When a cell is in a hypotonic solution, the concentration of dissolved substances (solutes) outside the cell is lower than the concentration inside the cell. This imbalance means the surrounding fluid is less concentrated with solutes compared to the cell’s internal fluid. This difference in solute concentration dictates the direction of water flow across the cell’s outer boundary, which ultimately determines the cell’s fate.
Osmosis The Driving Force
The movement of water in this scenario is driven by a process called osmosis, which is the diffusion of water molecules across a selectively permeable membrane. Water naturally moves from an area where it is in higher concentration (lower solute concentration) to an area where it is in lower concentration (higher solute concentration). In a hypotonic environment, the external solution has a higher concentration of water molecules.
Consequently, water rushes across the cell membrane and into the cell’s cytoplasm in an attempt to equalize the solute concentrations on both sides. This net movement of water is passive, requiring no energy expenditure from the cell, and is often facilitated by specialized protein channels in the membrane called aquaporins. The continuous influx of water generates a significant pressure inside the cell, known as osmotic pressure. The outcome of this water gain depends entirely on the cell’s physical structure, specifically whether it possesses a rigid outer wall.
Fate of Animal Cells Lysis
Animal cells, such as human red blood cells, lack a rigid outer cell wall, relying solely on a flexible plasma membrane to maintain their structural integrity. When these cells are placed into a hypotonic solution, the rapid and uncontrolled influx of water causes the cell to swell continuously. The plasma membrane is elastic, but its capacity to stretch is finite.
As the internal pressure mounts due to the rising volume of water, the delicate plasma membrane eventually reaches its breaking point. The cell then ruptures and bursts, a process known as cytolysis. If this phenomenon occurs specifically in red blood cells, it is termed hemolysis. This bursting is why administering pure distilled water intravenously is medically dangerous, as it would cause the patient’s blood cells to lyse.
Fate of Plant Cells Turgor Pressure
In contrast, plant cells possess a sturdy, rigid cell wall composed primarily of cellulose, which is located outside the plasma membrane. When a plant cell is exposed to a hypotonic solution, water moves into the cell through osmosis, similar to the animal cell response. The water first enters the cytoplasm and then the large central vacuole, causing the cell to swell dramatically.
The swelling plasma membrane pushes outward against the surrounding cell wall, creating an internal pressure known as turgor pressure. The rigid cell wall resists this expansion, preventing the cell from rupturing, thereby protecting it from lysis. A plant cell in this swollen, firm state is described as turgid. This turgidity is the optimal and healthy state for most plant tissues, providing the mechanical rigidity that allows non-woody plants to stand upright.
Practical Applications and Biological Context
Understanding the cellular response to hypotonic solutions is fundamental in medicine and environmental biology. In clinical settings, intravenous fluids must be carefully formulated to be isotonic, meaning they have the same solute concentration as human blood, to prevent red blood cells from lysing or shrinking. Hypotonic solutions, such as 0.45% sodium chloride, are sometimes administered to patients to rehydrate cells that are severely dehydrated, often seen in conditions like diabetic ketoacidosis.
In nature, many single-celled organisms, such as Paramecium, live in freshwater environments that are naturally hypotonic to their internal cytoplasm. These organisms have evolved specialized structures called contractile vacuoles that constantly collect and expel the excess water that rushes into the cell, preventing cytolysis. Plant life depends on hypotonic soil water to maintain the turgor pressure necessary for structural support and growth.