Question

Why do lettuce leaves in a salad wilt after a vinaigrette dressing containing salt is added?

Short answer

Lettuce leaves wilt because water leaves the cells due to the high salt concentration in the vinaigrette, reducing turgor pressure.

Step-by-step solution

Understanding Osmosis

Osmosis is the process by which water moves across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration.

Role of Salt in Vinaigrette

Vinaigrette dressing often contains salt. When salt is added to the vinaigrette, it creates a high solute concentration in the dressing.

Osmosis Effect on Lettuce Cells

When the vinaigrette dressing is added to the lettuce, the high concentration of salt outside the lettuce cells causes water to move out of the cells by osmosis to try to balance the concentration.

Resulting Wilting

As the water leaves the lettuce cells, the cells lose turgor pressure and become less firm, causing the lettuce leaves to wilt.

Key concepts

Turgor Pressure

Turgor pressure is the pressure of the cell contents against the cell wall in plants. It plays a crucial role in keeping plants upright and firm. When plant cells take in water, the cell membrane pushes against the cell wall. This pressure makes the plant rigid. It's like inflating a balloon inside a box—the balloon pushes outwards, making the structure firm. If the water leaves the plant cells, the pressure drops, causing the cells to become flaccid and the plant to wilt. This explains why lettuce in a salad wilts after adding a vinaigrette dressing. The water moves out of the lettuce cells due to osmosis, reducing turgor pressure.

Solute Concentration

Solute concentration refers to the amount of solute dissolved in a solvent. In plants, solutes can include salts, sugars, and other substances. When discussing osmosis, it's essential to know that water moves towards the higher solute concentration to balance the levels on both sides of a membrane. In the example of lettuce wilting in a salad, the vinaigrette dressing has a high salt concentration. This increase creates a higher solute concentration outside the lettuce cells compared to the inside. Thus, water exits the cells to balance the concentration difference, leading to loss of turgor pressure and wilting.

Semipermeable Membrane

A semipermeable membrane is a barrier that allows some molecules to pass while blocking others. It's like a sieve that lets water in but keeps larger particles out. In plant cells, the cell membrane acts as a semipermeable membrane. During osmosis, water moves through this membrane from an area of low solute concentration to high solute concentration. When lettuce with a vinaigrette dressing is observed, the salt in the dressing creates a high solute concentration outside the lettuce cells. The cell membrane allows water to move out but not the solute, leading to a loss of water in the cells, reduced turgor pressure, and wilting.