Transpiration Pull

What is Transpiration Pull?

Transpiration pull is one of the vital physiological processes associated with a plant. Evaporation of water from the surface of leaves i.e, transpiration, creates a negative pressure inside plant xylem vessels. The negative pressure of the water column results in the upward pull from the roots through the vascular system of the plant.

Another beneficial role is the effective delivery of water and dissolved nutrients from the soil, to other parts of the plant through a pull of transpiration. This will support the plant in maintaining its rigidity in nutrient uptake, cooling, and hydration. The system works in a way that there is an ongoing movement of water from the roots, where it is transported to the leaves, and as it evaporates into the atmosphere, it drives the movement of water throughout the plant.

Overview of Transpiration

Transpiration is a process by which water is absorbed by the roots of a plant from the soil, moved into a plant, and vapour leaves the plant into the atmosphere. Mainly, it is done through small openings called stomata.

This process is vital for the water balance within the plant and allows transportation of nutrients, thereby cooling the plant. While water evaporation occurs on the surface of the leaf, the negative pressure in the xylem vessels leads to the upward pull from the roots to maintain a constant flow of water and nutrients.

Types of Transpiration

The different types of transpiration are:

Stomatal Transpiration

• This is the most common type of transpiration. Stomata refer to a few minute openings, usually in the lower epidermis of leaves.

• This accounts for the greatest part of the water loss in plants and is crucial for the regulation of gas exchange and the water balance.

Cuticular Transpiration

• Water is lost in transpiration directly through the cuticle, a waxy boundary layer on the upper surfaces of the leaves and stem.

• Although it accounts for a minor fraction compared with the stomatal transpiration in the total amount of loss, this also helps reduce the loss and provides additional moisture regulation.

Lenticular Transpiration

• It occurs through lenticels. These are small openings in the bark of any woody plant.

• This type of transpiration is less significant relative to stomatal or cuticular transpiration. Although, in some plants, it has a part to play in the water regulation process.

Mechanism of Transpiration Pull

The mechanism is described below:

Cohesion-Tension Theory

• The cohesion-tension theory defines the flow of water through a plant as a result of cohesiveness and adhesiveness of water.

• Cohesion is an attractive force between water molecules, whereas adhesion is the force that the water has towards the sides of xylem vessels.

• These two forces act in combination, thus offering a continuous column of water from roots to leaves, something critical to the flow of water through a plant.

Water Potential Gradient

• Water potential refers to the potential energy of the water, which drives its movement.

• What follows is the continuous flow of water from high to low potential as a result of the gradient in the water potential between the soil, which has high water potential, and the atmosphere, which has low water potential.

• This gradient is established in plants by the evaporation of water from the leaves, thereby reducing the water potential in the leaf, and causing more water to be pulled up through the xylem.

Capillary Action In Xylem

• Capillary action is the rise of water in narrow tubes due to adhesive forces between water molecules and the walls of xylem vessels.

• The action, together with cohesion, provides for the transportation of water upward against gravity.

• Capillary action in xylem vessels aids in the efficient transport of water without resistance from the roots to the leaves.

Steps in the Transpiration Pull Process

The steps of the transpiration pull process are:

• Water is taken up into plant roots from the soil by osmosis. Osmosis raises the water potential inside the cells of the rainout the surrounding soil.

• The negative pressure produced as transpiration pulls out water from the leaves and the cohesive properties of water molecules pull water through the vessels of the xylem.

• As water evaporates, it leaves the stomata and creates some negative pressure inside the leaf, which pulls more water into it from the xylem. Hence, constantly water moves from roots to leaves.

Role and Importance of Transpiration Pull

The role and importance are described below:

Nutrient Transport

• Nutrient transport is the process whereby the minerals and nutrients are absorbed from the soil into the leaves of the plants.

• The transpiration pull mechanism accounts for far and away most of the effect.

• Due to the evaporation of water from the stomata, negative pressure is created that attracts water carrying all the dissolved minerals from the roots through xylem vessels.

• This upward movement helps ensure the nutrients reach different parts of the plants, allowing them to properly grow and develop.

Cooling Effect

• Transpiration helps in keeping the plants cool, which means that the temperature of plants is regulated by the process of evaporative cooling.

• When water evaporates from the leaf's surface, it consumes heat energy from the plant, cooling these organisms.

• This mechanism is vital to ensure that the enzyme activity and metabolism within these organisms work at optimum temperatures under hot conditions.

Turgor Pressure Maintenance

• The transpiration pull contributes in the maintenance of the turgor pressure within the plants cells.

• In effect, in addition to giving turgidity, it helps in maintaining the internal pressure within the plant cells called the turgor pressure.

• This turgor pressure infuses rigidity into the plant tissue and makes it upright, and turgid, thus performing the structural functions of stability and strength in plants.

Factors Affecting Transpiration Pull

There are several factors that affect transpiration pull:

Transpiration Pull NEET MCQs (With Answers & Explanations)

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

• Types of Transpiration

• Mechanism of Transpiration Pull

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

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