Plant Physiology: Absorption by Roots

1. Introduction to Water Absorption

The process by which plants absorb water and minerals from the soil is crucial for their survival and growth. This absorption primarily occurs through the roots, which are specialized structures adapted for this function. Understanding the mechanisms behind water absorption involves delving into various physical and biological processes.

1.1. Characteristics of Roots for Water Absorption

Roots are remarkably suited for their role in water absorption due to several key features:

• Vast Surface Area: The root system of a single plant can be incredibly extensive, with numerous branching roots and millions of root hairs. This vast network provides a massive surface area for water absorption.
• Semi-Permeable Membrane: The cell membranes of root hairs are selectively permeable, allowing water molecules to pass through while restricting the movement of larger solute molecules. This property is fundamental to the process of osmosis.
• High Solute Concentration: The cell sap within root hairs contains a higher concentration of salts and sugars compared to the surrounding soil water. This concentration gradient drives the movement of water into the roots via osmosis.

1.2. Structure of a Root Hair

A root hair is a microscopic, tubular extension of an epidermal cell of a root. Its structure is optimized for water absorption:

• Large Surface Area: The elongated shape of a root hair significantly increases the surface area available for absorbing water and mineral nutrients.
• Thin Cell Wall: The cell wall of a root hair is thin and permeable, allowing water to pass through easily.
• Semi-Permeable Cell Membrane: Located just inside the cell wall, the cell membrane controls the passage of substances into the cell.
• Vacuole: A large central vacuole contains cell sap, which has a high concentration of solutes, contributing to the osmotic gradient that draws water into the cell.

2. The Process of Water Absorption

The absorption of water by roots involves a combination of physical phenomena, including imbibition, diffusion, and osmosis.

2.1. Imbibition

Imbibition is the initial step in water absorption, where water is absorbed by the solid components of the root, such as the cell wall. This process is driven by the affinity of hydrophilic (water-attracting) substances for water.

• Mechanism: Water molecules adhere to the surfaces of cellulose and pectin in the cell wall, causing it to swell.
• Significance: Imbibition helps to wet the root surfaces, facilitating the subsequent movement of water into the root cells.

2.2. Diffusion

Diffusion is the net movement of molecules from an area of higher concentration to an area of lower concentration. In the context of water absorption, water molecules diffuse from the soil, where they are more concentrated, into the root cells.

• Mechanism: This movement is random and driven by the kinetic energy of the molecules.
• Significance: Diffusion plays a role in the movement of water across the cell wall and cytoplasm.

2.3. Osmosis

Osmosis is the primary mechanism by which roots absorb water. It is the movement of water across a semi-permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration).

• Mechanism: The semi-permeable cell membrane of the root hair allows water molecules to pass through but restricts the movement of larger solute molecules. The difference in water potential between the soil water and the cell sap creates an osmotic gradient, driving water into the root.
• Significance: Osmosis is responsible for the bulk of water uptake by the roots.

3. Water Potential and its Components

The movement of water in plants is governed by water potential, which is a measure of the potential energy of water. It is influenced by several factors:

• Osmotic Pressure: This is the pressure required to prevent the inward flow of water across a semi-permeable membrane. A higher solute concentration results in a more negative osmotic potential, increasing the tendency of water to move into the solution.
• Root Pressure: This is the positive pressure that develops in the xylem of the roots due to the active transport of minerals into the root cells. This influx of minerals lowers the water potential in the xylem, causing water to move in from the surrounding cortex cells. Root pressure is most evident at night when transpiration is low.
• Turgor Pressure: As water enters a plant cell, the cell contents swell and press against the cell wall. This pressure is known as turgor pressure. It provides structural support to the plant and is essential for cell expansion and growth.

4. Turgidity, Flaccidity, and Plasmolysis

The water balance within a plant cell determines its state:

• Turgidity: A cell is turgid when it is fully hydrated, and the cell membrane is pushed firmly against the cell wall. This is the normal state for most plant cells and is essential for maintaining the plant's form and function.
• Flaccidity: A cell becomes flaccid when it loses water, and the cell membrane is no longer pressed against the cell wall. This leads to wilting.
• Plasmolysis: If a plant cell is placed in a hypertonic solution (a solution with a higher solute concentration than the cell), it will lose water through osmosis. The cell membrane pulls away from the cell wall, and the cell is said to be plasmolyzed.
• Deplasmolysis: If a plasmolyzed cell is placed in a hypotonic solution (a solution with a lower solute concentration), it will regain water, and the cell will become turgid again. This process is called deplasmolysis.

5. Absorption of Water and Minerals

While water is absorbed primarily through osmosis, the absorption of mineral ions involves both passive and active transport.

• Passive Transport: Some mineral ions move into the root cells by diffusion, following their concentration gradient. This process does not require energy.
• Active Transport: Most mineral ions are present in the soil in very low concentrations. Therefore, they need to be actively transported into the root cells against their concentration gradient. This process requires energy in the form of ATP and involves specific carrier proteins in the cell membrane.

6. Ascent of Sap

Once water and minerals are absorbed by the roots, they need to be transported to the other parts of the plant. This upward movement of water and dissolved minerals from the roots to the leaves is called the ascent of sap. It occurs through the xylem, a specialized water-conducting tissue.

6.1. Forces Responsible for Ascent of Sap

Several forces work together to move water up the plant:

• Cohesion: Water molecules have a strong tendency to stick to each other due to hydrogen bonds. This property, known as cohesion, creates a continuous water column in the xylem.
• Adhesion: Water molecules also adhere to the walls of the xylem vessels. This adhesion helps to support the water column against the force of gravity.
• Transpirational Pull: The primary driving force for the ascent of sap is transpirational pull. Transpiration is the evaporation of water from the surfaces of leaves. As water evaporates, it creates a tension or pull on the water column in the xylem. This pull is transmitted all the way down to the roots, drawing water up the plant.

6.2. Experiments to Show Conduction of Water Through Xylem

A simple experiment can demonstrate that water is conducted through the xylem:

1. Take a young, leafy shoot of a balsam plant and place it in a beaker containing a colored solution (e.g., eosin or safranin).
2. After a few hours, observe the stem and leaves. You will see that the veins in the leaves have turned red, indicating that the colored water has been transported upwards.
3. If you take a transverse section of the stem, you will observe that only the xylem tissue is stained red, confirming that it is the pathway for water conduction.