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 due to water loss through osmosis.

Step 1: Understand Osmosis

Osmosis is the process in which water molecules move across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. This process aims to balance the solute levels on both sides of the membrane.

Step 2: Cucumber and Cellular Structure

Cucumbers are made up of cells that contain a lot of water. The cell membrane in cucumber cells acts as a semipermeable membrane, allowing water to move in and out of the cells.

Step 3: When Cucumber is Placed in Salt Solution

When a cucumber is placed in a concentrated salt solution, the solution outside the cucumber has a higher concentration of solute (salt) compared to the inside of the cucumber, which has more water (and less salt).

Step 4: Osmosis in Action

Due to osmosis, water molecules move from the inside of the cucumber cells, where there is a high concentration of water, to the outside in the salt solution, where there is a lower concentration of water. This causes the water to leave the cucumber cells.

Step 5: Resulting Shrivelled Appearance

As water leaves the cucumber cells, the cells lose their turgor pressure, resulting in the cells collapsing or shrivelling. This is why the cucumber appears shrivelled after being in the salt solution.

Key Concepts

Semipermeable Membrane

A semipermeable membrane is like a selective gate for molecules. Think of it as a net that lets tiny water molecules pass through but blocks bigger molecules like salt. This is essential in processes like osmosis. In cucumbers, the cell membrane serves as this semipermeable barrier. It helps maintain the balance of solutes and water inside and outside the cells. When the cucumber is submerged in a salt solution, osmosis occurs because the membrane doesn't allow salt to enter, but it permits water to move out of the cell. This movement is crucial for maintaining cell structure.
Understanding the functions and characteristics of semipermeable membranes can help explain many biological phenomena beyond just cucumbers, such as how nutrients are absorbed and waste products are expelled in different organisms.
Semipermeable membranes make sure that life processes run smoothly by regulating what goes in and out of cells.

Solute Concentration

Solute concentration is a measure of how much solute is present in a solution. In the case of the cucumber in saltwater, salt acts as the solute. Each cucumber cell has its own concentration of solvents and solutes that need to be balanced with their surroundings.
When the cucumber is placed in a highly concentrated salt solution, the solute concentration outside becomes higher than inside the cucumber's cells. This imbalance triggers osmosis, causing water to move out of the cucumber and into the salt water.

• The outward movement of water is due to the effort to balance solute concentration.
• The goal is to have equal solute concentration on both sides of the membrane.

Understanding solute concentration is important not only for explaining why cucumbers shrivel but also in fields like medicine, where it helps understand fluid movement in the body.

Turgor Pressure

Turgor pressure is the pressure exerted by water inside the cell against the cell wall. It gives plants their firm structure. Imagine a water balloon filled just right; it maintains its shape and is firm to touch. Similarly, when cucumbers have adequate water in their cells, they are firm and crunchy.
However, when placed in a salt solution, water from the cucumber cells moves out due to osmotic pressure, decreasing the internal water pressure, or turgor pressure.

• This loss of water causes the cucumber to lose its firmness.
• The cells become less rigid, leading to a shrivelled appearance.

Turgor pressure is essential for plant health and growth, affecting processes such as nutrient transport. Therefore, maintaining proper hydration is critical for plant life.