Lesson Video: Transport in the Xylem Biology

In this video, we will learn how to describe the mechanisms of water movement from the root to the leaves in the xylem


Video Transcript

In this video, we will learn how water moves from the roots through the plant to the leaves in a special type of vascular tissue, the xylem. We will learn how various scientists had various theories as to how exactly this transport happens and how one theory after the other was rejected until finally one was found that can explain how the plant does this, the cohesion-tension theory.

Water is essential for a plant’s ability to thrive and grow. Why is this the case? Water is a key reactant in photosynthesis, which is the process by which many plants are able to make their own food in the form of glucose. Photosynthesis requires light energy, which is usually supplied by the Sun. As a result, photosynthesis mostly takes place in the leaves, which are found above ground trying to absorb as much light as possible.

Plants require a lot of water, not only as a reactant for photosynthesis, but also because as photosynthesis proceeds, a large amount of water is lost directly to the atmosphere in the form of water vapor, mostly through pores, the majority of which are found on the underside of leaves, called stomata, or a singular stoma. This loss of water through evaporation into the atmosphere is called transpiration. It’s estimated that around 90 percent of the water that’s absorbed daily by plants is lost through transpiration. In this video, we’ll try to understand how transpiration can actually be a driving force of water transport in plants.

When watering a plant, we should always try to water the soil in which the plant grows, as by watering the soil, we are effectively watering the roots of the plant. The roots are the plant organs that are usually responsible for absorbing water molecules from the soil. But how does the water that’s absorbed in the roots make its way to the leaves where it’s needed for photosynthesis? It flows upward from the roots to the leaves through specialized vessels called xylem vessels. The main function of xylem vessels is to transport water in one direction 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 between the cells in xylem vessels. So the vessels form continuous open tubes that water can flow through easily and without much force. Xylem tissue starts as living cells, but as the cells mature, they die and form the hollow tubes that make up the xylem vessels. For water transport through the plant, the xylem vessel is a path of least resistance and probably the most direct pathway to the leaves. The xylem sap that runs through the xylem vessels mainly consists of water molecules. But it also contains some dissolved mineral ions that have been absorbed into the roots from the soil. But how does the water flow upwards from the roots to the leaves?

Gravity pulls water downward toward the earth. So there must be some sort of mechanisms that are stronger than the gravitational pull that are able to move the water upwards through the xylem vessels. The first theories of how water is transported in the plant focused on determining whether there were physical forces that work in the xylem vessels that may help move water upward against gravity. The first scientist to attempt to answer the question of how water is transported in plants focused on the similarities between xylem vessels and blood vessels. Noting that both had a very small diameter, it was suggested that water moved through the plant using capillary action.

Capillary action is the ability of water molecules to travel upward against the pull of gravity within a narrow space. A simple experiment using a bowl of water and a series of tubes with decreasing capillary sizes can demonstrate this. This phenomenon was first described by Leonardo da Vinci in the 17th century with this simple experiment. It showed that in plants, the xylem vessels could raise the level of the water to its intended destination by becoming smaller and smaller in diameter. This simple experiment with glass tubes demonstrates that the statement “The smaller the capillary, the higher the water rises” is true.

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

In 1874, the great botanist doctor Julius Sachs suggested a new view on water’s transport upward through plants based on water absorption. Sachs supported the view that imbibition, which is the absorption of water through the cell walls of the xylem vessels, is the physical force behind water transport in the plant. The word “imbibition” derives from the Latin word “imbibe,” meaning to drink. Given that plants absorb a lot of water, imbibition theory seemed to make sense. As imbibition is the absorption of water by the cell walls of plant cells, the cells found in dry seeds that are placed into water like these ones become swollen in appearance, which makes the whole seed become swollen. This process is typically observed when dry dormant seeds absorb water molecules from soil through imbibition. This is one of the factors that leads to seed germination.

Imbibition is important for seed germination, as it helps the seeds to increase in size and sprout. The water from soil is absorbed efficiently by the seed coat of a germinating seed. Upon closer inspection, the absorption of water occurs in the roots that are embedded in the soil and not along the entire length of the xylem vessel. The force of imbibition is also very weak. Therefore, despite its importance in seed germination, imbibition theory is not a reasonable explanation for the movement of water up through the xylem vessels.

If you cut the stem of a plant at its base, you will see the xylem sap ooze or flow up from the roots. This phenomenon is called exudation. In 1920, the observation of exudation in plants is what caused another plant biologist, doctor Priestley, to suggest that root pressure might be the physical force behind water transport in plants. You might recall that due to pressure gradients, liquids like water tend to flow from areas of high pressure to areas of comparatively lower pressure. It is this high pressure in the roots that causes water to flow up through the stem. And it’s the phenomenon that doctor Priestley used as the base of his theory.

Root’s pressure is a positive pressure that develops in the xylem vessels in the root. This occurs due to the absorption of water into the roots by osmosis. However, normally observed, root pressure is pretty low, and it’s unable to raise water to the tops of very tall trees. Furthermore, water transport does not always require the roots. This can be best observed with a fresh flower and two beakers of food coloring.

If a plant stem is split and each part of the split stem is placed into a different beaker of colored water, the color of the water in these beakers might start to show in the petals of the flowers from a few hours to a day later. This is due to the transport of water and the dissolved dyes through the xylem. In this simple 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.

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. In 1894, a botanist named Henry Dixon and a physicist named John Jolly developed the idea of cohesion tension. They suggested that the loss of water in the leaves through transpiration might account for the movement of water through the plant.

In order to understand their theory, let’s first review what transpiration is and how it occurs. This diagram shows a side-view cross section of some of the cells in a leaf. The leaves of many higher plants have a waxy cuticle through which water cannot diffuse to reduce water loss. However, this would also prevent the exchange of gases, like carbon dioxide and oxygen, from entering and leaving the plant’s leaves. Plants must absorb carbon dioxide as this is a key reactant in photosynthesis. So there are spaces in this waxy cuticle that we looked at earlier, called stomata, through which carbon dioxide can diffuse into the leaf. Once it’s in the leaf, carbon dioxide can diffuse into the cells where it’s needed for photosynthesis.

Photosynthesis releases the byproduct oxygen. Oxygen usually moves in the opposite direction to carbon dioxide, first diffusing out of the leaves’ photosynthesizing cells, then through the stomata, and into the atmosphere. However, when the stomata are open to allow this exchange of gases for photosynthesis, some water is also lost to the atmosphere in the form of water vapor. This happens at an incredibly fast rate through transpiration.

Transpiration might sound like a bad thing as the plant is losing its precious water. However, transpiration also starts a chain of events that helps to replace this lost water. Let’s simplify our diagram so that we can see how this happens more clearly. When water evaporates from the leaves and exits the stomata in the form of water vapor, a tension or pull on the water in the xylem vessels arises. This draws water upwards from the roots. It is the biochemical forces of the water molecules that allow water to move against the downward pull of gravity up through the plant.

The transpirational pull of water evaporation from the leaves creates a chain reaction of pulls all down the column of water molecules that line the xylem vessel. Let’s discuss the biochemical forces of these water molecules in more detail. The water molecules in the xylem vessel are strongly attracted to each other, which makes them stick together. This stickiness between water molecules is called cohesion, and it’s been represented in this diagram by red dashes between the water molecules. The cohesion between water molecules is due to the strong hydrogen bonds that form between different water molecules, causing them to stick together.

Cohesion between water molecules in the xylem vessels is complemented by adhesion. In this diagram, adhesion has been represented by orange dashes between the water molecules and the xylem vessel walls. The stickiness of hydrogen bonds in water also helps it to stick or adhere very tightly to the cell walls of the xylem vessels. Both cohesion and adhesion are related to sticking together, but the objects they’re sticking to are very different. While cohesion occurs between different water molecules, adhesion is the attraction between one water molecule and the xylem vessel walls.

The cohesion between different water molecules helps to form a continuous column of water, while adhesion helps this column of water to move upward against the downward force of gravity. Cohesion and adhesion therefore work together to pull water molecules from the roots, through the xylem vessels of the stem, and out of the plant through the stomata on the leaves. This is often referred to as the cohesion-adhesion or the cohesion-tension theory of water transport.

There are a few conditions that need to be met for the transpiration pull to occur in the xylem. The xylem vessel must act as a capillary tube. The tubes must be free of gas or air bubbles, and there must be no breaks in the xylem vessels. You may recall that four main environmental factors can affect the rate of transpiration. They are light, temperature, humidity, and wind. These factors can significantly impact the rate of transpiration. The more water that’s transpired, the greater the force of the transpirational pull at the top of the xylem vessels. For example, in the daytime, the sunlight will stimulate the opening of the stomata in the leaves, which causes water vapor to leave the stomata through diffusion, speeding up the transpiration rate.

Increasing the temperature supplies the water molecules with more thermal energy, which increases their kinetic energy. This increases the speed of the movement of the water molecules and therefore their rate of evaporation and subsequent transpiration through the stomata. Furthermore, a low humidity or a high wind speed can increase the rate of transpiration at the leaves by increasing the concentration gradient for water vapor between the inside of the leaf and the external environment. A steeper concentration gradient between the leaf and the atmosphere means that water will transpire faster.

Let’s see how much we’ve learned about transport in the xylem by applying our knowledge to a practice question.

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? (A) Vein, (B) phloem, (C) cell walls, or (D) xylem.

To answer this question, let’s go through the different answer options and discuss what the purpose of each of the different mentioned structures is.

A vein, when used in the context of transport vessels, is a type of blood vessel that’s found in animals like humans. Veins form part of the human circulatory system that are responsible for transporting blood back toward the heart from the body tissues. A different sort of vein can also be found in leaves. While leaf veins do play a role in transport, they are not transport vessels themselves. As the question is specifically asking us the transport vessel through which water moves, we can eliminate option (A).

While the phloem and the xylem are both transport vessels that are found in plants, the cell wall is a rigid structural layer that surrounds plant cells. Cell walls are responsible for providing structural support and protection to plant cells, but cell walls are not a transport vessel. So this option can also be eliminated.

As we mentioned, the phloem and the xylem are both transport vessels, but they differ in their structure and function. The phloem transports substances like sugars and amino acids both up and down the plant stem. While these sugars and amino acids will be dissolved in water, so some water will be involved in phloem transport, it is not the main function of the phloem to transport water.

The xylem, on the other hand, is responsible for transporting the majority of water molecules in a plant. It also transports dissolved mineral ions. The water molecules and ions are generally absorbed from soil into the roots of a plant and then are transported from the roots into the plant stem. Unlike in the phloem, where substances can move both up and down the plant, the xylem only transports water and mineral ions up the plant from the roots to the leaves and flowers and any other organs that might require them.

If we take a look back at the diagram provided by the question, we can see that water is only moving up the plant from the roots to the leaves. This helps us to confirm that the phloem is not the vessel through which the majority of water moves, as it transports sugars and amino acids both up and down the plant stem. Instead, the role of the xylem is to move the majority of water through the plant.

Let’s take a moment to wrap up the video by reviewing the key points that we’ve learned. Water is first absorbed usually from soil into a plant’s roots. And it then needs to be transported against the pull of gravity up the plant to its photosynthetic parts. This transport of water occurs in xylem vessels from the roots, through the stem, and to the leaves, where photosynthesis can occur.

How exactly this happened was a mystery for a long time, and several scientists have attempted to develop theories to explain the mechanism of water transport through the xylem vessels. But ultimately, the theories of capillary action, root pressure, and imbibition were all invalidated. The most validated theory was that of transpiration, producing an upward pull of the water in the xylem vessels. Finally, the cohesion- tension or cohesion-adhesion theory explained how the biochemical forces between individual water molecules and between the water molecules and the xylem vessel walls allowed their movement upward through the plant.

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