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6: Roots and the Movement of Water - How is water moved through a plant? - Biology

6: Roots and the Movement of Water - How is water moved through a plant? - Biology


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Learning Objectives

Content Objectives

  • Understand what influences the weird behavior of water, particularly tonicity, adhesion, and cohesion
  • Learn how transpiration drives the movement of water through a plant and which environmental factors influence transpiration rates

Skill Objectives

  • Predict the movement of water in different situations and explain why it moved the way it did
  • Draw and describe the flow of water as it moves through a plant from the soil environment to the atmosphere

Hydrological Cycle &ndash the Movement of Water from Ocean

Water covers about 73 per cent of the earth’s surface entirely, and it is a major constituent of the lithosphere and the atmosphere.

Water is also the most abundant component of the protoplasm, and therefore it is the major requirement of all living organisms.

In metabolism, water is the only source of hydrogen and one of the several sources of oxygen. The major pools of water occur in the oceans – 97.3% of the total for the biosphere (Bener and Bemer, 1987), the ice of the polar ice – caps and glaciers (2.06%), groundwater (0.67%) and in rivers and lakes (0.01%).

Water or Hydrological cycle is defined as the movement of water from ocean, by evaporation, to atmosphere and by precipitation to land and back via river flow, to ocean (Begon et al, 1996). It is a balanced continuous process of evaporation, transpiration, precipitation, surface runoff and ground water movements. Each year about 507 Tm 3 (one tetra cubic metre = one million cubic metres) are evaporated and the same quantity of water is precipitated over the whole surface of the earth, including the land and oceans.

The amount of water, which flows from the land to the sea in rivers and streams is about 44.5 Tm 3 per year and is available for the needs of man. Solar energy evaporates water from the soil, ground surface, vegetation, water surfaces and oceans into the atmosphere (Fig. 6.8). Subsequent cooling and condensation of water vapour at higher altitudes produces clouds and precipitation as rain, hail or snow returns the water to the hydrosphere. Natural evaporation from the seas exceeds precipitation by rain into the oceans by about 9%. The latter is eventually moved as water vapour over to the land surface, and so balances the hydrological cycle and provides additional water for man’s needs. Thus water as rain, hail or snow is precipitated over land and water surfaces (Fig. 6.8).

Water on land surfaces eventually percolates into the soil as soil or ground water. Within the ground there is always a natural water table or water level. The soil below the water table level is saturated, and water is sustained by the underlying clay and rock strata. However, ground water does not remain static but moves in various directions. It can move up above the water table by capillary, thus providing a continuous supply of water to the surface layers of soil, where it is absorbed by plant roots during the dry season.

Some ground water moves by filtering through the interstices of the soil or substratum in any direction. Water also emerges from the ground at lower altitude levels, and flows into streams, rivers and lakes, and helps to provide man’s water supplies.

In some regions of the earth water percolates through the porous rocks and forms underground reservoirs. These underground water bearing layers of porous rock, situated above impermeable rock strata, are called aquifers. They are important source of water, which can be extracted by sinking wells or boreholes and pumping it to the surface.

All the water precipitated on land does not percolate into the soil. Surface water or run-off flows into streams, rivers, lakes and catchment storage areas or reservoirs. Some of the ground water that is absorbed by plants passes out the leaf surfaces as water vapour by transpiration. This is an important process helping in conduction of water and dissolved minerals throughout the plant. Thus, through the natural process of hydrological cycle, water is exchanged between die atmosphere (troposphere), land, sea, all living plants and animals, and industrial plants.


Movement of Water in Roots: 3 Pathways (With Diagram)

This allows direct cytoplasm-to-cytoplasm flow of water and other nutrients along concentration gradients. In particular, it is used in the root systems to bring in nutrients from soil.

It moves these solutes from epidermis cells through the cortex into the endodermis and eventually the pericycle, where it can be moved into the xylem for long distance transport.

Specifically there are three pathways of water passage from root hairs to xylem inside the root:

(i) Apoplast Pathway:

Here water passes from root hair to xylem through the walls of intervening cells without crossing any membrane or cytoplasm. This pathway provides the least resistance to movement of water. However, it is interrupted by the presence of impermeable lignosuberin casparian strips in the walls of endodermal cells.

(ii) Non-vacuolar Symplast Pathway:

Water passes from cell to cell through their protoplasm. It does not enter cell vacuoles. The cytoplasm of the adjacent cells are connected through through bridges called plasmodesmata. For entering into symplast, water has to pass through plasma lemma (cell membrane) at-least at one place. It is also called trans-membrane pathway,

(iii) Vacuolar Symplast Pathway:

Here individual root cells function as tiny osmotic systems. The first osmotic system operates at root hair cell. It absorbs water from soil through osmosis. The pathway is cell wall, plasma-lemma, cytoplasm, tonoplast and central vacuole. From root hair eel, water passes into vacuoles of adjacent cortical cell through osmosis.

The process continues till water reaches the xylem parenchyma cells. However, vacuolar pathway provides a lot of resistance. It is used only when individual cells are to pick up water. Otherwise, the usual pathway of water movement is partly apoplastic and partly symplastic.


Movement of Water and Minerals in the Xylem

Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential (higher Gibbs free energy) to an area of lower total water potential. Gibbs free energy is the energy associated with a chemical reaction that can be used to do work. This is expressed as ΔΨ.

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–atmosphere interface it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion–tension theory of sap ascent.

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 this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells (Figure 4), 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. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.

Art Connection

–0.2 MPA in the root cells to

–0.6 MPa in the stem and from

–1.5 MPa in the highest leaves, to

–100 MPa in the atmosphere." />

Figure 4. The cohesion–tension theory of sap ascent is shown. Evaporation from the mesophyll cells produces a negative water potential gradient that causes water to move upwards from the roots through the xylem.

Which of the following statements is false?

  1. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the xylem. Transpiration draws water from the leaf.
  2. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the phloem. Transpiration draws water from the leaf.
  3. Water potential decreases from the roots to the top of the plant.
  4. Water enters the plants through root hairs and exits through stoma.

Transpiration—the loss of water vapor to the atmosphere through stomata—is a passive process, meaning that 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.

Control of Transpiration

The atmosphere to which the leaf is exposed drives transpiration, but 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 (Figure 5). 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.

Figure 5. Plants are suited to their local environment. (a) Xerophytes, like this prickly pear cactus (Opuntia sp.) and (b) epiphytes such as this tropical Aeschynanthus perrottetii have adapted to very limited water resources. The leaves of a prickly pear are modified into spines, which lowers the surface-to-volume ratio and reduces water loss. Photosynthesis takes place in the stem, which also stores water. (b) A. perottetii leaves have a waxy cuticle that prevents water loss. (c) Goldenrod (Solidago sp.) is a mesophyte, well suited for moderate environments. (d) Hydrophytes, like this fragrant water lily (Nymphaea odorata), are adapted to thrive in aquatic environments. (credit a: modification of work by Jon Sullivan credit b: modification of work by L. Shyamal/Wikimedia Commons credit c: modification of work by Huw Williams credit d: modification of work by Jason Hollinger)

Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and 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.


Capillary Action and Water

Plants and trees couldn't thrive without capillary action. Capillary action helps bring water up into the roots. With the help of adhesion and cohesion, water can work it's way all the way up to the branches and leaves. Read on to learn more about how this movement of water takes place.

Capillary Action

Capillary Action . in Action! Without capillary action, the water level in all tubes would be the same. Smaller diameter tubes have more relative surface area inside the tube, allowing capillary action to pull water up higher than in the larger diameter tubes

Even if you've never heard of capillary action, it is still important in your life. Capillary action is important for moving water (and all of the things that are dissolved in it) around. It is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension.

Capillary action occurs because water is sticky, thanks to the forces of cohesion (water molecules like to stay close together) and adhesion (water molecules are attracted and stick to other substances). Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. The surface tension acts to hold the surface intact. Capillary action occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules. The height to which capillary action will take water in a uniform circular tube (picture to right) is limited by surface tension and, of course, gravity.

Not only does water tend to stick together in a drop, it sticks to glass, cloth, organic tissues, soil, and, luckily, to the fibers in a paper towel. Dip a paper towel into a glass of water and the water will "climb" onto the paper towel. In fact, it will keep going up the towel until the pull of gravity is too much for it to overcome.

Capillary action is all around us every day

We know that no one will ever spill a bottle of Cherry Berry Go drink on the Mona Lisa, but if it happened, capillary action and paper towels would be there to help clean up the mess.

  • If you dip a paper towel in water, you will see it "magically" climb up the towel, appearing to ignore gravity. You are seeing capillary action in action, and "climbing up" is about right - the water molecules climb up the towel and drag other water molecules along. (Obviously, Mona Lisa is a big fan of capillary action!)
  • Plants and trees couldn't thrive without capillary action. Plants put down roots into the soil which are capable of carrying water from the soil up into the plant. Water, which contains dissolved nutrients, gets inside the roots and starts climbing up the plant tissue. Capillary action helps bring water up into the roots. But capillary action can only "pull" water up a small distance, after which it cannot overcome gravity. To get water up to all the branches and leaves, the forces of adhesion and cohesion go to work in the plant's xylem to move water to the furthest leaf.
  • Capillary action is also essential for the drainage of constantly produced tear fluid from the eye. Two tiny-diameter tubes, the lacrimal ducts, are present in the inner corner of the eyelid these ducts secrete tears into the eye. (Source: Wikipedia)
  • Maybe you've used a fountain pen . or maybe your parents or grandparents did. The ink moves from a reservoir in the body of the pen down to the tip and into the paper (which is composed of tiny paper fibers and air spaces between them), and not just turning into a blob. Of course gravity is responsible for the ink moving "downhill" to the pen tip, but capillary action is needed to keep the ink flowing onto the paper.

The proof is in the pudding . I mean, in the celery

You can see capillary action in action (although slowly) by doing an experiment where you place the bottom of a celery stalk in a glass of water with food coloring and watch for the movement of the color to the top leaves of the celery. You might want to use a piece of celery that has begun to whither, as it is in need of a quick drink. It can take a few days, but, as these pictures show, the colored water is "drawn" upward, against the pull of gravity. This effect happens because, in plants, water molecules move through narrow tubes that are called capillaries (or xylem).

Do you think you know a lot about water properties?
Take our interactive water-properties true/false quiz and test your water knowledge.


Movement of water and nutrients in the roots

Negative water potential continues to drive movement once water (and minerals) are inside the root Ψ of the soil is much higher than Ψ or the root, and Ψ of the cortex (ground tissue) is much higher than Ψ of the stele (location of the root vascular tissue). Once water has been absorbed by a root hair, it moves through the ground tissue 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.

By Jackacon, vectorised by Smartse – Apoplast and symplast pathways.gif, Public Domain, https://commons.wikimedia.org/w/index.php?curid=12063412

Water and minerals that move into a cell through the plasma membrane has been “filtered” as they pass 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 exclusive to 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 ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded.

This image was added after the IKE was open:

Water transport via symplastic and apoplastic routes. By Kelvinsong – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25917225

The cross section of a dicot root has an X-shaped structure at its center. The X is made up of many xylem cells. Phloem cells fill the space between the X. A ring of cells called the pericycle surrounds the xylem and phloem. The outer edge of the pericycle is called the endodermis. A thick layer of cortex tissue surrounds the pericycle. The cortex is enclosed in a layer of cells called the epidermis. The monocot root is similar to a dicot root, but the center of the root is filled with pith. The phloem cells form a ring around the pith. Round clusters of xylem cells are embedded in the phloem, symmetrically arranged around the central pith. The outer pericycle, endodermis, cortex and epidermis are the same in the dicot root. Image credit: OpenStax Biology.


Transport pathways in sugar translocation

Before we get into the details of how the pressure flow model works, let’s first revisit some of the transport pathways we’ve previously discussed:

  1. Diffusion occurs when molecules move from an area of high concentration to an area of low concentration. Diffusion does not require energy because the molecules move down their concentration gradient (from areas of high to low concentration).
  2. Proton pumps use energy from ATP to create electrochemical gradients, with a high concentration of protons on one side of a plasma membrane. This electrochemical gradient can then be used as a source of energy to move other molecules against their concentration gradients via co-transporters.
  3. Co-transporters are channels that perform a type of secondary active (energy-requiring) transport. Co-transporters move two molecules at the same time: one molecule is transported along (“down”) its concentration gradient, which releases energy that is used to transport the other molecule against its concentration gradient.
    1. Symporters are a type of co-transporter that transports two molecules in the same direction both into the cell, or both out of the cell.
    2. Antiporters are a type of co-transporter that transports two molecules in opposite directions one into the cell, and the other out of the cell.

    Symporters move two molecules in the same direction Antiporters move two molecules in opposite directions. Image credit: Khan Academy, https://www.khanacademy.org/science/biology/membranes-and-transport/active-transport/a/active-transportImage modified from OpenStax Biology. Original image by Lupask/Wikimedia Commons.

    Each of these transport pathways play a role in the pressure flow model for phloem transport.


    Transport in Plants

    Transportation is a process in which a substance either synthesized or absorbed in one part of the body reaches another. In living things, many substances such as food, gases, minerals salts, hormones, and waste products have to be transported from one part of the body to another. Plants require inorganic substances like nitrogen, phosphorous, magnesium, manganese, sodium, etc. Soil provides such substances. In this article, we shall study transport in plants by osmosis and diffusion.

    Plants and animals have a system of transporting substances throughout their body. In plants, water is the medium of transport. In higher plants (vascular plants) xylem conducts the water whereas the phloem conducts the food. All parts of the body are connected to these tissues.

    To demonstrate Transportation in Plants:

    Take a young herbaceous plant (e.g. sunflower) with roots and leaves. Wash the soil off the roots. Place it in a jar of water containing stains like eosin or red ink. After two days, cut sections of the stem and the root and observe it under a microscope. We see rings of red colour. This experiment shows transport of materials in plants.

    In plants, water transport minerals salts through special tubes called xylem. Plants have root hairs on their primary and secondary roots. Plants absorb water and minerals salt from the soil with the help of root hairs. They absorb water by the process of osmosis. Osmosis is the movement of water molecules (solvent) from a lower concentration solution to a higher concentration solution through a semi permeable membrane. The concentration of the water molecules is lower in the root hair than in the soil. So the water moves into the root hairs through osmosis. Thus the cell of root hairs become turgid and exert pressure on the adjacent cells. This pressure is called root pressure.

    The water and mineral diffuse from one layer to the next layer of cells and eventually reaches the xylem tubes in the centre of the root. Under the effect of root pressure, water and minerals reach xylem and continuously push forward. This root pressure is sufficient to lift water up in shrubs, small plants and small trees.

    Demonstration of Osmosis ( Abbe Nollet Experiment):

    A thistle funnel with a narrow long stem and wide mouth was taken. A semipermeable membrane is tied tightly around the wide mouth of the funnel. Now the stem of the funnel is filled with a sugar solution to a certain level. Then the thistle funnel is dipped in a beaker containing water with the help of an iron stand such that the broad mouth remains immersed in the water. The apparatus are left undisturbed for some time.

    After some time it is observed that there is an increase in the level of sugar solution in the thistle funnel. This shows that there is at the flow of water (solvent molecules) into the solution through the semipermeable membrane. To stop this flow of solvent molecules into solution, we have to apply excess sufficient pressure from stem side of the thistle funnel on the solution. This excess pressure is the osmotic pressure.

    Demonstration of Osmosis (Experiment – 2):

    An egg has a shell made up of calcium carbonate. Below shell, there is a layer of semipermeable membrane. The calcium carbonate can be dissolved in acid like HCl. Place an egg in a beaker containing acid. Make sure that the egg does not float in acid. Allow it to remain in the acid until the shell completely dissolves. Leave the egg with the dissolved shell in water overnight. Remove the egg the next day and see its size. We will find that the egg will be swollen considerably.

    If you carefully pierce the membrane of the egg with a needle, a jet of water will shoot into the air. It shows that the water from acid solution gets into the egg through a semipermeable membrane.

    Mineral and salts are absorbed by a process called diffusion. Diffusion is the movement of solute molecules of a solid, a gas or liquid from a region of high concentration region to that of low concentration region.

    Difference Between Osmosis and Diffusion:

    In osmosis, solvent moves from lower concentration region to higher concentration region. In diffusion, solute particles move from higher concentration region to lower concentration region.

    Ascent of Sap:

    After reaching xylem tubes water is conducted from the root to the stem and then to the leaves. The continuous columns of water in the xylem tubes do not break due to strong cohesive force between the water molecules. The upward movement of cell sap containing water and minerals salts in a plant is called ascent of sap.

    Transpiration:

    Water evaporates from the leaves through the opening on them called stomata There are two guard cells at the opening of stomata. These cells control opening and closing of stomata.

    The loss of water from the aerial parts of a plant is called transpiration. When transpiration takes place, the leaves lose water and become less turgid, or less swollen. They then absorb water from the xylem tubes in the veins. They pull water from the stem, which in turn pulls water from the roots. This process is called transpirational pull. Transpirational pull works due to the formation of a continuous column of water in the xylem.

    The significance of Transpiration:

    Minerals salts absorbed by the roots are taken along through this column of water during the ascent of sap and the essential elements needed by the plant reach the leaves and every part of the plant.

    When the temperature is high or the atmosphere is dry or when there is an air current, the rate of transpiration becomes high. If there is no sufficient water in the soil, the leaves become less turgid. Transpiration pull is very important for big plants. By this method, water and minerals reach the different part of the body of the big plant. In the night the rate of transpiration is low, hence in night water and mineral transportation takes place by root pressure.

    Translocation:

    Glucose (sugar) is made in the leaves during the day in presence of sunlight by the process called photosynthesis. This food is required to be transported to other parts of the plant. This transportation of food material takes place through a special tube like tissues phloem in upward and downward direction.

    During day glucose is formed and it is formed is converted into starch and stored in the leaves. As the day advances and manufacture of glucose (food) take place, the starch grains become more and more abundant. About the middle of the afternoon, the starch content reaches its maximum.

    At night, due to the absence of sunlight plants do not make food. Then the stored starch in the cells of the leaves gets converted back into glucose, which is soluble in water. This sugar solution and other substances move from the leaves to other parts of the plant all through the night. This movement of food substance from leaves to the different parts of the body in the phloem is called translocation.

    Before the daybreak, the food-making cells in the leaves are cleared of stored food and they become ready for food manufacture.


    How does Water Reach the Top of the Tree from the Roots?

    Without water, life would not be possible on earth. You already know that we, human beings, cannot survive without water. Whenever we feel thirsty, we drink water. Our body uses this water to regulate body temperature and to maintain other bodily functions. No living being can live without water.

    Not only us, but plants, too, need water to survive. If you don’t water the plant properly, you will see that plant becomes weak and dies after few days.

    Image Credit (left to right): coolgarden.me

    But, how do plants intake water? We drink water from a glass or a bottle. We store water in pots to be used for various purposes. But how do plants get water? You know that plants can’t drink water like us. Then, what do they do? Their process of consumption of water is unique. They have developed an effective system of water absorption, transportation to the body, storing and utilisation.

    Do you know from where plants get water and how do they absorb it? Let’s discuss this.

    We know that water is stored under the ground. And we also know that roots of a plant grow underground and hold them erect on the ground. Roots go inside the ground in search of water and nutrition stored in the soil and absorbs this water by its roots. Have you ever thought how?

    When we drink lemonade from a bottle, we suck the lemonade by a straw or pipe it is pulled upward through the pipe and lemonade reaches to our mouth. In the case plants, roots absorb the water and nutrients from the soil. This water and nutrients reach the whole body, branches and leaves of the plants. But how does this happen? This happens because there is a unique mechanism which helps plants to take up the water and nutrition from the soil to the leaves.

    To understand this mechanism, first, we have to discuss the factor that enables the suction of water and nutrients by the roots from the soil.

    In this respect, the leaves of the plant play a significant role. What is that? We know that there are enumerable tiny pores present in the leaves, mostly on the underside of the leaves. These are called stomata. Water from the plant evaporates as vapour through these tiny pores e.g. stomata. This is called transpiration, which is similar to the way you sweat in the summer. But, can you say why the transpiration is so important? This is because, firstly, the transpiration makes the plants cool, in the same way sweat cools your body down, and another is that, this process enables the plant to pull the water, that is absorbed by the roots, up to the leaves. The speed of absorption of water by the roots depends on the speed of transpiration by the leaves.

    Water from the soil is absorbed by root hair, present in the roots. Now, what is root hair? Root hairs are very thin hair-like growth just above the root tips. There are millions of root hair on the roots of a plant. Root hair helps to increase the surface area of the root. This enables the roots to absorb more water.

    Image Credit (left to right): wikipedia.org

    Root hair contains a solution called cell sap. The concentration of root sap is higher than the concentration of soil water. Root hair has thin membrane that allows absorption of water from the soil. The process by which soil water is absorbed is called osmosis.

    Image Credit (left to right): Nuts.com. BestHerbalHealth.com

    Now, I am sure you will want to know, what is osmosis? Osmosis is the process of carrying fluid through a semipermeable membrane from less concentrated to more concentrated fluid. Let’s try to understand this with the help of an example. If you soak raisins in water, after a few hours, you will see that they swell up. How does this happen? This is because the concentration of solution inside raisin is higher than that in water. So when raisins are kept in bowl of water, water starts to move inside the raisins through outer skin of the raisin. This process called osmosis

    The same thing happens in case of absorption of water by root hair. The concentration of fluid inside the root hair, i.e. cell sap, is higher than the concentration of water in the soil. By the process of osmosis, water from the soil moves inside the root hair through the thin cell membrane.

    Plants have excellent water transportation system. They have a huge network of tube-like channels. These are made up of two types of tissues – xylem and phloem tissue. Plants use these tube-like tissues to transport water and nutrients to the different parts of a plant. These pathways of tissues start in the roots. From there these extend up along the trunk of the plant, and then spread into the branches and then, further up to the leaves. Phloem tissues run alongside the xylem tissues.

    Now the question is, what causes the water to rise up? There are two forces that combine to move the water upwards through these tubes: transpiration and root pressure.

    Transpiration creates a pump-like action in leaves. When water vapour escapes the leaves, by the process of transpiration, it creates a vacuum, or need, for more water. To replace this lost water, a pulling action of water starts in the xylem tissues present in the leaves. This pulling action extends to the rest of the xylem tissues up to the roots. This is like as we suck lemonade by a straw. This process produces a flow of water and minerals from the roots to the leaves. As I have already mentioned, the speed of this upward water flow directly depends on the speed of transpiration.

    Root pressure occurs when water flows into the roots through osmosis due to differences in the concentrated solutes between the soil and roots. This higher pressure in the roots exerts a slight upward force on the fluid column. Combination of these two forces are enough to pull the water to the leaves.

    Thus we can see that plants have devised a distinctive yet simple technique to absorb the water nutrients that they need from the soil.


    Examining water uptake by the stem

    To examine water uptake by the stem.

    Apparatus

    • water
    • food colouring dye (available at supermarket)
    • white flower on a stem, e.g. Impatiens, carnation or chrysanthemum
    • scissors
    • two jars, cups or measuring cylinders
    • plastic tray
    • sticky tape

    Method

    1. Fill one jar with plain water, and one with water containing several drops of food colouring dye.
    2. Take the flower and carefully cut the stem lengthwise, either part way up the stem or right up to the base of the flower (try both, the results will be different!)
    3. Put one half of the stem into the jar containing plain water and one half of the stem into the jar containing food colouring dye. To make it easier to insert the stalks without breaking them, it helps to wedge paper underneath the jars so that you can tilt them towards each other. Tape the jars or cylinders down onto a tray so that they do not fall over.
    4. Observe the flowers after a few hours and the next day, and note where the dye ends up in the flower head. You can leave the flowers up to a week but be sure to make sure that they have enough water.

    Variation: Instead of using one cylinder with water and one with food dye, use two different colour food dyes (e.g. blue and red). At first the flower will show two separate colours, but as time goes by the whole flower will show both dyes. This is because water can move sideways between xylem vessels through openings along their length. The ability of water to move laterally between vessels is useful for when air becomes trapped in a vessel, causing a blockage. If you cut the stem right up to the base of the flower, this will limit movement between the xylem vessels.

    Variation: Try using celery stalks with leaves. Cut open the celery stalk (cross-section) and you will see darker-coloured little holes/ spots. These are the vessels.

    Results

    Record your observations and results

    Conclusions

    What did you conclude from this experiment?

    An example of this experiment with photographs can be found at:

    Investigation: To examine water uptake by the stem

    In this investigation learners are to write down their observations. The learners should observe that the flowers show the red or blue colour of the food colouring in their petals, especially in the veins. The celery stalk should have dark red / blue spots, where the xylem in the vascular bundles has been stained by the dye in the food colouring.

    CONCLUSIONS

    The class can then discuss their observations and conclusions, particularly if learners used different plants. Learners should conclude that the water in plants is transported by xylem in the veins. Xylem vessels branch throughout the flower petals, starting off as fairly large bundles of xylem vessels at the base of the petal and becoming finer bundles as it gets to the edges of the petals.

    Movement of manufactured food (ESG7W)

    Movement of manufactured food

    This section deals with how sugars are transported from the leaves to the rest of the plant via specialised phloem cells.

    Revise the concept of photosynthesis and that plants use carbon dioxide and water to manufacture glucose and oxygen is the waste product. Sunlight and enzymes are necessary for photosynthesis to occur. Once the food is manufactured in the leaves it needs to be distributed to the entire plant so that the glucose can be used by each cell for respiration (manufacture energy).

    Learners to understand that the glucose is manufactured mainly in the palisade cells and then passes into the phloem. Transport of food material from leaves to other parts of the plant is called translocation. This food may be stored in roots, stems or fruit.

    Plants use sunlight, carbon dioxide and water to manufacture glucose, yielding oxygen as a by-product. Sunlight or radiant light is captured by the green pigment chlorophyll inside of chloroplasts to provide the energy for photosynthesis to occur. Once the food is manufactured in the leaves it needs to be distributed to the entire plant so that the glucose can be used by each cell for respiration and some of the photosynthetic products are then stored for later use.

    The glucose is manufactured mainly in the palisade cells where there are more chloroplasts, and then passes into the phloem. Plants usually transport food in the form of the sugar sucrose because it is less reactive than glucose. Sucrose is transported to where it is needed in the the plant via phloem sap, and may be stored in roots, stems or fruit. Transport of food material from leaves to other parts of the plant is called translocation. Understanding the phloem structure is important to understanding how it transports food.

    Figure 5.28: Aphids feeding on phloem sap which is rich in glucose.

    How the phloem functions

    While the transport of water is usually unidirectional in xylem (upward or lateral), the movement of sugars in the phloem is multi-directional, and occurs by active transport, an energy-dependent process. Sucrose is actively transported against a concentration gradient into sieve-tube elements. The sieve-tube elements have no nuclei but the adjacent companion cells do. Companion cells are closely associated with sieve tubes and carry out all the cellular functions of the sieve tubes.

    The cytoplasm of sieve tubes and companion cells is connected through numerous channels called plasmodesmata. These cytoplasmic connections allow the companion cells to regulate the content and activity of the sieve tube cytoplasm. The companion cells also help load the sieve tube with sugar and the other metabolic products that they transport throughout the plant. This lowers the water potential of the sieve-tube element, causing water to move in by osmosis, creating a pressure that pushes the sap down the tube. The metabolising cells of the plant actively transport sugars out of sieve-tube elements, producing exactly the opposite effect. The diagram below illustrates how the overall process works.

    Figure 5.29: Diagram showing movement in the xylem and phloem vessels. Water movement is upwards in the xylem and lateral into and out of the phloem. Lateral movement also occurs into and out of the companion cells accompanying the phloem vessel.


    9.2.10 Outline four adaptations of xerophytes that help to reduce transpiration.

    Any four of the following:

    1) Reduced surface area of the plant - reduced leaves such as spines in cacti (modified leaves)

    2) Thick waxy cuticle covering the epidermis

    3) Reduced numbers of stomata

    4) Water storage tissues in roots, leaves and stems

    5) CAM physiology - Stomata open during the evening/night instead of during the day (when the temperature is at its highest) as the transpiration rate will be lower during cooler hours.


    Watch the video: Roots Manuva - Movements (July 2022).


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