Question

What important role does lignin play in vascular plants? a. It is the major component of the cuticle, which prevents plants from drying out. b. It forms the outer layer of pollen grains. c. It forms the tough, rigid cell walls of water-conducting cells. d. It nourishes the embryo in seed plants.

Short answer

Lignin plays an important role in vascular plants by forming the tough, rigid cell walls of water-conducting cells, allowing for effective transport of water and nutrients throughout the plant.

Step-by-step solution

Option A: Cuticle Component

Lignin is not the major component of the cuticle; the cuticle is primarily made up of waxes and cutin. Therefore, option A is incorrect.

Option B: Pollen Grain Layer

Lignin does not form the outer layer of pollen grains. The pollen grains' outer layer consists of a substance called sporopollenin. So, option B is incorrect.

Option C: Water-conducting Cell Walls

Lignin is an essential component in the cell walls of the vascular tissue, imparting strength and rigidity. This feature allows water-conducting cells to transport water and nutrients effectively throughout the plant. Therefore, this statement is accurate, and option C is the correct answer.

Option D: Nourishing Embryo

Lignin does not nourish the embryo in seed plants. Instead, it's the nutrients stored in the seed's endosperm or cotyledons that provide nourishment to the growing embryo. Thus, option D is incorrect. From the explanation above, the correct answer is: c. It forms the tough, rigid cell walls of water-conducting cells.

Key concepts

Lignin's Role in Plant Structure

Lignin is a critical organic compound found in the cell walls of many plants, particularly in vascular plants. It plays an important role in maintaining the structure and rigidity of these plants. By binding with cellulose fibers, lignin adds considerable strength and stability to the plant cell walls.

This is especially important in water-conducting cells, found in the vascular tissue. These cells need to be strong and rigid to effectively transport water and nutrients throughout the plant, often against gravity. Therefore, lignin is vital for ensuring that these cells can withstand the pressures of water transport. Without lignin, plants would struggle to maintain their upright structure, leading to issues in nutrient delivery and overall growth.

In addition to providing strength, lignin also makes the plant cell walls more water-resistant, helping to prevent the invasion of pathogens. This protective feature is crucial for plants to survive in various environmental conditions.

Water-Conducting Cells: Xylem

In vascular plants, the xylem tissue is responsible for conducting water and dissolved minerals from the roots to other parts of the plant. Water-conducting cells, also known as tracheary elements, are the specialized cells in the xylem that facilitate this process.

The two main types of water-conducting cells are:

• Tracheids: These are elongated cells found in all vascular plants. They have thickened walls and tapered ends to interconnect with each other, forming efficient pathways for water movement.
• Vessel elements: Found mainly in angiosperms (flowering plants), these cells are shorter and wider than tracheids and align end to end to form long tubes called vessels. Vessels allow faster water transport due to their larger diameter and efficient connectivity.

The lignin in their walls provides the necessary structural support for these cells, allowing the plants to thrive even under the weight of fluids being transported.

Additionally, the rigidity and durability of lignified walls help resist negative pressures created during water transportation—a phenomenon vital for the plant’s survival under drought conditions and in tall plants.

Understanding Plant Cell Walls

Plant cell walls are complex structures that offer support, protection, and rigidity to plant cells. They are made up of several components: primarily cellulose, hemicellulose, pectin, and lignin.

The primary cell wall, which forms during cell growth, is flexible and allows for expansion. It contains cellulose microfibrils intertwined with a matrix of hemicellulose and pectin, providing structural support.

The secondary cell wall, forming after the cell has reached its full size, is thicker and contains more lignin. This presence of lignin in the secondary wall provides additional strength and rigidity, facilitating the transport of water and nutrients through the vascular system.

In addition, plant cell walls serve other functions:

• Protection: They act as a protective barrier against physical damage and microbial invasion.
• Growth regulation: The cell wall regulates the direction and rate of cell growth.
• Communication: They play a role in cell signaling, allowing the plant cells to coordinate responses to environmental stimuli.

The integration of lignin within the plant cell walls is key to supporting these functions, ensuring plants maintain health and functionality across various environments.