the pull of gravity, and a high capillarity, which enables it to rise in narrow tubes. Both of these are necessary for the ascent of sap in tall trees.

The Role of Xylem

The process of photosynthesis, which is the plant’s way of producing food, requires water. The system of xylem vessels from the root to the leaf vein network supplies the needed water and nutrients absorbed by the roots from the soil.

Summary

The ascent of sap is an essential process for the survival and growth of plants. It involves the upward movement of water from the roots to the other parts of the plant through the xylem. This process is driven by transpiration from the leaves and the properties of cohesion and adhesion in water, creating a transpiration pull that helps the water defy gravity.

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SAQ-4 : “Transpiration is a necessary evil”. Explain

Introduction

Transpiration is the process by which water is lost from the plant through evaporation. Often described as a “necessary evil,” this term highlights the paradox where transpiration offers several crucial benefits to the plant but also poses potential drawbacks.

Benefits of Transpiration

1. Transpiration Pull: A key function of transpiration is creating the ‘transpiration pull’. This is the tension or pull exerted on the water column in the xylem due to water loss from the leaves, aiding in water absorption and transportation in plants.
2. Water Supply for Photosynthesis: Transpiration helps supply water required for photosynthesis, the process of converting light energy into chemical energy (glucose).
3. Transportation of Minerals: The transpiration pull also facilitates the transportation of minerals and nutrients from the soil to various plant parts.
4. Cooling Mechanism: Transpiration acts as a cooling mechanism. Similar to sweating in humans, water loss from plant surfaces leads to a cooling effect.
5. Maintains Plant Structure: By keeping plant cells turgid or firm, transpiration helps maintain the shape and structural integrity of plants.

Drawbacks of Transpiration

1. Risk of Flaccid Cells: Excessive transpiration can cause cells to become flaccid or soft, impacting plant structure and stability.
2. Potential for Retarded Growth: High rates of transpiration may lead to water stress, potentially slowing down plant growth.
3. High Water Loss: Transpiration can result in significant water loss, particularly detrimental in dry or drought conditions.
4. Limited Photosynthesis: High transpiration rates can deplete water needed for photosynthesis, indirectly limiting this process.

Summary

Transpiration is indeed a double-edged sword, a “necessary evil.” While it is essential for processes like transpiration pull, photosynthesis, nutrient transport, cooling, and structural maintenance, it also poses risks such as causing flaccid cells, retarding growth, leading to high water loss, and limiting photosynthesis. Thus, plants must maintain a careful balance in regulating transpiration to mitigate its adverse effects.

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SAQ-5 : Transpiration and photosynthesis a compromise. Explain.

Introduction

Photosynthesis and transpiration are two fundamental and interconnected processes in plants. The regulation of these processes often involves a compromise or trade-off, primarily centered around the opening and closing of stomata – the small openings on leaf surfaces.

Compromise between Transpiration and Photosynthesis

1. Need for Photosynthesis: Photosynthesis is the process where plants convert light energy into chemical energy. It requires carbon dioxide from the atmosphere and water from the soil, with carbon dioxide entering the leaves through stomata.
2. Role of Transpiration: Transpiration is where plants lose water to the atmosphere, mainly occurring through the stomata during gas exchange for photosynthesis. As water vapor exits, it creates a transpirational pull that draws more water and nutrients up from the roots.
3. The Compromise: Plants face a trade-off between acquiring sufficient carbon dioxide for photosynthesis and preventing excessive water loss through transpiration. In dry conditions, plants may close their stomata to conserve water, but this also limits carbon dioxide intake, reducing photosynthesis rates.
4. Adaptations: Plants have evolved adaptations for this compromise. Some open their stomata at night when temperatures are cooler and humidity is higher to reduce water loss. This adaptation is prevalent in plants in arid environments, like cacti and succulents.

Summary

The relationship between photosynthesis and transpiration demonstrates a crucial compromise for plants. They must open their stomata to absorb carbon dioxide for photosynthesis, which simultaneously leads to water loss through transpiration. To manage this balance and ensure survival, plants have developed various strategies and adaptations.

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SAQ-6 : Explain the structure and mechanism of opening and closing of stomata.

Introduction

Stomata are tiny pores on the surfaces of plants, especially leaves, playing a crucial role in regulating gas exchange and water loss. Each stoma is flanked by a pair of specialized cells called guard cells, which are responsible for the opening and closing of the stomata.

Structure

Guard Cells: In dicots, they are kidney or bean-shaped, while in monocots, they are dumbbell-shaped. These cells contain chloroplasts for photosynthesis, unlike other epidermal cells. The inner wall of the guard cell is thicker than the outer wall, which is essential for their functioning.

Mechanism of Opening and Closing

1. Levitt’s K+ Ion Pump Theory: This theory explains the turgidity of guard cells, which is controlled by the movement of potassium ions (K+) and changes in osmotic potential.
2. Daylight Photosynthesis: Potassium ions (K+) move into the guard cells from surrounding subsidiary cells. The influx of K+ decreases the water potential inside the guard cells, causing water to enter by osmosis. This results in guard cells becoming turgid, making the stomatal pore open.
3. Efflux of Hydrogen Ions (H+): This movement is accompanied by an efflux of H+ ions, increasing the pH level inside the guard cells.
4. Night or Stressful Conditions: At night or under stress, K+ ions are actively pumped out, leading to water efflux. This makes guard cells flaccid and closes the stomatal pore.
5. Abscisic Acid (ABA) in Water-Stressed Conditions: ABA is produced, prompting the efflux of K+ ions from guard cells, promoting stomatal closure to conserve water.

Summary

In certain succulent plants adapted to arid environments, an adaptation known as Crassulacean Acid Metabolism (CAM) photosynthesis occurs. Here, the accumulation of organic acids at night decreases the water potential of the guard cells, leading to stomatal opening during cooler, humid nights and closing during hot, dry days.