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:
Correct Answer: b. More water molecules will flow from the extracellular fluid into the SimCell than from the SimCell into the extracellular fluid.
Explanation (300 words):
To understand the movement of water molecules across the SimCell membrane, we must examine the concept of osmosis. Osmosis is the passive movement of water molecules across a selectively permeable membrane, from a region of lower solute concentration (more water) to a region of higher solute concentration (less water). This occurs until equilibrium is reached or unless restricted by membrane properties.
In this scenario, the SimCell membrane is permeable to water but not to solutes (such as hemoglobin and dextrose). Therefore, we must calculate the relative concentrations of solutes inside and outside the SimCell.
- Inside the SimCell:
- 20 hemoglobin molecules (solute)
- 480 water molecules
- Total molecules: 500
- Solute concentration = 20/500 = 4%
- Outside the SimCell (extracellular fluid):
- 40 dextrose molecules (solute)
- 960 water molecules
- Total molecules: 1000
- Solute concentration = 40/1000 = 4%
At first glance, both the SimCell and the extracellular fluid have equal solute concentrations (4%), which might suggest no net water movement. However, the actual number of water molecules is different. The extracellular fluid has 960 water molecules, whereas the SimCell has only 480.
Because water moves from areas of higher water concentration (lower solute concentration in terms of water ratio) to lower water concentration, the absolute number of water molecules matters too. In this case, more water molecules are outside the SimCell. This creates a gradient that drives net water influx into the SimCell.
Thus, more water molecules will move into the SimCell than out of it, leading to net osmosis toward the SimCell.
Answer (b) is correct.