Mechanism of Transpiration — Revision Notes

⚡ 30-Second Revision

Transpiration: — Evaporation of water from plant aerial parts.
Primary site: — Stomata (90-95%).
Driving force: — Transpiration pull (negative pressure).
Theory: — Cohesion-Tension theory.
Water properties: — Cohesion (H$_2$O-H$_2$O attraction), Adhesion (H$_2$O-xylem wall attraction).
Stomatal opening: — K$^+$ influx $\rightarrow$ $\Psi_{guard cell} \downarrow \rightarrow$ H$_2$O influx $\rightarrow$ Turgor $\uparrow \rightarrow$ Stoma open.
Stomatal closing: — K$^+$ efflux $\rightarrow$ $\Psi_{guard cell} \uparrow \rightarrow$ H$_2$O efflux $\rightarrow$ Turgor $\downarrow \rightarrow$ Stoma close.
Hormone: — ABA promotes stomatal closure.
Factors increasing rate: — High temperature, low humidity, wind, light.
Factors decreasing rate: — Low temperature, high humidity, still air, high CO$_2$, ABA.

2-Minute Revision

Transpiration is the evaporative loss of water from plants, primarily through stomata. This process generates a 'transpiration pull' or suction force, which is the main driver for the ascent of water and dissolved minerals from the roots to the leaves, a concept explained by the Cohesion-Tension theory.

Water molecules exhibit strong cohesion (attraction to each other) and adhesion (attraction to xylem walls), maintaining an unbroken water column. Stomata, flanked by guard cells, regulate the rate of transpiration.

Stomatal opening occurs when guard cells become turgid due to the active influx of K$^+$ ions, which lowers their water potential, causing water to enter by osmosis. This turgor causes the guard cells to bow outwards, opening the pore.

Conversely, K$^+$ efflux leads to water loss, flaccidity, and stomatal closure, often triggered by darkness, high CO$_2$, or the hormone abscisic acid (ABA). Environmental factors like high temperature, low humidity, and wind increase transpiration, while their opposites decrease it.

Transpiration is crucial for nutrient transport, cooling, and maintaining plant turgor, despite the water loss.

5-Minute Revision

The mechanism of transpiration is a critical aspect of plant physiology, explaining how plants manage water transport. It begins with water absorption by roots, driven by a water potential gradient. This water then moves into the xylem, forming a continuous column.

The 'transpiration pull' is the primary force for the upward movement of this water column. This pull originates from the evaporation of water from the moist surfaces of mesophyll cells into the intercellular air spaces of the leaf, and then out into the atmosphere through stomata.

The Cohesion-Tension theory explains how this pull is transmitted: water molecules are strongly cohesive (attracted to each other via hydrogen bonds), forming an unbroken chain, and adhesive (attracted to the xylem walls), preventing the column from breaking.

This allows the entire water column to be pulled upwards under tension.

Stomata, microscopic pores on the leaf surface, are the main sites of transpiration. Their opening and closing are regulated by specialized guard cells. Stomatal opening is an active process: light or low CO$_2$ triggers the active transport of K$^+$ ions into guard cells.

This influx lowers the guard cells' water potential, causing water to move in by osmosis. The increased turgor pressure makes the guard cells swell and bow outwards, opening the pore. Stomatal closure is the reverse: K$^+$ ions exit the guard cells, water follows by osmosis, the guard cells become flaccid, and the pore closes.

Abscisic acid (ABA) is a key hormone that promotes stomatal closure during water stress. Environmental factors like high temperature, low humidity, and wind increase the rate of transpiration by steepening the water potential gradient or removing saturated air.

Conversely, high humidity, low temperature, still air, and high CO$_2$ decrease it. Transpiration is essential for the ascent of sap, nutrient distribution, cooling the plant, and maintaining cell turgor.

Prelims Revision Notes

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Definition: — Transpiration is the evaporative loss of water from the aerial parts of a plant, primarily as water vapor.
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Main Site: — Stomata on leaves (90-95%). Minor sites: Cuticle (cuticular transpiration), Lenticels (lenticular transpiration).
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Driving Force: — Transpiration pull (negative pressure/tension) generated by water evaporation from leaves.
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Cohesion-Tension Theory: — Explains sap ascent.

* Cohesion: Mutual attraction between water molecules (due to H-bonds) forming a continuous column. * Adhesion: Attraction between water molecules and hydrophilic xylem walls, preventing column breakage. * Tension: Negative pressure created by transpiration, pulling the water column upwards.

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Water Potential Gradient: — Water moves from higher water potential (soil) to lower water potential (atmosphere) through the plant.

* $\Psi_{soil} > \Psi_{root} > \Psi_{stem} > \Psi_{leaf} > \Psi_{atmosphere}$

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Stomatal Mechanism (Guard Cells):

* Structure: Two guard cells surrounding a stomatal pore. Inner walls (facing pore) are thicker and less elastic; outer walls are thinner and more elastic. * Opening: * Light/low CO$_2$ $\rightarrow$ Active influx of K$^+$ ions into guard cells (along with Cl$^-$ or malate$^-$).

* K$^+$ influx $\rightarrow$ Decreased water potential (more negative) inside guard cells. * Water moves into guard cells from subsidiary cells by osmosis. * Guard cells become turgid $\rightarrow$ Outer walls bulge, pulling inner walls apart $\rightarrow$ Stoma opens.

* Closing: * Darkness/high CO$_2$/water stress $\rightarrow$ Efflux of K$^+$ ions from guard cells. * K$^+$ efflux $\rightarrow$ Increased water potential (less negative) inside guard cells. * Water moves out of guard cells by osmosis.

* Guard cells become flaccid $\rightarrow$ Inner walls move closer $\rightarrow$ Stoma closes. * Hormone: Abscisic Acid (ABA) promotes stomatal closure during water stress.

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Factors Affecting Transpiration Rate:

* Light: Increases (stimulates stomatal opening). * Temperature: Increases (increases evaporation rate). * Humidity: Decreases (reduces water potential gradient). * Wind Speed: Increases (removes saturated air layer). * **CO$_2$ Concentration:** High CO$_2$ decreases (causes stomatal closure). * Water Availability: Decreases under water stress (ABA production, stomatal closure).

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Significance: — Ascent of sap, mineral transport, cooling, turgor maintenance.

Vyyuha Quick Recall

Trees Pull Water Continuously Against Gravity Solely Keeping Open Channels.

Transpiration Pull: The driving force.
Water Column: Maintained by...
Cohesion: Water-water attraction.
Adhesion: Water-xylem wall attraction.
Gravity: Overcome by the pull.
Stomata: Primary site of water loss.
K$^+$: Ion responsible for stomatal Opening and Closing.