Free Slice Lessons: Root Hair Cells and Water Absorption

Location: Grow from cells in the epidermis (outer layer) of the root.
Function: Absorb water and minerals from the soil.

Objectives

Describe how root hair cells are adapted for the absorption of water and minerals.
Explain how water passes through the roots to the xylem, including the simplast and apoplast pathways.

Increased Surface Area: Densely packed root hairs increase the surface area to volume ratio of the root.
Thin Surface: Consists only of the cell wall and cell membrane, which facilitates rapid osmosis.
Active Transport: Root hair cells use active transport to move mineral ions (e.g., magnesium) into the cell.
- Results in lower water potential inside the root hair cell compared to the soil.
- Water moves into the cell by osmosis down the water potential gradient.

Through the Root Cortex to the Xylem
- Simplast Pathway:
  - Water moves through the cytoplasm of cells via plasmodesmata (microscopic channels).
  - Driven by water potential gradient from root hair cells to xylem.
  - Relatively slow due to obstructions by organelles.
- Apoplast Pathway:
  - Water moves within cell walls and spaces between cells.
  - Cell walls' open structure allows easy movement between cellulose fibers.
  - Less resistance than the simplast pathway.

Endodermis: Lies before the xylem.
- Contains the Casparian strip, a waterproof suberin band around cell walls.
- Forces water into the cytoplasm (simplast pathway) to control entry into the xylem.

Mechanism: Active transport by endodermis cells moves mineral ions into the xylem.
- Lowers water potential in the xylem, causing water to move in by osmosis.
- Known as root pressure.
Dependence on Respiration:
- Root pressure is an active process requiring respiration.
- Can be inhibited by metabolic poisons like cyanide or by excluding oxygen.

This summary is based on a lecture from "Free Slice Lessons."