Cells constantly interact with their surrounding fluids and substances. Understanding these interactions is central to comprehending how living systems maintain stability and perform diverse functions. The nature of a cell’s outer boundaries significantly determines how it responds to changes in its external world.
Cells Without Walls
Many cells possess a rigid outer layer called a cell wall, which provides structural support, protection, and helps maintain shape. However, not all cells have this robust external covering. Animal cells, including human cells like red blood cells, are prime examples of cells that naturally lack a cell wall. Instead, animal cells are enclosed by a flexible cell membrane.
This absence of a cell wall allows animal cells greater flexibility, enabling them to change shape, move, and specialize for various functions within complex tissues and organs. For instance, muscle contraction or nerve signal transmission relies on this flexibility. Some single-celled organisms, such as certain protozoa like amoebas, also lack a cell wall, relying on their flexible membrane for protection and movement. Additionally, certain bacteria, like Mycoplasma species and L-form bacteria, do not possess a cell wall, making them unique in the prokaryotic world. The lack of this rigid layer makes these cells susceptible to changes in their external environment, especially concerning water movement.
Hypotonic Solutions and Osmosis
To understand what happens to cells without a wall, it is necessary to grasp the concepts of hypotonic solutions and osmosis. A hypotonic solution has a lower concentration of dissolved substances (solutes) compared to the concentration of solutes inside a cell. This means it contains a higher concentration of water molecules than the cell’s internal environment.
Osmosis is the passive movement of water molecules across a selectively permeable membrane, like a cell membrane. This movement occurs from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). The cell membrane allows water molecules to pass through, sometimes aided by specialized protein channels called aquaporins, while restricting the movement of larger solute molecules. This tendency of water to equalize solute concentrations drives the events that occur when a cell is placed in a hypotonic environment.
The Outcome: Swelling and Lysis
When a cell that lacks a cell wall, such as an animal cell, is placed into a hypotonic solution, water rapidly moves from the surrounding solution into the cell. This inward movement is driven by osmosis, as the cell’s internal environment has a higher solute concentration than the external hypotonic solution. As water continuously enters, the cell begins to swell.
The cell membrane, though flexible, has a limited capacity to stretch. As water volume inside the cell increases, it exerts increasing pressure against the membrane. Without a rigid cell wall to provide structural counter-pressure and prevent over-expansion, the cell membrane cannot withstand this rising internal pressure.
Eventually, the membrane stretches beyond its breaking point and ruptures, leading to the cell bursting. This process is known as cytolysis or osmotic lysis. When this occurs specifically in red blood cells, it is termed hemolysis.
The Protective Function of a Cell Wall
In contrast to cells without walls, those with a cell wall respond differently when exposed to a hypotonic solution. Plant cells, fungal cells, and most bacterial cells have cell walls. When these cells are placed in a hypotonic environment, water still moves into the cell via osmosis, causing it to swell.
However, the rigid cell wall surrounding the cell membrane prevents excessive expansion and bursting. As water enters, the cell membrane is pushed firmly against the unyielding cell wall, generating an internal pressure known as turgor pressure. This turgor pressure maintains the cell’s shape and rigidity. Instead of lysing, the cell becomes turgid (firm and swollen), a state often beneficial for organisms like plants, as it helps them maintain their upright structure. This comparison highlights the protective role a cell wall provides against osmotic lysis.