Overview
This lecture explains how root hair cells are adapted for absorbing water and minerals, and details how water moves through the root to the xylem via the symplast and apoplast pathways.
Adaptations of Root Hair Cells
- Root hair cells are extensions of the root epidermis, increasing the surface area for absorption.
- Dense packing of root hairs increases the surface area to volume ratio.
- Root hair surfaces are thin, consisting only of the cell wall and membrane, which speeds up osmosis.
- Mineral ions are taken up by active transport, lowering the water potential inside the cell.
- Lower water potential inside root hair cells causes water to enter from the soil by osmosis.
Movement of Water in Roots
- Water enters root hair cells by osmosis due to a water potential gradient.
- Water travels from root hair cells through the cortex towards the xylem.
The Symplast Pathway
- Water moves through the cytoplasm of adjacent cells via microscopic channels called plasmodesmata.
- This pathway follows the water potential gradient from root hair cell to xylem.
- The cytoplasmic route is slower due to obstruction by organelles.
The Apoplast Pathway
- Water travels in cell walls and spaces between cells, moving easily due to the open cellulose structure.
- Water molecules show cohesion, helping their flow along the pathway.
- The apoplast pathway allows faster water movement with less resistance.
The Endodermis and Casparian Strip
- Before entering the xylem, water must pass through the endodermis.
- The Casparian strip is a waterproof band of suberin in endodermal cell walls.
- The Casparian strip blocks the apoplast pathway, forcing water through the symplast route (cell membranes and cytoplasm).
- This control point allows selective uptake of substances into the xylem.
Root Pressure
- Endodermal cells use active transport to move mineral ions into the xylem, lowering its water potential.
- Water enters the xylem by osmosis, generating root pressure.
- Root pressure requires energy from respiration and stops if respiration is inhibited or oxygen is excluded.
Key Terms & Definitions
- Root hair cell — Extension of root epidermal cell for increased absorption.
- Osmosis — Movement of water from high to low water potential through a semi-permeable membrane.
- Active transport — Movement of substances against their concentration gradient using energy.
- Water potential — Measure of the potential energy in water; determines the direction of osmosis.
- Symplast pathway — Route through the cytoplasm and plasmodesmata of cells.
- Apoplast pathway — Route through cell walls and intercellular spaces.
- Plasmodesmata — Channels connecting the cytoplasm of adjacent cells.
- Endodermis — Inner cell layer in root cortex with the Casparian strip.
- Casparian strip — Waterproof band of suberin in endodermal cell walls.
- Root pressure — Pressure generated by osmotic movement of water into the xylem.
Action Items / Next Steps
- Review key concepts of osmosis, active transport, and water potential.
- Prepare for the next lesson on transpiration.