Transpiration

What is Transpiration?

Transpiration is when plants absorb water via their roots and transport it up the plant through the xylem, losing it as water vapour through small pores called stomata, on the undersides of leaves. This cools the plant down while also being significant in nutrient uptake and turgor in plants.

Essentially, it is the mechanism of the plant for the balance of water and the rise of minerals upward from the soil. A continuous flow of water from roots to leaves makes it possible to scatter all the essential nutrients within plants promoting the development of growth.

Transpiration in Plants

Plants drive the uptake and distribution of minerals and nutrients from the soil through a process called transpiration. There is transpirational cooling through evaporative cooling to prevent overheating and maintain the optimum internal temperature of plants.

Transpiration also maintains turgor pressure, which is responsible for maintaining the shape and rigidity of plant cells. It throws light upon the entire elaborate mechanism of transpiration, covering its mechanisms, types, factors affecting it, and its significance.

Process of Transpiration

Transpiration is a vital activity in plant biology, involving the movement of water from the soil to the plant and finally into the atmosphere. In explaining this process, one would need to trace the various mechanisms, pathways, and structures involved.

Mechanism of Transpiration

• Water evaporates from the cell walls of the spongy mesophyll into the intercellular spaces within the leaf.

• It is then diffused out to the atmosphere through stomata, moving down a concentration gradient.

Water Movement Pathways

• Apoplastic Pathway: Water passes through the cell walls and the intercellular spaces, not crossing any membranes.

• Symplastic Pathway: The water moves from cell to cell through the cytoplasm using plasmodesmata, crossing the cell membrane once.

Role of Stomata in Transpiration

• Stomata are small pores on the surface of the leaf, which are surrounded by guard cells. Guard cells close or open the stomata depending on their shape.

• The guard cells open the stomata or close them by changing their turgor pressure by the gaining or losing of potassium ions.

• Usually, stomata are open during the day to photosynthesize but close at night to avoid excessive loss of water.

Types of Transpiration

Transpiration occurs through different pathways which all add up to result in loss of water from plants.

Stomatal Transpiration

• It is the primary pathway of water loss in most plants.

• It loses the maximum amount of water vapours from the plant.

• The opening and closing of the stomata by the guard cells strike a balance between the loss of water and the intake of carbon dioxide for photosynthesis.

Cuticular Transpiration

• It occurs through the cuticle. A waxy layer which covers the epidermis on leaves and stems.

• It is less significant than stomatal and typically contributes to a minor portion of total water loss.

• It increases when stomata are closed. It becomes more significant when stomata are closed, for example during drought conditions.

Lenticular Transpiration

• It happens through lenticels, small openings in the bark of woody stems.

• Minor contribution to total transpiration and fairly small part, when contrasted with stomatal and cuticular transpiration

• It is found in woody stems, in trees and bushes, which helps exchange gases.

Factors Affecting Transpiration

The rate of transpiration is affected by both internal and external factors. All these combined factors play an important role in ascertaining the amount of water loss.

Internal Factors

• A larger leaf area increases the surface through which water may be lost.

• Features that reduce transpiration include a thicker cuticle or trichomes, small hairs or other outgrowths on a leaf surface.

• An increase in the number of stomata might allow for increased transpiration rates, the overall distribution and regulation would factor into this process, however.

External Factors

• Light will increase transpiration by warming the leaf, hence opening the stomata.

• The higher the temperature, the greater the rate of evaporation and diffusion of water vapour.

• A lower relative humidity will steepen the concentration gradient outside the leaf and thus raise the rate of transpiration.

• Wind removes moist air lying immediately above the leaf surface and hence improves the diffusion gradient for water vapour.

• A good supply of soil water ensures a high rate of continued transpiration, but this is lowered in times of drought.

Transpiration and Water Relations in Plants

Understanding the relationship between transpiration and water movement in plants holds a central place in plant physiology.

Water Potential (Ψw)

• Water flows from an area of higher to lower water potential.

• Solute potential, osmotic potential, pressure potential and turgor pressure are its prime components.

• Relation: Ψw = Ψs + Ψp

Transpiration Stream

• The continuous flow of water from roots to leaves is driven by the evaporation of water from leaf surfaces.

• Differences in the water potential between soil, root, and atmosphere ensure upward water movement.

Cohesion-Tension Theory

• Water molecules stick together cohesion and to the walls of xylem vessel adhesion.

• Based on cohesive properties of water and tension created by evaporation.

• The pull from evaporation at the leaf surface creates a tension that draws water upward.

Role of Transpiration in Plants

Transpiration is not simply a water loss process; it has various crucial functions essential for the survival of plants.

Cooling Effect

• Evaporative cooling lowers the leaf temperature.

• Prevents overheating and protects against heat damage of enzymes and other cellular structures.

Nutrient Uptake

• Nutrients are dissolved in the soil water and are taken up by the roots.

• Distributed in the plant through the transpiration stream.

Turgor Pressure

• Maintains turgidity and functions of the cells.

• Plant structure is maintained. Wilting, leading to drooping habit or prostrate habit of the plant, is avoided.

Transpiration NEET MCQs (With Answers & Explanations)

The key concepts to be covered under this topic for different exams are:

• Process of Transpiration

• Types of Transpiration

Practice Questions for NEET

Q1. The consequence of a high rate of transpiration is

1. Death of some plants due to excessive loss of water

2. Plants suffer from loss of turgidity

3. Lower concentration of water in the cell sap

4. All of these

Correct answer: 4) All of these

Explanation:

Some plants die as a result of excessive water loss: A plant may get dehydrated and even die if it loses too much water through transpiration, or the evaporation of leaves.

The pressure inside plant cells that keeps them firm is called turgidity, and it is lost in plants. The plant wilts when its cells shrink due to excessive water loss.

Reduced water content in cell sap: When a plant's cells contain less water, the fluids within—known as sap—become more concentrated with other materials, such as salts.

Hence, the correct answer is option 4)All of these.

Q2. Water rises in trees through a small tube called an xylem. This is because of :

1. Cohesion

2. Adhesion

3. Both 1 and 2

4. None of these

Correct answer: 3) Both 1 and 2

Explanation:

In trees, water rises via a tiny tube known as the xylem. Both cohesion and adhesion are to blame for this.

• Cohesion: A continuous column of water forms in the xylem as a result of hydrogen bonds between water molecules.

• Adhesion: Water molecules adhere to the xylem walls which helps to defy gravity and keep the water flowing upward.

Hence, the correct answer is option 3) Both 1 and 2.

Q3. Transpiration and root pressure cause water to rise in plants by:

1. Pushing it upward

2. Pushing and pulling it, respectively

3. Pulling it upward

4. Pulling and pushing it, respectively

Correct answer: 4) Pulling and pushing it, respectively

Explanation:

Transpiration creates the environment for the passive absorption of water by creating a pulling force, while root pressure develops because of the active absorption of solutes and it pushes the water up the stem. Transpiration primarily occurs through stomata and is driven by evaporation, which creates a negative pressure that pulls water from the roots through the plant. Root pressure, on the other hand, is generated when roots actively absorb minerals and water from the soil, causing a positive pressure that pushes water upwards. Both processes contribute to the movement of water through the plant, maintaining turgor and facilitating nutrient transport. While transpiration is the dominant force under normal conditions, root pressure can provide additional support, especially in the early morning or at night.

Hence, the correct answer is option 4) pulling and pushing it, respectively.

Also Read:

• MCQs on Transpiration in Plants

• Facilitated Diffusion

• Symport, Antiport, Uniport

Recommended Video on Transpiration