Absorption of Water and Ascent of Sap

ABSORPTION OF WATER AND ASCENT OF SAP

 

  • Absorption of water refers to the process by which plant roots take in water from the soil. 
  • Roots, particularly the root hairs, are crucial for water absorption. 
  • Absorption occurs through diffusion, where water moves from regions of higher concentration in the soil to lower concentration in the root cells. 
  • Thin-walled extensions of root epidermal cells that significantly increase the surface area for absorption. 
  • Root hairs absorb water along with mineral solutes purely through diffusion. 
  • The experiment with a twig in coloured water highlights the movement of water through vascular bundles, specifically the xylem. 
  • Understanding the mechanism of long-distance transport in plants is crucial for comprehending nutrient and water distribution. 
  • Diffusion is insufficient for long-distance transport due to its slow pace. 
  • Over short distances, diffusion is effective, but for extensive movements, additional mechanisms are required. 
  • The movement of a molecule across a typical plant cell takes about 2.5 seconds for a distance of 50 µm. Calculate the time required for a 1 m distance by diffusion alone. 
  •  Water, minerals, and food are transported by mass or bulk flow systems. 
  • Mass flow involves substances moving en masse due to pressure differences, unlike diffusion where substances move independently. 
  • Bulk movement of substances through plant vascular tissues is termed translocation. 
  • Xylem and phloem are highly specialized vascular tissues associated with translocation. 
  • Xylem primarily translocates water, mineral salts, some organic nitrogen, and hormones from roots to aerial plant parts. 
  • Xylem ensures efficient upward movement of substances against gravity. 
  • Ascent of sap refers to the upward movement of water through the plant, particularly from the roots to the aerial parts. 
  • Phloem translocates a variety of organic and inorganic solutes, predominantly from leaves to other plant parts. 
  • Two-Way Transport: Phloem facilitates bidirectional movement, supporting the overall nutrient distribution. 
  • Vascular Tissue Specialization:

- Roots, Stems, and Leaves: Cross-sections reveal the specialized vascular system in higher plants. 

- Xylem and Phloem: These tissues play distinct roles in nutrient and water transport.

 

Ascent of sap 

  • Ascent of sap refers to the upward movement of water through the plant, particularly from the roots to the aerial parts. 
  • Xylem, the specialized vascular tissue, is responsible for the upward transport of water, minerals, and some organic nitrogen and hormones. 
  • Vessels and tracheids within the xylem facilitate the bulk flow of water. 
  • Roots are the primary sites for water absorption in plants. 
  • Root hairs, present at the tips of roots, play a crucial role in water and mineral absorption. 
  • Root hairs, extensions of root epidermal cells, significantly increase the surface area for absorption. 
  • Water and minerals are absorbed by root hairs through diffusion. 
  • Water Movement in Roots:

- Apoplast Pathway: Water moves through the system of adjacent cell walls (apoplast), excluding the casparian strips of the endodermis. This pathway involves mass flow and relies on water's adhesive and cohesive properties. 

- Symplast Pathway: Water travels through interconnected protoplasts via plasmodesmata, involving cytoplasmic strands. Movement is relatively slower, occurring down a potential gradient.

 

 

 

  • Apoplastic movement is dominant due to loosely packed cortical cells. 
  • Casparian strip in the endodermis prevents water movement through the apoplast, directing it through the symplast. 
  • Water enters xylem vessels and tracheids, which are non-living conduits and part of the apoplast.

 

 

 

  • Symbiotic association of fungus with the root system, enhancing water and mineral absorption through a vast fungal surface area. Mutualistic exchange involves minerals and water provided by the fungus and sugars by the plant roots. 
  • Some plants, like Pinus, depend on mycorrhizae for germination and establishment. 
  • Driving Forces for Ascent of Sap:

- Root Pressure: Pressure exerted by water entering the roots creates a force propelling water upwards. 

- Capillarity: Adhesive and cohesive properties of water enable capillary action, aiding the rise of water through narrow tubes like xylem vessels. 

- Transpiration Pull: Water loss through transpiration creates tension, resulting in a negative pressure that pulls water upwards.

 

Transpiration pull

  • Water ascends through the xylem at impressive rates, reaching up to 15 meters per hour. 
  • Unlike animals with a circulatory system, plants lack a heart, yet they efficiently transport water. 
  • The mechanism behind water movement—whether it is 'pushed' or 'pulled'—has long intrigued researchers. 
  • Transpiration from leaves is identified as the primary force propelling water upwards. 
  • Cohesion-Tension-Transpiration Pull Model:

- The prevailing model for water transport, emphasizing the cohesive and tensional properties of water coupled with transpiration. 

- The cohesion between water molecules and tension generated by transpiration collaboratively contribute to the pull.

 

 

 

  • Despite its transient nature, less than 1% of water reaching leaves is utilized for photosynthesis and plant growth. 
  • Most water is lost through stomata in leaves, a phenomenon recognized as transpiration.

 

 

 

  • Transpiration experiment: Enclosing a healthy plant in a polythene bag to observe water droplets formed inside. 

 

 

  • Cobalt chloride paper, turning color upon absorbing water, serves as a tool to study water loss from leaves.