Question

Assume that you place a freshwater plant into a saltwater solution and examine it under a microscope. What happens to the plant cells? What if you placed a saltwater plant in pure water? Explain. Draw pictures to illustrate your explanations.

Short answer

When a freshwater plant is placed in a saltwater solution, the plant cells lose water due to osmosis, become dehydrated, and shrink in a process called plasmolysis. On the other hand, when a saltwater plant is placed in pure water, the plant cells take in water due to osmosis and swell up, potentially bursting in a process called lysis. These processes demonstrate why some plants are adapted to live in freshwater or saltwater environments. Illustrations should clearly show the changes in plant cell size and shape during these scenarios.

Step-by-step solution

Scenario 1: Freshwater Plant in Saltwater Solution

When a freshwater plant is placed in a saltwater solution, the water inside the plant cells is less concentrated in solutes than the surrounding saltwater. Due to osmosis, water molecules will move from the plant cells (lower solute concentration) to the saltwater (higher solute concentration) through the plant cell's semi-permeable membrane. This will cause the plant cells to lose water, become dehydrated, and eventually shrink due to the loss of water pressure. This phenomenon is called plasmolysis. To illustrate this, draw a plant cell with a normal, turgid shape at the beginning. Then, as the water moves out of the cell, draw the cell shrinking and the cell membrane pulling away from the cell wall.

Scenario 2: Saltwater Plant in Pure Water

When a saltwater plant is placed in pure water, the water inside the plant cells has a higher solute concentration than the surrounding pure water. In this case, osmosis causes water molecules to move from the pure water (lower solute concentration) to the saltwater plant cells (higher solute concentration) through the cell membranes. As a result, the saltwater plant cells will take in water and swell up. If the pressure from the absorbed water is too much, the cell can burst. This is called lysis. To illustrate this, draw a saltwater plant cell with a normal shape at the beginning. As the water moves into the cell, draw the cell enlarging and possibly bursting if the pressure becomes too great. In conclusion, the differential effects of osmosis in these provided scenarios help explain why certain plants are adapted to live in freshwater or saltwater environments. In both scenarios, the illustrations should clearly demonstrate the changes happening to the plant cells as emphasized in the step-by-step explanations provided.

Key concepts

Plasmolysis

Plasmolysis is a process that occurs when plant cells lose water in a hypertonic solution—a solution with a higher concentration of solutes than the cell's cytoplasm. This can happen when a freshwater plant is placed in saltwater, as the saline environment causes water to move out of the cells to balance the solute concentration on both sides of the cell membrane.

During plasmolysis, the cell shrinks, and its plasma membrane pulls away from the cell wall. Imagine a balloon deflating within a rigid box—the balloon represents the plasma membrane, and the box is analogous to the cell wall. This shrinkage can be severe in extreme conditions and lead to cell damage. Illustrating plasmolysis typically involves showing the cell before and after the water loss, highlighting how the internal structure collapses inward and away from the plant cell wall.

Cell Membrane Permeability

The semi-permeable nature of a plant cell's membrane is critical in regulating what substances enter and exit the cell. This permeability allows the cell to maintain homeostasis by controlling the flow of water and solutes. For instance, when plant cells are placed in differing solutions like saltwater or freshwater, osmosis occurs because the membrane permits the passage of water molecules while restricting most solutes.

Cells rely on this selective permeability to ensure that essential materials can enter, metabolic products can leave, and potentially harmful substances are kept out. Understanding this concept is key to grasping why plant cells behave differently in various environmental conditions, such as the contrasting responses to saltwater and freshwater outlined in the provided scenarios.

Saltwater and Freshwater Plant Adaptations

Adaptations for saltwater and freshwater environments in plants are essential for survival and involve a complex interplay of physiological and biochemical strategies. Freshwater plants typically have mechanisms to prevent the excessive intake of water and to store necessary solutes. In contrast, saltwater plants, often referred to as halophytes, have adaptations that allow them to cope with high salt concentrations, such as specialized salt excretion systems or compartments for storing excess salt.

These adaptations result in differing responses to a change in external osmotic conditions, as illustrated by our two scenarios. Freshwater plants are not adapted to withstand the high solute concentrations found in saltwater, leading to plasmolysis, whereas saltwater plants could suffer from lysis in pure water due to their cells taking in too much water.

Lysis

Lysis is the bursting of a cell due to excessive internal pressure, typically occurring when a cell takes in too much water. For saltwater plants, which are adapted to environments where they are typically losing water, immersion in pure water presents a challenge. Here, the surrounding water is less concentrated than the cell's interior, leading to an influx of water through osmosis.

If the cell wall cannot contain the swelling, the cell may rupture, similar to overfilling a water balloon until it pops. In the illustration for this process, it is important to depict the cell as it expands progressively more and finally bursts. This demonstrates the fragility of cell membrane permeability balances and the potential harm that an uncontrolled environment can cause to a cell.