Problem 199 - Objective Biology for NEET

Question

Assertion: Tracheids are dead cells. Reason: Tracheids have lignified cell wall.

Step-by-Step Solution

Answer

Both the assertion and reason are correct. Tracheids are indeed dead cells, and this is because they have a lignified cell wall.

Step 1: Understanding Tracheid Cells

Tracheids are long, slender cells found in the xylem of vascular plants. They are part of the system that transports water and dissolved minerals upwards from the roots. They do not contain protoplasm at maturity which means they are no longer living cells.

Step 2: Correlating Tracheids and Lignified Cell Wall

The cell wall of tracheid is strengthened by a substance called lignin. This makes the cell walls rigid and hard, consequently, the cells lose their protoplasm and die. A lignin cell wall is, indeed, a characteristic of a dead cell.

Step 3: Final Evaluation

From the first two steps, it can be concluded that tracheid cells are dead cells because during their maturity their cell wall lignifies, which eventually leads to them being void of protoplasm. Therefore, both the assertion and reason given in the problem are correct, and the reason is a valid explanation of the assertion.

Key Concepts

Xylem in Vascular Plants

The xylem is an essential component of vascular plants, serving as the primary conduit through which water and nutrients move from the roots to other parts of the plant. This complex tissue comprises various specialized cells, with one of the most critical being tracheid cells. Tracheids are elongated and interconnected through their ends, allowing them to form a networked pathway for the crucial flow of nutritional elements.

Particularly in gymnosperms and lower plant types like ferns, tracheids are the sole type of water-conducting cells in the xylem. Angiosperms usually have another type called vessel elements, which are shorter and wider than tracheids, and combine to form long tubes known as vessels. Nonetheless, even in plants with vessels, tracheids play a vital role in support due to their mechanical strength and also contribute to water conduction.

Lignified Cell Wall

The process of lignification is fundamental to the physical characteristics that allow plant cells, particularly tracheid cells, to fulfill their role in water transport and structural support. Lignin is a complex organic polymer that is deposited in the cell walls of plants, providing rigidity and water resistance. This process occurs as tracheids develop and mature.

Lignification essentially strengthens the cell wall, making it stiff and woody, which in return prevents collapse under the tension created by water transpiration and also provides resistance against pathogens and mechanical damage. However, the presence of lignin is also what renders these cells dead at maturity, as they lose their internal cellular components, leaving behind a robust yet hollow tube perfect for water conduction.

Water Transport in Plants

One of the most fascinating aspects of plant biology is how water, an essential resource for survival, is transported from roots to leaves against gravity. This movement occurs in the xylem, facilitated primarily by tracheids in many plants. Water transport relies on the cohesion between water molecules and the adhesion of these molecules to the walls of the xylem cells.

The driving force behind this upward flow is a combination of root pressure, capillary action, and the evaporation of water from leaves, known as transpiration. As water evaporates from the surface of the leaves, it pulls other water molecules along due to its cohesive properties. The lignified cell walls provide the necessary strength to withstand the negative pressures generated by this process, ensuring efficient water transport from the roots to the highest leaves.