Problem 152 - Objective Biology for NEET

Question

Assertion: The water and mineral uptake by root hairs from the soil occurs through apoplast until it reaches the endodermis. Reason: Casparian strips in endodermis are suberized.

Step-by-Step Solution

Answer

Yes, the assertion and reason are correct. The apoplast pathway allows the movement of water and minerals until it reaches solid barrier presented by the Casparian strips in the endodermis, forcing the water and minerals into the cells.

Step 1: Understanding the Role of Apoplast

The Apoplast pathway is one of the pathways through which water and minerals are transported in plants. This involves the movement of water along the cell walls and the spaces between the cells. This is a fast, non-selective process and continues without hindrance until it reaches the endodermis.

Step 2: Role of Casparian Strips

The Casparian strip is a part of the endodermal cells and is made up of Suberin, a waxy substance. This strip acts like a barrier for apoplastic pathway as Suberin is impervious to water and dissolved minerals. Hence when water and minerals reach the endodermis via Apoplast, it has to move into the cells due the barrier created by Casparian strips.

Step 3: Linking the Apoplast and Casparian Strips

Water and mineral uptake by root hairs from the soil occurs through the apoplast pathway until it reaches the endodermis, where the Casparian strips block this pathway and water has to move into the cells. Thus, the assertion is linked to the reason; both are correct and the reason correctly explains the assertion.

Key Concepts

Water and Mineral Uptake

Water and mineral uptake in plants is a vital process that allows them to absorb the necessary nutrients and moisture from the soil. This process begins at the root hairs, which are thin, hair-like extensions of root epidermal cells. These hairs significantly increase the surface area for absorption and play a crucial role in the uptake of water and dissolved minerals.

Water uptake from the soil is driven by the concentration gradient and involves both active and passive transport mechanisms. Minerals, on the other hand, require specific transport proteins for active uptake into root cells because they typically move against a concentration gradient.

The absorbed water and minerals are then transported through the plant via two main pathways: apoplast and symplast. The apoplast pathway allows movement along the cell walls and intercellular spaces without entering the cytoplasm of the cells, while the symplast pathway involves the passage from one cell to another through the plasmodesmata, which are tiny channels in the cell walls.

Casparian Strips

Casparian strips are key structures found in the endodermis, which is the innermost layer of cells in the root cortex. They form a continuous band around each endodermal cell, made of Suberin—a waxy, waterproof substance. The presence of these strips creates a selective barrier within the plant root system.

Because Suberin is waterproof, Casparian strips effectively block the apoplastic pathway for water and minerals, preventing them from freely passing beyond the endodermis. This forces the water and minerals to enter the cytoplasm of endodermal cells, transitioning to the symplastic pathway. This transition is crucial for the selective uptake of nutrients, as the cell membranes of the endodermal cells can actively regulate which minerals are absorbed into the vascular system of the plant.

The regulation provided by the Casparian strips is essential for maintaining ionic balance in plants and ensuring that harmful substances do not enter the vascular tissues.

Root Hairs Absorption

Root hairs are specialized structures that significantly increase the root surface area and enhance the efficiency of water and nutrient absorption from the soil. These tiny extensions work by actively absorbing water and minerals from the soil through both diffusion and active transport mechanisms.

Due to their proximity to the soil particles, root hairs can easily access the nutrient-rich water film surrounding the soil particles. They have thin walls and a large surface area to volume ratio, which facilitates the rapid intake of water and dissolved substances.

The effectiveness of root hair absorption contributes to the success of the apoplast pathway in the initial stages of water and mineral transport in plants. Their role as the primary site of nutrient uptake underscores their importance in the overall growth and health of the plant.

Plant Transport System

The plant transport system is an intricate network that ensures the movement of water, minerals, and nutrients throughout the plant. This system includes both the apoplast and symplast pathways. The xylem and phloem are two sets of vascular tissues that play a central role in transport.

The xylem is responsible for the upward movement of water and soluble minerals from the roots to the leaves. It operates mainly on physical forces, including capillary action and the transpiration pull. Phloem, on the other hand, distributes sugars and other organic nutrients made in the leaves to different parts of the plant through a process known as translocation.

Together, these pathways and vascular tissues create a dynamic and flexible system capable of meeting the plant's needs in various environmental conditions. They not only facilitate the distribution of vital substances but also contribute to plant stability and structure.