RBSE Solutions Class 12 Biology Chapter 6 Absorption of Water and Ascent of Sap in Plants

RBSE Class 12 Biology Chapter 6 Multiple Choice Questions

Question 1. Soil water which is available to the plants is?
(a) Capillary water
(b) Runaway water
(c) Hygroscopic water
(d) Gravitational water
Answer: (a) Capillary water
In simple words: Plants can only absorb capillary water from the soil because it forms a thin film around soil particles and stays in the small spaces between them. Other types of water are either too tightly held or drain away too quickly.

🎯 Exam Tip: Remember that capillary water is the most important type of water for plants because it is available in the soil for a longer period and is easily accessible to root hairs.

Question 2. Which part of roots performs absorption?
(a) Root cap
(b) Zone of elongation
(c) Zone of maturation
(d) Root hair zone
Answer: (d) Root hair zone
In simple words: The root hair zone has many tiny hairs that stick out, making the surface area much bigger for the root to take in water and nutrients from the soil. This greatly increases the efficiency of absorption.

🎯 Exam Tip: Root hairs are delicate and short-lived, constantly being replaced as the root grows, which highlights their active role in absorption.

Question 3. Which path creates maximum resistance during lateral movement of absorbed water up to Xylem?
(a) Apoplast
(b) Symplast
(c) Membrane path
(d) Vacuolar path
Answer: (b) Symplast
In simple words: The symplast pathway involves water moving from cell to cell through the cytoplasm, crossing cell membranes multiple times, which makes it a slower path with more resistance compared to moving through cell walls. This constant crossing of membranes requires energy and specific channels.

🎯 Exam Tip: Understand the differences between apoplast and symplast pathways; the symplast route requires crossing the cell membrane, making it more regulated but slower.

Question 4. Cohesion tension principle of the ascent of sap was proposed by?
(a) Godlewski
(b) J.C. Bose
(c) Strasburger
(d) Dixon and Jolly
Answer: (d) Dixon and Jolly
In simple words: The idea that water moves up plants because water molecules stick to each other (cohesion) and are pulled up by transpiration (tension) was first suggested by Dixon and Jolly. This theory is widely accepted today to explain how water reaches great heights in trees.

🎯 Exam Tip: Dixon and Jolly's cohesion-tension theory is a cornerstone of plant physiology; remember the names associated with key theories.

Question 5. During the ascent of sap in plants through which tissue water rises up?
(a) Cortex
(b) Vessels only
(c) Tracheids only
(d) Both tracheids and vessels
Answer: (d) Both tracheids and vessels
In simple words: Water moves upwards in a plant through both tracheids and vessels, which are parts of the xylem tissue. These structures form continuous tubes that act like straws, carrying water from the roots to the leaves.

🎯 Exam Tip: Remember that xylem is the primary water-conducting tissue, and it contains both tracheids and vessels to efficiently transport water and dissolved minerals.

RBSE Class 12 Biology Chapter 6 Very Short Answer Questions

Question 1. Name the different paths of the lateral flow of water.
Answer: The different paths for the lateral flow of water in a plant are:
1. Apoplast pathway
2. Symplast pathway
3. Vacuolar pathway
These pathways allow water to move from the root surface to the xylem in the center.
In simple words: Water can move sideways through a plant's cells in three main ways: by going around the cells (apoplast), by going through the living parts of the cells (symplast), or by passing through the large water-filled sacs inside cells (vacuolar path).

🎯 Exam Tip: Distinguish clearly between the apoplast (non-living parts like cell walls) and symplast (living parts like cytoplasm) pathways, as they differ in resistance and regulation.

Question 2. What is meant by capillary water?
Answer: Capillary water is the soil water that forms a continuous film around soil particles and fills the small spaces between them. This water is held by capillary forces and is available for absorption by plant roots. It is the most important type of water for plant growth.
In simple words: Capillary water is like tiny bits of water that stick to soil particles and fill the small gaps, which plants can easily drink up with their roots.

🎯 Exam Tip: Focus on the "available to plants" aspect when defining capillary water, as other forms of soil water might not be accessible.

Question 3. What is meant by root hair?
Answer: Root hairs are tiny, thin, tubular outgrowths from the outermost layer of root cells (called epiblema or epidermis). They extend into the soil, greatly increasing the root's surface area for absorbing water and minerals. The cells of epiblema that produce root hair are called trichoblasts.
In simple words: Root hairs are like tiny, fuzzy extensions on a root, which help the plant soak up more water and food from the soil by covering a larger area.

🎯 Exam Tip: Highlight the function of root hairs in increasing surface area for absorption; this is their primary significance.

Question 4. Sir J.C. Bose worked on which plant?
Answer: Sir J.C. Bose experimented on the Indian telegraph plant, also known as Desmodium gyrans. He used this plant to study rhythmic pulsation activity in its cells, supporting his pulsation theory of sap ascent.
In simple words: Sir J.C. Bose studied the Indian telegraph plant to learn how water moves up in plants.

🎯 Exam Tip: When asked about specific scientists, remember the plant or experiment they are famously associated with, such as J.C. Bose and Desmodium gyrans.

Question 5. Name the theory proposed by Dixon and Jolly.
Answer: Dixon and Jolly proposed the Transpiration pull-Cohesion tension theory. This theory explains how water is pulled up through the plant from roots to leaves due to the cohesive forces between water molecules and the tension created by transpiration from leaves.
In simple words: Dixon and Jolly came up with the idea that water goes up plants because water drops stick to each other and are pulled by the leaves losing water to the air.

🎯 Exam Tip: The cohesion-tension theory is the most widely accepted explanation for water transport in tall trees; emphasize both "cohesion" and "tension" in your answer.

RBSE Class 12 Biology Chapter 6 Short Answer Questions

Question 1. Draw the diagram of the path of lateral movement of water.
Answer: The diagram below illustrates the three main pathways for lateral water movement in plant roots: the Apoplast, Symplast, and Vacuolar pathways. The apoplast pathway involves water moving through cell walls and intercellular spaces, while the symplast pathway involves movement through the cytoplasm of adjacent cells via plasmodesmata. The vacuolar pathway involves water moving through the cell vacuoles in addition to the cytoplasm.

In simple words: Imagine water moving sideways through plant cells. It can go through the cell walls without entering the cells (apoplast), or it can enter each cell and pass through the living part (cytoplasm) to the next cell (symplast). Sometimes, it even goes through the big water sacs (vacuoles) inside the cells (vacuolar).

🎯 Exam Tip: When drawing, use different line types or colors to clearly distinguish each pathway and label the cellular components accurately.

Question 2. Name different parts of the root and describe the part which performs absorption of water.
Answer: The root typically has two main parts: the young part and the mature part. The young part is further divided into:
1. Root cap
2. Meristematic region or region of cell division
3. Region of cell elongation
4. Root hair region
5. Mature region
The part which mainly performs the absorption of water and minerals is the **Root hair region**. This region is located just after the zone of elongation. In this area, the outer cells (epidermis or epiblema) stop dividing and begin to change, forming long, hair-like extensions called root hairs. These root hairs have thin, flexible walls and reach into the tiny spaces in the soil, greatly increasing the surface area for taking in water.
In simple words: A root has young parts and old parts. The young part has a tiny tip (root cap), a growing area, an area where cells get longer, and an area with tiny hairs. The tiny hair area is the main part that soaks up water and food for the plant because its hairs reach far into the soil.

🎯 Exam Tip: Focus on the root hair region as the primary site of absorption due to its large surface area and thin cell walls, which facilitate efficient uptake of water and minerals.

Question 4. Discuss any two factors which affect water absorption.
Answer: Water absorption by plants is affected by several factors. Here are two important ones:
1. **Available soil water:** The amount of water available in the soil directly impacts absorption. Plants absorb capillary water, which is found as a film around soil particles and in small spaces. The rate of water absorption is highest when the soil water is at its ideal level, known as field capacity. If there is less capillary water, absorption decreases.
2. **Soil aeration:** Good air circulation in the soil is crucial for water absorption. In well-aerated soils (like loam), water absorption occurs rapidly. However, if the soil is waterlogged, it lacks air, which reduces the oxygen supply for root cells. This lack of oxygen slows down or even stops the metabolic activities of roots, significantly decreasing water absorption.
Other factors like soil temperature and soil solution concentration also play a role in water absorption.
In simple words: How much water a plant can take in depends on two main things: first, how much easy-to-reach water is actually in the soil, and second, how much air is in the soil. If there's enough water and air, the plant drinks well; if not, it struggles.

🎯 Exam Tip: When discussing factors, always explain the mechanism by which each factor influences water absorption, rather than just listing it.

Question 5. Define the ascent of sap.
Answer: Ascent of sap is the process by which water and dissolved minerals, absorbed by the roots, are transported upwards to the highest parts of the plant, against the force of gravity. This movement happens through the specialized water-conducting tissues, primarily the xylem elements, which act as continuous pipelines.
In simple words: Ascent of sap is when water and minerals travel up from the roots to the very top of a plant, like a drink going up a straw, even though gravity tries to pull it down.

🎯 Exam Tip: Emphasize "upward movement," "against gravity," and "through xylem" in your definition for a complete answer.

Question 6. The principles proposed to explain the ascent of sap is divided into how many parts? Name these.
Answer: The principles or theories proposed to explain the ascent of sap have been divided into three main parts:
(i) **Vital force theories:** These theories suggest that living cells in the plant actively contribute to pushing water upwards. Examples include Godlewski's Relay Pump Theory and J.C. Bose's Pulsation Theory.
(ii) **Root pressure theory:** This theory proposes that a positive pressure developed in the roots helps to push water up the xylem, though its effect is limited to shorter distances.
(iii) **Physical force theories:** These theories explain water movement based on physical properties of water and plant structures. The most accepted one is the Transpiration Pull-Cohesion Tension Theory by Dixon and Jolly.
In simple words: Scientists have three main ideas about how water goes up plants: some think living plant cells push it up, some think pressure from the roots pushes it a little, and others think water gets pulled up from the leaves because of strong water-to-water connections.

🎯 Exam Tip: Remember the three broad categories of theories for sap ascent (vital, root pressure, physical) and a key proponent or concept for each.

RBSE Class 12 Biology Chapter 6 Essay Type Questions

Question 1. Write an account of the mechanism of absorption of water by plants.
Answer: Plants absorb water through two primary mechanisms: active absorption and passive absorption.

1. **Mechanism of Active Absorption:**
This process involves forces within the root cells actively taking up water. Only a small percentage (2-4%) of the total water is absorbed this way. Active absorption usually happens during humid nights when transpiration is very low. Root cells play a significant role here, creating a positive force called root pressure in the xylem. This process requires metabolic energy from the plant, obtained through respiration, as water moves against a concentration gradient (DPD). In this method, the roots actively participate in absorbing water.

2. **Mechanism of Passive Absorption:**
This is the main method, accounting for 96-98% of water absorption. In passive absorption, the driving force comes from the transpiring surfaces of the plant, mainly the leaves, rather than the roots. When leaves lose water through transpiration, it creates a shortage of water, leading to a tension or "pull" in the water column of the xylem. This transpiration pull is then transmitted down to the stem and finally to the roots, drawing soil water into the root hairs. This tension is a negative pressure, also known as suction force. Importantly, passive absorption does not require active participation or metabolic energy from the root cells; it is driven by the physical process of transpiration. The stronger the transpiration rate, the faster the water absorption.
In simple words: Plants take in water in two main ways: "active" means the root cells use their own energy to pull water in, especially at night. "Passive" means the leaves losing water (like sweating) creates a pull that draws water all the way up from the roots, and this is how most water is absorbed without the roots using much energy themselves.

🎯 Exam Tip: Clearly differentiate between active and passive absorption based on energy requirement, the part of the plant generating the force, and the percentage of water absorbed by each method.

Question 2. Describe in detail the mechanism of the ascent of sap in trees.
Answer: The ascent of sap, which is the upward movement of water and dissolved minerals from the roots to the leaves against gravity, is explained by several theories. These are broadly categorized into Vital Force Theories, Root Pressure Theory, and Physical Force Theories.

1. **Vital Force Theories:**
These theories suggest that living cells of the plant actively contribute to the upward movement of sap.
* **Relay Pump Theory (Godlewski, 1884):** This theory proposes that living cells in the xylem parenchyma and medullary rays undergo rhythmic changes in their osmotic pressure, creating a pumping action that pushes water upwards.
* **Strasburger (1891):** He proved experimentally that sap ascent is not solely dependent on living cells, as it still occurs in plants where living cells have been killed. However, living cells might provide conditions favorable for ascent.
* **Pulsation Theory (Sir J.C. Bose, 1923):** Bose strongly believed that rhythmic pulsations in the innermost layer of cortex cells, just outside the endodermis, pump water upwards from cell to cell. He demonstrated this using the Indian telegraph plant (Desmodium gyrans).

2. **Root Pressure Theory:**
This theory suggests that a positive pressure, called root pressure, develops in the xylem vessels of the root. This pressure arises from the active accumulation of water in root parenchyma cells, creating hydrostatic pressure that pushes water upwards. Root pressure can raise water up to about 2.0 atmospheres, lifting water up to 20 meters. However, it has limitations:
* Very tall woody trees require a much higher pressure (around 12 atm) which root pressure cannot provide.
* Root pressure is not observed in all plants, especially in very tall trees like gymnosperms.
* It is generally observed when transpiration is low (e.g., at night) and plays a minor role in overall water transport.

3. **Physical Force Theories:**
These theories explain water movement based on physical principles, with the Transpiration Pull-Cohesion Tension Theory being the most widely accepted.
* **Transpiration Pull-Cohesion Tension Theory (Dixon and Jolly, 1894):** This is the most accepted explanation. It states that water forms a continuous column from the roots to the leaves through the xylem ducts. When water evaporates from the leaves during transpiration, it creates a negative pressure (tension or suction pull). Due to the cohesive forces (attraction between water molecules) and adhesive forces (attraction between water molecules and xylem walls), this tension pulls the entire water column upwards like a single rope. This continuous column of water can sustain significant tension, allowing water to reach the highest parts of even the tallest trees.
The diagram below illustrates a setup to demonstrate root pressure.

In simple words: Water goes up trees because of a few ideas. Some thought living cells pushed it up, like a tiny pump. Another idea was that pressure from the roots pushes water a little way. But the best idea is that leaves losing water creates a strong pull, and because water drops stick together very strongly, this pull lifts the whole column of water from the roots all the way to the top of the tree.

🎯 Exam Tip: For essay questions, always structure your answer with clear headings for each theory. For the Transpiration Pull-Cohesion Tension theory, highlight the roles of cohesion, adhesion, and transpiration in creating a continuous water column.