Скачать или смотреть Class 11 Biology – Transport in Plants | Full Explanation

1. Introduction

Plants need to move water, minerals, food, and hormones to different parts. Unlike animals, they don’t have a circulatory system but depend on specialized tissues (**xylem and phloem**) and physical processes.

2. Types of Transport

1. *Short Distance Transport (Cell level):*

Movement of molecules across cell membranes.
Involves **diffusion, facilitated diffusion, active transport, osmosis**.

2. *Long Distance Transport (Tissue level):*

Transport through *xylem* (water & minerals) and *phloem* (food).
Called **translocation**.

3. Transport at Cellular Level

*Diffusion* → Passive movement of molecules from high to low concentration.
*Facilitated diffusion* → With the help of carrier proteins.
*Active transport* → Uses ATP energy, against concentration gradient.
*Osmosis* → Diffusion of water through selectively permeable membrane.

Key terms:

*Plasmolysis* – Shrinking of cytoplasm when water leaves cell.
*Water potential (Ψ)* – Determines direction of water movement (higher → lower).
*Osmotic potential & Pressure potential* regulate water flow.

4. Water Transport in Plants

(i) Absorption

Water absorbed by *root hairs* via osmosis.
Moves via two pathways:

*Apoplast pathway* → Through cell walls.
*Symplast pathway* → Through cytoplasm via plasmodesmata.

(ii) Ascent of Sap

Water moves upward in xylem due to:

1. *Root Pressure* – Positive pressure in roots (minor role).
2. *Transpiration Pull* – Evaporation of water from leaves creates negative pressure.
3. *Cohesion-Tension Theory (Dixon & Joly)* – Cohesion (water-water), Adhesion (water-xylem wall), Transpiration pull.

5. Mineral Transport

Absorbed as ions (active & passive).
Transported through xylem along with water (mass flow).

6. Transpiration

*Loss of water as vapor* mainly through stomata.
Types: Stomatal, Cuticular, Lenticular.
Functions: Creates transpiration pull, cooling effect, maintains water movement.
Controlled by *stomatal opening/closing* (guard cells, turgor pressure, light, CO₂).

7. Phloem Transport (Translocation of Food)

Food (sucrose) moves from *source (leaves)* → **sink (roots, fruits, storage organs)**.
*Pressure Flow Hypothesis (Münch):*

Loading of sucrose at source → Osmosis → High pressure.
At sink, sucrose unloaded → Low pressure.
Creates pressure gradient that drives bulk flow.

8. Comparison of Xylem vs Phloem Transport

| Feature | Xylem | Phloem |
| --------- | ------------------------------------ | -------------------------------- |
| Material | Water + minerals | Organic food (sucrose, hormones) |
| Direction | Mostly upward | Both upward & downward |
| Mechanism | Transpiration pull, cohesion-tension | Pressure-flow hypothesis |
| Tissue | Tracheids, vessels | Sieve tubes, companion cells |

---

✅ *In summary:*

Water & minerals → Xylem (root → shoot, driven by transpiration).
Food → Phloem (source → sink, driven by pressure flow).
Cellular transport relies on diffusion, osmosis, active transport.