RBSE Solutions Class 12 Biology Chapter 7 Transpiration

Rbse Class 12 Biology Chapter 7 Multiple Choice Questions

 

Question 1. Transportation does not occur from which of the following parts?
(a) Leaf
(b) stem
(c) Root
(d) Unripe fruit
Answer: (c) Root
In simple words: Water loss from plants, called transpiration, happens mostly through leaves and stems. Roots are used to absorb water, not release it.

🎯 Exam Tip: Remember that transpiration is primarily a process of water loss from aerial parts of the plant, not from underground structures like roots.

 

Question 2. Does maximum transpiration occur from?
(a) Stomata
(b) Lenticels

 

Question 3. Hydathodes are found situated?
(a) At the margin of the leaf
(b) On the upper surface of leaves
(c) On the lower surface of leaves
(d) In the bark
Answer: (a) At the margin of the leaf
In simple words: Hydathodes are special pores on plants, usually found at the edges of leaves, which release water droplets during guttation. They are like tiny taps for excess water.

🎯 Exam Tip: Distinguish hydathodes from stomata; hydathodes are always open and involved in guttation, while stomata regulate gas exchange and transpiration.

 

Question 4. The fluid secreted out through hydathodes in plants is?
(a) Pure water
(b) Only excreted substances
(c) Water and \( \{CO\}_{2} \)
(d) Water containing organic and inorganic substances
Answer: (d) Water containing organic and inorganic substances
In simple words: The liquid released by hydathodes is not pure water. It contains water mixed with other things like minerals and sugars that the plant is getting rid of.

🎯 Exam Tip: Remember that guttation fluid is often not pure water, but a dilute solution, which helps distinguish it from dew.

Rbse Class 12 Biology Chapter 7 Very Short Answer Questions

 

Question 1. How many types of transpiration is found? Name these.
Answer: Transpiration, the process of water loss from plants, occurs in three main ways:

• Stomatal transpiration
• Cuticular transpiration
• Lenticular transpiration

Each type depends on the specific plant structure through which water vapor is released.
In simple words: There are three kinds of transpiration: stomatal (through small holes), cuticular (through the waxy layer), and lenticular (through tiny pores on bark).

🎯 Exam Tip: Clearly list the three types of transpiration and briefly explain where each occurs for full marks.

 

Question 2. Maximum transpiration takes place by which processes and how much is it?
Answer: Most of the transpiration, the process where plants lose water vapor, happens through stomata. About 80-90% of all water lost by a plant through transpiration occurs this way. Stomata are tiny pores mostly on the underside of leaves that regulate gas exchange.
In simple words: Most water loss from plants happens through tiny holes called stomata. This accounts for 80-90% of all the water lost.

🎯 Exam Tip: Remember the stomata are the primary sites for water loss and state the percentage range to show a comprehensive understanding.

 

Question 4. Name the scientist(s) who proposed the principle of active \( \{K\}^{+} \) transport.
Answer: The idea of active potassium ion \( (\{K\}^{+}) \) uptake was first suggested by scientists Imamura and Fujino in 1959. Later, in 1974, Levitt made changes and additions to this theory, refining our understanding of how stomata open and close. This theory helps explain how plants regulate water loss.
In simple words: Imamura and Fujino first thought of the active \( \{K\}^{+} \) transport idea in 1959, and Levitt updated it in 1974.

🎯 Exam Tip: Accurately recall the names of both Imamura & Fujino and Levitt, along with their respective contributions, to answer this question fully.

 

Question 5. Write one benefit of transpiration.
Answer: One important benefit of transpiration is that it helps in the ascent of sap. This means the upward movement of water and minerals from the roots to the leaves and other parts of the plant relies on the pull created by transpiration. Without this pull, water would struggle to reach the top of tall plants.
In simple words: Transpiration helps pull water and nutrients up from the roots to all parts of the plant.

🎯 Exam Tip: Focus on the "transpiration pull" mechanism as a key benefit, linking it to water and mineral transport.

 

Question 6. What is the state of guard cells at the time of opening of stomata?
Answer: When stomata open, the guard cells surrounding the pore are in a turgid state. This means they are swollen with water, which makes them bend outwards and create an opening for gases to pass through. Turgidity is essential for the mechanical movement of guard cells.
In simple words: When stomata open, the guard cells are full of water and swollen, called the turgid state.

🎯 Exam Tip: The key term here is "turgid state"; ensure you use it correctly and understand why it leads to stomatal opening.

Rbse Class 12 Biology Chapter 7 Short Answer Questions

 

Question 1. Define transpiration.
Answer: Transpiration is the process where plants lose water in the form of water vapor. This water vapor is released into the air from the parts of the plant that are above the ground, such as leaves and stems. It is a vital part of the plant's water cycle and helps cool the plant.
In simple words: Transpiration is when plants release water vapor into the air from their leaves and stems.

🎯 Exam Tip: Include "water vapour" and "aerial parts of the plant body" in your definition to ensure accuracy.

 

Question 2. Define guttation.
Answer: Guttation is a process where plants release small drops of liquid water and dissolved substances from specialized pores called hydathodes. These hydathodes are usually found at the edges and tips of leaves. Unlike transpiration, this water loss occurs as liquid, not vapor, and often happens when the air is humid.
In simple words: Guttation is when plants release small drops of liquid water and other substances from special pores at their leaf edges.

🎯 Exam Tip: Clearly state that guttation involves liquid water droplets and occurs via hydathodes, distinguishing it from water vapor lost during transpiration.

 

Question 3. Draw a diagram of the stomatal apparatus.
Answer: Here is a simple diagram showing the structure of the stomatal apparatus, which includes two guard cells surrounding a pore, with thickened inner walls:

In simple words: The stomatal apparatus includes two bean-shaped guard cells that surround a central opening called the pore. These cells control whether the pore is open or closed, which affects water loss.

🎯 Exam Tip: When drawing, clearly label the guard cells, the stomatal pore, and indicate the thickened inner walls of the guard cells.

 

Question 4. Describe one external factor which affects transpiration.
Answer: Temperature is an important external factor that changes the rate of transpiration. When the temperature goes up, the air's ability to hold water increases, so its relative humidity drops. This makes the plant lose more water vapor, causing the rate of transpiration to increase. Warmer temperatures directly cause more water molecules to evaporate from the leaf surface.
In simple words: Higher temperatures make plants lose more water through transpiration because the air gets drier and can hold more water.

🎯 Exam Tip: When explaining temperature's effect, clearly link increasing temperature to decreasing relative humidity, which in turn increases the transpiration rate.

Rbse Class 12 Biology Chapter 7 Essay Type Questions

 

Question 1. Describe the types of transpiration.
Answer: Transpiration, the process of water evaporating from plants into the atmosphere, mainly occurs in three ways, depending on where the water vapor exits the plant:
1. Stomatal transpiration: This is the most common type. Stomata are tiny openings found mostly on the surface of leaves and on young stems. About 90-97% of the total water a plant absorbs is lost through these small pores. They open and close to control gas exchange and water loss.
2. Cuticular transpiration: This happens through the cuticle, which is a waxy layer covering the outer walls of the epidermal cells on the aerial parts of plants. This type of transpiration occurs continuously, day and night, but it usually accounts for a smaller amount of water loss compared to stomatal transpiration.
3. Lenticular transpiration: Lenticels are small, raised pores found on the bark of woody stems and fruits. These pores are filled with loosely arranged cells. Water loss through lenticels is very small, accounting for only about 0.1% of the total transpiration, and also happens continuously.
In simple words: Plants lose water in three main ways: through stomata (tiny leaf pores, most common), through the waxy skin (cuticle), and through small openings on woody stems (lenticels).

🎯 Exam Tip: For each type of transpiration, clearly mention the specific plant structure involved and the approximate percentage of water loss associated with it.

 

Question 2. Explain the mechanism of opening and closing of stomata.
Answer: The opening and closing of stomata are complex processes regulated by changes in the turgidity of guard cells. Several theories explain this mechanism:

1. Starch-Sugar Hypothesis (Sayre, 1923):
This theory suggests that during the day, photosynthesis in guard cells uses carbon dioxide, causing the pH to rise (around 7.0). This higher pH activates an enzyme called phosphorylase, which converts insoluble starch into soluble sugar (glucose-1-phosphate). The increase in sugar concentration inside the guard cells raises their osmotic potential, causing water to enter them (endosmosis), making them turgid and thus opening the stomata.

During the night or in conditions of low pH (around 5.0), the reverse happens. Sugar is converted back to starch, lowering the osmotic potential. Water leaves the guard cells (exosmosis), making them flaccid and closing the stomata.

2. Steward's Hypothesis (1964):
Steward's theory refines the starch-sugar hypothesis. It states that an increase in the pH of guard cells during the day (due to \( \{CO\}_{2} \) consumption in photosynthesis) leads to the conversion of starch into glucose-1-phosphate, then into glucose-6-phosphate, and finally into glucose. Glucose and glucose-6-phosphate are highly soluble in water compared to glucose-1-phosphate. This increase in soluble sugars raises the osmotic concentration within the guard cells, drawing water in and causing the stomata to open.

Here's a simplified sequence of reactions:
During Day Time:
Starch \( + \) iP \( \xrightarrow{\text{Phosphorylase pH=7.0}} \) Glucose-1-phosphate
Glucose-1-phosphate \( \xrightarrow{\text{Phosphoglycomutase}} \) Glucose-6-phosphate
Glucose-6-phosphate \( \xrightarrow{\text{Phosphatase}} \) Glucose \( + \) iP

During Night Time:
Glucose \( + \) ATP \( \xrightarrow{\text{Hexokinase}} \) Glucose-1-phosphate
Glucose-1-phosphate \( \xrightarrow{\text{pH=5.0}} \) Starch \( + \) iP

3. Active Potassium Ion Transport Theory (Imamura & Fujino, 1959; Levitt, 1974):
This is currently the most widely accepted theory. It focuses on the movement of potassium ions \( (\{K\}^{+}) \).

During Day Time (Stomata Opening):
1. Malic acid is formed in guard cells in the presence of light, which then breaks down into malate and hydrogen ions \( (\{H\}^{+}) \).
2. The \( \{H\}^{+} \) ions move out of the guard cells into adjacent subsidiary cells.
3. To balance the charge, potassium ions \( (\{K\}^{+}) \) from subsidiary cells move actively into the guard cells.
4. Inside the guard cells, \( \{K\}^{+} \) combines with malate to form potassium malate.
5. This increases the osmotic concentration of the guard cells.
6. Water enters the guard cells from surrounding cells (endosmosis), making them turgid.
7. The turgidity causes the guard cells to bend, leading to the opening of the stomata.

During Night Time (Stomata Closing):
1. Photosynthesis stops, causing an increase in \( \{CO\}_{2} \) concentration in the substomatal cavity.
2. This rise in \( \{CO\}_{2} \) activates abscisic acid (a hormone), which promotes the efflux of \( \{K\}^{+} \) ions from the guard cells.
3. The decrease in \( \{K\}^{+} \) ions lowers the osmotic concentration of the guard cells.
4. Water moves out of the guard cells (exosmosis), making them flaccid.
5. The flaccid guard cells cause the stomata to close.

Here is a simplified flow of the Active Potassium Ion Transport mechanism:
Guard Cells During Day Time:
Malic acid \( \longrightarrow \) Malate \( + \) H\(^{+}\)
H\(^{+}\) Efflux from guard cells \( \longrightarrow \) Influx of \( \{K\}^{+} \) by active transport into guard cells
\( \{K\}^{+} \) \( + \) Malate \( \longrightarrow \) Potassium malate
Potassium malate \( \longrightarrow \) O.P. increases \( \longrightarrow \) Endosmosis \( \longrightarrow \) Guard cells become turgid \( \longrightarrow \) Stomata open

Guard Cells During Night:
Phtosynthesis stops \( \longrightarrow \) \( \{CO\}_{2} \) concentration increases in substomatal cavity \( \longrightarrow \) Abscisic acid (Hormone) becomes active
Abscisic acid \( \longrightarrow \) Efflux of \( \{K\}^{+} \) from guard cells
Efflux of \( \{K\}^{+} \) \( \longrightarrow \) Decrease in osmotic concentration of guard cells \( \longrightarrow \) Exosomosis \( \longrightarrow \) Guard cells become flaccid \( \longrightarrow \) Stomata close
In simple words: Stomata open and close because guard cells gain or lose water. During the day, cells make sugar or take in potassium, which makes them swell with water and open the stomata. At night, they lose sugar or potassium, release water, shrink, and close the stomata.

🎯 Exam Tip: When explaining stomatal movement, clearly outline each hypothesis with its key chemical changes, ion movements, and how these affect guard cell turgidity.

 

Question 3. Describe the theory related to stomatal movement.
Answer: The most accepted theory for explaining how stomata open and close is the Active Potassium Ion Transport theory. According to this theory, during the day, light causes malic acid to form in guard cells, which breaks into malate and hydrogen ions \( (\{H\}^{+}) \). These \( \{H\}^{+} \) ions move out, and potassium ions \( (\{K\}^{+}) \) move into the guard cells from subsidiary cells. This influx of \( \{K\}^{+} \) increases the guard cells' osmotic concentration, causing water to enter and make them turgid, thus opening the stomata. At night, the process reverses, leading to stomatal closure. This is also called \( \{K\}^{+} \) ion exchange or photoactive proton transport theory.
In simple words: The most common idea for how stomata open and close involves tiny pumps that move potassium ions into or out of the guard cells, which then makes water move and changes their size.

🎯 Exam Tip: When describing the Active Potassium Ion Transport theory, emphasize the role of light, malic acid formation, \( \{K\}^{+} \) ion movement, and the resulting change in guard cell turgidity.

 

Question 4. Transpiration is a necessary evil for plants. Discuss this statement.
Answer: The statement "transpiration is a necessary evil for plants" means it has both good and bad effects. While plants lose a lot of water through this process, which can be harmful, they also need it for several important functions.

Benefits of Transpiration:

• The upward movement of water and dissolved minerals (ascent of sap) in plants largely depends on the transpiration pull created by water loss from leaves.
• It helps in the absorption of water and minerals from the soil and their transportation to different parts of the plant.
• Transpiration has a cooling effect on leaves and the entire plant body, much like sweating cools animals, helping to prevent overheating.

Harmful Aspects of Transpiration:

• Excessive water loss can lead to wilting of the shoot, where the plant droops and loses its firmness.
• Millions of plants can die each year if the rate of water loss through transpiration is much higher than the rate of water absorption by the roots.

Therefore, plants need transpiration for essential processes, but too much of it can be damaging. It's a trade-off for survival.
In simple words: Transpiration is called a "necessary evil" because plants need it to pull water and keep cool, but it also causes them to lose a lot of water, which can be bad if they don't get enough water from the soil.

🎯 Exam Tip: To effectively discuss this statement, clearly present both the advantages (e.g., sap ascent, cooling) and disadvantages (e.g., wilting, water stress) of transpiration.

 

Question 5. Describe the factors affecting transpiration.
Answer: Many factors influence the rate at which plants lose water through transpiration. These factors can be broadly divided into external (environmental) and internal (plant-specific) categories:

(1) External or Atmospheric Factors:
1. Light: Light affects transpiration in two ways. Directly, it causes stomata to open, increasing water loss. Indirectly, light increases temperature, which further raises the transpiration rate. No light causes stomata to close and reduces transpiration.
2. Wind: Faster wind removes humid air from around the leaves and replaces it with dry air, which increases the rate of transpiration. Slow wind allows humid air to stay, reducing water loss.
3. Available soil water: When there is plenty of water in the soil, plants can absorb more, leading to higher transpiration. If soil water is low, plants reduce water loss by closing stomata.
4. Temperature: Higher temperatures generally increase the rate of transpiration because water molecules evaporate more quickly from the leaf surface and the air can hold more moisture.
5. Atmospheric pressure: A reduction in atmospheric pressure can increase the rate of transpiration because water molecules find it easier to escape from the leaf surface into the less dense air.
6. Humidity: High humidity in the atmosphere reduces the rate of transpiration because the air is already saturated with water vapor, making it harder for the plant to release more. Lower humidity increases transpiration.

(2) Internal or Plant Factors:
1. Leaf structure: Certain leaf features, like sunken stomata (stomata recessed below the leaf surface), a thick waxy cuticle, or a covering of hairs, help to reduce water loss. Leaves with many stomata per unit area will transpire more.
2. Leaf orientation: Leaves positioned directly facing the sun (at a right angle to sunlight) experience higher rates of transpiration due to increased heat absorption and direct light on stomata.
3. Root-shoot ratio: A higher ratio of root mass to shoot (stem and leaves) mass means the plant can absorb more water than it loses, which generally supports higher transpiration rates without wilting. A balanced ratio is important for water management.
In simple words: Transpiration is affected by outside things like light, wind, temperature, humidity, and how much water is in the soil. It also depends on plant parts like leaf shape, how leaves are angled, and the balance between roots and shoots.

🎯 Exam Tip: Structure your answer by clearly separating external and internal factors, and for each factor, briefly explain how it affects the rate of transpiration.

 

Question 6. Write a note on guttation.
Answer: Guttation is the process where plants release small droplets of liquid water, often mixed with dissolved substances, from specialized pores called hydathodes. These hydathodes are usually found at the tips and margins of leaves.

Here are some key points about guttation:

• De Barry first observed this process in 1869.
• It involves the exudation (oozing out) of water droplets from the tips and margins of leaves.
• This water loss occurs through tiny openings called hydathodes, which are located at the ends of leaf veins in some plants.
• The pore of a hydathode is sometimes called a water pore and is an incompletely differentiated stoma, meaning it doesn't function exactly like a regular stoma.
• Guttation happens mostly at night or in the early morning when the air is humid, and transpiration rates are low. This is when root pressure builds up due to higher water absorption than transpiration.
• When roots absorb more water than the leaves can release through transpiration, root pressure increases, pushing water from the xylem vessels into the epithem tissue.
• Eventually, this water is exuded as droplets through the hydathode pores.
• The fluid released during guttation is not pure water; it contains dissolved minerals and organic substances.
• If these minerals and organic substances deposit on the leaf surface, they can sometimes cause harm or create a breeding ground for diseases.

In simple words: Guttation is when plants leak small drops of water from their leaf edges, especially when it's humid. This happens because roots push more water into the plant than leaves can release as vapor, and special pores called hydathodes let the extra water out.

🎯 Exam Tip: Clearly differentiate guttation (liquid water, hydathodes, high humidity, root pressure) from transpiration (water vapor, stomata, low humidity, transpiration pull).

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