A hypotonic solution describes a liquid environment that contains a lower concentration of dissolved particles, known as solutes, compared to another solution or a cell’s internal environment. This particular concentration difference is fundamental to understanding how water moves across biological membranes within living systems.
Comparing Solutions
The designation “hypotonic” is always a comparative term, indicating that a solution has a lower solute concentration when measured against a different solution or a cell’s internal fluid. For instance, pure water would be considered hypotonic relative to most living cells because it contains virtually no dissolved solutes. This difference in concentration drives a process called osmosis, which is the passive movement of water molecules through a selectively permeable membrane. Water naturally moves from an area where its concentration is higher (lower solute concentration) to an area where its concentration is lower (higher solute concentration), aiming to equalize concentrations on both sides.
Solutions can also be isotonic or hypertonic. An isotonic solution has a solute concentration equal to that of the cell’s internal environment, resulting in no net water movement. Conversely, a hypertonic solution possesses a higher solute concentration than the cell, causing water to move out of the cell. The prefix “hypo-” signifies “low” or “under,” which helps in remembering that a hypotonic solution has a comparatively lower solute concentration. This relative difference in solute levels dictates the direction of water flow.
How Hypotonic Solutions Affect Cells
When cells are exposed to a hypotonic solution, water moves into the cell through osmosis due to the higher solute concentration inside the cell compared to the external environment. The response of a cell to this influx of water depends on its structural characteristics, particularly the presence or absence of a rigid cell wall.
Animal cells, which lack a cell wall, are particularly susceptible to hypotonic environments. For example, a red blood cell placed in a hypotonic solution will absorb water. As water continues to enter, the cell swells, and its flexible cell membrane cannot withstand the increasing internal pressure. This leads to the cell bursting, a process known as lysis or hemolysis for red blood cells.
In contrast, plant cells are equipped with a rigid cell wall. When a plant cell is in a hypotonic solution, water moves into the cell, causing the central vacuole to expand and push the cell membrane against the cell wall. This internal pressure, called turgor pressure, makes the cell firm or “turgid.” The strong cell wall prevents the plant cell from bursting, allowing it to maintain its shape and rigidity. High turgor pressure is important for plants to remain upright.