Transport in Plants
1. Need for Transport in Plants
Plants need to transport:
- WATER and MINERALS from roots to ALL parts
- FOOD (glucose) from leaves to storage organs and growing regions
'Unlike animals, plants do NOT have a circulatory system with a heart. Instead, they have TWO types of vascular tissues — xylem and phloem.'
2. Vascular Tissues
| Tissue | Direction | What it Carries | Cells |
|---|---|---|---|
| Xylem | UPWARD (roots → leaves) | Water + dissolved MINERALS | Tracheids, vessels |
| Phloem | UP and DOWN (leaves → everywhere) | FOOD (sucrose, amino acids) | Sieve tubes, companion cells |
Xylem: Made of DEAD cells (at maturity). Thick CELLULOSE walls. Forms CONTINUOUS tubes from roots to leaves.
Phloem: Made of LIVING cells. Sieve tubes have PERFORATED ends (sieve plates). Companion cells provide ENERGY.
3. Absorption by Roots
Plants absorb water and minerals through ROOT HAIRS — tiny extensions of root epidermal cells.
Mechanism of Water Absorption:
| Process | Description |
|---|---|
| Osmosis | Water moves from SOIL (higher water concentration) into ROOT HAIR (lower water concentration) through a SELECTIVELY PERMEABLE membrane |
| Diffusion | Mineral ions move from SOIL (higher concentration) into ROOT (lower concentration) |
Pathway of water: Root hair → Root cortex → Endodermis → Xylem vessels → Up the stem → Leaves
4. Root Pressure
Root pressure is the FORCE generated by the roots that PUSHES water UP the xylem.
Demonstration: Cut the stem of a plant near the base. Water OOZES out from the cut end. This is root pressure.
Role: Root pressure is IMPORTANT for:
- Starting the upward movement of water
- GUTTATION — water drops from leaf tips (in humid conditions)
5. Transpiration
The loss of water in the form of WATER VAPOUR from the AERIAL parts of the plant (mainly LEAVES).
Transpiration occurs through:
- Stomata (leaf surfaces) — MOST important
- Lenticels (stem bark)
- Cuticle (waxy layer — very little)
6. Factors Affecting Transpiration
| Factor | Effect |
|---|---|
| Temperature | HIGHER temperature → FASTER transpiration |
| Humidity | HIGHER humidity → SLOWER transpiration |
| Wind speed | HIGHER wind → FASTER transpiration |
| Light | MORE light → stomata OPEN → FASTER transpiration |
| Soil water | LESS water → stomata CLOSE → SLOWER transpiration |
7. Experiments on Transpiration
Bell Jar Experiment
- A potted plant is covered with a BELL JAR.
- The inner surface becomes MOIST (water vapour from transpiration condenses).
- Control: Cover the SOIL with a rubber sheet → moisture is ONLY from the plant.
Cobalt Chloride Paper
- COBALT CHLORIDE paper is BLUE when DRY and PINK when WET.
- Attach to both sides of a leaf.
- The LOWER surface turns pink FASTER → MORE stomata on the LOWER surface.
8. Transpiration Pull
As water EVAPORATES from leaf cells (transpiration), it creates a SUCTION (pull) that draws water UP the xylem from the roots.
'Transpiration pull is the MAIN force that moves water up tall trees — even to heights of 100 m or more!'
9. Significance of Transpiration
| Advantage | Disadvantage |
|---|---|
| CREATES transpiration pull for water movement | EXCESSIVE water loss (wilting) |
| COOLS the plant (evaporative cooling) | |
| Helps in MINERAL absorption | |
| Maintains TURGOR (cell firmness) |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Xylem carries food' | Xylem carries WATER and MINERALS UPWARD. Phloem carries FOOD in ALL directions |
| 'Transpiration and evaporation are the same' | Transpiration is a CONTROLLED (by stomata) biological process — evaporation is a PHYSICAL process |
| 'Root hairs absorb only water' | Root hairs absorb BOTH water (by osmosis) and minerals (by diffusion/active transport) |
| 'Transpiration is wasteful' | Transpiration is NECESSARY for water transport and cooling, though excessive loss can be harmful |
ICSE Exam Focus (5–6 marks)
- 2-mark questions: Define transpiration, root pressure, osmosis
- 3-mark questions: Differences between xylem and phloem
- 4-mark questions: Experiments on transpiration with controls
- 5-mark questions: Factors affecting transpiration and their effects
Self-Test
Q1. Name the two conducting tissues in plants and their functions. A1. (1) XYLEM — transports water and minerals from roots to leaves. (2) PHLOEM — transports food from leaves to all parts of the plant.
Q2. What is root pressure? How can it be demonstrated? A2. Root pressure is the FORCE generated by roots that pushes water upward. Demonstration: Cut the stem near the base — water OOZES out from the cut surface.
Q3. List three factors that increase the rate of transpiration. A3. (1) HIGH temperature. (2) LOW humidity. (3) HIGH wind speed. (4) BRIGHT light.
Q4. Why does a plant wilt on a hot summer day? A4. On a hot day, transpiration is VERY HIGH. The plant loses water FASTER than the roots can absorb it. Cells LOSE turgor and the plant WILTS.
Q5. How can you prove that transpiration occurs mainly through the lower surface of a leaf? A5. Use COBALT CHLORIDE paper. Attach DRY blue paper to both leaf surfaces. The paper on the LOWER surface turns PINK faster, proving MORE stomata (and therefore more transpiration) on the lower surface.
Q6. What is the role of transpiration in the upward movement of water? A6. Transpiration creates a SUCTION (transpiration pull) in the leaf cells. This pull is transmitted DOWN the xylem, drawing water up from the roots in a CONTINUOUS column.
