Problem 63 - Objective Biology for NEET

Question

What will be the direction of movement of water, when solution \(\mathrm{A}\) having water potential of \(-9\) bars and another solution \(\mathrm{B}\) of \(-4\) bars is separated by a semi-permeable membrane? (a) \(\mathrm{B}\) to \(\mathrm{A}\) (b) \(\mathrm{A}\) to \(\mathrm{B}\) (c) Both directions (d) None

Step-by-Step Solution

Answer

Water will move from solution B to solution A.

Step 1: Determining the Solution with Higher Water Potential

Among the two given solutions A and B, with water potentials of -9 and -4 bars respectively. The solution with a higher water potential is solution B as water potential of -4 bars is less negative compared to -9 bars.

Step 2: Direction of Water Movement

Water tends to move from a region of higher water potential to a region of lower water potential. Therefore, water will move from solution B (water potential of -4 bars) to solution A (water potential of -9 bars).

Key Concepts

Osmosis

Osmosis is a fundamental biological process that describes the movement of water molecules across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This movement aims to equalize solute concentrations on the two sides, leading to an equilibrium state. In osmosis, water moves freely, but the solutes do not, as the membrane is selectively permeable to only water molecules.

Understanding osmosis is vital for comprehending a wide range of biological and chemical processes, such as cell hydration, nutrient absorption, and the maintenance of blood pressure. For students, a useful analogy is to imagine a tea bag in hot water; the flavor (solute) gradually disperses until uniform taste is achieved throughout the water – similar to how water moves during osmosis.

Semi-Permeable Membrane

A semi-permeable membrane, also known as a selectively permeable membrane, acts as a barrier that permits certain substances to pass through while blocking others. In the context of osmosis, this type of membrane allows only water molecules to move across it but not solute particles such as salts, sugars, or ions.

To further clarify through a real-world example, think of a garden sieve that holds back the larger stones but allows the fine soil to pass through. Similarly, semi-permeable membranes allow for the selective movement of molecules, thereby controlling the passage of substances in and out of cells. This selectivity plays a critical role in maintaining cellular environments and is essential for proper cellular function.

Direction of Water Movement

The direction of water movement in osmosis is governed by differences in water potential. Water potential is a measure that combines the effects of solute concentration and pressure. In simple terms, water moves from areas of higher water potential (less negative value) to areas of lower water potential (more negative value). Water potential can be influenced by solute concentration, pressure, gravity, and matrix potential (in cases involving soil or other porous media).

In the exercise provided, water will move from solution B to solution A because solution B has a water potential of -4 bars (higher/less negative) compared to solution A's -9 bars (lower/more negative). Situations like this are crucial in everything from plant root absorption to the principles of dialysis used in medicine.