Question

A cell increases in volume if the external medium is (a) Hypotonic (b) Hypertonic (c) Isotonic (d) None of these

Short answer

The cell increases in volume when placed in a hypotonic medium. So the correct answer is (a) Hypotonic.

Step-by-step solution

Understanding Hypotonic Solution

A hypotonic solution has a lower concentration of solutes than the cytoplasm of the cell. This means that water molecules, following the process of osmosis, tend to move from the area of lower solute concentration (the solution) into an area of higher solute concentration (the cell). This leads to the cell absorbing water and swelling in size, i.e., its volume increases. This means the answer could be (a) Hypotonic.

Understanding Hypertonic Solution

A hypertonic solution has a higher concentration of solutes than the cell’s cytoplasm. This also leads water molecules to follow the process of osmosis, but this time they move from the cell (lower solute concentration) into the solution (higher solute concentration). This results in the cell losing water, leading to a decrease in its volume. So, the answer is not (b) Hypertonic.

Understanding Isotonic Solution

An isotonic solution has an equal concentration of solutes as the cell's cytoplasm. This means that the movement of water molecules into and out of the cell is balanced, leading to no overall change in cell volume. Consequently, answer (c) Isotonic will not make the cell volume increase.

Evaluating answer (d) None of these

Since we have already identified that the cell volume increases in a hypotonic solution, answer (d) None of these is incorrect.

Key concepts

Hypotonic Solution

When a cell is placed in a hypotonic solution, it encounters an environment where the outside liquid has fewer dissolved substances, also known as solutes, compared to the cell's interior. This difference in solute concentration sets the stage for a natural phenomenon known as osmosis, where water moves from an area of lower solute concentration into one of higher concentration to balance out the differences.

This inbound movement of water causes the cell to take on water, leading to an increase in its volume. Over time, if the surrounding solution remains hypotonic, the cell may continue to absorb water until it reaches a point of distress, sometimes leading to the cell bursting, which is referred to as lysis. This effect is crucial to understand, especially in medical treatments and biology, where the balance of fluid volumes is often critical.

Hypertonic Solution

In contrast to a hypotonic solution, a hypertonic solution features a higher concentration of solutes outside the cell than inside. This creates a gradient which, due to osmosis, causes water molecules to migrate out of the cell towards the external solution.

As the cell loses water, its volume diminishes, which can lead to crenation in red blood cells, or shriveling in plant cells. This process is often utilized in food preservation methods such as salting or sugaring to create environments that inhibit bacterial growth by drawing out moisture, thereby reducing the risk of spoilage.

Isotonic Solution

An isotonic solution is one where the solute concentration is equal on both sides of the cell membrane. Here, equilibrium is achieved as water moves in and out of the cell at the same rate, with no net gain or loss of cellular volume. Medical treatments, including intravenous drips, often use isotonic solutions to avoid causing cells to swell or shrink, maintaining the integrity of tissues.

Understanding isotonic conditions is not only important for health care but also for athletes, as they need to replenish their fluids with solutions that match the body’s natural osmolality to avoid cellular stress during strenuous activities.

Osmosis

The term osmosis defines the movement of water molecules across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. This fundamental process plays an integral role in maintaining life, as it regulates the hydration of cells and the overall balance of fluids within an organism.

Osmosis does not require energy, as it is a passive process driven by the inherent kinetic energy of water molecules. This biological mechanism allows organisms to control their internal environments, ensuring that vital processes like nutrient absorption and waste removal are executed efficiently.