Water is fundamental for all living organisms, playing a central role in countless biological processes. Inside every cell, water acts as a solvent, facilitating chemical reactions and transporting substances. Cells are enveloped by a cell membrane, a sophisticated barrier that meticulously controls what enters and exits, including the movement of water molecules necessary for cellular life.
The Fundamental Process: Osmosis
Water primarily enters and leaves cells through osmosis, a passive mechanism. This process involves the net movement of water molecules across a selectively permeable membrane, which allows water to pass freely but restricts dissolved substances, known as solutes. Water moves from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration), continuing until water potential is equalized on both sides. The water potential gradient drives osmosis, moving water from a region of higher to lower potential. Osmosis is a specialized type of diffusion, specifically referring to the movement of water.
Specialized Channels: Aquaporins
While osmosis can occur through the lipid bilayer of the cell membrane, the process is significantly accelerated by specialized protein channels called aquaporins. These integral membrane proteins form pores that selectively allow water molecules to pass through rapidly, and are often referred to as “water channels” because they facilitate water transport much faster than simple diffusion alone. This accelerated transport is particularly important for cells that require quick adjustments to their water balance, such as kidney cells in animals or root cells in plants. These channels ensure that only water molecules pass through, largely preventing the movement of ions and other solutes. Some aquaporins, known as aquaglyceroporins, can also transport other small uncharged molecules, including glycerol.
Cellular Outcomes of Water Movement
The movement of water into or out of a cell depends on the external solute concentration, a concept known as tonicity.
Isotonic Solution
When a cell is in an isotonic solution, the solute concentration outside the cell is equal to that inside. In this balanced state, there is no net movement of water, and the cell maintains its normal shape and size.
Hypotonic Solution
In a hypotonic solution, the external environment has a lower solute concentration than the cell’s interior. Water therefore moves into the cell through osmosis, causing it to swell. Animal cells, lacking a rigid cell wall, can burst (lysis) if too much water enters. Plant cells, however, are protected by their strong cell walls; they become firm and rigid, a state known as turgid, as water pushes against the cell wall.
Hypertonic Solution
Conversely, a hypertonic solution has a higher solute concentration outside the cell compared to its inside. This causes water to move out of the cell, leading to cellular shrinkage. Animal cells in a hypertonic solution shrivel (crenation) due to water loss. In plant cells, the plasma membrane pulls away from the cell wall as water exits (plasmolysis).