Classroom Techniques to Illustrate Water Transport in Plants

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1 Classroom Techniques to Illustrate Water Transport in Plants Author(s): Mohamed Lakrim Source: The American Biology Teacher, 75(8): Published By: National Association of Biology Teachers URL: BioOne ( is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne s Terms of Use, available at Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.

HOW TO DO IT Classroom Techniques to Illustrate Water Transport in Plants MOHAMED LAKRIM ABSTRACT The transport of water in plants is among the most difficult and challenging concepts to explain to

2 HOW TO DO IT Classroom Techniques to Illustrate Water Transport in Plants MOHAMED LAKRIM ABSTRACT The transport of water in plants is among the most difficult and challenging concepts to explain to students. It is even more difficult for students enrolled in an introductory general biology course. An easy approach is needed to demonstrate this complex concept. I describe visual and pedagogical examples that can be performed quickly and easily during class to assist students in their understanding of transport in plants. Key Words: Water transport; plant; visual learning; practical examples. Water is not only a component of living organisms, but plays a tremendous role in many metabolic processes within and outside the cell. Plants use water to transport nutrients and to maintain structure through turgidity, and water supplies the electrons and protons needed for the light reaction during photosynthesis. However, most of the water that travels through plants will evaporate. It is estimated that more than 99% of water just travels through plants and is lost as vapor in a process called transpiration (the loss of water by conversion from liquid to a gaseous state); the remaining 1% is used for other purposes, such as photosynthesis (Raven et al., 2005). Water is absorbed through the root hairs, travels through the cortex, and crosses the endodermis (Casparian strip) before reaching the xylem, where it generates root pressure, pushing xylem sap, water, and dissolved minerals. The xylem conducts water through its cells, mainly the tracheid and vessel elements, to the various compartments of the plant, and most of it will eventually escape through the stomata, openings regulated by guard cells through transpiration. This process has been widely investigated (Schrock, 1982; Ford, 1998; Evert et al., 2005; Hodson & Acuff, 2006). Water also moves through phloem to the developing parts of the plant and circulates from the phloem back to the xylem (Raven et al., 2005). Plants use water to transport nutrients and to maintain structure through turgidity. This brief description seems simple, but the mechanism of plant water transport involves complex biophysical and chemical concepts. The most classical explanations for the transport of water and minerals found in textbooks (Raven et al., 2005) are based on cohesion tension theory of water molecules. Following root pressure, capillary action involving adhesion to the tiny cells of xylem adds to the movement of water. The water tension, and negative pressure created by evaporation at the surface of the leaf, generates a transpiration pull and forms a continuous column of water. The water movement continues in two ways: apoplastic, along the cell wall, and symplastic, through the cytoplasm. The results of exams, quizzes, and reports have shown that students find the concepts related to water movement in plants troublesome. The difficulty in understanding seems to be associated with the lack of connection to more evident life experiences. Lectures and laboratory experiences are currently improved by introducing new technologies such as animations and video clips (Reece et al., 2009). The efficiency of these classroom enhancements depends on the instructor s ability to use them appropriately in order to develop student learning. The visuals are very useful in attracting and motivating students, provided that the instructor follows some basic rules, such as Cook s (2012) seven suggestions for helping students get the most out of visuals. There are always ways to demonstrate difficult concepts so that students are able to better comprehend biological concepts. Visualization and the use of practical, real-life examples add meaning so that students are better able to conceptualize principles. Here, I introduce a pedagogical approach using practical, visual examples designed to help students understand the complex topic of water transport in plants. These examples are easy to use, inexpensive, fast, and effective, and some of them can be conducted in the lecture room. The American Biology Teacher, Vol. 75, No. 8, pages ISSN , electronic ISSN by National Association of Biology Teachers. All rights reserved. Request permission to photocopy or reproduce article content at the University of California Press s Rights and Permissions Web site at DOI: /abt THE AMERICAN BIOLOGY TEACHER VOLUME 75, NO. 8, OCTOBER 2013

Demonstration 1: Cohesion Theory of Water Molecules By passing a sharp object such as a knife or key through water, students are able to see that the water can be dragged and stay attached because of

Demonstration 2: Adhesion of Water to the Cell Wall & Apoplastic Movement of Water The apoplastic movement of water can be observed by using a piece of paper and a small amount of water dropped on a

3 Demonstration 1: Cohesion Theory of Water Molecules By passing a sharp object such as a knife or key through water, students are able to see that the water can be dragged and stay attached because of the constant forming and reforming of hydrogen bonds between water molecules (Figure 1). Demonstration 2: Adhesion of Water to the Cell Wall & Apoplastic Movement of Water The apoplastic movement of water can be observed by using a piece of paper and a small amount of water dropped on a hard surface. Because paper is made of cellulose, it mimics the cell wall, which also contains cellulose. Water attaches by adsorption and moves through and up the paper as shown in Figure 2. Demonstration 3: The Contribution of Adhesion & Cohesion to the Formation of a Water Column Adhesion cohesion can be demonstrated by dipping a strip of paper (that is cellulose) into a container, such as a small beaker, containing water or a solution of methylene blue. Water will attach by adhesion to the cellulose, and other molecules of water will follow by cohesion. The movement of water will be ascending, additionally carrying other molecules of water with dissolved substances or minerals (Figure 3). Figure 1. A small quantity of water is dropped on a solid surface, such as a tabletop. (A) An office key is passed through the drop. (B) The water cannot be cut because the bonds between the water molecules are reformed as soon as the instrument is passed. Figure 2. (A) A piece of paper towel is dipped in the small quantity of water. (B) Within a few seconds, one can see the rapid movement of water through the cellulose to which water molecules stick. THE AMERICAN BIOLOGY TEACHER WATER TRANSPORT IN PLANTS 567

Figure 3. A piece of paper is placed in (A) water or (B) methylene blue solution. The water column forms within seconds. The methylene blue dye represents dissolved nutrients. Figure 4.

(B) As soon as the two capillary tubes are placed in the dye solution, the tube with the smaller diameter will conduct water more quickly and the dye level in the capillary tube will be higher.

4 Figure 3. A piece of paper is placed in (A) water or (B) methylene blue solution. The water column forms within seconds. The methylene blue dye represents dissolved nutrients. Figure 4. (A) The movement of water depends on the diameter of the capillary and the force of the gravity. (B) As soon as the two capillary tubes are placed in the dye solution, the tube with the smaller diameter will conduct water more quickly and the dye level in the capillary tube will be higher. Demonstration 4: Capillary Action After a brief explanation of capillary action on the board, the demonstration is conducted using two capillarity tubes of different sizes to demonstrate capillary action. The smaller the diameter, the higher the movement will be through the capillary tubes that simulate the tiny xylem vessels (Figure 4). As mentioned above, capillary action explains in part the movement of water through the plant. Demonstration 5: Adhesion Cohesion Principle & Importance in the Formation of a Water Column The adhesion cohesion principle can be demonstrated through the transport of water inside the xylem in a celery stalk. An incision is made in the celery branch to break the water column, and the stalk is placed in a methylene blue solution. The xylem will become distinctly stained from other tissue by the methylene blue, and the water column can now be observed (Figure 5A). This continuum cannot be broken naturally and is demonstrated by the continuous movement of water through the xylem (Figure 5B). Demonstration 6: Tension & Negative Pressure Created by Transpiration Pull Tension and negative pressure can be demonstrated using a straw to draw water into the mouth in the same way one would use a straw to drink any beverage (Figure 6; a pipette with a bulb could substitute for straw and beverage). The negative pressure caused by transpiration is analagous to that created when consuming liquid through a straw. 568 THE AMERICAN BIOLOGY TEACHER VOLUME 75, NO. 8, OCTOBER 2013

(B) This shows that the water column is a continuous pipeline in celery, a petiole with xylem in channels with collenchyma cells to one side (beneath epidermis) and sclerenchyma cells in the vascular

Demonstration 7: Water Regulation by Stomata The movement of the guard cells of stomata can be shown on the board or through the movement of one s fingers as shown in Figure 7.

5 Figure 5. (A) When a celery stalk is dipped in the methylene blue solution, the dye will obviously not cross the portion with the incision, while on the other side it will keep moving up through the stalk. (B) This shows that the water column is a continuous pipeline in celery, a petiole with xylem in channels with collenchyma cells to one side (beneath epidermis) and sclerenchyma cells in the vascular bundle. Figure 6. Pulling a liquid with a straw, simulating the negative pressure caused by transpiration. Demonstration 7: Water Regulation by Stomata The movement of the guard cells of stomata can be shown on the board or through the movement of one s fingers as shown in Figure 7. The guard cells, which are modified epidermal cells, can be located either on the model (in the lab) or on a diagram (in the lecture). The water pressure and availability, along the hormone abscisic acid (ABA), control the opening and closing of the stomatal pore. Figure 7. (A) The thumb and the index finger of the hands form a round circle, simulating the two individual guard cells. When turgid, the two hands are put side to side, the space in the middle simulating the stoma. (B) The stoma collapses and closes when the guard cells are flaccid because of the lack of water. THE AMERICAN BIOLOGY TEACHER WATER TRANSPORT IN PLANTS 569

6 Conclusions These demonstrations are conducted during the lesson and require little time and space. They can be performed in a lecture room as well as in a laboratory to support the explanations of these difficult concepts. They have allowed students to more readily reach the learning objectives on water transport through plants. Students show better retention as well as comprehension and relate to these demonstrations as a strong way to understand the concepts. On the basis of demonstrations presented here, instructors can develop additional visual examples for other concepts using endless creativity to improve their ways of teaching. References Cook, M. (2012). Teaching with visuals in the science classroom. Science Scope, 35, Evert, R.F., Eichhorn, S.E. & Russin, W.A. (2005). Laboratory Topics in Botany. New York, NY: W.H. Freeman. Ford, R.H. (1998). A transpiration experiment requiring critical thinking skills. American Biology Teacher, 60, Hodson, R.C. & Acuff, J. (2006). Water transport in plants: anatomy and physiology. In M.A. O Donnell (Ed.), Tested Studies for Laboratory Teaching, Vol. 27 (pp ). Proceedings of the 27th Workshop/ Conference of the Association for Biology Laboratory Education. Raven, P.H., Evert, R.F. & Eichhorn, S.E. (2005). Biology of Plants, 7 th Ed. New York, NY: W.H. Freeman. Reece, J.B., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V. & Jackson, R.B. (2009). Campbell Biology, 9 th Ed. San Francisco, CA: Benjamin Cummings. Schrock, G.F. (1982). A laboratory exercise to assess transpiration. American Biology Teacher, 44, , 255. MOHAMED LAKRIM is Associate Professor of Biological Sciences at Kingsborough Community College, 2001 Oriental Blvd., Brooklyn, NY mlakrim@kbcc.cuny.edu. The finest detailed, most accurate and affordable reproductions of human and animal skeletal anatomy available today. Visit our website to see our complete line of skulls, skeletons and over 1500 other fine products. 570 THE AMERICAN BIOLOGY TEACHER VOLUME 75, NO. 8, OCTOBER 2013

Bio Factsheet. Transport in Plants. Number 342

Bio Factsheet. Transport in Plants. Number 342 Number 342 Transport in Plants This Factsheet: Explains why plants need a transport system Describes what plants transport Describes the tissues which carry out transport Outlines the position of the xylem