Question

Balancing osmotic pressures. Consider a membrane-enclosed vesicle that contains within it a protein species \(A\) that cannot exchange across the membrane. This causes an osmotic flow of water into the cell. Could you reverse the osmotic flow by a sufficient concentration of a different nonexchangeable protein species \(B\) in the external medium?

Short answer

Yes, introducing a high enough concentration of protein B outside can reverse the osmotic flow.

Step-by-step solution

Understand Osmotic Pressure

Osmotic pressure is the pressure required to prevent the flow of water across a semipermeable membrane. It is influenced by solute concentration on each side of the membrane.

Evaluate the Scenario

Inside the vesicle, there is a protein species A that cannot cross the membrane, causing water to flow into the vesicle due to osmotic pressure.

Introduce an External Protein Species

To counteract the osmotic flow into the vesicle, an external protein species B, which also cannot cross the membrane, is introduced into the medium surrounding the vesicle.

Balance the Osmotic Pressures

By adding a sufficient concentration of protein species B externally, you increase the osmotic pressure outside the vesicle. This would counterbalance the osmotic pressure inside the vesicle caused by species A.

Reverse the Osmotic Flow

If the concentration of protein B outside the vesicle is high enough, this would reverse the net osmotic flow, either stopping water from entering the vesicle or even causing it to leave the vesicle.

Key concepts

Semipermeable Membrane

A semipermeable membrane is a barrier that allows certain particles to pass through while blocking others. In biological contexts, these membranes often let water move freely but block larger molecules or ions. This selectivity is essential for establishing gradients that drive important physiological processes.

In the given exercise, the membrane of the vesicle is semipermeable, allowing water to pass but not protein species A or B. This selectivity sets the stage for osmotic pressure to come into play. Understanding how water moves through this membrane, while proteins stay confined, is crucial for grasping the resulting osmotic dynamics.

The behavior of semipermeable membranes can be observed in various real-world examples, such as plant cell walls, kidney filtration, and even dialysis treatment for kidney failure.

• Remember, semipermeable membranes are not an absolute barrier—they are selective based on size, charge, and other factors of the particles.
• These membranes are fundamental in both natural and artificial systems, regulating the internal environment of cells, and other compartments.

Solute Concentration

Solute concentration refers to the amount of solute—like salts, proteins, or other molecules—dissolved in a solvent, usually water. Higher solute concentration means more particles are present in the solution. This is an important factor in determining osmotic pressure.

In the exercise, the vesicle contains protein species A creating a high solute concentration inside. According to colligative properties, osmotic pressure increases with rising solute concentration. This higher concentration of A inside the vesicle draws water in through the semipermeable membrane to balance the internal and external environments.

When an external protein species B is introduced into the surrounding medium, it adds to the solute concentration outside the vesicle. If the concentration of B is sufficiently high, it can increase the external osmotic pressure to counteract or even reverse the water flow into the vesicle.

• Think of solute concentration like adding more sugar to a cup of tea—the more you add, the sweeter it becomes, similar to how increased concentration affects osmotic pressure.
• In practical scenarios like medicine, managing solute concentrations is critical for treatments involving intravenous fluids to ensure cells maintain their proper function.

Osmotic Balance

Osmotic balance is the state when the osmotic pressures on either side of a semipermeable membrane are equal, resulting in no net movement of water. Achieving osmotic balance is key for cellular homeostasis, ensuring cells neither swell up nor shrink.

In the exercise scenario, the initial imbalance caused by protein A inside the vesicle creates a net influx of water. By introducing a sufficient concentration of protein B externally, the osmotic pressures can be balanced. This balance occurs when the external osmotic pressure (from protein B) matches the internal osmotic pressure (from protein A), stopping the net inflow of water.

If the concentration of B is higher than that of A, the external pressure exceeds the internal pressure, potentially reversing the water flow and causing the vesicle to lose water.

• Osmotic balance is essential for the proper functioning of biological cells. Imbalances can lead to conditions like swelling (edema) or dehydration of cells.
• In medical treatments, maintaining osmotic balance is crucial. For instance, the fluids used for IV drips must be carefully balanced to match the body's osmotic conditions.