Elodea, an aquatic plant commonly known as “water weed,” is a fixture in introductory biology laboratories worldwide. The plant’s thin, translucent leaves offer an unobstructed view of living plant cells under a microscope. This transparency makes the Elodea cell an ideal specimen for observing fundamental structures and dynamic processes. Studying this cell provides insight into phenomena like photosynthesis, internal transport, and the cell’s response to its watery environment.
Key Structural Components
The individual cells of an Elodea leaf present a distinct, rectangular structure defined by a rigid cell wall. This thick, non-living layer of cellulose provides mechanical support and maintains the box-like shape. Just inside this barrier is the cell membrane, a thin, selectively permeable layer that regulates the passage of substances.
The most dominant feature is the large central vacuole, which can occupy up to 90% of the cell’s total volume. This water-filled sac exerts internal pressure, pushing the living contents (the protoplast) into a thin layer against the periphery. Pressed against the cell wall in this narrow cytoplasmic layer are numerous, small, bright green organelles called chloroplasts, which are the sites of photosynthesis.
The Dynamic Motion Inside the Cell
The most captivating characteristic of a living Elodea cell is the constant, organized movement of its contents, called cytoplasmic streaming or cyclosis. This process involves the active, flowing movement of the cytoplasm around the perimeter of the cell, circulating along the boundary of the central vacuole. The movement is a visible manifestation of the cell’s internal transport system, powered by motor proteins moving along actin microfilaments.
As the cytoplasm flows, it carries chloroplasts and other organelles, making the process easily observable. This streaming effectively distributes nutrients and metabolic waste products throughout the cell volume. This directional circulation ensures that chloroplasts are regularly moved into positions where they can absorb light for photosynthesis, maximizing energy production.
How Elodea Cells React to Water
The structure of the Elodea cell dictates its reaction to different water environments through osmosis. When immersed in a hypotonic solution (like freshwater), water moves into the cell because the solute concentration is lower outside the cytoplasm. The influx of water causes the central vacuole to swell, pressing the protoplast firmly against the rigid cell wall and generating turgor pressure. This pressure keeps the plant cells firm and maintains the plant’s shape.
A change occurs when the Elodea cell is exposed to a hypertonic solution, such as concentrated salt water. Water rapidly leaves the cell via osmosis, moving out of the central vacuole and across the cell membrane. The loss of water causes the vacuole to shrink, and the entire protoplast pulls away from the fixed cell wall. This separation is called plasmolysis, demonstrating the distinct roles of the flexible cell membrane and the cell wall.