Problem 10 - Animal Physiology

Question

The cell membranes of mammalian red blood cells are permeable to urea. If red blood cells are dropped into a solution of urea that is identical in osmotic pressure (isosmotic) to the cytoplasm of the cells, although the cells do not swell and burst as quickly as when they are dropped simply into pure water, they eventually swell and burst. Explain. Also discuss how you would design a solution into which red cells could be placed without ever swelling. (Hint: Think about whether urea will stay on the outside of the cells and the implications for osmotic pressures.)

Step-by-Step Solution

Answer

Urea diffuses into cells, disrupting the osmotic balance and causing water influx and subsequent cell swelling. To prevent this, cells should be placed in a solution with substances to which the cell membrane is impermeable and that has the same osmotic pressure as the cells, ensuring such osmotic imbalance cannot occur.

Step 1: Understanding Osmosis and Cell Membranes

Osmosis is the process where water molecules move from an area of low solute concentration to an area of high solute concentration, across a semi-permeable membrane. This movement aims to equalize the solute concentrations on the two sides of the membrane. Cell membranes, being semi-permeable, only allow certain substances through.'

Step 2: Understanding the Effect of Urea

Red blood cells are permeable to urea. Thus, when they are placed in a urea solution that initially has the same osmotic pressure as the cytoplasm, urea molecules diffuse into the cells over time, increasing the intracellular solute concentration. This creates a situation where the inside of the cell has a higher solute concentration than the outside, inducing water to diffuse into the cell to balance solute concentrations.

Step 3: Explaining the Swelling and Bursting

Following the laws of osmosis, water molecules move from the urea solution into the cells, directed by the new osmotic gradient created by the urea entering the cell. As more water enters the cells, they begin to swell. Since the cell membrane can only stretch to a certain point, continuous water influx leads to bursting of the cells.

Step 4: Designing a non-impactful solution

To avoid cell swelling, cells need to be placed in a solution where passive movement of solutes or water across the cell membrane does not disrupt the osmotic balance. This can be achieved by using a solution that comprises of substances to which the cell membrane is impermeable and that has the same osmotic pressure as the cytoplasm of the cells. This ensures no net movement of water into or out of the cells, maintaining their size.

Key Concepts

Osmosis

Osmosis is a fundamental process in cell biology. It refers to the movement of water molecules across a semi-permeable membrane, such as the cell membrane.
The direction of water movement is driven by differences in solute concentration on either side of the membrane. Water moves from areas with lower solute concentration to areas with higher solute concentration.
This movement continues until the solute concentration on both sides of the membrane is equal, or until the cell reaches its maximum capacity. Osmosis is crucial for maintaining cell structure and function.

Red Blood Cells

Red blood cells (RBCs) are essential for transporting oxygen throughout the body. They have a flexible, biconcave shape that increases their surface area and allows them to squeeze through tiny blood vessels.
Their membranes are semi-permeable, meaning they allow certain small molecules to pass through, but not all.
This permeability is vital; however, it also makes RBCs susceptible to changes in their environment. If the surrounding solution changes, especially in solute concentration, it can impact the RBCs. This can lead to situations where water influx causes the cells to swell and potentially burst.

Urea Diffusion

Urea is a small, nitrogen-containing waste product that results from the breakdown of proteins in the body. It is freely permeable across red blood cell membranes due to its small size.
When RBCs are placed in a urea solution, urea molecules can diffuse easily into the cells. This diffusion increases the intracellular solute concentration, creating an imbalance.
As a result, it triggers osmosis where water moves into the cells to re-establish an equilibrium. Understanding how urea diffuses is crucial to predicting how cells will behave in various solutions.

Osmotic Pressure

Osmotic pressure refers to the pressure required to prevent the movement of water across a semi-permeable membrane. It reflects the concentration of solutes in a solution.
In biological contexts, osmotic pressure is critical as it governs water movement across cell membranes.
If the osmotic pressure is the same, or isotonic, on both sides of the cell membrane, there is no net movement of water, and the cells maintain their shape. However, when the balance tips, and one side has a higher osmotic pressure, water moves to that side, potentially causing cells to swell and burst.