Lesson Explainer: Transport in the Xylem Biology

In this explainer, we will learn how to describe how water is moved through the xylem using cohesion–tension.

Water is central to a plant’s ability to thrive and grow. It is the key reactant in photosynthesis, which is how plants make their own food. Yet despite the plants’ dependence on water, they keep less than 5% of the water absorbed by their roots. As most plant enthusiasts will tell you, while the roots are what we water, they are not the most important site for water. In a plant, water is most useful when it is able to move from the roots to the leaves, which is the site of photosynthesis.

Plants require a lot of water because as photosynthesis proceeds, a large amount of water is lost directly to the atmosphere in a process called transpiration. You may remember that, in photosynthesis, plants produce their own nutrition through sunlight and water. Plants must absorb carbon dioxide from the atmosphere through small pores in their leaves called stomata. When stomata open to absorb carbon dioxide, water is lost to the atmosphere at an incredible rate through transpiration. Transpiration is the evaporation of water from leaves in the form of water vapor. So, as plants proceed with photosynthesis to make their own nutrition, they also lose a substantial amount of water due to transpiration.

Definition: Transpiration

Transpiration is the loss of water through evaporation from the leaves of a plant into the atmosphere.

To keep the plant from dying, transpiration also starts a chain of events that helps replace the evaporated water. As the water is evaporated at the leaves, more water is transported into the leaves through the xylem vessels that provide a direct pathway to the water in the soil to the leaves as shown in Figure 1.

The main function of the xylem vessels is to transport water in one direction, upward from the roots, through the stem, and to the leaves. Xylem vessels are very long tubelike structures made of individual cells that are stacked end to end. There are no end walls on xylem cells, so the vessels form continuous open tubes that water can move through easily and without force as shown in Figure 2. Xylem tissue starts as living cells, but as the cells mature, they die and form the hollow tubes that make the xylem vessels.

Definition: Xylem Vessels

Xylem vessels are very long tubelike structures joined end to end that transport the majority of water from the roots to the leaves.

Example 1: Describing the Transport Vessels for Water in the Plant

The diagram provided shows a simplified outline of the movement of water through a plant. Through what transport vessel does the majority of water move?

  1. Xylem
  2. Phloem
  3. Vein
  4. Cell walls

Answer

Water is central to a plant’s ability to thrive and grow. It is the key reactant in photosynthesis, which is how plants make their own food. Yet while the roots are what we water, they are not the most important site for water. Plants require a lot of water because as photosynthesis proceeds, a large amount of water is lost directly to the atmosphere in a process called transpiration. So, in a plant, water is most useful when it is able to move from the roots to the leaves where photosynthesis takes place.

In photosynthesis, plants produce their own nutrition through sunlight and water. Plants must absorb carbon dioxide from the atmosphere through small pores in their leaves called stomata. So, as plants proceed with photosynthesis to make their own nutrition, they also lose a substantial amount of water due to transpiration. Transpiration is the evaporation of water from leaves in the form of water vapor.

As the water is evaporated at the leaves, more water is pulled into the leaves from the xylem vessels, which directly connect the water in the soil to the water in the leaves. Xylem vessels are very long tubelike structures made of individual cells that are stacked end to end. There are no end walls on xylem cells, so the vessels form continuous open tubes that water can move through easily and without force. Xylem tissues start as living cells, but as the cells mature, they undergo ordered deconstruction to help form the hollow tubes that make up the xylem vessels. The main function of the xylem vessels is to transport water in one upward direction from the roots, through the stem, and to the leaves.

Therefore, in the diagram provided the majority of water moves through a plant using the xylem as the transport vessel.

So, for water transport through the plant, the xylem vessel is the path of least resistance and probably the longest, but most direct, pathway to the leaves. This means that unlike the phloem sap (which is mostly sucrose and amino acids), the xylem sap consists mainly of water and some mineral nutrients.

Even after scientists discovered that it is the xylem vessels that transport water throughout the plant, the mechanics of how the transport worked took even more time to figure out. The first theories on how water is transported in the plant focused on determining whether it was the physical forces at work in the xylem vessels that may help move water upward against gravity.

The first scientist to attempt to answer how water is transported in plants focused on the similarities between xylem vessels and human blood vessels. Noting that both have a very small diameter, it was suggested that water moved through the plant using capillary action. Capillary action is the ability for water to travel upward against gravity in a narrow space. So, in plants, the xylem vessels would “raise” the water to its intended destination by becoming smaller and smaller. You can see in Figure 3 that the smaller the capillary, the higher the water rises.

However, after a series of calculations by various other scientists, it was shown that even the smallest of capillaries would not be able to lift water to heights above 150 centimetres. As most trees and plants are well over 150 centimetres in height, capillary action cannot explain the movement of water up the xylem vessel. Additionally, for capillary action to work best, there must be direct contact between the xylem and the water in the soil. Since the math and experiments both failed to support this theory, it quickly fell out of favor.

Definition: Capillary Action

Capillary action is the ability for water to travel upward against gravity in a narrow space.

Another plant scientist, Dr. Sachs, suggested a new view of water’s movement upward in plants based on water absorption in 1874. Sachs supported the view that imbibition, or absorption of water, through the walls of xylem vessels is the physical force behind water transport in the plant. Imbibition is the process of water absorption by the cell walls that causes them to have a swelled appearance. It is most typically seen with seeds, which swell with water through imbibition during germination.

The word imbibition comes from the Latin word imbibe, meaning “to drink.” Given that plants absorb a lot of water, imbibition theory seemed to make sense. Yet upon closer inspection, the absorption of water occurs in the roots embedded in soil, not all along the entire length of the xylem vessel, and the force of imbibition is very weak. Imbibition is important for seed germination as it helps the seeds increase in size to sprout. The water from the soil is absorbed efficiently by the seed coat of a germinating seed. Yet despite its importance in seed germination, imbibition theory is not a reasonable explanation for the movement of water up the xylem vessel.

Definition: Imbibition

Imbibition is the process of water absorption by the cell walls that causes them to have a swelled appearance.

If you cut the stem of a plant at the base, you will see the sap ooze or flow up from the roots, which is a phenomenon called exudation. Observation of exudation is what caused another plant biologist, Dr. Priestley, to suggest that root pressure may be the physical force behind water transport in plants. You may recall that due to pressure gradients, liquids flow from areas of high pressure to areas of low pressure. It is this high pressure in the roots that causes water to flow up the stem, and it is the phenomenon Dr. Priestly used as the base of his theory. Root pressure is a positive pressure that develops in the xylem vessels in the root during active water absorption of some plants.

However, normally observed, root pressure is generally low and is unable to raise water to the tops of very tall trees, like conifers. Also, water transport does not always require the roots. This is best observed with fresh flowers and food coloring, as shown in the picture below.

Daffodil with a split stem

Figure4

In the picture above, we see a flower with a split stem that has been placed in a beaker of colored water. Due to the transport of water up the xylem, the color of the water will begin to show in the flower petals a few days later, which we can see in the picture above. From this small experiment, we can see that the transport of water has occurred in this cut flower even in the absence of roots. So, if water still moves up the stem, even in the absence of roots, root pressure is not enough to explain how water is transported upward in plants.

Definition: Root Pressure

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

Example 2: Describing the Transport Vessels for Water in the Plant

Which of the following best explains the theory of water movement by root pressure?

  1. Root pressure is the force that pushes water from the roots vertically up the xylem.
  2. Root pressure is the force that pulls water into the xylem from the roots.
  3. Root pressure forces water into the cell walls of the phloem to be transported around the plant.
  4. Root pressure is the bonding of water molecules to the walls of the root hair cells.

Answer

Even after scientists discovered it was the xylem vessels that transport water throughout the plant, the mechanics of the transport process needed to be determined. One theory on how water is transported in the plant focused on determining whether it was the physical forces at work in the xylem vessels that may help move water upward against gravity. One physical force that was considered was root pressure.

When you cut the stem of a plant at the base, you can see the sap flow up from the roots. From this observation, another plant biologist, Dr. Priestley, suggested that root pressure may be the physical force behind water transport in plants. You may recall that due to pressure gradients, liquids flow from areas of high pressure to areas of low pressure. It is this high pressure in the roots that causes water to flow up the stem, and it is the phenomenon Dr. Priestly used as the base of the root pressure theory. Root pressure is a positive pressure that develops in the xylem vessels in the root during active water absorption of some plants.

However, normally observed, root pressure is generally low and is unable to raise water to the tops of very tall trees. Also, water transport does not always require the roots. This is best observed with fresh flowers and food coloring. When cut flowers are placed in a beaker of colored water, due to the transport of water up the xylem, the color of the water will begin to show in the flower petals a few days later. From this, we can see that the transport of water has occurred in these cut flowers even in the absence of roots! So, the transport of water up the stem in the absence of roots cannot be explained by the root pressure theory.

Therefore, the best explanation of the theory of water movement by root pressure is that root pressure is the force that pushes water from the roots vertically up the xylem.

Finally, an unexpected team of a botanist and a physicist were able to figure out the best explanation for how water transport in plants occurs. Henry H. Dixon, an Irish botanist, and John Joly, a physicist, developed the idea of cohesion–tension in 1894. Dixon and Joly suggested that the loss of water in the leaves through transpiration created a “pulling” effect on the water in the xylem vessel, drawing more water upward from the roots. It is the biochemical forces of the water molecules that allowed water to move against the downward force of gravity.

Transpiration and the force of water evaporation at the leaf creates negative pressure at the surface. This creates a “pulling” force on the water in the rest of the xylem vessels. The “transpirational pull” of water evaporation creates a chain reaction of “pulls” all down the column of water molecules that line the xylem vessel.

The water molecules in the xylem vessels are strongly attracted to each other, which makes them “stick together.” This “stickiness” between water molecules is called cohesion and is due to the formation of strong hydrogen bonds between different water molecules.

Cohesion between the water molecules in the xylem vessel is complemented by the adhesion of water to xylem walls. The stickiness of the hydrogen bonds in water also helps it stick, or adhere, very tightly to the cell walls of xylem vessels. This property of water is called adhesion. Both cohesion and adhesion are related to “sticking together,” but the objects they are sticking to are very different.

Definition: Cohesion

Cohesion is the strong attractive force between different water molecules that causes them to stick together.

Definition: Adhesion

Adhesion is the attractive force between water molecules and the cell walls of the xylem vessels.

The difference between the terms cohesion and adhesion is illustrated in Figure 5. Cohesion occurs between two different water molecules, while adhesion is the attraction between one water molecule and the xylem vessel walls. The cohesion between water molecules in the xylem vessels helps form a continuous column of water, while adhesion allows the column of water to move against the downward force of gravity.

Example 3: Defining the Attractive Forces between Water Molecules

The cohesion–tension theory of water movement aims to explain how water is transported through the xylem. Which of the following best explains what is meant by cohesion in water movement?

  1. Cohesion refers to the forces of attraction between the molecules of water inside the xylem.
  2. Cohesion refers to the forces of attraction that occur between the water molecules and the walls of the xylem.
  3. Cohesion refers to the pull of water through the xylem, as water vapor diffuses from the stomata.
  4. Cohesion refers to the movement of water from the soil into the root, from an area of low solute concentration to an area of high solute concentration.

Answer

As the water is evaporated at the leaves, more water is pulled into the leaves from the xylem vessels, which directly connect the water in the soil to the water in the leaves. Xylem vessels are very long tubelike structures made of individual cells that are stacked end to end. There are no end walls on xylem cells, so the vessels form continuous open tubes that water can move through easily and without force. Xylem tissues start as living cells, but as the cells mature, they undergo ordered deconstruction to help form the hollow tubes that make up the xylem vessels. The main function of the xylem vessels is to transport water in one upward direction from the roots, through the stem, and to the leaves.

The water molecules in the xylem vessels are strongly attracted to each other, which makes them “stick together.” This “stickiness” between water molecules is called cohesion and is due to the formation of strong hydrogen bonds between different water molecules. The cohesive property between water molecules causes water to form a column, with one molecule of water following the next, in the xylem vessel. Cohesion occurs between two different water molecules, while adhesion is the attraction between one water molecule and the xylem vessel walls.

Therefore, the best explanation for what is meant by cohesion in water movement is A: cohesion refers to the forces of attraction between the molecules of water inside the xylem.

Cohesion and adhesion work together to pull water molecules from the roots, through xylem vessels of the stem, and out of the plant through the stomata on the leaf. This is often referred to as the cohesion–adhesion, or cohesion–tension theory of water transport.

Key Term: Cohesion–Adhesion or Cohesion–Tension Theory of Water Movement

The cohesion–adhesion theory, sometimes also called cohesion–tension theory, describes the two forces in the xylem vessels that help transport water upward against gravity, from the roots to the leaves.

The cohesive property between water molecules causes water to form a column, with one molecule of water following the next, in the xylem vessel. This continuous column of water in the xylem vessels stretches from the roots, through the stem, and into the leaves. At the “top” of the xylem, as water transpires, cohesion keeps the column of water intact, while adhesion of water molecules to the xylem walls prevents breakage of the water column. Figure 6 shows the relationship between transpiration, cohesion, and adhesion in a tree, which helps transport water from the roots to the leaves.

There are a few conditions that need to be met for the transpiration pull to occur in the xylem:

  1. The vessel must act as a capillary tube.
  2. The tubes must be free of gas/air bubbles.
  3. There must be no breaks in the xylem.

Since transpiration impacts the amount of water in the plant, the rate of transpiration can lead to changes in the size of the stem (or trunk) of the plant. You may remember that four main environmental factors can affect the rate of transpiration. They are

  • light,
  • temperature,
  • humidity,
  • wind.

These factors can significantly impact the rate of transpiration. The more water is transpired, the greater the transpirational pull at the “top” of the xylem. So, in the daytime, the sunlight stimulates the opening of the stomata of the leaf, which causes water to leave the stomata by diffusion, speeding up transpiration. An increase in temperature increases the kinetic energy of the water molecules, meaning they evaporate faster and subsequently increase the rate of transpiration. Additionally, low humidity and high wind speed can also increase the rate of transpiration at the leaves by increasing the concentration gradient between the inside of the leaf and its surroundings.

While the environmental factors that increase transpiration may not seem like such impactful factors, the negative pressure, or tension, on the water in the leaf is so great that it changes the actual size of tree trunks, every day!

At night, there is less light, lower temperature, high humidity, and little wind. In response to the change in these environmental factors, the stomata shut and transpiration stops. But thanks to the cohesion–adhesion of water in the plant, water is held in the stem and in the leaf by the adhesion of water to the cell walls of the xylem vessels and the cohesion of water molecules to each other. This also means that at night, there is less tension in the xylem, which enables the diameter of the trunk to increase.

This relationship can be summarized as follows: when the rate of transpiration is high, the trunk will shrink. When the rate of transpiration is low, the trunk will expand.

Example 4: Describing the Rate of Transpiration in Plants

The graph provided shows the relationship between the time of day and the rate of water flow through a plant. Between which hours was the rate of transpiration the greatest?

  1. 12 pm and 6 pm
  2. 2 am and 8 am
  3. 10 am and 2 pm
  4. 2 am and 6 pm
  5. 8 pm and 10 pm

Answer

The four main environmental factors can affect the rate of transpiration: light, temperature, humidity, and air. In the daytime, these environmental factors help the plant transpire very quickly, increasing the amount of water used by the plant during the day. And the rate of transpiration is related to the flow of water through the plant.

These factors can significantly impact the rate of transpiration. The more water is transpired, the greater the transpirational pull at the “top” of the xylem. So, in the daytime, the sunlight stimulates the opening of the stomata of the leaf, speeding up transpiration. An increase in temperature increases the kinetic energy of the water molecules, meaning they evaporate faster and subsequently increase the rate of transpiration. Additionally, low humidity and high wind speed can also increase the rate of transpiration at the leaves by increasing the concentration gradient between the inside of the leaf and its surroundings.

For example, light stimulates the opening of the stomata and increases the temperature of the leaf, which speeds up transpiration. This is because water evaporates more rapidly as the temperature rises. As more water is rapidly transpired, more water is needed by the plant to cool the plant and be used in photosynthesis. Also in the daytime, humidity is lower and wind speed is higher. This means the surrounding air is drier and there is more air movement around the stomata, which helps water transpire more rapidly, again, increasing the amount of water flowing through the plant. However, at night, there is less light, lower temperature, high humidity, and little wind. In response to the change in these environmental factors, the stomata shut and transpiration stops.

In the graph, we see the rate of water flow is being tracked with the time of day.

Starting from the first time point on the left, at 12 am, we see the rate of water flow is very low. The low rate of water flow continues from 12 am to about 10 am. Starting at 10 am, we can see the increase in water flow through the plant until about 12 pm, which is indicated by the upward slope of the line in the graph. This indicates that the rate of water flow is rising between 10 am and 12 pm.

The rate of water flow stays steadily high from 12 pm to around 6 pm. This means that the rate of water flow is constant and high during this period. At 6 pm, the graph line begins to fall until 10 pm.

We can assess the choices provided to identify a time period where the rate of water flow is high. The first choice is 12 pm and 6 pm. From the graph, we see that the rate of water flow is very high.

Therefore, based on the graph provided, which shows the relationship between the time of day and the rate of water flow through a plant, the rate of transpiration is the greatest between 12 pm and 6 pm.

Example 5: Understanding How Tension Changes with the Rate of Transpiration in Plants

The graph provided shows the relationship between the time of day and the rate of water flow through a plant. Between which hours would the tension in the xylem be highest?

  1. 12 pm and 6 pm
  2. 2 am and 8 am
  3. 6 am and 12 pm
  4. 6 pm and 10 pm
  5. 4 am and 10 am

Answer

The environmental factors that increase transpiration can significantly impact the negative pressure, or tension, on the water in the leaf. The four main environmental factors can affect the rate of transpiration: light, temperature, humidity, and air. With the increase in the amount of water transpired, more water will flow through the plant.

So, during the daytime, these environmental factors help the plant transpire very quickly, increasing the amount of water used by the plant during the day. The more a plant transpires, the more tension there is on the surface of the leaf. The increase of tension in the xylem is so strong that it can decrease the diameter of the trunk!

At night, there is less light, lower temperature, high humidity, and little wind. In response to the change in these environmental factors, the stomata shut and transpiration stops. In the plants, at night, water is held in the stem and in the leaf by the adhesion of water to the cell walls of the xylem vessels and the cohesion of water molecules to each other. When there is less tension in the xylem, the diameter of the trunk increases!

This relationship can be summarized as follows: when the rate of transpiration is high, tension is high, and the trunk will shrink. When the rate of transpiration is low, the tension is low, and the trunk will expand.

In the graph, we see the rate of water flow is being tracked with the time of day. Starting from the first time point on the left, at 12 am, we see the rate of water flow is very low, which continues from 12 am to about 10 am. With a low rate of water flow, the rate of transpiration is also low, so tension is also low.

At 10 am, the water flow increases until about 12 pm, which is indicated by the upward slope of the line in the graph. As the rate of transpiration increases, tension increases. Then the rate of water flow stays steadily high from 12 pm to around 6 pm. This means that the rate of water flow is constant and high during this period, so tension is constant. At 6 pm, the graph line begins to fall until 10 pm, as the rate of water flow and tension decrease.

The question asks that we identify a time period where the tension in the xylem is highest. High tension is caused by a high rate of transpiration. When transpiration is high, there is a high flow of water to rapidly replace the water lost in transpiration. So, the correct answer in the given choices is the time period where the rate of water flow is highest. From reading the graph, the high rate of water flow is between 12 pm and 6 pm.

Therefore, based on the graph provided that shows the relationship between the time of day and the rate of water flow through a plant, tension is highest in the xylem between 12 pm and 6 pm.

Let’s take a look at what we have learned in this explainer.

Key Points

  • Most of the water in plants passes through the plant and directly into the atmosphere in a process called transpiration.
  • The transport of water from the soil to the leaves occurs in the xylem vessels.
  • Several theories (e.g., capillary action, root pressure, and imbibition) attempted to explain the movement of water up the xylem vessel, but ultimately, all of these theories were invalidated.
  • In the xylem vessels, water molecules are strongly attracted to each other and exhibit a “stickiness” called cohesion that is due to the formation of hydrogen bonds between different water molecules.
  • The “stickiness” between water molecules and the xylem vessel walls is called adhesion.
  • Cohesion and adhesion work together in the xylem to pull water molecules from the roots, up the stem, and out of the plant through the stomata.
  • The flow of water transport is impacted by the plant rate of transpiration.
  • The rate of transpiration can also affect the size of the plant, such that when transpiration is high, tension is high, and the plant size shrinks. Conversely, when the rate of transpiration is low, the tension is low, and the plant size expands.

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