Problem 101 - Objective Biology for NEET

Question

The path of water from soil up to secondary xylem is (a) Soil \(\rightarrow\) Root hair cell wall \(\rightarrow\) Cortex \(\rightarrow\) Endodermis \(\rightarrow\) Pericycle \(\rightarrow\) Protoxylem \(\rightarrow\) Metaxylem (b) Metaxylem \(\rightarrow\) Protoxylem \(\rightarrow\) Cortex \(\rightarrow\) Soil \(\rightarrow\) Root hair (c) Cortex \(\rightarrow\) Root hair \(\rightarrow\) Endodermis \(\rightarrow\) Pericycle \(\rightarrow\) Protoxylem \(\rightarrow\) Metaxylem (d) Pericycle \(\rightarrow\) Soil \(\rightarrow\) Root hair \(\rightarrow\) Cortex \(\rightarrow\) Endodermis \(\rightarrow\) Protoxylem \(\rightarrow\) Metaxylem

Step-by-Step Solution

Answer

\nThe correct path of water from soil to secondary xylem is option (a), which is Soil → Root hair → Cortex → Endodermis → Pericycle → Protoxylem → Metaxylem.

Step 1: Understand the steps in Water Absorption

Water in the plant primarily moves from the soil through the roots to the xylem tissue. The root hair absorbs water from the soil, then it moves passively into the root cortex, including the endodermis. It continues its movement through the pericycle, then progresses into the xylem tissue - First the protoxylem and then the metaxylem.

Step 2: Match the correct sequence

You have to identify the sequence that exactly matches the process described in step 1.

Step 3: Select the Correct Option

Upon comparing the sequences provided in the options with the process of water absorption, you find that the logical and correct sequence is given in option (a), which is Soil → Root hair → Cortex → Endodermis → Pericycle → Protoxylem → Metaxylem.

Key Concepts

Xylem Function

The xylem is an essential component of the plant vascular system, playing a crucial role in the transport of water and nutrients. It is a complex tissue composed of various cell types, including tracheids and vessel elements, which work together to conduct water from the roots to other parts of the plant.

Tracheids are long, thin cells that help in the support and transport of water and minerals. Vessel elements are shorter and wider, forming continuous tubes that allow for more efficient water movement. The xylem also contains fibers and parenchyma cells which provide structural support.

In addition to water transport, the xylem also has a secondary function of mechanical support. As xylemic cells develop, they deposit lignin, a substance that strengthens their cell walls and helps the plant maintain its upright position.

Root Anatomy

Understanding root anatomy is crucial to comprehend how plants absorb water. The root system begins with the root hairs, which significantly increase the root's surface area for maximum water absorption. These hair-like extensions originate from the epidermis, which is the outermost layer of root cells.

Proceeding inward from the epidermis is the cortex, made up of loosely packed parenchyma cells. This structure facilitates the storage of starch and the movement of water towards the center of the root. The endodermis acts as a selective barrier with the Casparian strip, a waxy layer that regulates the flow of substances into the vascular cylinder.

The Vascular Cylinder and Pericycle

The core of the root's anatomy is the vascular cylinder, which includes the pericycle. This layer gives rise to lateral roots and is involved in secondary growth. The vascular cylinder houses the xylem and phloem—the two primary vascular tissues responsible for transport within the plant.

Plant Water Transport

Water transport in plants is a vital process involving the movement of water from the soil, through the roots, and up to every part of the plant. This movement relies predominantly on transpiration, a phenomenon where water evaporates from the leaves, creating a negative pressure that draws more water upward through the xylem.

Water uptake begins at the root hairs, which absorb water from the surrounding soil due to the higher concentration of solutes within the root cells. This water then passes through the cortex and reaches the endodermis, where the Casparian strip ensures that water enters the vascular cylinder in a controlled manner.

Once inside the vascular cylinder, water enters the protoxylem and subsequently flows into the metaxylem, where it is hoisted to all plant parts. This journey is not just essential for plant hydration but also for the movement of nutrients and maintenance of plant structure through turgor pressure in the cells.