RBSE Solutions Class 12 Biology Chapter 5 Plant Water Relations

S. No.	Hypertonic solution	Hypotonic solution
1.	This is more concentrated than the concentration of cell sap.	This has concentration less than the concentration of cell sap.
2.	Osmotic pressure of this is higher than O.P. of the cell sap.	Osmotic pressure of this is much less than the O.P. of the cell sap.
3.	When a cell is dipped in this solution, exosmosis takes place.	When a cell is dipped in this solution, endosmosis takes place.
4.	Cells dipped in this become flaccid.	Cells dipped in this become turgid.

Question 4. Differentiate between hypertonic and hypotonic solution.
Answer: A hypertonic solution has a higher concentration of solutes and a lower water potential compared to the cell's sap. When a cell is placed in it, water moves out, causing exosmosis and making the cell flaccid. Conversely, a hypotonic solution has a lower concentration of solutes and a higher water potential than the cell's sap. If a cell is put into it, water moves in, leading to endosmosis and making the cell turgid. These differences are key to how cells gain or lose water, impacting their overall health and function.
In simple words: A hypertonic solution has more salt outside the cell, so the cell loses water. A hypotonic solution has less salt outside, so the cell gains water.

🎯 Exam Tip: Focus on the relative solute concentration and water potential of the solution compared to the cell's interior to determine if it's hypertonic or hypotonic, and predict water movement.

 

Question 5. Explain the diffusion pressure gradient.
Answer: A diffusion pressure gradient (DPG) is the difference in diffusion pressure between two regions. Diffusion pressure represents the tendency of molecules to diffuse. When the DPD (Diffusion Pressure Deficit) of one cell (A) is higher than that of an adjacent cell (B), cell (A) will draw water from cell (B). This movement continues until the DPD of both cells becomes equal, reaching a state of equilibrium. This difference in water-absorbing capacity due to varying DPD values creates the diffusion pressure gradient, driving water movement in plants. This gradient is the driving force behind water absorption and transport.
In simple words: A diffusion pressure gradient is a difference in how strongly two areas pull water. Water moves from the area that pulls less to the area that pulls more, until they are balanced.

🎯 Exam Tip: Relate the diffusion pressure gradient directly to DPD. Water always moves down a DPG, meaning from lower DPD to higher DPD, or from higher water potential to lower water potential.

 

At normal temperature and atmospheric pressure, the difference between the diffusion pressure of a solution and the diffusion pressure of pure water is called DPD. In fact, DPD of a cell is the measure of the force by which the cell can draw water, hence it is also called suction pressure.

 

Question 7. Giving example, explain imbibition.
Answer: Imbibition is a physical process where solid or semi-solid organic substances, which are hydrophilic (water-loving), adsorb water from their surroundings and swell up without dissolving to form a true solution. The pressure developed due to imbibition is called imbibition pressure (IP), and it is equal to osmotic pressure, being an exothermic process. This phenomenon is crucial in several biological contexts.
Examples:

• Wooden doors and windows often swell during the rainy season due to imbibition, making them stick.
• Seeds absorb water through imbibition before germination, which breaks the seed coat.
• Epiphytes (plants growing on other plants) use the hygroscopic nature of their roots to draw water through imbibition.
• In many dry fruits and sporangia, dehiscence (splitting open) occurs due to imbibition, helping to release seeds or spores.
• Historically, the immense pressure generated by imbibition has been used to break large rocks.

Imbibition is a key initial step for water uptake in many plant processes.
In simple words: Imbibition is when dry, water-loving things like wood or seeds soak up water and swell a lot, like a sponge, but don't fully dissolve. This process is strong enough to split things apart.

🎯 Exam Tip: When explaining imbibition, always mention "adsorption by hydrophilic solids" and provide at least two clear examples, such as seed germination or wood swelling.

 

RBSE Class 12 Biology Chapter 5 Essay Type Questions

 

Question 1. Describe osmotic potential, pressure potential and water potential and explain their mutual relationship.
Answer: The concept of water potential, first proposed by Slatyer and Taylor in 1960, describes the free energy of water, which indicates its ability to move. In any system, free energy allows components to do work. Water potential (\( \Psi_w \)) is the potential energy of water in a system compared to pure water at standard conditions, which has a water potential of zero. Water always moves from an area of higher water potential to an area of lower water potential. The key components of water potential are:

• **Osmotic Potential (\( \Psi_s \)):** Also called solute potential, this is the reduction in water potential caused by the presence of solutes. Solutes reduce the free energy of water, making \( \Psi_s \) always negative in a solution. The more solutes present, the more negative the osmotic potential. It is equal in magnitude but opposite in sign to osmotic pressure (OP), which is positive.
• **Pressure Potential (\( \Psi_p \)):** This is the potential due to physical pressure exerted on water. In plant cells, this is primarily turgor pressure, the pressure exerted by the cell's protoplast against the cell wall. It can be positive (turgor pressure), negative (tension in xylem), or zero (flaccid cell). The cell wall helps prevent the cell from bursting.
• **Matrix Potential (\( \Psi_m \)):** This refers to the potential energy of water due to its adsorption onto surfaces of colloidal materials like cell walls or soil particles. While significant in soil, it is usually negligible in cellular osmotic systems and is often ignored for simplicity in cell-to-cell water movement calculations.

**Mutual Relationship:**
The water potential of a system is the sum of its component potentials. For a plant cell, the primary relationship is:
\( \Psi_w = \Psi_s + \Psi_p \)
(If matrix potential is considered significant, it would be \( \Psi_w = \Psi_s + \Psi_p + \Psi_m \)).
Key points about their relationship:

• Pure water has \( \Psi_w = 0 \), since there are no solutes (\( \Psi_s = 0 \)) and no applied pressure (\( \Psi_p = 0 \)).
• Adding solutes makes \( \Psi_s \) negative, thus lowering \( \Psi_w \).
• Increasing turgor pressure makes \( \Psi_p \) positive, thus increasing \( \Psi_w \).
• A flaccid cell has \( \Psi_p = 0 \), so \( \Psi_w = \Psi_s \).
• In a fully turgid cell, water potential (\( \Psi_w \)) often approaches zero because the positive pressure potential (\( \Psi_p \)) balances the negative osmotic potential (\( \Psi_s \)). This balance prevents further water intake. This dynamic interaction between potentials drives water movement from the soil into roots and throughout the plant.
  

In simple words: Water potential tells us how much water wants to move. It's affected by two main things: how much stuff is dissolved in the water (osmotic potential, which makes water want to move in) and how much pressure is pushing on the water (pressure potential, which can push water out or keep it in). These three work together to decide where water goes in a plant.

🎯 Exam Tip: For essay questions, define each potential clearly with its sign (positive/negative/zero), then provide the master equation (\( \Psi_w = \Psi_s + \Psi_p \)) and explain how each component affects the overall water potential, especially in different cell states (turgid, flaccid).

 

Question 2. Defining the process of osmosis and describe an experiment by which it may be demonstrated.
Answer:
**Osmosis:** Osmosis is a special type of diffusion that involves the movement of solvent (usually water) molecules. It is defined as the net movement of solvent molecules from a region of their higher concentration (or higher water potential) to a region of their lower concentration (or lower water potential) across a semi-permeable or selectively permeable membrane. This movement occurs to equalize the solute concentrations on both sides of the membrane. Osmosis is fundamental for water absorption by roots and cell-to-cell water transport in plants.

**Demonstration with a Thistle Funnel Experiment:**
The process of osmosis can be clearly demonstrated using a simple thistle funnel experiment:

1. **Setup:** Take a thistle funnel and securely tie a semi-permeable membrane (like parchment paper or an animal bladder) across its broad mouth.
2. **Solution in Funnel:** Fill the inverted thistle funnel with a concentrated sugar solution and mark its initial level on the stem.
3. **Beaker:** Place the thistle funnel in a beaker filled with pure water, ensuring the water level in the beaker is below the level of the sugar solution in the funnel stem.
4. **Observation:** After some time (a few hours), it will be observed that the level of the sugar solution in the stem of the thistle funnel rises.
5. **Explanation:** Water molecules, having a higher concentration (higher water potential) in the beaker, move across the semi-permeable membrane into the sugar solution in the funnel, which has a lower water concentration (lower water potential). Sugar molecules, being larger, cannot pass through the semi-permeable membrane. This net influx of water into the funnel causes the volume of the sugar solution to increase, making its level rise in the stem.

**Types of Osmosis:**
1. **Endosmosis:** This is the inward movement of water molecules into a cell or system through a semi-permeable membrane. It happens when a cell is placed in a hypotonic solution (one with lower solute concentration) or pure water. As water enters, the cell swells and becomes turgid.
* **Example:** Raisins swelling when placed in water, or root hair cells absorbing soil water.
2. **Exosmosis:** This is the outward movement of water molecules from a cell or system through a semi-permeable membrane. It occurs when a cell is placed in a hypertonic solution (one with higher solute concentration). As water leaves, the cell shrinks and becomes flaccid.
* **Example:** Grapes shrinking when placed in a concentrated sugar solution, or the adverse effect of excessive chemical fertilizer on crop plants, causing them to lose water.
In simple words: Osmosis is when water moves through a special skin (semi-permeable membrane) from where there's lots of water to where there's less. The thistle funnel experiment shows this by having sugar water inside the funnel and plain water outside. The plain water moves into the funnel, making the sugar water level rise. There are two types: endosmosis (water goes in, cell swells) and exosmosis (water goes out, cell shrinks).

🎯 Exam Tip: Clearly define osmosis by mentioning the semi-permeable membrane and direction of water movement. For the experiment, describe the setup, observation, and explanation. For types of osmosis, provide examples for each.

 

Question 3. Describing briefly the process of osmosis, diffusion and imbibition, explain their importance in plant physiology.
Answer: Plants rely on several physical processes to manage water and nutrient uptake, transport, and gas exchange. The key processes are osmosis, diffusion, and imbibition, each with distinct roles in plant physiology.

**1. Osmosis:**
Osmosis is the net movement of solvent molecules (typically water) across a semi-permeable membrane from a region of higher water potential to a region of lower water potential. Solute molecules are generally too large to pass through the membrane.
* **Demonstration:** The thistle funnel experiment (described in Q2) effectively demonstrates osmosis. A sugar solution in a funnel with a semi-permeable membrane at its mouth, placed in pure water, shows the water level rising in the funnel stem due to water entering from the beaker.
* **Types:**
* **Endosmosis:** Water moves into the cell when placed in a hypotonic solution, causing it to swell (e.g., raisins in water).
* **Exosmosis:** Water moves out of the cell when placed in a hypertonic solution, causing it to shrink (e.g., grapes in concentrated sugar).
* **Importance in Plant Physiology:**
* Root hairs absorb water from the soil primarily through osmosis.
* It helps in cell-to-cell movement of water and its transportation within the plant.
* Maintains cell turgidity, which is essential for the growth of young cells and the structural support of non-woody plants.
* Regulates the opening and closing of stomata, controlling gas exchange and transpiration.
* Contributes to the specific shape and form of leaves, flowers, and fruits.
* Helps plants resist freezing and desiccation by maintaining internal water balance.
* Facilitates seed germination by initial water uptake.

**2. Diffusion:**
Diffusion is the net movement of molecules (solids, liquids, or gases) from a region of higher concentration to a region of lower concentration, occurring spontaneously due to the kinetic energy of molecules. It does not require a membrane.
* **Factors Affecting Diffusion:**
* **Temperature:** Higher temperature increases kinetic energy, thus increasing the rate of diffusion.
* **Density:** Diffusion rate is inversely proportional to the square root of the substance's density; lighter molecules diffuse faster.
* **Pressure:** Higher pressure increases the rate of diffusion as molecules move faster from high to low pressure zones.
* **Medium:** Diffusion is slower in denser mediums (e.g., water) compared to sparse mediums (e.g., air).
* **Importance in Plant Physiology:**
* Gas exchange (CO2 uptake for photosynthesis, O2 release for respiration) in leaves occurs via diffusion through stomata.
* Loss of water vapor during transpiration from leaves into the atmosphere is a diffusion process.
* Passive absorption of minerals by roots occurs to some extent through diffusion.
* Aids in the translocation of food materials (sugars) from leaves to other parts of the plant.
* Distribution of plant hormones throughout the plant body.

**3. Imbibition:**
Imbibition is the process of adsorption of water by solid or colloidal substances (hydrophilic substances) without forming a true solution, leading to a significant increase in volume. This is a type of diffusion where water moves into the solid matrix.
* **Characteristics:**
* Colloidal substances like cellulose, pectin, and lignin in plant cell walls are hydrophilic and absorb large amounts of water.
* Requires an affinity between the imbibant (substance) and water.
* A large imbibition pressure can develop, which is strong enough to split rocks or break seed coats.
* **Importance in Plant Physiology:**
* Essential for the initial stages of water absorption by dry seeds, leading to germination.
* Contributes to the intake of soil water by root hairs, especially in combination with osmosis.
* Believed to be an important force in the ascent of sap (water movement up the plant stem).
* Helps in the rehydration of resurrection plants, which can survive extreme desiccation by rapidly absorbing water.

These three processes, though distinct, often work together, ensuring efficient water balance, nutrient distribution, and structural integrity vital for plant life.
In simple words: Plants use osmosis, diffusion, and imbibition to stay alive. Osmosis moves water across cell skins, helping roots drink water and keep cells firm. Diffusion moves gases like air and helps water vapor leave the plant. Imbibition is when dry plant parts, like seeds, soak up water and swell a lot, which helps them grow. All three are very important for a plant's everyday functions.

🎯 Exam Tip: For each process, provide a clear definition, explain its mechanism, and list at least two specific physiological roles in plants. Highlight how they are distinct but often interconnected.

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