Ascent of Sap and Transpiration
Root pressure, cohesion–tension theory, stomatal regulation and transpiration pull. (Biology › Transport in Plants and Mineral Nutrition, NEET UG syllabus.)
What is Ascent of Sap and Transpiration?
The upward movement of water and dissolved minerals from the roots to the aerial parts of a plant.
Key formula / rule: Water Potential
Key points
- Explain the mechanism of ascent of sap, including root pressure and transpiration pull.
- Describe the cohesion-tension-transpiration pull model in detail.
- Define transpiration and list factors affecting its rate.
- Explain the structure and function of stomata and guard cells in regulating transpiration.
Common exam trap
Confusing root pressure as the primary force for ascent of sap in all plants.
Definitions
- Term
Ascent of Sap
- Meaning
The upward movement of water and dissolved minerals from the roots to the aerial parts of a plant.
- Term
Transpiration
- Meaning
The evaporative loss of water by plants, primarily through stomata in the leaves.
- Term
Cohesion
- Meaning
The mutual attraction between water molecules, forming a continuous column in the xylem.
- Term
Adhesion
- Meaning
The attraction of water molecules to the polar surfaces (e.g., xylem vessel walls).
- Term
Transpiration Pull
- Meaning
The negative pressure or tension created in the xylem sap due to water evaporation from leaves, which pulls water upwards.
- Term
Root Pressure
- Meaning
A positive pressure developed in the xylem sap of roots due to active absorption of ions, pushing water up to a small extent.
- Term
Guttation
- Meaning
The loss of water in the form of liquid droplets from the margins of leaves in small herbaceous plants, typically occurring when transpiration is low and root pressure is high.
- Term
Stomata
- Meaning
Microscopic pores on the epidermis of leaves, regulated by guard cells, through which gaseous exchange and transpiration occur.
Learning objectives
Explain the mechanism of ascent of sap, including root pressure and transpiration pull.
Describe the cohesion-tension-transpiration pull model in detail.
Define transpiration and list factors affecting its rate.
Explain the structure and function of stomata and guard cells in regulating transpiration.
Differentiate between ascent of sap, transpiration, and guttation.
Relate water potential to water movement in plants.
Formulae
- Name
Water Potential
- Note
Where Ψ is total water potential, Ψs is solute potential (osmotic potential), and Ψp is pressure potential (turgor potential). Water moves from higher to lower water potential.
- Expression
Ψ = Ψs + Ψp
Prerequisites
Basic understanding of plant cell structure (cell wall, vacuole).
Knowledge of osmosis and diffusion.
Familiarity with plant tissues, especially xylem and epidermis.
Concept of water potential.
Common mistakes
Confusing root pressure as the primary force for ascent of sap in all plants.
Not understanding the role of cohesion and adhesion in maintaining the water column.
Incorrectly identifying factors that cause stomatal opening or closing.
Confusing guttation with dew formation or transpiration.
Keywords
Ascent of Sap
Transpiration
Cohesion
Adhesion
Transpiration Pull
Root Pressure
Xylem
Stomata
Guard Cells
Water Potential
Guttation
Practice preview
An increase in the atmospheric humidity generally leads to which of the following effects on the rate of transpiration?…
medium
If a plant is subjected to severe water stress (e.g., drought conditions), what is the most immediate and common physiological response to conserve water?…
medium
Despite the absence of a pumping organ, water can reach great heights in tall trees. Which of the following statements best explains this phenomenon according to the cohesion-tension model?…
hard