Prep Guide: Absorption by Roots

Key Concepts (Recall & Understanding)

Root Hairs: Microscopic extensions of root epidermal cells that vastly increase the surface area for absorption.
Mechanisms of Absorption:
- Imbibition: Initial absorption of water by the hydrophilic cell wall.
- Diffusion: Movement of molecules from high to low concentration.
- Osmosis: The primary mechanism; movement of water across a semi-permeable membrane from a region of high water potential to low water potential.
Tonicity:
- Hypotonic Solution: Lower solute concentration; causes endosmosis (water enters the cell), leading to turgidity.
- Hypertonic Solution: Higher solute concentration; causes exosmosis (water leaves the cell), leading to plasmolysis.
- Isotonic Solution: Equal solute concentration; no net water movement, cell becomes flaccid.
Active Transport: Movement of mineral ions against their concentration gradient, requiring energy (ATP).
Ascent of Sap: The upward movement of water and minerals from the roots to the leaves through the xylem.
Forces for Ascent of Sap:
- Root Pressure: Positive pressure developed in the xylem due to active transport of minerals.
- Cohesion: Attraction between water molecules.
- Adhesion: Attraction of water molecules to the xylem walls.
- Transpirational Pull: The main driving force; evaporation of water from leaves creates a tension that pulls the water column upwards.

Application Corner

Why does a freshly picked lettuce leaf wilt when left in the sun, but become crisp again when placed in cold water?
- Answer: In the sun, the leaf loses water through transpiration faster than it can absorb it, causing the cells to become flaccid and the leaf to wilt. When placed in cold water (a hypotonic solution), the cells absorb water through osmosis, become turgid, and the leaf becomes crisp again.
If you add a large amount of salt to the soil around a plant, it will eventually die. Explain this phenomenon using the principles of osmosis.
- Answer: Adding salt to the soil makes the soil water a hypertonic solution compared to the cell sap in the plant's roots. This causes water to move out of the roots and into the soil via exosmosis. The plant cannot absorb water, leading to severe plasmolysis and eventual death from dehydration.

Analytical Thinking

Odd One Out: Imbibition, Diffusion, Osmosis, Active Transport.
- Odd One: Active Transport.
- Category: The rest are forms of passive transport that do not require metabolic energy.
Scenario: A balsam plant shoot is placed in a beaker of water with a layer of oil on top. What is the purpose of the oil layer in this experiment, which is designed to demonstrate the ascent of sap?
- Answer: The oil layer is to prevent the loss of water from the beaker due to evaporation. This ensures that any decrease in the water level in the beaker is solely due to the water being absorbed by the plant shoot and transported upwards (ascent of sap).

Key Case Study

The Challenge of Saline Soils in Agriculture

In many coastal and arid regions, soil salinity is a major agricultural problem. High concentrations of salt in the soil create a hypertonic environment. This makes it extremely difficult for crop plants to absorb water through their roots via osmosis. In fact, the osmotic potential of the soil water can be so low that it draws water out of the plant roots, leading to plasmolysis, wilting, and crop failure. To overcome this, scientists are working on developing salt-tolerant crop varieties that can either exclude salt from their roots or accumulate it in specific tissues to maintain a favorable water potential gradient for water absorption.

Assertion-Reason Practice

Assertion (A): The ascent of sap in tall trees is primarily due to transpirational pull. Reason (R): The cohesive and adhesive properties of water create an unbroken, continuous water column in the xylem.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A). The transpirational pull can only work to draw water up the tree because the cohesion and adhesion of water molecules maintain the integrity of the water column in the xylem, allowing the pull to be transmitted from the leaves down to the roots.

HOTS (Higher-Order Thinking Skills) Question

Root pressure is a relatively weak force and is often absent during the day. How then can a plant ensure a continuous supply of water to its leaves, especially at the top of a very tall tree, on a hot, sunny day?

Answer: On a hot, sunny day, the rate of transpiration is very high. This creates a strong transpirational pull, which is the main driving force for the ascent of sap, especially in tall trees. This pull, or tension, is far more powerful than root pressure. The cohesion of water molecules (sticking to each other) and their adhesion to the xylem walls create a continuous, unbroken water column that is pulled up from the roots to the leaves, ensuring a constant supply of water to replace what is lost through transpiration. Therefore, even without root pressure, the plant can efficiently transport water to great heights.

Specimen Prep - Absorption by Roots