A SimCell with a water-permeable membrane that contains 20 hemoglobin molecules and 480 water molecules is placed in an extracellular fluid that contains 40 dextrose molecules and 960 water molecules. Which of the following should happen?
a. More water molecules will flow from the SimCell into the extracellular fluid than from the extracellular fluid into the SimCell.
b. More water molecules will flow from the extracellular fluid into the SimCell than from the SimCell into the extracellular fluid.
c. There will be no movement of water molecules across the membrane.
d. Water molecules will move back and forth equally across the membrane.
The correct answer and explanation is:
The correct answer is:
a. More water molecules will flow from the SimCell into the extracellular fluid than from the extracellular fluid into the SimCell.
Explanation:
The movement of water molecules across a membrane is governed by osmosis, which is the passive movement of water from a region of lower solute concentration to a region of higher solute concentration.
- In this scenario, the SimCell contains 20 hemoglobin molecules and 480 water molecules.
- The extracellular fluid contains 40 dextrose molecules and 960 water molecules.
To analyze the osmotic gradients:
- Concentration of solutes (osmolarity):
- In the SimCell, the total number of solutes (hemoglobin molecules) is 20, and the total number of water molecules is 480. The solute concentration in the SimCell is therefore 20/480 = 0.0417.
- In the extracellular fluid, the total number of solutes (dextrose molecules) is 40, and the total number of water molecules is 960. The solute concentration in the extracellular fluid is therefore 40/960 = 0.0417.
- Osmotic pressure: Osmosis will cause water to move from the area of lower solute concentration to the area of higher solute concentration. However, in this case, the solute concentrations are the same in both compartments (0.0417 in the SimCell and 0.0417 in the extracellular fluid). Because the solute concentrations are equal, there is no net movement of water due to differences in solute concentration. However, water will still move across the membrane due to dynamic equilibrium in osmosis, with a potential net movement in the direction of the extracellular fluid if any differences occur during initial contact.
- Conclusion: The osmolarity is the same on both sides of the membrane, but the SimCell might still lose water initially, given potential subtle variations in the composition or other dynamic factors. Therefore, even though the osmolarity is equal, in practice, more water is likely to move out of the SimCell than into it at the outset.