Question

A cucumber placed in concentrated brine (saltwater) shrivels into a pickle. Explain.

Short answer

The cucumber shrivels due to osmosis, where it loses water to the concentrated brine.

Step-by-step solution

Understanding Osmosis

Osmosis is the movement of water across a semi-permeable membrane from a region of lower solute concentration (more water) to a region of higher solute concentration (less water) until equilibrium is reached.

Saltwater's Effect on Cucumber

The brine solution, which is concentrated saltwater, contains a high solute concentration compared to the inside of the cucumber. This creates a concentration gradient across the cucumber's membrane.

Water Movement

Water inside the cucumber moves out through its semi-permeable membrane into the brine to balance the solute concentration on both sides of the membrane, following the principle of osmosis.

Resulting State of the Cucumber

As water leaves the cucumber, it loses turgor pressure, causing the cucumber to shrivel and become firmer, thereby converting it into a pickle over time.

Key concepts

Semi-permeable Membrane

A semi-permeable membrane is a critical concept in understanding osmosis. Imagine it as a thin barrier that only lets certain substances pass through, specifically small molecules like water. Large solutes, such as the salt in our example with the cucumber, cannot move across the membrane. This selective permeability is essential for the process of osmosis to occur.

In the case of the cucumber in the brine solution, the skin of the cucumber acts as this semi-permeable membrane. It allows the movement of water molecules between the inside of the cucumber and the saltwater environment outside. However, the salt itself is too large to pass through, maintaining the salt concentration outside of the cucumber. This difference is vital for the whole process of osmosis, which is the driving force behind the shrinking of the cucumber into a pickle.

Understanding this barrier function helps explain many biological processes where balance is necessary, such as nutrient absorption in cells and the regulation of water content in organisms.

Concentration Gradient

The concentration gradient is a somewhat tricky but important idea to grasp. It refers to the difference in solute concentration across a solution. In simpler terms, it's the variation in how crowded solute molecules, like salt, are on one side of the membrane compared to the other.

In our cucumber scenario, the inside contains a lower concentration of salt compared to the outside brine. This gradient is essential for osmosis to occur. Because the external salt concentration is much higher, it attracts water molecules from inside the cucumber to move outwards, helping balance the concentrations on either side of the membrane.

The flow from high water concentration inside the cucumber to the lower concentration in the brine is a natural attempt to reach equilibrium. When equilibrium is approached, though never perfectly reached due to the membrane's nature, the flow slows down. This phenomenon emphasizes why substances spread out in solutions, influencing numerous biological functions.

Turgor Pressure

Turgor pressure is what keeps plant cells firm and rigid. It's the pressure of the cell contents against the cell wall. In an upright plant, strong turgor pressure keeps leaves stiff and the plant standing tall.

However, in the pickling process, this concept explains why the cucumber becomes limp and shrivels. As water exits the cucumber due to the concentration gradient created by the brine, the internal pressure decreases. This loss in pressure causes the cells to lose firmness—a direct symptom of reduced turgor pressure.

Think of it like deflating a balloon; as air leaks out, it goes limp and loses shape. Similarly, the decline in turgor pressure means the cucumber cells can't maintain their turgid state, resulting in a shriveled pickle. This process highlights how crucial water balance is to maintaining the structure and vitality of plant tissues.