- About Organismal Biology
- Phylogenetic Trees and Geologic Time
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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
- Ion and Water Regulation and Nitrogenous Wastes in Animals
- The Mammalian Kidney: How Nephrons Perform Osmoregulation
- Plant and Animal Responses to the Environment
- 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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Water Uptake and Transport in Vascular Plants
Why Do Plants Need So Much Water?
If water is so important to plant growth and survival, then why would plants waste so much of it? The answer to this question lies in another process vital to plants — photosynthesis. To make sugars, plants must absorb carbon dioxide (CO 2 ) from the atmosphere through small pores in their leaves called stomata (Figure 1). However, when stomata open, water is lost to the atmosphere at a prolific rate relative to the small amount of CO 2 absorbed; across plant species an average of 400 water molecules are lost for each CO 2 molecule gained. The balance between transpiration and photosynthesis forms an essential compromise in the existence of plants; stomata must remain open to build sugars but risk dehydration in the process.
From the Soil into the Plant
Essentially all of the water used by land plants is absorbed from the soil by roots. A root system consists of a complex network of individual roots that vary in age along their length. Roots grow from their tips and initially produce thin and non-woody fine roots. Fine roots are the most permeable portion of a root system, and are thought to have the greatest ability to absorb water, particularly in herbaceous (i.e., non-woody) plants (McCully 1999). Fine roots can be covered by root hairs that significantly increase the absorptive surface area and improve contact between roots and the soil (Figure 2). Some plants also improve water uptake by establishing symbiotic relationships with mycorrhizal fungi, which functionally increase the total absorptive surface area of the root system.
Roots of woody plants form bark as they age, much like the trunks of large trees. While bark formation decreases the permeability of older roots they can still absorb considerable amounts of water (MacFall et al . 1990, Chung & Kramer 1975). This is important for trees and shrubs since woody roots can constitute ~99% of the root surface in some forests (Kramer & Bullock 1966).
Roots have the amazing ability to grow away from dry sites toward wetter patches in the soil — a phenomenon called hydrotropism. Positive hydrotropism occurs when cell elongation is inhibited on the humid side of a root, while elongation on the dry side is unaffected or slightly stimulated resulting in a curvature of the root and growth toward a moist patch (Takahashi 1994). The root cap is most likely the site of hydrosensing; while the exact mechanism of hydrotropism is not known, recent work with the plant model Arabidopsis has shed some light on the mechanism at the molecular level (see Eapen et al . 2005 for more details).
Roots of many woody species have the ability to grow extensively to explore large volumes of soil. Deep roots (>5 m) are found in most environments (Canadell et al . 1996, Schenk & Jackson 2002) allowing plants to access water from permanent water sources at substantial depth (Figure 3). Roots from the Shepard's tree ( Boscia albitrunca ) have been found growing at depths 68 m in the central Kalahari, while those of other woody species can spread laterally up to 50 m on one side of the plant (Schenk & Jackson 2002). Surprisingly, most arid-land plants have very shallow root systems, and the deepest roots consistently occur in climates with strong seasonal precipitation (i.e., Mediterranean and monsoonal climates).
Through the Plant into the Atmosphere
Flow = Δψ / R ,
which is analogous to electron flow in an electrical circuit described by Ohm's law equation:
i = V / R ,
where R is the resistance, i is the current or flow of electrons, and V is the voltage. In the plant system, V is equivalent to the water potential difference driving flow (Δψ) and i is equivalent to the flow of water through/across a plant segment. Using these plant equivalents, the Ohm's law analogy can be used to quantify the hydraulic conductance (i.e., the inverse of hydraulic R ) of individual segments (i.e., roots, stems, leaves) or the whole plant (from soil to atmosphere).
Upon absorption by the root, water first crosses the epidermis and then makes its way toward the center of the root crossing the cortex and endodermis before arriving at the xylem (Figure 4). Along the way, water travels in cell walls (apoplastic pathway) and/or through the inside of cells (cell to cell pathway, C-C) (Steudle 2001). At the endodermis, the apoplastic pathway is blocked by a gasket-like band of suberin — a waterproof substance that seals off the route of water in the apoplast forcing water to cross via the C-C pathway. Because water must cross cell membranes (e.g., in the cortex and at apoplastic barriers), transport efficiency of the C-C pathway is affected by the activity, density, and location of water-specific protein channels embedded in cell membranes (i.e., aquaporins). Much work over the last two decades has demonstrated how aquaporins alter root hydraulic resistance and respond to abiotic stress, but their exact role in bulk water transport is yet unresolved.
Once in the xylem tissue, water moves easily over long distances in these open tubes (Figure 5). There are two kinds of conducting elements (i.e., transport tubes) found in the xylem: 1) tracheids and 2) vessels (Figure 6). Tracheids are smaller than vessels in both diameter and length, and taper at each end. Vessels consist of individual cells, or "vessel elements", stacked end-to-end to form continuous open tubes, which are also called xylem conduits. Vessels have diameters approximately that of a human hair and lengths typically measuring about 5 cm although some plant species contain vessels as long as 10 m. Xylem conduits begin as a series of living cells but as they mature the cells commit suicide (referred to as programmed cell death), undergoing an ordered deconstruction where they lose their cellular contents and form hollow tubes. Along with the water conducting tubes, xylem tissue contains fibers which provide structural support, and living metabolically-active parenchyma cells that are important for storage of carbohydrates, maintenance of flow within a conduit (see details about embolism repair below), and radial transport of water and solutes.
When water reaches the end of a conduit or passes laterally to an adjacent one, it must cross through pits in the conduit cell walls (Figure 6). Bordered pits are cavities in the thick secondary cell walls of both vessels and tracheids that are essential components in the water-transport system of higher plants. The pit membrane, consisting of a modified primary cell wall and middle lamella, lies at the center of each pit, and allows water to pass between xylem conduits while limiting the spread of air bubbles (i.e., embolism) and xylem-dwelling pathogens. Thus, pit membranes function as safety valves in the plant water transport system. Averaged across a wide range of species, pits account for >50% of total xylem hydraulic resistance. The structure of pits varies dramatically across species, with large differences evident in the amount of conduit wall area covered by pits, and in the porosity and thickness of pit membranes (Figure 6).
After traveling from the roots to stems through the xylem, water enters leaves via petiole (i.e., the leaf stalk) xylem that branches off from that in the stem. Petiole xylem leads into the mid-rib (the main thick vein in leaves), which then branch into progressively smaller veins that contain tracheids (Figure 7) and are embedded in the leaf mesophyll. In dicots, minor veins account for the vast majority of total vein length, and the bulk of transpired water is drawn out of minor veins (Sack & Holbrook 2006, Sack & Tyree 2005). Vein arrangement, density, and redundancy are important for distributing water evenly across a leaf, and may buffer the delivery system against damage (i.e., disease lesions, herbivory, air bubble spread). Once water leaves the xylem, it moves across the bundle sheath cells surrounding the veins. It is still unclear the exact path water follows once it passes out of the xylem through the bundle sheath cells and into the mesophyll cells, but is likely dominated by the apoplastic pathway during transpiration (Sack & Holbrook 2005).
Mechanism Driving Water Movement in Plants
Stephen Hales was the first to suggest that water flow in plants is governed by the C-T mechanism; in his 1727 book Hales states "for without perspiration the [water] must stagnate, notwithstanding the sap-vessels are so curiously adapted by their exceeding fineness, to raise [water] to great heights, in a reciprocal proportion to their very minute diameters." More recently, an evaporative flow system based on negative pressure has been reproduced in the lab for the first time by a ‘synthetic tree' (Wheeler & Stroock 2008).
When solute movement is restricted relative to the movement of water (i.e., across semipermeable cell membranes) water moves according to its chemical potential (i.e., the energy state of water) by osmosis — the diffusion of water. Osmosis plays a central role in the movement of water between cells and various compartments within plants. In the absence of transpiration, osmotic forces dominate the movement of water into roots. This manifests as root pressure and guttation — a process commonly seen in lawn grass, where water droplets form at leaf margins in the morning after conditions of low evaporation. Root pressure results when solutes accumulate to a greater concentration in root xylem than other root tissues. The resultant chemical potential gradient drives water influx across the root and into the xylem. No root pressure exists in rapidly transpiring plants, but it has been suggested that in some species root pressure can play a central role in the refilling of non-functional xylem conduits particularly after winter (see an alternative method of refilling described below).
Disruption of Water Movement
Water transport can be disrupted at many points along the SPAC resulting from both biotic and abiotic factors (Figure 8). Root pathogens (both bacteria and fungi) can destroy the absorptive surface area in the soil, and similarly foliar pathogens can eliminate evaporative leaf surfaces, alter stomatal function, or disrupt the integrity of the cuticle. Other organisms (i.e., insects and nematodes) can cause similar disruption of above and below ground plant parts involved in water transport. Biotic factors responsible for ceasing flow in xylem conduits include: pathogenic organisms and their by-products that plug conduits (Figure 8); plant-derived gels and gums produced in response to pathogen invasion; and tyloses, which are outgrowths produced by living plant cells surrounding a vessel to seal it off after wounding or pathogen invasion (Figure 8).
Abiotic factors can be equally disruptive to flow at various points along the water transport pathway. During drought, roots shrink and lose contact with water adhering to soil particles — a process that can also be beneficial by limiting water loss by roots to drying soils (i.e., water can flow in reverse and leak out of roots being pulled by drying soil). Under severe plant dehydration, some pine needle conduits can actually collapse as the xylem tensions increase (Figure 8).
Water moving through plants is considered meta-stable because at a certain point the water column breaks when tension becomes excessive — a phenomenon referred to as cavitation. After cavitation occurs, a gas bubble (i.e., embolism) can form and fill the conduit, effectively blocking water movement. Both sub-zero temperatures and drought can cause embolisms. Freezing can induce embolism because air is forced out of solution when liquid water turns to ice. Drought also induces embolism because as plants become drier tension in the water column increases. There is a critical point where the tension exceeds the pressure required to pull air from an empty conduit to a filled conduit across a pit membrane — this aspiration is known as air seeding (Figure 9). An air seed creates a void in the water, and the tension causes the void to expand and break the continuous column. Air seeding thresholds are set by the maximum pore diameter found in the pit membranes of a given conduit.
Fixing the Problem
Failure to re-establish flow in embolized conduits reduces hydraulic capacity, limits photosynthesis, and results in plant death in extreme cases. Plants can cope with emboli by diverting water around blockages via pits connecting adjacent functional conduits, and by growing new xylem to replace lost hydraulic capacity. Some plants possess the ability to repair breaks in the water columns, but the details of this process in xylem under tension have remained unclear for decades. Brodersen et al . (2010) recently visualized and quantified the refilling process in live grapevines ( Vitis vinifera L.) using high resolution x-ray computed tomography (a type of CAT scan) (Figure 10). Successful vessel refilling was dependent on water influx from living cells surrounding the xylem conduits, where individual water droplets expanded over time, filled vessels, and forced the dissolution of entrapped gas. The capacity of different plants to repair compromised xylem vessels and the mechanisms controlling these repairs are currently being investigated.
References and Recommended Reading
Agrios, G. N. Plant Pathology . New York, NY: Academic Press, 1997.
Beerling, D. J. & Franks, P. J. Plant science: The hidden cost of transpiration. Nature 464, 495-496 (2010).
Brodersen, C. R. et al . The dynamics of embolism repair in xylem: In vivo visualizations using high-resolution computed tomography Plant Physiology 154 , 1088-1095 (2010).
Brodribb, T. J. & Holbrook, N. M. Water stress deforms tracheids peripheral to the leaf vein of a tropical conifer. Plant Physiology 137 , 1139-1146 (2005)
Canadell, J. et al . Maximum rooting depth of vegetation types at the global scale. Oecologia 108, 583-595 (1996).
Choat, B., Cobb, A. R. & Jansen, S. Structure and function of bordered pits: New discoveries and impacts on whole-plant hydraulic function. New Phytologist 177, 608-626 (2008).
Chung, H. H. & Kramer, P. J. Absorption of water and "P through suberized and unsuberized roots of loblolly pine. Canadian Journal of Forest Research 5, 229-235 (1975).
Eapen, D. et al . Hydrotropism: Root growth responses to water. Trends in Plant Science 10, 44-50 (2005).
Hetherington, A. M. & Woodward, F. I. The role of stomata in sensing and driving environmental change. Nature 424, 901-908 (2003).
Holbrook, N. M. & Zwieniecki, M. A. Vascular Transport in Plants . San Diego, CA: Elsevier Academic Press, 2005.
Javot, H. & Maurel, C. The role of aquaporins in root water uptake. Annals of Botany 90, 1-13 (2002).
Kramer, P. J. & Boyer, J. S. Water Relations of Plants and Soils . New York, NY: Academic Press, 1995.
Kramer, P. J. & Bullock, H. C. Seasonal variations in the proportions of suberized and unsuberized roots of trees in relation to the absorption of water. American Journal of Botany 53, 200-204 (1966).
MacFall, J. S., Johnson, G. A. & Kramer, P. J. Observation of a water-depletion region surrounding loblolly pine roots by magnetic resonance imaging. Proceedings of the National Academy of Sciences of the United States of America 87 , 1203-1207 (1990).
McCully, M. E. Roots in Soil: Unearthing the complexities of roots and their rhizospheres. Annual Review of Plant Physiology and Plant Molecular Biology 50, 695-718 (1999).
McDowell, N. G. et al . Mechanisms of plant survival and mortality during drought: Why do some plants survive while others succumb to drought? New Phytologist 178, 719-739 (2008).
Nardini, A., Lo Gullo, M. A. & Salleo, S. Refilling embolized xylem conduits: Is it a matter of phloem unloading? Plant Science 180, 604-611 (2011).
Pittermann, J. et al . Torus-margo pits help conifers compete with angiosperms. Science 310, 1924 (2005).
Sack, L. & Holbrook, N. M. Leaf hydraulics. Annual Review of Plant Biology 57, 361-381 (2006).
Sack, L. & Tyree, M. T. "Leaf hydraulics and its implications in plant structure and function," in Vascular Transport in Plants , eds. N. M. Holbrook & M. A. Zwieniecki. (San Diego, CA: Elsevier Academic Press, 2005) 93-114.
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Transport systems - Plants Transporting water
Multicellular organisms require transport systems to supply their cells and remove waste products. Plants transport substances through xylem and phloem.
Plants require transport systems to move water, dissolved food and other substances around their structures in order to stay alive.
Plants require water for two major reasons:
- For photosynthesis. In most flowering plants this happens in mesophyll cells in the leaves.
- To transport materials, eg minerals close minerals Naturally occurring, inorganic chemical substances. Minerals are necessary for both plant and animal health. .
Water taken up by the roots of a plant is transported through a plant to the leaves where some of it passes into the air. The stages of the process are:
1. Soil to xylem
- Water enters root hair cells close root hair cell A specialised cell that increases the surface area of the root epidermis to improve the uptake of water and minerals. : tiny hairs covering the ends of the smallest roots. They provide a large surface area for the absorption of water by the process of osmosis.
- Water then moves from cell to cell through the root cortex by osmosis down a concentration gradient. This means that each cell has a lower water concentration than the one before it.
- In the centre of the root the water enters the xylem vessels - vein-like tissues that transport water and minerals up a plant.
2. Xylem to leaf to air
Water molecules move up the xylem vessels close xylem vessels Narrow, hollow, dead tubes with lignin, responsible for the transport of water and minerals in plants. to the leaves where they exit and move from cell to cell. Water moves from the xylem vessels into the mesophyll cells where it can be used for photosynthesis.
Some of the water evaporates into the surrounding air spaces inside the leaf and then diffuses out through the stomata close stomata Tiny holes in the epidermis (skin) of a leaf. They control gas exchange by opening and closing and are involved in loss of water from leaves. Singular is stoma. into the surrounding air. The opening and closing of the stomata is controlled by guard cells in the epidermis.
Watch the video below to see how to prepare a leaf slide to investigate stomata under a microscope.
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February 8, 1999
11 min read
How do large trees, such as redwoods, get water from their roots to the leaves?
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Last week we presented a general outline of how trees lift water. Donald J. Merhaut of Monrovia Nursery Company, headquartered in Azusa, Calif., has provided a more detailed reply:
Original answer posted on February 1, 1999
Xylem: Structure, Function, and Importance in Plant Physiology
- Editor Desk
- June 14, 2023
Xylem is a vital component of the plant’s vascular system responsible for the transport of water and nutrients from the roots to other parts of the plant. Composed of specialized cells, including tracheids and vessel elements, xylem plays a crucial role in maintaining plant hydration and providing mechanical support.
Structure of Xylem
The xylem is a complex tissue composed of several types of cells, each with unique structural characteristics. The main cell types in xylem are tracheids and vessel elements. Tracheids are elongated cells with tapered ends and secondary cell walls, while vessel elements are shorter, wider cells with perforated end walls. Both cell types are dead at maturity and contain lignin, a rigid compound that provides structural support to the plant.
Water Transport Mechanism
The primary function of xylem is the upward transport of water from the roots to the rest of the plant. This process, known as transpiration, relies on several physical principles. Water molecules evaporate from the stomata in leaves, creating a negative pressure gradient within the xylem. This negative pressure, known as tension, pulls water upward through the xylem vessels, similar to a chain of straws. Cohesion, the attraction between water molecules, and adhesion, the attraction between water and the xylem cell walls, aid in maintaining a continuous column of water.
Role in Nutrient Transport
In addition to water transport, xylem also plays a crucial role in the movement of dissolved minerals and nutrients throughout the plant. As water is pulled upward through the xylem, dissolved nutrients, such as nitrogen, phosphorus, and potassium, are carried along. This process ensures the efficient distribution of essential elements required for plant growth and development.
Significance in Plant Physiology
Xylem is essential for maintaining the plant’s overall health and survival. It provides structural support, preventing wilting and maintaining turgidity. The continuous water transport through the xylem also helps regulate the temperature of the plant by cooling the leaves through evaporation. Furthermore, xylem contributes to the transport of signaling molecules, hormones, and defense compounds, allowing plants to respond to environmental cues and defend against pathogens.
Xylem is a specialized tissue that forms an integral part of the plant’s vascular system. It facilitates the upward movement of water, nutrients, and other vital substances, playing a fundamental role in plant hydration, nutrition, and overall physiology. Understanding the structure and function of xylem provides valuable insights into the remarkable adaptation and survival strategies of plants.
FAQs about xylem
What is xylem.
Xylem is a plant tissue responsible for transporting water, nutrients, and other substances from the roots to other parts of the plant.
How does xylem work?
Xylem works through a process called transpiration, where water evaporates from the leaves, creating a negative pressure that pulls water upward through the xylem vessels.
What are tracheids and vessel elements?
Tracheids and vessel elements are specialized cells found in xylem. Tracheids are elongated cells with tapered ends, while vessel elements are shorter, wider cells with perforated end walls.
What is the role of xylem in plants?
Xylem plays a vital role in maintaining plant hydration, providing structural support, and facilitating the transport of water, nutrients, and signaling molecules throughout the plant.
Can xylem transport nutrients apart from water?
Yes, xylem also transports dissolved minerals and nutrients, such as nitrogen, phosphorus, and potassium, along with water, ensuring their distribution to various parts of the plant.
How does xylem help plants survive?
Xylem contributes to the overall health and survival of plants by preventing wilting, maintaining turgidity, regulating plant temperature, and aiding in the transport of defense compounds and hormones.
How does xylem differ from phloem?
While xylem transports water and nutrients upward from the roots, phloem is responsible for transporting sugars and other organic compounds produced during photosynthesis throughout the plant.
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How Water Moves Through Plants
Two Environmental Factors That Affect Transpiration
The importance of plants in everyday life cannot be understated. They provide oxygen, food, shelter, shade and countless other functions.
They also contribute to the movement of water through the environment. Plants themselves boast their own unique way of taking in water and releasing it into the atmosphere.
TL;DR (Too Long; Didn't Read)
Plants require water for biological processes. The movement of water through plants involves a pathway from root to stem to leaf, using specialized cells.
Water Transportation in Plants
Water is essential to the life of plants at the most basic levels of metabolism. In order for a plant to access water for biological processes, it needs a system to move water from the ground to different plant parts.
The chief water movement in plants is through osmosis from the roots to the stems to the leaves. How does water transportation in plants occur? Water movement in plants occurs because plants have a special system to draw water in, conduct it through the body of the plant and eventually to release it to the surrounding environment.
In humans, fluids circulate in bodies via the circulatory system of veins, arteries and capillaries. There is also specialized network of tissues that aids the process of nutrient and water movement in plants. These are called xylem and phloem .
What Is Xylem?
Plant roots reach into the soil and seek water and minerals for the plant to grow. Once the roots find water, the water travels up through the plant all the way to its leaves. The plant structure used for this water movement in plants from root to leaf is called xylem.
Xylem is a kind of plant tissue that is made out of dead cells that are stretched out. These cells, named tracheids , possess a tough composition, made of cellulose and the resilient substance lignin . The cells are stacked and form vessels, allowing water to travel with little resistance. Xylem is waterproof and has no cytoplasm in its cells.
Water travels up the plant through the xylem tubes until it reaches mesophyll cells, which are spongy cells that release the water through miniscule pores called stomata . Simultaneously, stomata also allow for carbon dioxide to enter a plant for photosynthesis. Plants possess several stomata on their leaves, particularly on the underside.
Different environmental factors can rapidly trigger stomata to open or close. These include temperature, carbon dioxide concentrate in the leaf, water and light. Stomata close up at night; they also close in response to too much internal carbon dioxide and to prevent too much water loss, depending on the air temperature.
Light triggers them to open. This signals the plant’s guard cells to draw in water. The guard cells’ membranes then pump out hydrogen ions, and potassium ions can enter the cell. Osmotic pressure declines when the potassium builds up, resulting in water attraction to the cell. In hot temperatures, these guard cells do not have as much access to water and can close up.
Air can also fill the xylem’s tracheids. This process, named cavitation , can result in tiny air bubbles that could impede water flow. To avoid this problem, pits in xylem cells allow for water to move while preventing gas bubbles from escaping. The rest of the xylem can continue moving water as usual. At night, when stomata close up, the gas bubble may dissolve into the water again.
Water exits as water vapor from the leaves and evaporates. This process is called transpiration .
What Is Phloem?
In contrast to xylem, phloem cells are living cells. They make up vessels as well, and their main function is to move nutrients throughout the plant. These nutrients include amino acids and sugars.
Over the course of the seasons, for example, sugars may be moved from the roots to the leaves. The process of moving nutrients throughout the plant is called translocation .
Osmosis in Roots
The tips of plant roots contain root hair cells. These are rectangular in shape and have long tails. The root hairs themselves can extend into the soil and absorb water in a process of diffusion called osmosis.
Osmosis in roots leads to water moving into root hair cells. Once water moves into the root hair cells, it can travel throughout the plant. Water first makes its way to the root cortex and passes through the endodermis . Once there, it can access the xylem tubes and allow for water transportation in plants.
There are multiple paths for water’s journey across roots. One method keeps water between cells so that the water does not enter them. In another method, water does cross cell membranes . It can then move out of the membrane to other cells. Yet another method of water movement from the roots involves water passing through cells via junctions between cells called plasmodesmata .
After passing through the root cortex, water moves through the endodermis, or waxy cellular layer. This is a sort of barrier for water and shunts it through endodermal cells like a filter. Then water can access the xylem and proceed toward the plant’s leaves.
Transpiration Stream Definition
People and animals breathe. Plants possess their own process of breathing, but it is called transpiration .
Once water travels through a plant and reaches its leaves, it can eventually release from the leaves via transpiration. You can see evidence of this method of “breathing” by securing a clear plastic bag around a plant’s leaves. Eventually you'll see water droplets in the bag, demonstrating transpiration from the leaves.
The transpiration stream describes the process of water transported from the xylem in a stream from root to leaf. It also includes the method of moving mineral ions around, keeping plants sturdy via water turgor, making sure leaves have enough water for photosynthesis and allowing the water to evaporate to keep leaves cool in warm temperatures.
Effects on Transpiration
When plant transpiration is combined with evaporation from land, this is called evapotranspiration . The transpiration stream results in approximately 10 percent of moisture release into the atmosphere of the Earth.
Plants can lose a significant amount of water through transpiration. Even though it is not a process that can be seen with the naked eye, the effect of water loss is measurable. Even corn can release as much as 4,000 gallons of water in a day. Large hardwood trees can release as much as 40,000 gallons daily.
Rates of transpiration vary depending on the status of the atmosphere around a plant. Weather conditions play a prominent role, but transpiration is also affected by soils and topography.
Temperature alone greatly affects transpiration. In warm weather, and in strong sun, the stomata are triggered to open and release water vapor. However, in cold weather, the opposite situation occurs, and the stomata will close up.
The dryness of the air directly affects transpiration rates. If the weather is humid and the air full of moisture, a plant is less likely to release as much water via transpiration. However, in dry conditions, plants readily transpire. Even the movement of wind can increase transpiration.
Different plants adapt to different growth environments, including in their rates of transpiration. In arid climates such as deserts, some plants can hold onto water better, such as succulents or cacti.
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About the Author
J. Dianne Dotson is a science writer with a degree in zoology/ecology and evolutionary biology. She spent nine years working in laboratory and clinical research. A lifelong writer, Dianne is also a content manager and science fiction and fantasy novelist. Dianne features science as well as writing topics on her website, jdiannedotson.com.
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Which Organs or Parts of the Plant Are Involved in Transpiration?
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How does water move in plants?
Have you ever wondered how plants are able to pull water out of the ground? It’s not like they have a heart to pump water around or even a digestion system to extract the water from the soil!
In fact, water movement in plants doesn’t rely on energetically expensive biological pumps or even magic. It relies on some pretty basic physical principles operating within unique plant structures, and anyone can understand it. We’ll see how in this home experiment.
3 glass or plastic cups (sturdy enough not to tip over) 300 g room-temperature water Food coloring Metric scale Fan Medium-to-large sealable plastic box (tall enough to fit an upright stalk of celery inside) 2 small squares of plastic wrap 2 stalks celery, leaves attached
- Pick two celery stalks that they have similar amounts of leaves. (Hint: If you can’t find celery with leaves attached in your grocery store, buy a head of celery. The small inner stalks usually still contain leaves.) Cut off the bases of the stalks so that they are roughly the same height.
- Place one glass on the scale and tare it: press the “zero” button so that the cup registers as “0 g”. Fill with 150 g of room-temperature water; gently place two drops of food coloring in the water; stir. Be careful not to spill any colored water! Repeat with the second glass.
- Place one celery stalk in each glass, leaf–end at the top.
- Wrap one square of the plastic wrap around the top of each glass and the celery stalk. This is to prevent any colored water evaporating into the air directly from the glass.
- Fill the bottom of the plastic box with roughly one inch of room-temperature water. Place one of the cups with the celery stalk inside the box and seal the lid to create a humid, closed environment.
- Place both the boxed celery and the naked celery in front of a fan, and turn it on the lowest setting. Record the time: _________
- Wait 24 hours.
- Boxed celery:___________________________________________
- Naked celery:___________________________________________
- Place the third glass cup on the scale, and tare it again so that the scale reads “0 g”.
- Boxed celery:____________
- Naked celery:____________
- Celery cross-section:____________________________
How does water move up the stalk?
Although plants don’t have circulatory systems like animals, they do have something quite similar—a network of small tubes called xylem , used for carrying water.
Xylem is composed of long, hollow tubes formed by overlapping cells. As these cells grow, they stretch out and elongate, die, and leave behind hollow cavities that are all interconnected to form one long tube. Plants contain many xylem vessels stretching from the roots to the tips of the leaves, just like a series of drinking straws. When you sliced the celery in half and saw colored dots in the cross-section of the stalk, you were actually looking at the xylem vessels!
Xylem works within some basic physical principles to bring water from the ground up into the rest of the plant. The whole process starts out in the leaves: when the plant is photosynthesizing, it opens tiny holes in the underside of the leaf called stomata . The plant does this so that carbon dioxide can flow in, but it also has a downside: water also diffuses out of the stomata at the same time, drying out the inside of the leaf ever so slightly.
As the plant dries out from the leaves, it brings more water in from the xylem due to some interesting chemical properties. Water is a polar molecule, meaning that it’s slightly “sticky”—it forms temporary hydrogen bonds with itself. This creates cohesion ; small quantities of water will tend to stick together rather than scattering and spreading everywhere (think of dew drops on grass). Water also sticks to the inside of small tubes due to a property called capillary action . These two properties allow the water to travel in one unbroken column through the xylem from the roots to the leaves.
What factors affect how water moves through the plant?
Water moves through plants thanks to a few basic principles, but none of these can work without the first step in the process: water loss from the leaves. This process, called transpiration , happens faster when humidity is low, such as on a hot, windy day. This causes water to evaporate quickly, so the plant needs to suck up more water from the ground (or from the cup) to catch up!
When you put the celery stalk inside the plastic box with water, it increased the humidity in the box, so the celery didn’t lose very much water from the leaves. On the flip side, when you placed the naked celery stalk in front of the fan, it was losing a lot of water! It needed to catch up, so it sucked up more water, and food coloring with it.
When you measured the amount of water left in the glasses at the end of the experiment, you found that the naked celery actually did suck up more water. And, in case you didn’t believe the numbers, you could actually observe that the naked celery had a lot more food coloring within its leaves.
Normally, we can’t see transpiration and water transport happening within plants, but rest assured: as long as it’s above freezing, this process is always happening on a mass scale all over the world!
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Lindsay graduated with a master’s degree in wildlife biology and conservation from the University of Alaska Fairbanks. She also spent her time in Alaska racing sled dogs, and studying caribou and how well they are able to digest nutrients from their foods. Now, she enjoys sampling fine craft beers in Fort Collins, Colorado, knitting, and helping to inspire people to learn more about wildlife, nature, and science in general.
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Water Movement Through Xylem
September 8, 2016 By Janice VanCleave
Do Plants Suck Up Water?
The cartoon diagram shows a flower using a straw to drink water from an underground stream. I’ve never seen a flower drinking through a straw, but water underground is pulled to the surface of plant leaves and flower petals in much the same way. Instead of a straw, plants have tube-like structures from the roots to the leaves and flower petals. These tubes are called xylem.
Xylem tubes are very small. Water moves up small tubes because the water molecules are attracted to the cellulose chemical in the walls of the plant tubes. This attraction between unlike molecules is called adhesion. Water molecules also have a strong attraction for each other, which is called cohesion. In the diagram, animated diagrams of water molecules are shown. The molecules walking up the walls of the narrow tube are pulling water molecules up the center of the tube. When you look at the surface of water in a thin tube, you will notice that the water moves up the sides and sinks down in the center. This downward dip in the water level is called the meniscus. The movement of the water through tubes or spaces because adhesion and cohesion is called capillary action.
Capillary action moves water up narrow tubes, such as xylem tubes in plants.
Capillary action occurs due to the cohesive force of attraction between water molecules and the adhesive force of attraction between water molecules and the molecules in the walls of the tube.
Due to gravity, capillary action can only raise water a short distance up the xylem tubes in plants. Another process called transpiration pulls the water to the top of the xylem where it moves in to the cells of leaves, stems, flowers, and other organs.
The process of traspiration is similar to the process of drinking through a straw. Both processes raise the height of a liquid in a tube. When you suck the air out of a straw, you decrease air pressure pushing down on the liquid inside the straw.Air pressure outside the straw is still pushing down on the surface of the liquid in the glass. Thus, air pressure is pushing the liquid up the straw to your mouth.
In plants, liquid water moves from the roots to small openings in the surface of leaves and flower petals called stomata. At the surface, liquid water evaporates when a stoma is open. Evaporation of the water creates a low pressure at the top of the xylem tube. The higher pressure on the water at the bottom of the xylem pushes the water up. As long as there is available water for the roots, the xylem remains filled with water.
Nutrients in the soil that dissolve in water are carried from a plant’s roots up xylem tubes to different parts of the plant, such as leaves and flowers.
You now know why and how, but this is all secondary research . While secondary research, done by others, can be accepted as true, encourage kids to investigate in order to “see it for themselves.” In other words, experimentally prove that nutrients dissolved in water move from the roots through a plant’s stem to its leaves. This is called primary research.
Problem: Prove that nutrients dissolved in water move through xylem in a plant’s stem.
tap water measuring cup 2-1 pint glass jars red food color 2 innerstems from a stalk of celery (Pull the outter stems off and use the inner pale celery stems and leaves) lighter colored inner stems.) * knife (adult use only) cutting board magnifying lens camera (optional)
Preparation of Materials by an Adult Pour 1-cup (250 mL) of tap water into each glass jar. • Add 20 drops of food color to the water in one of the glasses. • Place the celery stems on the cutting board and with a knife cut across the bottom of each stem. Cut each stem again, cutting off a small slice.
Tip: Scissors can crush the tiny vessels in the stem, thus affecting the transport of water.
• Without delay, stand 1 celery stem in the colored water and the remaining celery stem in the glass of uncolored water.
Procedure 1. After placing the celery stems in the two glasses, kids should observe and record the appearance of the surface of the celery slices. Look for the xylem tubes around the outside edge of each slice. The diagram shows the location of these tubes.
Recording observations: Colored drawings with word descriptions. In addition to student descriptions, use a camera to take a picture of the two celery slices.
2. Observe and record the color of the leaves on each celery stalk. As before, make a colored drawing.
Hypothesis-1: Ask kids to predict what they think the leaves on the celergy stalks will look like in three days. They are to record their hypothesis by drawing colored pictures.
3. After 24 hours, observe and record the appearance of the leaves. Lift each stalk and observe and record the bottom cut surface. Note: Having kids to make colored drawings is not busy work. Instead, the time it takes to draw and color their observations helps to keep their mind focused on the topic being studied. As they make their drawings on the first day, 24 hours after the experiment started, ask them to give ideas about how the color moved from the colored water in the glass to the leaves? Older kids can do research to discover on their own how the coloring is transferred. Provide clues by giving them this list of terms: capillary action, transpiration, xylem
4. Conclusion: Describe the results. State your hypothesis and whether the experimental results did or didnot support your hypothesis.
Science Fair Project Ideas
Follow the previous procedure using white long stemmed carnations instead of celery stalks. Hold the flower stems under water and cut at an angle. Measure the time it takes for the color to reach the end of the flower petals. This will be the rate of petal coloration or the flow rate of water through xylem.
The results of the experiment will provide data used as a control for a science project. Your science project experiment will have only one of the variables in the experiment changed. Examples of variables that can be changed are:
- Length of stem
- Using a different white flower, such as a white daisy.
- Use different concentrations of the food coloring–higher and lower.
- Multicolored flower–block part of the stem with Vaseline to determine is only part of the flower can be colored.
- Use distilled water instead of tap water
- Method of cutting the stems–Angle of cut, cutting underwater, cutting in the air.
Examples of a Science Project Experiment Question 1. What affect does the length of the stem have on the rate of petal coloration? 2. What affect would different types of flowers have on the rate of petal coloration?
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36.2.1: Movement of Water and Minerals in the Xylem
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- Outline the movement of water and minerals in the xylem
Movement of Water and Minerals in the Xylem
Most plants obtain the water and minerals they need through their roots. The path taken is: soil -> roots -> stems -> leaves. The minerals (e.g., K+, Ca2+) travel dissolved in the water (often accompanied by various organic molecules supplied by root cells). Water and minerals enter the root by separate paths which eventually converge in the stele, or central vascular bundle in roots.
Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf, or atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. However, this value varies greatly depending on the vapor pressure deficit, which can be insignificant at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata close and transpiration stops, the water is held in the stem and leaf by the cohesion of water molecules to each other as well as the adhesion of water to the cell walls of the xylem vessels and tracheids. This is called the cohesion–tension theory of sap ascent.
The cohesion-tension theory explains how water moves up through the xylem. Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to the internal air space and the water on the surface of the cells evaporates into the air spaces. This decreases the thin film on the surface of the mesophyll cells. The decrease creates a greater tension on the water in the mesophyll cells, thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that form via a process called cavitation. The formation of gas bubbles in the xylem is detrimental since it interrupts the continuous stream of water from the base to the top of the plant, causing a break (embolism) in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water in a continuous column, increasing the number of cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.
Control of Transpiration
Transpiration is a passive process: metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled. The atmosphere to which the leaf is exposed drives transpiration, but it also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.
Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.
Plants have evolved over time to adapt to their local environment and reduce transpiration. Desert plant (xerophytes) and plants that grow on other plants ( epiphytes ) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.
Xerophytes and epiphytes often have a thick covering of trichomes or stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and other substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants.
- The cohesion – tension theory of sap ascent explains how how water is pulled up from the roots to the top of the plant.
- Evaporation from mesophyll cells in the leaves produces a negative water potential gradient that causes water and minerals to move upwards from the roots through the xylem.
- Gas bubbles in the xylem can interrupt the flow of water in the plant, so they must be reduced through small perforations between vessel elements.
- Transpiration is controlled by the opening and closing of stomata in response to environmental cues.
- Stomata must open for photosynthesis and respiration, but when stomata are open, water vapor is lost to the external environment, increasing the rate of transpiration.
- Desert plants and plants with limited water access prevent transpiration and excess water loss by utilizing a thicker cuticle, trichomes, or multiple epidermal layers.
- cohesion–tension theory of sap ascent : explains the process of water flow upwards (against the force of gravity) through the xylem of plants
- cavitation : the formation, in a fluid, of vapor bubbles that can interrupt water flow through the plant
- trichome : a hair- or scale-like extension of the epidermis of a plant
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How does water travel through a plant: the fascinating journey.
Are you curious about how water travels through a plant? It’s a fascinating process that allows plants to grow and thrive. Plants depend on water to carry out crucial functions in their lives, such as photosynthesis. Understanding how water travels through a plant can help us appreciate the intricacies of the natural world.
The Journey Begins: Water’s Entry into the Plant
Water enters a plant through its roots, which absorb water from the soil. Roots are equipped with tiny root hairs that increase the surface area of the root, helping it absorb more water. The water is then transported to the stem, which acts as a conduit for water movement.
Roots: The Primary Entry Point
The roots of a plant are the primary points of entry for water. The roots are covered in tiny root hairs that increase the surface area of the root, enabling it to absorb more water and nutrients from the surrounding soil. These root hairs are incredibly sensitive and can detect even small changes in moisture and nutrient levels in the soil. This allows the root to adjust its absorption rate and ensure it is getting enough water and nutrients to thrive.
The Stem: A Conduit for Water Movement
Once the water is absorbed by the roots, it is transported through the stem of the plant. The stem acts as a conduit for water movement and is equipped with specialized cells that help move water upwards to the leaves. These cells are called xylem and are arranged in long, tube-like structures that run the length of the stem. Xylem cells are incredibly strong and can withstand high levels of pressure, which helps move water against gravity.
The Journey Continues: How Water Moves through the Plant
Now that we understand how water enters a plant, let’s take a closer look at how it moves through the plant to reach its final destination in the leaves.
The Mechanism of Water Movement
The movement of water through a plant is driven by a process called transpiration. Transpiration occurs when water evaporates from the leaves of the plant, creating a negative pressure that pulls water up through the stem and into the leaves. This process is similar to the way water is drawn up into a straw when you suck on it.
The Role of the Leaves in Water Movement
Leaves play a crucial role in water movement through a plant. The leaves are equipped with tiny pores called stomata that allow for the exchange of gases. When the stomata open to allow for gas exchange, water vapor is released into the air. This release of water vapor creates a negative pressure that draws water up from the roots and through the stem to the leaves.
The Importance of Xylem Cells in Water Movement
Xylem cells play a critical role in water movement through a plant. These cells are arranged in long, tube-like structures that run the length of the stem, allowing water to be transported upwards to the leaves. The walls of xylem cells are incredibly strong and can withstand high levels of pressure, which helps move water against gravity. The cohesive and adhesive properties of water also play a crucial role in water movement through xylem cells.
The Advantages and Disadvantages of Water Movement in Plants
While water movement is crucial for plant growth and survival, it also comes with its own set of advantages and disadvantages. Let’s take a closer look at these below.
Water movement through a plant is vital for its growth and survival. It helps transport nutrients, minerals, and other essential substances throughout the plant, ensuring that all parts of the plant receive the necessary resources to thrive. Water movement also helps cool the plant and protect it from overheating, particularly on hot days. Additionally, water movement helps to maintain the turgor pressure of the plant cells, which is necessary for maintaining the structural integrity of the plant.
Despite its many advantages, water movement through a plant also comes with some disadvantages. One of the most significant disadvantages is the risk of water loss through transpiration. As water is transported from the roots to the leaves, some of it is lost through evaporation, which can be a significant issue in hot and dry environments. Additionally, if the plant absorbs water that is contaminated with harmful substances such as heavy metals or pesticides, these substances can be transported throughout the plant, potentially causing harm.
Frequently Asked Questions
What is the role of root hairs in water absorption.
Root hairs increase the surface area of the root, allowing it to absorb more water and nutrients from the surrounding soil.
What is the mechanism of water movement through a plant?
Water movement through a plant is driven by a process called transpiration, which occurs when water evaporates from the leaves of the plant, creating a negative pressure that pulls water up through the stem and into the leaves.
What is the role of xylem cells in water movement?
Xylem cells are specialized cells that help move water upwards to the leaves. These cells are arranged in long, tube-like structures that run the length of the stem.
What is the importance of turgor pressure in plants?
Turgor pressure is necessary for maintaining the structural integrity of the plant. It helps the plant to maintain its shape and prevents it from collapsing under its weight.
What are the advantages of water movement in plants?
Water movement in plants ensures that all parts of the plant receive the necessary resources to thrive, helps cool the plant, and maintains the turgor pressure of plant cells.
What are the disadvantages of water movement in plants?
The risk of water loss through transpiration and the potential transport of harmful substances are the most significant disadvantages of water movement in plants.
How does water get from the roots to the leaves?
Water is transported from the roots to the leaves through specialized cells called xylem.
What is transpiration?
Transpiration is the process by which water evaporates from the leaves of a plant, creating a negative pressure that pulls water up through the stem and into the leaves.
How do plants cool themselves?
Plants cool themselves by releasing water vapor through stomata in their leaves.
Why are xylem cells important?
Xylem cells are critical for helping transport water and minerals throughout the plant.
What is the role of the stem in water movement?
The stem acts as a conduit for water movement and is equipped with specialized cells called xylem that help move water upwards to the leaves.
What are stomata?
Stomata are small pores on the surface of a plant’s leaves that allow for the exchange of gases.
How do plants absorb water from the soil?
Plants absorb water from the soil through their roots, which are covered in tiny root hairs that increase the surface area of the root, allowing it to absorb more water and nutrients from the surrounding soil.
Why is water movement important for plant growth?
Water movement is vital for plant growth because it helps transport nutrients, minerals, and other essential substances throughout the plant, ensuring that all parts of the plant receive the necessary resources to thrive.
What is the risk of water loss through transpiration?
The risk of water loss through transpiration is that it can cause dehydration and plant wilting, particularly in hot and dry environments.
Conclusion: A Journey Worth Understanding
Water movement through plants is a fascinating process that is essential for their growth and survival. Plants have evolved intricate mechanisms for transporting water from their roots to their leaves, ensuring that all parts of the plant receive the necessary resources to thrive. While water movement comes with its own set of challenges, understanding the complexity of this process can help us appreciate the natural world around us.
If you want to learn more about water movement in plants, consider taking a botany or plant biology class at your local university. You can also read more about this topic in scientific journals or textbooks.
The information presented in this article is intended for educational purposes only and should not be used as a substitute for professional medical or scientific advice. Always consult a qualified expert before making changes to your plant care routine or attempting to diagnose plant health issues.
Watch Video:How Does Water Travel Through a Plant: The Fascinating Journey
Science in School
How water travels up trees teach article.
Author(s): Clare van der Willigen
Why do giant redwoods grow so tall and then stop? It all has to do with how high water can travel up their branches.
The redwoods of northern California, Sequoia sempervirens , are the tallest trees in the world and can grow to heights of more than 110 m. However, what finally limits their height is still debated.
The most popular theory is the ‘hydraulic limitation hypothesis’ ( Ryan & Yoder, 1997 ), which suggests that as trees grow taller, it becomes more difficult to supply water to their leaves. This hydraulic limitation results in reduced transpiration and less photo-synthesis, causing reduced growth.
In tall trees, water supply can be limited by two factors: distance and gravity. Tall trees have a longer path- way of transport tissue – known as xylem – which increases the difficulty of water to travel, something we call hydraulic resistance. In addition, not only is the xylem pathway long, but the trees are tall and the water has to overcome gravity. Increased force is necessary to pull the water up to the highest leaves. This situation differs from a long hosepipe lying along the ground: it would have high resistance due to its length, but not the additional difficulty of being upright.
Fast-growing trees often have shorter life spans. To achieve their rapid growth, pioneer trees have wider xylem vessels, increasing their hydraulic efficiency but also increasing the risk of embolisms (air locks). Air locks in xylem vessels prevent water from being able to travel through them.
In contrast, very tall trees are often very long-lived. It is thought that this is partly because they are more likely to adopt a safe hydraulic design, with multiple narrow xylem vessels instead of a few wider ones.
This increased safety is counteracted by a decreased efficiency of water transport, which consequently limits growth rates. Tree height, therefore, may also be limited by the safety versus efficiency trade-off in xylem function ( Burgess et al, 2006 ).
The following two activities explore the trade-off that plants make between being efficient with water transport and having a safe design. Both activities can be adapted for students aged 15–18 with a wide range of abilities, but you should assess whether the students can perform all of the experiments or whether it is safer for the teacher to do the cutting. Each activity will take about 50 minutes.
Estimating maximum xylem vessel lengths
Comparing the lengths of the xylem vessel will allow students to predict their relative resistance to water flow.
- Selection of recently cut branches from a tree or shrub, including any leaves or side branches, up to 2 m in initial length. If the experiment is to be performed within a few hours of harvesting, keep the plant material in a plastic bag to avoid excessive water loss.
- Rubber/silicon tubing
- Cable ties or jubilee clips
- Sharp pruning shears or scissors
- 60 cm 3 syringes
- Large basin of tap water
- Cut a length of branch over 1 m, making sure the cut is clean and the end of the branch is not crushed. The branch will be much longer than the xylem vessels inside.
- Attach a 60 cm3 syringe, filled with air, to the proximal (wider) end of the branch using silicon tubing and cable ties as required.
- Pressurise the air in the syringe and branch by compressing the volume of air in the syringe by about half (e.g. from 60 cm3 of air to 30 cm3). This pressure must be maintained through steps 4–6.
- Hold the distal end of the branch under water.
- Use a hand lens to see if a steady stream of bubbles can be detected from the distal end of the branch.
- Progressively cut the distal end of the branch back by about 1 to 5 cm at a time, making sure each time that the end of the branch is not crushed and has a clean cut.
- When a stream of bubbles is observed, the length of the branch gives an approximate maximum length of the xylem vessels.
Students should be warned about the safety precautions necessary when using sharp objects. See also the general safety note .
Students could compare maximum xylem vessel lengths in a variety of different plants or different parts (roots, main and side branches) of the same plant. It is common for fast-growing plants to have longer xylem vessels and therefore fewer breaks between xylems. Can the students suggest why this might be?
About what happens
A branch contains several xylem vessels linked together. Between the xylem vessels are perforated wall plates. The fewer of these divisions there are, the lower the resistance and the faster water can travel.
A detailed study of vessel length in Chrysanthemum stems ( Nijsse et al, 2001 ) and in a wide range of shrubs and trees ( Jacobsen et al, 2012 ) can be used for cross-reference.
Measuring xylem hydraulic conductivity
Measurements of xylem hydraulic properties show how well plants can supply water to their leaves. It is possible to measure the hydraulic conductance of stems, branches and roots in the classroom with some simple, inexpensive equipment. To measure hydraulic conductivity, the branch length should be longer than the mean length of the xylem vessels (see previous activity).
- Selection of recently cut branches from a tree or shrub investigated in the previous experiment. Ensure that the pieces are longer than the longest xylem vessels measured. If the experiment is to be performed within a few hours of harvesting, keep the plant material in a plastic bag to avoid excessive water loss.
- Sharp secateurs, scissors or a large scalpel
- Chopping board
- Large basin of water
- Reservoir of degassed, distilled water in a container with a tap at the bottom. Degas the water by boiling it or using a vacuum pump for approximately 1h until all the gas has been expelled from the water. Air bubbles in water that is not degassed may block the xylem vessels.
- Hydrochloric acid
- 1 cm 3 pipette (a pipette with a 90o bend is most effective. A standard glass pipette can be bent in a very hot flame)
- 50 cm 3 plastic beaker
- Retort stand and clamp
- Balance (precision of at least 0.01 g)
- Stop watch or stop clock
1. Set up the apparatus as illustrated in the diagram above:
a Add hydrochloric acid to the degassed, distilled water to give a final concentration of 0.01 M. For example, add 0.5 cm 3 of 0.1 M HCl to 5 dm 3 degassed, distilled water. Hydrochloric acid prevents the long-term decline in conductance by reducing microbial growth in the xylem.
Remember to always add acid to water, not water to acid.
b Fill the reservoir with the acidified water. Insert a piece of tubing, sealed at one end with a bung, into the top of the reservoir. The open tubing ensures a constant pressure head because even if the water level drops, the effective height of the reservoir will remain the same.
c To the tap of the reservoir, add some tubing, fill with water from the reservoir, seal the open end and place into the large basin of water.
d Close the tap.
e Submerge the proximal end of the branch in the large basin of water. This is the end of the branch that was nearest to the main stem of the plant.
f Cut approximately 3 cm off the proximal end of the branch under water to ensure that no air pockets remain in the xylem. Shave off the end of the cut using a sharp blade.
g Connect the newly cut end of the branch to the water-filled tubing attached to the reservoir under water. If the bark is very rough, it can be stripped back prior to connection. A water-tight seal should be achieved using cable ties or jubilee clips if necessary, however do not over-tighten and compress the xylem vessels.
h Submerge the other end of the branch in the tub of water.
i Cut approximately 3 cm off the end of the branch under water to ensure that no air pockets remain in the xylem. Shave off the end of the cut using a sharp blade.
j Measure and record the length of the branch. Ensure it is longer than the maximum xylem vessel length (see previous experience).
k Connect the bent pipette to more rubber tubing and sub- merge into the basin of water.
l Connect the newly cut end of the branch to the water-filled tubing attached to the pipette as above.
m Fill the 50 cm 3 beaker with water and place on the pan balance.
n Take the branch end and pipette out of the basin of water with the end of the pipette sealed.
o Place the end of the pipette in the 50 cm 3 beaker on the balance.
p Use the retort stand and clamp to hold the pipette in place. The tip of the pipette should not lean on the bottom of the beaker, but should be below the water level. This ensures that as the water drips through the branch, there is a smooth increase in the mass of water in the beaker.
2. Open the tap from the reservoir.
3. Measure the mass of water every 30s for 3 min.
4. Measure the effective height of the reservoir using the metre rule. This is the height from the bottom of the open tubing in the reservoir to the proximal end of the branch.
Students should be warned about the safety precautions necessary when using sharp objects and acids. See also the general safety note .
Hydraulic conductivity is measured as the mass of water flowing through the system per unit time per unit pressure gradient (Tyree & Ewers, 1991). The hydraulic conductivity of the branch, kh, is calculated using the following formula:
k h = (flow rate x branch length)/hydrostatic pressure head
where the flow rate is measured in kilograms per second (kg/s); branch length in metres (m); and the pressure head in megaPascals (MPa). To calculate the flow rate, plot the mass of water (in kg) measured in step 3 against time (in s). The flow rate will be the gradient of the line of best fit (in kg/s). See table 2 and figure 1 for a worked example.
The hydrostatic pressure head is found by multiplying the effective height of the reservoir, measured in step 4, with the density of liquid and the acceleration due to gravity. The density of the acidified water can be assumed to be 1000 kg/m 3 (at room temperature) and a value of 9.81 m/s 2 can be used for acceleration due to gravity. Thus, with an effective height of the reservoir of 1m, the hydrostatic pressure head would be 1000 x 9.81 x 1 = 9810 Pa or 0.00981 MPa.
Remember, maximum hydraulic conductivity is only achieved if none of the xylem vessels are embolised (filled with air). To try to prevent this, branches can be flushed with water at a pressure of approximately 200 kPa for 20 min before measuring conductivity. Alternatively, ensure that branches are selected from well-watered trees and that the leaves are covered in a large plastic bag prior to measurement.
Investigations on different levels of water stress on the same, or similar, branches would give an indication of plants that are more vulnerable to cavitation, or air bubbles. Hydraulic conductivity can change de- pending on environmental conditions, and the same species of plant that have adapted to different environments could be tested in the laboratory or in the field. Compare branch cross-sections of different diameter or those supporting different leaf areas.
Students could observe the effect on hydraulic conductivity of changing the branch length and relate this to the height of the plant. They could also investigate the effect on the flow rate of changing the height of the reservoir. The reservoir height (pushing force) could be considered as equal, but opposite, to the pulling force created by the low water potential in xylem vessels.
Did you know?
Xylems are essentially porous filters, and scientists think that they could be used to filter water and make it safe to drink. Earlier this year, a group at the Massachusetts Institute of Technology in the USA showed that a 3cm 3 piece of pine branch could act as a filter and remove 99.9 % of bacteria from water, at a rate of several litres a day. The technique isn’t perfect yet: viruses and chemical contamination can’t be stopped by twigs, but the work by Boutilier et al. (2014) suggests a cheap way to purify water in developing countries.
- Boutilier M.S.H., Lee J., Chambers V., Venkatesh V., Karnik R. (2014) Water Filtration Using Plant Xylem. PLoS ONE 9(2) : e89934
- Burgess S.S., Pittermann J., Dawson T.E. (2006) Hydraulic efficiency and safety of branch xylem increases with height in Sequoia sempervirens (D. Don) crowns . Plant, Cell and Environment 29(2) : 229-239. doi: 10.1111/j.1365-3040.2005.01415.x
- Jacobsen A.L., Pratt R.B., Tobin M.F., Hacke U.G., Ewers F.W. (2012) A global analysis of xylem vessel lengths in woody plants . American Journal of Botany 99 : 1583-1591 doi: 10.3732/ajb.1200140
- Nijsse J., van der Heijden G.W.A.M., van Leperen W., Keijzer C.J., van Meeteren U. (2001) Xylem hydraulic conductivity to conduit dimensions along chrysanthemum stems. Journal of Experimental Botany 52 : 319-327 doi: 10.1093/jexbot/52.355.319
- Ryan M.G., and Yoder B.J. (1997) Hydraulic limits to tree height and tree growth . BioScience 47(4) : 235-242 doi: 10.2307/1313077
- Tyree M.T., Ewers F.W. (1991) The hydraulic architecture of trees and other woody plants. New Phytologist 19 : 345-360
- Koch G.W., Sillett S.C., Jennings G.M., Davis S.D. (2004) The limits to tree height . Nature 428 : 851–854. doi: 10.1038/nature02417; freely available
Clare van der Willigen has an MSc and a PhD in plant physiology from the University of Cape Town, South Africa. Following postdoctoral research on water stress in plants and aquaporins, she pursued her passion for teaching. She has worked in South Africa, France, The Netherlands and the United Kingdom, and is currently a senior examiner and teacher of many years’ experience.
The article describes two experiments that can easily be conducted in science classrooms or laboratories to study water movement in plants.
Although the procedures are easy to carry out, the concepts and knowledge that are explored aren’t so simple, but are appropriate for upper secondary-school students (aged 15-18). In my experience, there are not very many procedures that consider water movement for this age group, so many science teachers will welcome this article.
There are also relevant opportunities for interdisciplinary teaching involving mathematics in particular. It would be quite interesting to use this experiment as a starting point to introduce students to the development of a database and subsequent statistical analysis (not too complex). For example, students could estimate maximum xylem vessel lengths and measure xylem hydraulic conductivity of different plants and at different times (e.g. winter vs. summer). This database could be extended from year to year with other students. Such a strategy could help students to understand science as a collaborative activity – not only between different disciplines but also between different ‘generations’ of scientists.
Betina Lopes, Portugal
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The Three Pathways of Water Movement in Tree Roots (With Diagram)
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Tree roots absorb water, but have you ever wondered how the water is absorbed? There are three pathways the water can take. These are the apoplast, symplastic, or transmembrane pathways.
At a basic level, water travels into a tree’s fine roots. The water must then travel inside the root to the xylem . There are three different paths water can take to get to the xylem. This article will cover each pathway in depth.
Before reading this article, you may want to brush up on your root anatomy knowledge. This article will cover each piece of anatomy in-depth. And this article is an extensive guide to roots as a whole, with a brief look at their anatomy.
Osmosis refers to the diffusion of water through a semipermeable membrane NOT the diffusion of information through impermeable skulls
The correct pathway of water movement in plant roots is
Soil Water => Root Hair Cell => Cortical Cells => Passage Cells => Pericycle Xylem
When using the apoplast pathway, water will travel between cells. There will be tiny gaps between each cell. Water will move between these gaps.
Water must navigate these gaps as they move deeper inside the tree root. Eventually, the water will reach the endodermis. The endodermis is the inner skin of the tree root.
The endodermis controls how much water and nutrients move further inside the cell. The Casparian Strip helps with this process.
The Casparian Strip is a corky substance that prevents foreign material from getting deeper into the root. Something can only pass through the strip if that something is inside a cell.
So, to get past the Casparian Strip, water and nutrients need to enter a cell. Substances can enter a cell by moving through the cell’s plasma membrane.
After passing through the endodermis, the water can reach the tree’s vascular system.
Between plant cells, there is something called the plasmodesmata . The plasmodesmata are like tiny bridge that connects cells. This bridge connects the cytoplasm of one cell to the cytoplasm of neighbouring cells.
The cytoplasm is the gel-like liquid inside a cell.
The plasma membrane covers all cells, and the plasmodesmata is an extension of this membrane. So, one big, continuous plasma membrane surrounds all the connected cells.
The symplast or symplasm is the continuum of communication cytoplasm, which is created by the intracellular connections. From the Principles of Soil and Plant Water Relations, 2005:
So, the symplast is the connected plasma membrane.
Water must cross the plasma membrane to travel the symplastic pathway. Once the molecules are inside the membrane, they can travel through the cell’s cytoplasm.
The water molecules will reach the end of the cell. From here, the molecules can use the plasmodesmata (bridge) to cross to the next cell.
Water molecules will continue this process until they enter the xylem. Remember, water needs to be inside a cell to cross the endodermis. When using the symplastic pathway, the molecules are already inside a cell.
The transmembrane pathway is a kind of mix between the two previous paths.
When molecules travel along the transmembrane pathway, the molecules enter and exit each cell by crossing the plasma membranes.
Water molecules will travel into each cell. But, rather than use the plasmodesmata to enter the next cell, the molecules will hop through the cell walls.
So, water will travel through the symplast by moving through interconnected cytoplasms. But, the water will also travel through the apoplast by moving through cell walls and gaps between cell walls.
Thus, the transmembrane pathway combines both apoplast and symplastic paths.
The Water Potential
The water potential of a system is the measure of the potential energy in water. In other words, it’s a measure of movement in the water – specifically, movement between two different systems.
Water potential is denoted by the Greek letter psi (Ψ) and is expressed in units of pressure called megapascals (MPa).
The water potential for pure water is 0 MPa (even though pure water can have all sorts of energy that we don’t need to account for), but for the root, stem or leaf of a plant, it would be higher than 0 MPa because it has already lost some of its pressure through contact with soil.
Ψsystem = Ψs + Ψp [ Ψs = solute potential, and Ψp = pressure potential ]
The Role of Aquaporins in Water Movement
Aquaporins are integral membrane proteins that serve as water channels in living cells. They are found in all organisms and are essential for a variety of biological processes, including cell growth, development, and homeostasis. Aquaporins facilitate water transport across cell membranes and help regulate the hydration of cells. There are three types of aquaporins: (PMA),(TLA) and (ERLAs)
- plasma membrane aquaporins (PMA)
- tonoplast-localized aquaporins (TLA)
- endoplasmic reticulum-localized aquaporins (ERLAs)
PMA proteins are found in the plasma membrane and play a role in regulating cell hydration and volume. TLA proteins are located in the tonoplast, which is the membrane that surrounds vacuoles. ERLAs are found in the endoplasmic reticulum and play a role in regulating calcium homeostasis. Aquaporins are involved in a variety of water transport processes, including osmosis, transcellular transport, and ion channels. Osmosis is the diffusion of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration. Transcellular transport is the movement of water across cell membranes through specialized proteins called transporters. Ion channels allow for the movement of ions, such as calcium, sodium, and potassium, across cell membranes. Aquaporins play an important role in plant physiology. They are involved in processes such as water uptake, stomatal opening and closing, and transpiration. Aquaporins are also integral to the functioning of animal organs, such as the lungs, kidneys, and intestines.
In these organs, aquaporins facilitate the diffusion of water along osmotic gradients and regulate the passage of ions across the cell membrane. Without aquaporins, many essential physiological processes would not be possible.
Factors Affecting the Rate of Water Movement in Roots
The factors that affect the rate of water movement in roots are the surface area of the root, the level of water in the soil, and the type of soil.
The surface area of the root is important because it determines how much water can be absorbed by the root.
The level of water in the soil is important because it affects the amount of water that is available to the roots. The type of soil is important because it affects the rate at which water moves through the soil.
Water plays a critical role in the life of a plant, and it is essential for its survival. Roots are an important part of this process, as they act as conduits to bring water up from the soil into the rest of the tree or shrub.
Understanding how water moves through roots can help us better understand the physiology and ecology of plants, and why certain species may be more resistant to drought or flooding than others.
With further exploration into pathways of water movements in roots, we will have even greater insight into these complex processes.
Hi, I created this site to share my views on various aspects of trees to help beginners and semi-experts with pruning, planting, and cultivating trees, shrubs, and woody plants and their health assessments. I hope you find it useful.