Problem 104 - Objective Biology for NEET

Question

By what mechanism does water moves through vessels and tracheids of root stem and leaves? (a) Osmotic flow (b) Bulk flow (c) Gravity flow (d) Imbibition

Step-by-Step Solution

Answer

The correct answer is (b) Bulk flow. It represents the mechanism through which water moves through vessels and tracheids of root stem and leaves in plants.

Step 1: Understand the Options

Each option represents a biological process. Understanding is crucial: \n\n- Osmotic flow refers to the movement of water across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration. \n\n- Bulk flow or mass flow involves the movement of water and solutes together due to a pressure gradient. \n\n- Gravity flow is simply the downward movement of substances due to gravity. \n\n- Imbibition is the diffusion of water into a solid colloidal substance causing it to swell.

Step 2: Analyze the Process

Water in plants moves primarily through a process known as Transpiration pull. It creates tension or negative pressure allowing water to rise against the force of gravity. The cohesive and adhesive properties of water are crucial for this process. It starts from the leaves and extends to the roots.

Step 3: Make the Conclusion

Considering the details of the water-moving mechanism in plants, it is clear that this process corresponds to Bulk Flow. This is because Transpiration pull facilitates the mass movement of water and dissolved substances from the roots to the leaves due to a pressure gradient.

Key Concepts

Transpiration pull

In the complex world of plant biology, transpiration pull is a fascinating process that plays a vital role in the movement of water from the roots to the leaves. It starts in the leaves, where water evaporates from tiny openings called stomata. This evaporation creates a vacuum or negative pressure in the leaf. Think of it like sipping a drink through a straw.

This negative pressure pulls water up through the plant's vascular system, a bit like how liquid rises in a straw when you suck on it. This suction effect is what we refer to as the transpiration pull. It's a critical component in the process of water transport, ensuring that all parts of the plant, from roots to leaves, receive the necessary water and nutrients to thrive.

Transpiration pull is fueled by environmental factors and operates against the natural pull of gravity, which makes it both efficient and fascinating. Without it, plants wouldn't be able to grow to their majestic heights or spread their vibrant leaves.

Cohesion and adhesion

To understand how water moves so effectively in plants, we must delve into the properties of water, particularly cohesion and adhesion. These forces might sound technical, but they are key to understanding water transport.

Cohesion refers to the attraction between water molecules themselves. It occurs because of hydrogen bonding, which is the same reason why water droplets form beads on a leaf. These cohesive forces help to hold the water column intact as it moves up through the plant.

On the other hand, adhesion is the attraction between water molecules and the walls of the plant's xylem vessels, which are like tiny tubes. This adhesive interaction helps counteract gravity, sticking the water molecules to the walls and preventing them from slipping back down.

Together, cohesion and adhesion work in unison to enable water to move upward in a continuous stream, crucial for sustaining plant life.

Pressure gradient

The concept of a pressure gradient in plant water transport is akin to a stepping stone for understanding how water moves effortlessly through a plant's vascular system.

When we talk about a pressure gradient, we're discussing the difference in pressure from one part of the plant to another. In plants, there's typically higher pressure at the roots and lower pressure near the leaves due to transpiration.

This pressure difference creates a gradient, like the slope of a hill, which drives the bulk flow of water, carrying solutes along with it through the plant.

The pressure gradient enables the plant to transport not just water, but also essential minerals and nutrients from the soil to different plant parts. This process is fundamental for the plant's growth and development, effectively acting as an internal transport system powered by natural pressure differences.