Problem 107 - Objective Biology for NEET

Question

The continuity of water column in xylem is maintained due to the (a) Presence of air bubbles (b) Cohesive property of water (c) Evaporation power of water (d) None of these

Step-by-Step Solution

Answer

The continuity of water column in xylem is maintained due to the cohesive property of water.

Step 1: Understand the Role of Xylem in Plants

Xylem is a complex vascular tissue that helps to transport water and dissolved substances from roots to different parts of the plants. Xylem forms a continuous network of vessels and tracheids, from roots to the leaves, maintaining a water column.

Step 2: Study the Properties of Water

In addition to being a solvent, water also possesses certain important properties like cohesion (water molecules sticking to each other), adhesion (water molecules sticking to other substances) and high evaporation heat.

Step 3: Identify the Important Factor for Water Continuity in Xylem

Among the properties of water, it is the cohesive property that plays a key role in maintaining the water column's continuity in the xylem. Cohesion holds the water molecules together and allows them to move as a continuous stream within the plant.

Key Concepts

Cohesive Property of Water

The cohesive property of water is a vital characteristic that allows water molecules to stick to one another. This sticking is due to hydrogen bonding, which creates a pulling force. In plants, cohesion is crucial because it helps create a continuous stream of water within the plant's transport system. This continuity is fundamental for transporting water from the roots to various parts, including the leaves.

Without cohesion, the water column would break, making it impossible for water to travel efficiently through the plant. The cohesive forces are strong enough to counteract gravity, allowing water to climb upwards through the plant's tissues, even reaching leaves at higher elevations. Thus, the cohesive property is indispensable for efficient water transport in plants.

Vascular Tissue

Vascular tissues are specialized plant tissues responsible for the transport of water, nutrients, and sugars throughout the plant. In vascular plants, the two main types of vascular tissue are xylem and phloem. Xylem is specifically tasked with transporting water and dissolved minerals derived from the soil.

It consists of vessels and tracheids, which are tube-like structures that facilitate the movement of water by forming a continuous network throughout the plant. This network extends from the roots to the leaves. Xylem’s structure provides both support and a pathway for water movement. In contrast, phloem transports the products of photosynthesis, such as sugars, to different parts of the plant for energy and growth.

Together, xylem and phloem ensure that plants maintain their internal balance and proper function.

Water Transport in Plants

Water transport in plants is a complex process driven by physical and chemical forces. Key to this process is the movement of water from the roots, where it is absorbed from the soil, up to the leaves. This upward movement is supported by several mechanisms:

• Cohesion-tension theory: This theory explains how water is pulled up through the xylem due to tension created by evaporation (transpiration) at the leaf surfaces. Water molecules stick together (due to cohesion) and are pulled upward.
• Adhesion: While cohesion keeps water molecules together, adhesion helps them to stick to the walls of xylem vessels, facilitating upward movement.
• Root pressure: Generated in the roots, this pressure can also push water upwards, although it is a less significant force compared to transpiration pull.

Being very efficient, these mechanisms allow water to reach all plant parts, maintaining hydration and nutrient transport, critical for plant survival and growth. Together, these processes enable plants to grow, photosynthesize, and thrive in various environments.