Question

Red blood cells placed in a \(5 \%\) salt solution would: swell/stay the same/ shrink?

Short answer

Red blood cells placed in a \(5\%\) salt solution would shrink due to the hypertonicity of the solution.

Step-by-step solution

Understanding Osmosis

Osmosis is the process by which water molecules pass through a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. In this context, the solute is the salt in the solution and the solvent is the water.

Understanding Tonicity

Tonicity refers to the salt concentration of the solution compared to the cytoplasm of the red blood cells. If the solution's salt concentration is equal to that of the cell's cytoplasm, the solution is said to be isotonic, and no net movement of water occurs. If the solution's salt concentration is lower, the solution is hypotonic, and water tends to enter the cell, causing it to swell. If the solution's salt concentration is higher, the solution is hypertonic, and water tends to leave the cell, causing it to shrink.

Apply Tonicity to Our Situation

Since the salt concentration of a red blood cell is about \(0.9\%\) salt and the provided solution is \(5\%\), which is higher, the solution is considered hypertonic. Therefore, water will leave the red blood cells to try to equalize the salt concentration inside and outside the cell;

Key concepts

Tonicity

Tonicity is a crucial concept in understanding the behavior of red blood cells in different environments. It refers to the relative concentration of solutes, like salt, in a solution compared to the inside of a cell. If the concentrations are equal, the solution is isotonic , and no significant movement of water occurs between the cell and the environment. When the solution outside the cell has a lower concentration of solutes, it is called hypotonic . In such a scenario, water tends to enter the cell, potentially causing it to swell. Conversely, if the outside solution is hypertonic , with a higher solute concentration compared to the inside of the cell, water will exit the cell, leading it to shrink. Understanding tonicity helps in predicting how cells like red blood cells will react in different solutions.

Hypertonic Solution

A hypertonic solution is one where the concentration of solutes is higher outside the red blood cell than inside the cytoplasm. This difference in concentration drives osmosis. Water, which naturally moves from areas of lower solute concentration to higher solute concentration, will thus move out of the cell. This movement is an attempt to balance the solute levels on both sides of the cell membrane. As water exits the red blood cell, the cell starts to shrink in a process called crenation. This can affect the cell's function because cell structure is crucial for its biological roles. Recognizing a hypertonic solution, like a 5% salt solution , helps in understanding potential impacts on cells and is important in fields like medicine and biology.

Red Blood Cell Cytoplasm

The cytoplasm of a red blood cell is the gel-like substance enclosed within the cell membrane. It contains water, salts, enzymes, and various organic molecules. The typical salt concentration within the cytoplasm is around 0.9% . This balance of solutes and water is essential for maintaining the cell's shape and function. Changes in external solute concentration, like exposure to hypertonic solutions, can disrupt this balance, causing the cell to lose water and shrink. This can affect the red blood cell's ability to transport oxygen effectively since its shape, known as the biconcave form, may change.

Semi-Permeable Membrane

A semi-permeable membrane is a barrier that allows certain molecules, like water, to pass through while blocking others, such as salts. This selective permeability is essential for processes like osmosis. In the context of a red blood cell, the cell membrane serves as the semi-permeable barrier, allowing water movement but not the larger salt particles. This characteristic helps maintain homeostasis within the cell, even when external conditions change. The semi-permeable nature ensures that changes in solute concentration outside the cell lead to movement of water in or out to adjust the internal environment, vital for cell function and survival.