how does water travel up xylem

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• Prokaryotes: Bacteria & Archaea
• Eukaryotes and their Origins
• Land Plants
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• Animal Reproductive Strategies
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• Animal Development I: Fertilization & Cleavage
• Animal Development II: Gastrulation & Organogenesis
• Plant Reproduction
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• Plant Development II: Primary and Secondary Growth
• Intro to Chemical Signaling and Communication by Microbes
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• Nervous Systems
• Animal Sensory Systems
• Motor proteins and muscles
• Motor units and skeletal systems
• Nutrient Needs and Adaptations
• Nutrient Acquisition by Plants

Water Transport in Plants: Xylem

• Sugar Transport in Plants: Phloem
• Nutrient Acquisition by Animals
• Animal Gas Exchange and Transport
• Animal Circulatory Systems
• The Mammalian Cardiac Cycle
• Animal Ion and Water Regulation (and Nitrogen Excretion)
• The Mammalian Kidney: How Nephrons Perform Osmoregulation
• Plant and Animal Responses to the Environment

Learning Objectives

• Explain water potential and predict movement of water in plants by applying the principles of water potential
• Describe the effects of different environmental or soil conditions on the typical water potential gradient in plants
• Identify and differentiate between the three pathways water and minerals can take from the root hair to the vascular tissue
• Explain the three hypotheses explaining water movement in plant xylem, and recognize which hypothesis explains the heights of plants beyond a few meters
• Define transpiration and identify the source of energy that drives transpiration

Water Potential and Water Transport from Roots to Shoots

The information below was adapted from OpenStax Biology 30.5

The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and products of photosynthesis throughout the plant. The phloem is the tissue primarily responsible for movement of nutrients and photosynthetic produces, and xylem is the tissue primarily responsible for movement of water). Plants are able to transport water from their roots up to the tips of their tallest shoot through the combination of water potential, evapotranspiration, and stomatal regulation – all without using any cellular energy!

Water potential is a measure of the potential energy in water based on potential water movement between two systems. Water potential can be defined as the difference in potential energy between any given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter Ψ ( psi ) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψ pure H2O ) is defined as zero (even though pure water contains plenty of potential energy, this energy is ignored in this context).

Water potential can be positive or negative, and water potential is calculated from the combined effects of  solute concentration   (s) and  pressure (p) . The equation for this calculation is Ψ

An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered. Water is lost from the leaves via transpiration (approaching Ψ p  = 0 MPa at the wilting point) and restored by uptake via the roots.

This video provides an overview of water potential, including solute and pressure potential (stop after 5:05):

And this video describes how plants manipulate water potential to absorb water and how water and minerals move through the root tissues:

Impact of Soil and Environmental Conditions on the Plant Water Potential Gradient

As noted above, Ψ soil  must be > Ψ root  > Ψ stem  > Ψ leaf  > Ψ atmosphere in order for transpiration to occur (continuous movement of water through the plant from the soil to the air without equilibrating. This continuous movement of water relies on a water potential gradient , where water potential decreases at each point from soil to atmosphere as it passes through the plant tissues. However, this gradient can become disrupted if the soil becomes too dry, which can result in both decreased solute potential (due to the same amount of solutes dissolved in a smaller quantity of water) as well as decreased pressure potential in severe droughts (resulting from negative pressure or a “vacuum” in the soil due to loss of water volume). If water potential becomes sufficiently lower in the soil than in the plant’s roots, then water will move out of the plant root and into the soil.

Pathways of Water and Mineral Movement in the Roots

Once water has been absorbed by a root hair, it moves through the ground tissue and along its water potential gradient through one of three possible routes before entering the plant’s xylem:

• the  symplast : “sym” means “same” or “shared,” so symplast is “shared cytoplasm”.  In this pathway, water and minerals move from the cytoplasm of one cell in to the next, via plasmodesmata that physically join different plant cells, until eventually reaching the xylem.
• the  transmembrane  pathway: in this pathway, water moves through water channels present in the plant cell plasma membranes, from one cell to the next, until eventually reaching the xylem.
• the  apoplast : “a” means “outside of,” so apoplast is “outside of the cell”. In this pathway, water and dissolved minerals never move through a cell’s plasma membrane but instead travel through the porous cell walls that surround plant cells.

Water and minerals that move into a cell through the plasma membrane has been “filtered” as it passes through water or other channels within the plasma membrane; however water and minerals that move via the apoplast do not encounter a filtering step until they reach a layer of cells known as the endodermis which separate the vascular tissue (called the stele in the root) from the ground tissue in the outer portion of the root. The endodermis is present only in roots, and serves as a checkpoint for materials entering the root’s vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip , forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This process ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded.

Movement of Water Up the Xylem Against Gravity

How is water transported up a plant against gravity, when there is no “pump” or input of cellular energy to move water through a plant’s vascular tissue? There are three hypotheses that explain the movement of water up a plant against gravity. These hypotheses are not mutually exclusive, and each contribute to movement of water in a plant, but only one can explain the height of tall trees:

• Root pressure  pushes water up
• Capillary action draws water up within the xylem
• Cohesion-tension pulls water up the xylem

We’ll consider each of these in turn.

Root pressure relies on positive pressure that forms in the roots as water moves into the roots from the soil. Water moves into the roots from the soil by osmosis, due to the low solute potential in the roots (lower Ψs in roots than in soil). This intake o f water in the roots increases Ψp in the root xylem, “pushing” water up. In extreme circumstances, or when stomata are closed at night preventing water from evaporating from the leaves, root pressure results in guttation , or secretion of water droplets from stomata in the leaves. However, root pressure can only move water against gravity by a few meters, so it is not sufficient to move water up the height of a tall tree. 

Capillary action  (or capillarity) is the tendency of a liquid to move up against gravity when confined within a narrow tube (capillary). You can directly observe the effects of capillary action when water forms a meniscus when confined in a narrow tube. Capillarity occurs due to three properties of water:

• Surface tension , which occurs because hydrogen bonding between water molecules is stronger at the air-water interface than among molecules within the water.
• Adhesion , which is molecular attraction between “unlike” molecules. In the case of xylem, adhesion occurs between water molecules and the molecules of the xylem cell walls.
• Cohesion , which is molecular attraction between “like” molecules. In water, cohesion occurs due to hydrogen bonding between water molecules.

On its own, capillarity can work well within a vertical stem for up to approximately 1 meter, so it is not strong enough to move water up a tall tree.

This video provides an overview of the important properties of water that facilitate this movement:

The cohesion-tension  hypothesis is the most widely-accepted model for movement of water in vascular plants. Cohesion-tension combines the process of capillary action with transpiration or the evaporation of water from the plant stomata. Transpiration is ultimately the main driver of water movement in xylem, combined with the effects of capillary action. The cohesion-tension model works like this:

• Transpiration (evaporation) occurs because stomata in the leaves are open to allow gas exchange for photosynthesis. As transpiration occurs, evaporation of water deepens the meniscus of water in the leaf, creating negative pressure (also called tension or suction).
• The tension created by transpiration “pulls” water in the plant xylem, drawing the water upward in much the same way that you draw water upward when you suck on a straw.
• Cohesion (water molecules sticking to other water molecules) causes more water molecules to fill the gap in the xylem as the top-most water is pulled toward end of the meniscus within the stomata.

Transpiration results in a phenomenal amount of negative pressure within the xylem vessels and tracheids, which are structurally reinforced with lignin to cope with large changes in pressure. The taller the tree, the greater the tension forces (and thus negative pressure) needed to pull water up from roots to shoots.

Follow this link to watch this video on YouTube for an overview of the different processes that cause water to move throughout a plant (this video is linked because it cannot be directly embedded within the textbook; if needed, the video url is https://www.youtube.com/watch?v=8YlGyb0WqUw )

Transpiration Energy Source

The term “ transpiration ” has been used throughout this reading in the context of water movement in plants. Here we will define it as: evaporation of water from the plant stomata resulting in the continuous movement of water through a plant via the xylem, from soil to air, without equilibrating.

Transpiration is a passive process with respect to the plant, meaning that ATP is not required to move water up the plant’s shoots. The energy source that drives the process of transpiration is the extreme difference in water potential between the water in the soil and the water in the atmosphere. Factors that alter this extreme difference in water potential can also alter the rate of transpiration in the plant.

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The Pathway and Movement of Water into the Roots and Xylem (A-level Biology)

The pathway and movement of water into   the roots and xylem, water movement, in the root.

• Water is a transport system . Water is essential in plants as it it used as a transport system for nutrients and minerals across a water potential gradient .
• Water potential is higher within the soil than the root hair cell . Water is taken up by the roots of a plant and through the endodermis , before being moved into xylem tissue, which is in the centre of the root. The water potential inside the soil is higher than that of that root hair cells. This is because of the dissolved substances in the cell sap.
• Root hair cells increase surface area . Root hair cells function to increase the surface area in order to be pumped across against the concentration gradient .

Water can reach the cortex of the xylem vessels via two pathways:

• Symplast , where water moves between the cytoplasm of neighbouring cells.
• Apoplast , where water can moves directly through the permeable cell walls and intercellular spaces of neighbouring cells.

The symplast pathway allows water to enter the cytoplasm via the plasma membrane, where it travels between cells through plasmodesmata .

Plasmodesmata are tiny channels which cross the cell walls of neighbouring plant cells in order to be able to connect their cytoplasm, creating a large multinucleate mass of plant cells.

The apoplast pathway doesn’t need to travel through the plasma membranes in order for the water to move through the spaces between the cellulose molecules. Because of this, it can carry dissolved mineral ions and salts.

Once the water reaches the endodermis of the root, a layer of suberin (known as the Casparian strip ) stops the water’s path. This is because it cannot be penetrated by water.

To be able to cross the endodermis, the moving water can now use the symplast pathway by going down the water potential gradient to reach a pit in the xylem vessel. This is where water enters the vessel.

In the Xylem

Moving down the water potential gradient, water gets removed from the top of xylem vessel into mesophyll cells.

Root pressure, where the endodermis uses active transport to move minerals into the xylem, pushes the water upwards, into the xylem by osmosis.

Cohesion-Tension Theory

The cohesion-tension theory explains how water moves up the xylem.

• Water evaporates from the mesophyll cells within the leaf . This is known as transpiration , which is the evaporation of water from a plant.
• As water molecules are cohesive, tension is created . Water molecules stick together because the molecules can form hydrogen bonds with one another.
• This cohesion and tension causes more water to be drawn up into the xylem . The water is pulled up through the xylem via osmosis so   that the vessel is filled with an uninterrupted column of water. This column will make its way up through the xylem until it evaporates from the mesophyll cell’s walls. The water then diffuses from air sacs in the leaf, through open stoma.

Cohesion is the attraction of same molecules. Hydrogen bonds are formed between water molecules.

Adhesion is the attraction of unlike molecules. Hydrogen bonds are formed between water and surfaces. An example of a surface used in adhesion it the pores in mesophyll cells.

Water moves into plants through the roots and into the xylem, which is responsible for transporting water and dissolved minerals from the roots to the rest of the plant.

Water moves into the roots of a plant through osmosis, which is the movement of water molecules from an area of high concentration to an area of low concentration. In the case of plants, water moves from the soil into the roots due to a lower concentration of water inside the roots.

Transpiration is the process by which water is lost from the leaves of a plant through small pores called stomata. This water loss creates a negative pressure in the xylem that pulls water up from the roots and into the rest of the plant.

Root pressure is a natural force that results from the accumulation of water and minerals in the roots. This pressure helps to push water up into the xylem and into the rest of the plant.

The structure of the xylem plays a critical role in the movement of water into plants. The xylem is composed of long, narrow tubes that run from the roots to the leaves, and the continuous column of water in the xylem is maintained by the cohesive forces of the water molecules.

Capillarity is the process by which water is drawn up into small tubes, such as the xylem in plants. The combination of capillarity and transpiration creates a continuous column of water in the xylem, which provides a pathway for water to move from the roots to the leaves.

The pathway and movement of water into plants is a critical concept for A-Level Biology students to understand because it highlights the interconnectedness of different plant structures and functions. Additionally, this knowledge provides a foundation for further studies in botany, plant physiology, and related fields.

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