State that water is absorbed by root hair cells.

8.1 Transport in Plants

Learning Objective

State that water is absorbed by root‑hair cells.

Key Concepts

• Root hairs are thin‑walled extensions of epidermal cells that greatly increase the surface area for water uptake.
• Water moves from the soil into root‑hair cells by **osmosis** because the water potential of the soil is higher (less negative) than that inside the cell.
• After entering a root‑hair cell, water travels radially through the cortex and reaches the vascular tissue (xylem).
• Xylem – long, dead, lignified tubes (vessels and tracheids) that conduct water and dissolved minerals upward and give mechanical support.
• Phloem – living tubes (sieve‑tube elements with companion cells) that transport organic nutrients, mainly sucrose, from source to sink.

Structure and Function of Xylem and Phloem

Position of Vascular Tissues in Roots, Stems and Leaves (Dicots)

Step‑by‑Step Process of Water Absorption and Transport

1. Water potential gradient: Soil water has a higher (less negative) water potential than the interior of root‑hair cells.
2. Osmosis into root hairs: Water crosses the plasma membrane of the root‑hair cell into the cytoplasm.
3. Radial movement through the cortex (two possible routes):
   • Apoplast route: through cell walls and intercellular spaces (no membranes crossed).
   • Symplast route: through the cytoplasm linked by plasmodesmata (membranes crossed).
4. Casparian strip in the endodermis blocks the apoplast, forcing water to enter the symplast and thus pass the selective membrane.
5. Entry into the xylem: From the pericycle, water moves into the xylem vessels of the root.
6. Root pressure (optional): Active uptake of ions creates an osmotic gradient that can push water upward a short distance.
7. Transpiration pull in the stem: Water is drawn upward through the continuous xylem column by the cohesion‑tension mechanism.
8. Leaf uptake: Water reaches the leaf veins, exits the xylem into mesophyll cells, and evaporates from the spongy mesophyll into air spaces.
9. Transpiration: Water vapour diffuses out of the leaf through open stomata.

Comparison of Cortex Pathways

Factors Influencing Water Uptake

• Soil water potential – drier soil reduces the gradient.
• Root‑hair density and length – more surface area = greater uptake.
• Temperature – higher temperature increases kinetic energy of water molecules.
• Root pressure – active ion uptake creates an osmotic gradient that pushes water into the xylem.
• Transpiration rate – creates the pull that moves water through the xylem.

Practical Investigation: Tracing the Water Pathway

• Place a young plant in a solution of coloured dye (e.g., food colouring) or iodine.
• After 1–2 h, cut thin transverse sections of the root, stem and leaf.
• Observe the sections under a low‑power microscope; the coloured water will highlight the route:
  root‑hair → cortex → endodermis → xylem → stem → leaf veins → mesophyll.
• Discuss how the observed pattern matches the textbook pathway and the role of the Casparian strip.

Common Misconceptions

1. “Water is sucked into the plant.” – Water enters root hairs by diffusion (osmosis). The upward movement is a pull generated by transpiration, not suction.
2. “Only the root tip absorbs water.” – While the tip is important for growth, the majority of water uptake occurs along the length of the root where root hairs are present.

Check Your Understanding

1. Explain why root hairs are essential for water absorption.
2. Describe how the Casparian strip influences the route water takes to reach the xylem.
3. State the direction of water movement in terms of water potential.
4. Identify the primary functions of xylem and phloem, including a brief description of their structure.
5. Sketch (or label) a simple cross‑section of a root, stem and leaf showing the relative positions of xylem, phloem, pericycle and endodermis (root).