Problem 94 - Chemistry and Chemical Reactivity

Question

Explain why a cucumber shrivels up when it is placed in a concentrated solution of salt.

Step-by-Step Solution

Answer

The cucumber shrivels up due to osmosis, where water leaves its cells to dilute the concentrated external salt solution.

Step 1: Understanding Osmosis

Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. In biological systems, such as in plants and cucumbers, the cell membranes act as semipermeable barriers.

Step 2: Identifying the Solution

A concentrated salt solution has a high concentration of salt and a low concentration of water compared to the inside of the cucumber, which is mostly water.

Step 3: Direction of Water Movement

Because the salt solution is more concentrated with salt compared to the inside of the cucumber, water will move from inside the cucumber, where there is more water and less salt, to the outside environment of the salt solution.

Step 4: Consequence of Water Movement

As water leaves the cucumber's cells due to osmosis, the cells lose water and thus volume. This loss of water causes the cucumber to shrivel up, as its cells become dehydrated.

Key Concepts

Semipermeable Membrane

In the world of biology, a semipermeable membrane is like a selective bouncer for molecules. This special type of membrane allows certain small molecules, like water, to pass through, while keeping larger ones or compounds like salt out. In plant cells, the cell membrane plays this crucial role.
This is why plant cells can control what enters and exits, maintaining balance within their structure. The semipermeable nature of the membrane is vital for processes like osmosis.
Without semipermeable membranes, cells could not regulate their internal environment, and life as we know it would not be possible.

Water Movement

Water is constantly on the move in living organisms, thanks to a process known as osmosis. This natural movement occurs through semipermeable membranes and is driven by concentration differences.
In osmosis, water travels from areas of low solute concentration to areas of high solute concentration. This means water naturally flows from regions where there is often more of it and fewer dissolved substances, to areas where there are more dissolved substances and less water.
Water's movement is critical to many biological processes, aiding nutrient absorption in plants and maintaining fluid balances in animals.

Concentrated Solution

A concentrated solution has a high amount of solute compared to the amount of solvent. In our example, the salt solution around the cucumber is the concentrated solution because it contains a lot of salt relative to the water present.
Such a solution affects osmosis by creating a large difference in concentration across the semipermeable membrane. This difference causes water to move out of the cucumber to dilute the salt-rich environment around it.
Understanding concentrated solutions helps explain how substances in different environments interact and the effects these interactions can have, such as the shriveling of a cucumber.

Plant Cells

Plant cells have unique structures that make them well-suited to their environment. They contain rigid cell walls, large vacuoles, and, as we've discussed, semipermeable membranes. These structures support the plant and help regulate its internal processes.
When placed in a concentrated solution, plant cells will lose water because of osmosis. As they dehydrate, the vacuoles shrink and the cells lose turgor pressure, which causes them to shrivel.
This shriveling is visible in the cucumber as it loses water. Thus, understanding plant cells' response to osmosis is essential for explaining phenomena like wilting.