Absorption and movement of water: transpiration, ascent of sap, transpiration pull

1. Introduction

1. Water is essential for plant growth, photosynthesis, and nutrient transport.
2. Plants absorb water through their roots and transport it to different parts.
3. Water movement in plants occurs via transpiration, ascent of sap, and transpiration pull.

2. Absorption of Water

1. Water absorption mainly occurs through root hairs.
2. Water moves from the soil into the roots by osmosis.
3. There are two pathways for water absorption:
   • Apoplast Pathway: Water moves through the cell walls without entering the cytoplasm.
   • Symplast Pathway: Water moves through the cytoplasm and plasmodesmata.
4. Active absorption: Requires energy (ATP) and is driven by root pressure.
5. Passive absorption: Driven by transpiration pull and does not require energy.

3. Transpiration

1. Transpiration is the loss of water vapor from the aerial parts of a plant.
2. It mainly occurs through the stomata on leaves.
3. Types of transpiration:
   • Stomatal transpiration: Through stomata (major type).
   • Cuticular transpiration: Through the cuticle (waxy layer).
   • Lenticular transpiration: Through lenticels in stems.
4. Factors affecting transpiration:
   • Temperature: Higher temperature increases transpiration.
   • Humidity: High humidity decreases transpiration.
   • Wind: Increases transpiration by removing water vapor.
   • Light: More light opens stomata, increasing transpiration.
5. Functions of transpiration:
   • Helps in the cooling of plants.
   • Creates transpiration pull, aiding water movement.
   • Facilitates the transport of minerals from roots to leaves.

4. Ascent of Sap

1. Ascent of sap is the upward movement of water from roots to leaves through xylem.
2. It occurs due to:
   • Root pressure: Pressure generated by root cells pushing water upwards.
   • Capillarity: Water rises in narrow tubes due to cohesion and adhesion.
   • Transpiration pull: The major driving force behind water movement.

5. Transpiration Pull

1. Transpiration pull is the force that pulls water up through the xylem.
2. It is created by water loss from leaves through stomatal transpiration.
3. The cohesion-tension theory explains how transpiration pull works.
4. Cohesion: Water molecules stick together due to hydrogen bonding.
5. Adhesion: Water molecules stick to the xylem walls.
6. This results in a continuous column of water being pulled up from roots to leaves.

6. Importance of Water Movement in Plants

1. Maintains turgidity of cells, preventing wilting.
2. Supplies water for photosynthesis.
3. Facilitates transport of nutrients and minerals from the soil.
4. Regulates temperature through transpiration cooling.

7. Differences Between Transpiration, Ascent of Sap, and Transpiration Pull

8. Conclusion

1. Plants absorb water through root hairs via osmosis.
2. Transpiration helps cool plants and drives water movement.
3. Ascent of sap transports water upward through xylem.
4. Transpiration pull is the main force behind water transport in tall plants.
5. These processes are essential for plant survival and growth.