Problem 169 - Objective Biology for NEET

Question

Assertion: Pure water has greatest water potential. Reason: Pure water contain maximum free water molecule.

Step-by-Step Solution

Answer

Yes, the assertion and reasoning are correct. Pure water has the highest water potential because it contains the maximum amount of free water molecules.

Step 1: Clarify the Concept of Water Potential

Water potential is the potential energy of a water molecule in comparison to pure water, which is always set at zero standard conditions. It previews the direction and movement of water molecules - from regions of high (less negative) to regions of low (more negative) water potential.

Step 2: Understand the Condition of Pure Water

Pure water is defined as water without solute molecules. Since there are no solute molecules to bind to water molecules or create a solute potential, all water molecules are free to move.

Step 3: Deduce the Result

Since water molecules are free to move in pure water and there are no solutes present to create a pressure potential or solute potential, therefore, pure water has maximum water potential. This can also be explained as water will always move towards solutions with lower water potential from the region of pure water.

Key Concepts

Pure Water

Pure water is often referred to as water in its most natural and uncontaminated state. It's essentially H₂O with no additional substances mixed in. This purity means that there are no solutes like salt, sugar, or other substances dissolved in it.
When considering water potential, pure water is always the standard reference point. Its water potential is defined as zero in this condition, serving as a baseline. The reason for zero water potential is simple: without any solute particles, there are no disruptions in the water's natural behavior or movement. This absence of solutes lends pure water its maximum potential for movement.
Here are some key characteristics of pure water:

• Contains no solute molecules.
• Exhibits the highest water potential (set as zero under standard conditions).
• All water molecules are free to move and are unaffected by solutes.

Remember, pure water only exists under ideal laboratory conditions since in nature almost all water has some solutes.

Movement of Water Molecules

Water molecules are constantly in motion due to their kinetic energy. This movement is key to understanding why water potential is so important in biological and ecological systems.
These molecules will move from areas of higher water potential to areas of lower water potential. Essentially, this means they travel from less negative potential to more negative potential environments. In simpler terms, water naturally flows from areas where it is relatively abundant to areas where it is less available.
Movement of water molecules is influenced by:

• Concentration gradients – water tends to move toward higher concentrations of solutes.
• Temperature – higher temperatures increase the kinetic energy and movement speed of water molecules.
• Physical barriers – such as cell membranes, which can regulate the flow of water through processes like osmosis.

This movement is vital for processes such as nutrient transportation in plants and maintaining cell turgor pressure.

Solute Potential

Solute potential, often termed osmotic potential, is a crucial component of water potential. It defines the effect solutes have on the overall water potential. The presence of solute particles lowers the water potential because they bind water molecules, making them less free to move.
When a solute is dissolved in water, it creates what's called an 'osmotic pressure'. This pressure makes it more challenging for the water molecules to escape from the solution. As a result, solutions with dissolved solutes have a more negative water potential compared to pure water.
Important points regarding solute potential include:

• It is always negative since adding solutes decreases the free energy of water.
• The more solute present, the more negative the solute potential will be.
• It drives osmosis, causing water to move into or out of cells to balance solute concentrations.

Understanding solute potential helps explain how plants maintain water uptake and balance their internal hydration levels, which is essential for their survival and growth.