Water uptake
Function: Their primary function is to absorb water and mineral ions from the soil.
Adaptation for Function:
| Feature | Explanation |
|---|---|
| Large surface area | The long, hair-like projection significantly increases the surface area-to-volume ratio compared to a flat cell. This allows for maximum contact with soil water, increasing the rate of uptake. |
| Thin cell wall | Reduces the diffusion/osmosis distance for water entering the cell. |
Building on this structure, the large surface area is critical because it maximizes the efficiency of osmosis (the net movement of water from a region of higher water potential to lower water potential across a partially permeable membrane).
- Root hair cells: Water enters here via osmosis.
- Root cortex cells: Water moves across the root tissue (via cell walls or cytoplasm) towards the center.
- Xylem: Water enters the xylem vessels in the stele (center) of the root. The xylem acts as the continuous pipeline.
- Stem xylem: Water travels up the stem.
- Leaf mesophyll cells: Water exits the xylem in the leaf and enters the mesophyll cells, where it is used for photosynthesis or evaporates during transpiration.
Key Concept: The xylem is a dead, hollow tube that provides both structural support and transport. It is the only tissue in this pathway that transports water over long distances.
Methodology:
- Place a celery stalk in water containing a suitable stain (red dye).
- Leave for several hours to allow uptake.
- Cross-sections: Cut thin slices across the stem. Observe under a microscope. The red stain will appear in specific spots, identifying the location of the xylem vessels within the vascular bundle.
- Longitudinal sections: Cut lengthwise along the stem. This reveals that the stained xylem forms continuous tubes running from the base to the top, proving water moves upwards in a continuous column.
Linking Root and Stem: While this experiment demonstrates transport in the above-ground parts (stem/leaf), it relies on the initial uptake by root hair cells described earlier. The stain enters the root equivalent of the celery stalk's base, travels through the cortex, and enters the xylem, mirroring the pathway outlined in Learning Objective 3.
Error: Stating that 'root cortex cells' absorb mineral ions from the soil.
Correction: Only root hair cells are in direct contact with the soil solution and are adapted for absorption. Cortex cells transport ions after they have entered the root hair.
Mistake 2: Wrong Mechanism for Water Entry
Error: Saying water enters root hair cells by 'diffusion' or 'active transport'.
Correction: Water moves by osmosis. Active transport is used for mineral ions, not water. You must specify the mechanism (osmosis) when asked how water enters.
Mistake 3: Vague Stain Results
Error: Saying 'the whole stem turns red'.
Correction: Be precise. The stain is found only in the xylem vessels. Other tissues remain unstained.
Why examiners accept this: Examiners look for specific anatomical terms. General terms like 'tubes' or 'cells' are often insufficient if a more precise term is available.
Correct Usage Example:
- Question: Describe how water moves from the soil to the leaf.
- Good Answer: Water enters root hair cells, passes through root cortex cells, and enters the xylem. It travels up the stem xylem to the mesophyll cells in the leaf.
- Bad Answer: Water goes from roots to leaves through tubes.
Tip for Stain Experiments: When describing the result of a stain experiment, explicitly state that the red color indicates the presence of water in the xylem. This shows you understand that the dye travels with the water and identifies the transport tissue.
- Enters root hair cells.
2. Moves through root cortex cells.
3. Enters xylem (and travels to leaves/mesophyll).