Lesson Explainer: Transpiration Biology

In this explainer, we will learn how to describe the process of transpiration and the effect of limiting factors on the rate of transpiration.

Water is essential to the survival of all living organisms. In plants, through the process of photosynthesis, water and carbon dioxide are converted into glucose and oxygen. Glucose is the fuel that plants use to obtain the energy they need to grow and survive. Oxygen is a by-product of photosynthesis but is still very useful to plants as it can be used as a key reactant, along with glucose, in cellular respiration to release energy in the plant’s cells.

Definition: Photosynthesis

Photosynthesis is the process by which green plants convert carbon dioxide and water into sugars such as glucose and oxygen in the presence of sunlight.

Plants absorb water from the soil through their roots and return water to the atmosphere through a process called transpiration, as represented in Figure 1. Transpiration can be described as the loss of water from a plant into the atmosphere, through evaporation from the leaves. Water evaporates from tiny pores in the epidermis of leaves called stomata. These pores have special structures that enable them to be either open or closed, depending on certain conditions.

Definition: Transpiration

Transpiration is the loss of water through evaporation from the aerial parts of a plant such as leaves into the atmosphere.

Definition: Stomata

Stomata (singular: stoma) are the pores in the epidermis of leaves through which gas exchange takes place with the atmosphere.

Let’s take a closer look at stomata to understand how they help in transpiration.

Stomata (singular: stoma) are tiny pores in the epidermis of the leaves, which is the outermost layer of cells. Usually, leaves have more stomata on their lower surface than their upper surface. Each stoma is made up of two bean-shaped cells called guard cells, which enclose the stomatal pore. These guard cells are responsible for regulating the opening and closing of the stoma. Since all the cells that make up the stomata are leaf cells, they contain chloroplasts, which make them capable of performing photosynthesis. Photosynthesis provides these cells with sugars that can be metabolized to provide energy for the opening of the stomata.

In Figure 2 below, you can see two stomata. The stoma on the left is open, and the stoma on the right is closed.

The inner walls of the two guard cells are thick and elastic, while the outer walls are thinner. When water enters the guard cells, the pressure within these cells increases and they become turgid. The pressure causes the walls of the guard cells to bulge outward, as shown on the left side of Figure 2. This creates the stomatal pore through which water vapor can evaporate. When the guard cells lose water, they lose their turgidity and become flaccid, which causes the stomatal pore to close, as shown on the right side of Figure 2.

Aside from their role in transpiration, stomata are also involved in gaseous exchange between plants and the atmosphere, allowing the diffusion of gases like oxygen and carbon dioxide.

Example 1: Explaining How Water Is Lost from the Leaves of a Plant

How is water lost from the leaves of a plant?

  1. It diffuses out of the spongy mesophyll.
  2. It evaporates through the waxy cuticle.
  3. It evaporates from the stomata.
  4. It moves back to the roots by osmosis.

Answer

Plants need water for several of their life processes. Water is taken up from the soil by the roots, carrying dissolved minerals and nutrients. From the roots, it is transported upward to all the other parts of the plant. When water reaches the leaves, some of it is used as a reactant for photosynthesis, while some of it is lost from the surfaces of the leaves into the atmosphere, in the form of water vapor.

The process of loss of water from the surfaces of leaves through evaporation is called transpiration. Water evaporates from tiny pores on the surfaces of leaves called stomata.

Stomata are tiny pores in the epidermis of leaves, which is the outermost layer of cells. Usually, leaves have more stomata on their lower surfaces than their upper surfaces. Each stoma is made up of two bean-shaped cells called guard cells, which enclose the stomatal pore. These guard cells are responsible for regulating the opening and closing of the stoma. The inner walls of the two guard cells are thick and elastic, while the outer walls are thinner.

When water enters the guard cells, the pressure within these cells increases and they become turgid. The thin outer walls bulge, drawing the thicker inner walls outward. This creates the stomatal pore through which water vapor can evaporate. When the guard cells lose water, they lose their turgidity and become flaccid, which causes the stomatal pore to close.

Therefore, water is lost from the leaves of a plant through evaporation from the stomata.

Now that we have understood the structure and function of stomata, let’s trace the path that water takes through the different structures of the plant, from the roots to the leaves, and outline the physical processes associated with the movement of the water.

As we know, plants absorb water from the soil through their roots. Roots have specialized structures called root hair cells, which extend into the soil and maximize the surface area for absorption of water. This is represented in Figure 3.

Key Term: Root Hair Cells

Root hair cells are fine hairlike structures that extend into the soil and maximize the surface area of roots for the absorption of water and nutrients.

Water is absorbed into the root hair cells by a process called osmosis. Osmosis is the movement of water molecules from a region of high concentration to a region of low concentration of water, through a selectively permeable membrane.

Definition: Osmosis

Osmosis is the movement of water from an area of low solute concentration to an area of high solute concentration across a semipermeable membrane.

The soil in which the plant grows usually has a higher water concentration than the root hair cells do. This causes water to be pushed through the cell membranes of the root hair cells by osmosis, as represented in Figure 3.

The osmotic “push” of water molecules from the soil into the roots causes an upward pressure called the root pressure. Because of this pressure, the water absorbed from the soil is pushed upward along the xylem tissue of the stem, as shown in Figure 4. Xylem is the vascular tissue in plants responsible for transporting water and dissolved minerals from the roots to the stem and leaves of a plant.

Key Term: Root Pressure

Root pressure is the upward pressure developed in the roots by the push of water molecules into the roots through osmosis.

Key Term: Xylem

Xylem is a vascular tissue in plants that transports water and dissolved mineral ions from the roots to other parts of the plant.

Example 2: Identifying the Structures and Processes Involved in Transpiration

The diagram given shows the process of transpiration in a plant.

  1. By what process do the plant roots absorb water from the soil?
    1. Diffusion
    2. Osmosis
    3. Active transport
    4. Translocation
  2. What is the scientific term given to the pores in the leaf that allow water vapor to diffuse out of the plant?
    1. Guard cells
    2. Stomata
    3. Air spaces
    4. Palisade

Answer

Part 1

Plants need to move water and other substances over very long distances. Water absorbed from the soil has to be transported to the aerial parts of a plant, and the glucose produced in the leaves through photosynthesis has to reach all the other plant parts.

An important factor involved in the transport of substances in plants is the concentration gradient. The concentration gradient is the difference in concentration of a molecule between two regions. Molecules can move from a region of high concentration to a region of low concentration, along the concentration gradient, without requiring any energy. On the other hand, the movement of a molecule from a region of low concentration to a region of high concentration, against the concentration gradient, requires energy.

Osmosis is a means of transport that relies solely on the concentration gradient. Osmosis is the movement of water molecules from a region of high concentration to one of lower concentration, through a selectively permeable membrane. In plants, water is absorbed from the soil into the roots, via the root hair, by osmosis.

Therefore, roots absorb water from the soil through osmosis.

Part 2

The process of loss of water from the surfaces of leaves through evaporation is called transpiration. Water evaporates from tiny pores on the surfaces of leaves called stomata.

Stomata are tiny pores in the epidermis of leaves, which is the outermost layer of cells. Each stoma is made up of two bean-shaped cells called guard cells, which enclose the stomatal pore. These guard cells are responsible for regulating the opening and closing of the stoma. The inner walls of the two guard cells are thick and elastic, while the outer walls are thinner.

When water enters the guard cells, the pressure within these cells increases and they become turgid. The thin outer walls bulge, drawing the thicker inner walls outward. This creates the stomatal pore through which water vapor can evaporate. When the guard cells lose water, they lose their turgidity and become flaccid, which causes the stomatal pore to close.

Therefore, the scientific term for the pores in the leaf that allow water vapor to diffuse out of the plant is stomata.

Water needs to be transported upward, against the pull of gravity, from the roots to the aerial parts of the plant, which are the stem and leaves. In some plants, this distance can be very long. Coast redwood trees, for example, can grow up to a height of 115 metres! Root pressure, however, only provides enough force to push the column of water in the xylem vessels up to a height of about a metre.

Transpiration, the evaporation of water from the stomata of the leaves, provides the majority of the force needed by plants for water transport. While root pressure “pushes” water through the xylem tissues, transpiration exerts an upward “pull” on the column of water traveling upward from the roots. This is called the transpiration pull.

Key Term: Transpiration Pull

Transpiration pull is the upward pull on the column of water carried in the xylem when transpiration occurs in the leaves.

As we know, transpiration takes place in tiny pores in the epidermis of leaves called stomata. Water evaporates in the leaf cells forming water vapor. The air in the atmosphere surrounding the plant has a lower concentration of water vapor molecules than the space within the stomatal pore. The water vapor molecules therefore move from the stomata into the atmosphere. As water vapor is a gas, it moves by diffusion instead of osmosis. You can see this process occurring in Figure 5 below.

Definition: Diffusion

Diffusion is the movement of particles from a region of high concentration to a region of low concentration.

This diffusion creates a “pull,” causing more water molecules to move from the xylem in the stem into the xylem in the leaves, to replace those that evaporate. A strong upward force is therefore applied to the column of water carried in the xylem. Due to this force, plants can transport water upward from the roots to the different parts of the plant, carrying dissolved minerals from the soil. This is represented in Figure 6. The transpiration pull can create enough force to transport a xylem-sized column of water over 130 metres high!

Transpiration is also involved in several other plant processes. By providing the force that pulls water molecules into the leaves, transpiration is the process that supplies leaves with the water they need for photosynthesis. Further, transpiration plays an important role in regulating the temperature of a plant. Evaporation of water molecules from the surface of leaves provides a cooling effect, which is useful in maintaining an optimum temperature for the plant to grow and survive. This is called evaporative cooling.

Sweating in humans provides a nice parallel to transpiration in plants, partly due to this evaporative cooling process. Both processes involve the loss of water from an organism’s body through specific pores on the body surface, causing a cooling effect that helps to maintain internal temperature. There are some distinct differences between the two processes however. For example, while sweat pores are located over the whole human body and this process can occur during both day and night, stomata are mainly found on the leaves and water loss occurs mostly during the daytime. Furthermore, the water in human sweat is excreted onto the skin as a liquid, whereas transpiration causes water from a plant to be lost as a gas: water vapor.

The rate of transpiration in plants is affected by four main limiting factors: light intensity, temperature, humidity, and wind speed.

Definition: Limiting Factor

A limiting factor is an environmental factor whose increase or decrease affects the rate of a process.

As we have learned, transpiration depends on the opening and closing of stomata. In the daytime, when light intensity is high, plant cells carry out photosynthesis, converting carbon dioxide and water into glucose and oxygen. The stomata therefore open to allow the diffusion of carbon dioxide from the atmosphere into the leaves. At the same time, since the stomata are open, transpiration takes place, which also helps pull more water molecules from the xylem into the other leaf cells for photosynthesis. Therefore, as light intensity increases, the rate of transpiration increases.

We know that transpiration is the loss of water from plants through evaporation, which is the conversion of water into water vapor. Evaporation increases with higher temperatures, as water vapor molecules diffuse faster from the stomata into the atmosphere. Therefore, the rate of transpiration also increases when the temperature is high.

If plants are exposed to high temperatures, they could undergo excessive water loss through transpiration. The loss of huge amounts of water to the atmosphere would damage the plant, as there would not be enough water for the plant to survive and grow. To prevent this, the stomata on the surface of leaves close when the temperature increases. Plants may also wilt to conserve water, as wilting collapses the leaves and reduces the surface area available for transpiration to occur. Plants growing in hot climates, such as cacti, may display certain adaptations to conserve water; for example, their leaves may be reduced to spines, with a thick waxy cuticle. These adaptations, which are summarized in Figure 7 below, help the plant prevent excessive transpiration.

Example 3: The Effect of Temperature on the Rate of Transpiration

Which of the following best explains the relationship between temperature and the rate of transpiration?

  1. As temperature increases, the rate of transpiration increases because water molecules are actively transported out of the leaf faster.
  2. As temperature increases, the rate of transpiration decreases because water molecules diffuse from the leaf slower.
  3. As temperature decreases, the rate of transpiration increases because water molecules are actively transported out of the leaf faster.
  4. As temperature increases, the rate of transpiration increases because water molecules diffuse from the leaf faster.

Answer

In plants, water is taken up from the soil by the roots, carrying dissolved minerals. From the roots, it is transported upward to all the other parts of the plant. When water reaches the leaves, some of it is used as a reactant for photosynthesis, while some of it is lost from the surfaces of the leaves into the atmosphere, in the form of water vapor.

The process of loss of water from the surfaces of leaves through evaporation is called transpiration. Water evaporates from tiny pores on the surfaces of leaves called stomata.

The rate of transpiration in plants is affected by four main limiting factors: light intensity, temperature, humidity, and wind speed. A limiting factor is an environmental factor whose increase or decrease affects the rate of a process.

We know that transpiration is the loss of water from plants through evaporation—the conversion of liquid water into water vapor. Evaporation increases with higher temperatures, as water vapor molecules diffuse faster from the stomata into the atmosphere. Therefore, the rate of transpiration increases when the temperature is high.

The best explanation of the relationship between temperature and the rate of transpiration is therefore the following: as temperature increases, the rate of transpiration increases because water molecules diffuse from the leaf faster.

We have learned that water evaporates from the surfaces of leaves when the concentration of water in the surrounding atmosphere is lower than in the leaf. In conditions of high humidity, when there is a high concentration of water vapor in the atmosphere, the rate of transpiration therefore decreases. This is because the concentration gradient between the leaf and the atmosphere reduces.

Key Term: Humidity

Humidity is the concentration of water vapor in the air.

To understand this concept clearly, let’s take a look at Figure 8.

The left side represents air that has high humidity, containing a large number of gaseous water molecules (water vapor). Since leaves are constantly performing photosynthesis and respiration, they always have a large number of water molecules. In this case, therefore, there is not a very steep difference between the concentration of water molecules in the surrounding air and in the leaf. Therefore, the rate of transpiration will be low.

In the diagram on the right side representing low humidity, however, there are fewer gaseous water molecules in the surrounding air. The concentration gradient between the leaf and the air is higher since the surrounding air has fewer water molecules than the leaf. In this case, the rate of transpiration will be higher.

Example 4: The Effect of Humidity on the Rate of Transpiration

The diagram shows a leaf in high and low humidity.

  1. Complete the statement with “high” or “low”: If there is a lot of water vapor in the air, it is said to have humidity.
    1. high
    2. low
  2. Complete the statement with “higher” or “lower”: A difference in concentration will be present if there is a lot of water vapor in the stomata and not much in the surrounding air.
    1. higher
    2. lower
  3. Fill in the blanks: Leaves in areas of humidity have a steeper concentration gradient, so water diffuses out of the leaf .
    1. low, faster
    2. high, faster
    3. high, slower
    4. low, slower

Answer

Part 1

The air in the atmosphere contains water vapor. Humidity is defined as the concentration of water vapor in the air. Air with a lot of water vapor is said to have high humidity, while air with less water vapor is said to have low humidity.

Therefore, if there is a lot of water vapor in the air, it is said to have high humidity.

Part 2

In plants, water is taken up from the soil by the roots, carrying dissolved minerals and nutrients. From the roots, it is transported upward to all the other parts of the plant. The process of loss of water from the surfaces of leaves through evaporation is called transpiration. Water evaporates from tiny pores on the surfaces of leaves called stomata.

The concentration difference is important for the diffusion of water vapor into the atmosphere from the stomata. Water vapor can diffuse when the concentration difference is high, which means that there is a lot of water vapor in the stomata and not much in the surrounding air.

Therefore, a higher difference in concentration will be present if there is a lot of water vapor in the stomata and not much in the surrounding air.

Part 3

In plants, water is taken up from the soil by the roots, carrying dissolved minerals and nutrients. From the roots, it is transported upward to all the other parts of the plant. When water reaches the leaves, some of it is used as a reactant for photosynthesis, while some of it is lost from the surfaces of the leaves into the atmosphere, in the form of water vapor.

The process of loss of water from the surfaces of leaves through evaporation is called transpiration. Water vapor evaporates from tiny pores on the surfaces of leaves called stomata.

The rate of transpiration in plants is affected by four main limiting factors: light intensity, temperature, humidity, and wind speed. A limiting factor is an environmental factor whose increase or decrease affects the rate of a process.

The concentration gradient is the driving force for transpiration—water evaporates from the surface of leaves when the concentration of water vapor in the surrounding atmosphere is lower than in the leaf. In conditions of high humidity, when there is a high level of water vapor in the surrounding atmosphere, the rate of transpiration decreases. This is because the concentration gradient between the leaf and the surrounding atmosphere reduces. On the other hand, when the humidity is low, the concentration gradient is steeper, which means that water diffuses out of the leaf faster and the rate of transpiration increases.

Therefore, leaves in areas of low humidity have a steeper concentration gradient, so water diffuses out of the leaf faster.

An increase in wind speed increases the rate of transpiration. This is also related to the concept of the concentration gradient of water vapor. Higher wind speeds cause the water vapor in the immediate vicinity of the plant to be blown away at a higher rate. This increases the concentration gradient of water vapor between the leaves and the surrounding atmosphere.

An instrument called a potometer is a useful way of demonstrating transpiration and how the rate of transpiration can be affected by different environmental factors. Let’s take a look at the setup of this instrument and how it can be used.

Key Term: Potometer

A potometer is an instrument used to demonstrate and measure the rate of transpiration.

A potometer consists of a capillary tube with a scale for measurement, a reservoir, and a tube in which the shoot of a plant may be held. This is represented in Figure 9.

The process of using a potometer is described below.

How To: Using a Potometer

  1. Fill the entire potometer with water and submerge the bent end of the capillary tube in a beaker of water.
  2. Cut a plant shoot underwater to avoid any air bubbles being introduced into the xylem that would prevent water uptake and place the bottom of the shoot in the water in the left tube.
  3. Ensure there is a tight seal around the plant shoot so that no water can leak out of the potometer and that no water bubbles can enter.
  4. Lift the bent end of the capillary tube out of the beaker for a few seconds so that an air bubble may enter the system.
  5. Submerge the end of the tube again so that the capillary tube can fill completely with water.
  6. Measure the distance that the air bubble moves using the ruler over a set period of time.
  7. Measure the speed of the air bubble using the equation below, which gives us an approximate value for the rate of water uptake by the plant shoot: speedofbubbledistancemovedbythebubbletimetakentomovethisdistance=.

The potometer works on the principle of upward movement of water due to the transpiration pull of the plant. The air bubble introduced into the capillary tube should gradually move toward the plant, demonstrating transpiration. The distance traveled by the air bubble in a certain amount of time can be measured using the scale on the ruler adjacent to the capillary tube, from which the speed of the air bubble can be calculated. This can be used as a measurement for the rate of transpiration. The higher the rate of transpiration, the larger the distance the air bubble moves toward the plant in a certain period of time.

As we have learned, there are various environmental factors that affect the rate of transpiration. Using a potometer, these effects can be demonstrated by using the same procedure described above, but with few specific differences that we will look at now.

A fan may be placed next to the leaves of the plant shoot in the apparatus. The speed at which the air bubble moves through the capillary tube will represent the rate of transpiration at higher wind speeds.

A heater may be placed beside the apparatus, which means that the rate of transpiration can be measured at higher temperatures.

The initial position of the air bubble can be noted, and the apparatus can then be placed in the dark for a certain amount of time. The distance traveled by the air bubble during this time will be a measure of the rate of transpiration at a low intensity of light.

If the plant is sprayed with water and a plastic bag is tied over the plant shoot, the rate of transpiration at higher humidity can be measured.

Note that when one of these factors are being measured using a potometer, all of the other factors should be kept constant so that only the factor being measured is likely to affect the rate of transpiration.

It is also important to note that while a potometer provides us with an approximate value for the rate of transpiration, this value will not be entirely accurate. This is because not all of the water taken up by the leafy shoot will be transpired, as some of the water might be used in the plant for photosynthesis or maintaining cell turgidity.

Example 5: Using a Potometer to Calculate the Rate of Transpiration

A student investigates the effect of wind speed on the rate of transpiration. They place a fan next to the leaves of a plant in a potometer and record how far the air bubble moves in 60 s.

Their results are shown below.

Fan Speed Distance of Bubble Movement (mm) Rate of Water Uptake (mm/s)
1100.17
216
325

  1. Calculate the rate of water uptake for fan speed 2 and give your answer to two decimal places.
  2. Calculate the rate of water uptake for fan speed 3 and give your answer to two decimal places.

Answer

Part 1

In a potometer, the rate of water uptake can be measured by calculating the speed at which the air bubble moves through the capillary tube.

At fan speed 2, according to the data provided, the air bubble moves 16 mm in 60 s.

The speed of the air bubble can be calculated as follows: speeddistancetimespeedoftheairbubblemmsmms==1660=0.27/.

Therefore, the rate of water uptake at fan speed 2 is 0.27 mm/s.

Part 2

In a potometer, the rate of water uptake can be measured by calculating the speed at which the air bubble moves through the capillary tube.

At fan speed 3, according to the data provided, the air bubble moves 25 mm in 60 s.

The speed of the air bubble can be calculated as follows: speeddistancetimespeedoftheairbubblemmsmms==2560=0.42/.

Therefore, the rate of water uptake at fan speed 3 is 0.42 mm/s.

Let’s summarize everything we have learned in this explainer.

Key Points

  • In plants, water is taken up from the soil by the roots and is transported upward through the xylem to the other parts of the plant.
  • Transpiration is the loss of water from the leaves of a plant through evaporation.
  • Water is lost from tiny pores on the surfaces of leaves called stomata. Stomata are made of two guard cells, which are responsible for opening and closing the stomatal pore.
  • The force needed to transport water against the pull of gravity from the roots to the leaves is provided by root pressure and transpiration pull.
  • The rate of transpiration is affected by four limiting factors: light intensity, temperature, humidity, and wind speed.
  • A potometer is an instrument used to measure the effects of different environmental factors on the rate of transpiration.

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