Problem 61 - Objective Biology for NEET

Question

The plants face water stress due to the usc of excessive fertilizers because of (a) Exosmosis (b) Endosmosis (c) Imbibitions (d) None of these

Step-by-Step Solution

Answer

The correct answer is (a) Exosmosis. Excessive use of fertilizers leads to a hypertonic soil condition, thus causing exosmosis which in turn results in water stress in the plants.

Step 1: Definition of Exosmosis

Exosmosis is the outward movement of water from a cell when it is placed in a hypertonic solution. In this scenario, water moves out of the cell causing it to shrink, a process known as plasmolysis.

Step 2: Definition of Endosmosis

Endosmosis is the inward movement of water into a cell when it is placed in a hypotonic solution. Here, water moves into the cell making it turgid.

Step 3: Definition of Imbibition

Imbibition is the process of absorption of water by solids-colloids leading to an enormous increase in the volume. In the case of plants, water is absorbed by the seeds and dry wood as examples of imbibitions.

Step 4: Linking concepts to Fertilizer Use

When excessive fertilizers are used, they dissolve into the soil water creating a hypertonic solution. This causes water to flow out of the plant cells and into the soil due to exosmosis, leading to water stress in the plants.

Key Concepts

Exosmosis

When a plant cell is submerged in a hypertonic solution, which has higher solute concentration than the cell's cytoplasm, it undergoes exosmosis. This process involves the outward movement of water from the cell's interior through the semi-permeable cell membrane to the surrounding environment. As a result, the cell loses water and starts to shrink, appearing flaccid. This effect can be particularly noticeable when excessive fertilizers are dissolved in soil water. The increased solutes around the roots create a hypertonic situation, promoting exosmosis and causing the plant to experience water stress.

Endosmosis

Endosmosis is the reverse of exosmosis, where water enters the cell. This happens when the plant cell finds itself in a hypotonic solution, which is less concentrated than the cell's own fluids. During endosmosis, water moves from the solution into the cell, passing through the cell membrane, causing the cell to swell or become turgid. This mechanism is critical in maintaining the cell's structure and is essential for growth processes like germination, where the water uptake is vital.

Imbibition

Imbibition is a special type of water absorption by substances, causing them to swell without necessarily dissolving. It is not a cell-based process like osmosis; instead, it occurs in porous materials. The most common example in plants is the swelling of seeds as they absorb water to begin germination. Dry wood soaking up water and swelling is another example. This process is driven by the adhesive forces between the water molecules and the colloidal substances that are higher than the cohesive forces between the water molecules.

Plasmolysis

The phenomenon of plasmolysis can be observed when a plant cell loses water through exosmosis to the point where the cell membrane pulls away from the cell wall. It is a clear indicator that the cell is in a state of water stress and will be unable to function normally. Plasmolysis is reversible if the cell is placed back in an isotonic or hypotonic solution, which allows endosmosis to rehydrate the cell. However, prolonged plasmolysis may lead to cell death and, consequently, damage the plant's overall health.

Hypertonic Solution

A hypertonic solution contains a higher concentration of solutes compared to the inside of a plant cell. When plant roots are exposed to a hypertonic solution, such as soil with excessive fertilizer, water stress occurs through exosmosis. The cells lose water to the surrounding hypertonic solution, leading to plasmolysis. Mitigating this requires careful fertilizer management to prevent the soil from becoming excessively hypertonic and ensuring plants have sufficient water to avoid dehydration and stress.