Plants as inspirational contexts 2 – Water transport in plants
While the importance of water transport in plants in our everyday lives is obvious to us all (no one wants the flowers in a vase to wilt or the lettuce in a salad to lose its crunch), it can be difficult to devise inspirational and memorable ways of teaching and learning about it. In this article, using content from an online course developed by SAPS and STEM Learning, we will reflect on some of the ways in which students can use models, animations and practical investigations to gain meaningful experience of water transport in plants.
To find out more about the value of adding inspiring contexts and to explore doing this with the topic of osmosis, read Plants as inspirational contexts 1 – Osmosis.
Animated biology
Animations provide a valuable way to help students grasp biological concepts which involve movement over time.
Researchers have found that students are able to retain information better when animations were used in class and then during individual study (McClean, 2005).
Molecular and cellular biology animations: development and impact on student learning.
McClean P, Johnson C, Rogers R, Daniels L, Reber J, Slator BM, Terpstra J, White A Cell Biol Educ. 2005 Summer; 4(2):169-79.
The following animation shows the movement of water through a whole plant as well as the movement of sugars. Each section of the plant can be magnified to see how the water is moving between cells. The animation has no commentary. There are accompanying documents that can be used to explore what is being shown in each part of the animation.
To use this animation, click the buttons on the left side of the animation screen.
A quick quiz
This quiz is based upon the above animation.
Decide which responses are correct. Use the drop-down menu to check your answers. There may be more than one correct answer for each question.
1. Water molecules enter the root hair cells by osmosis because: (pick two)
a) There is a higher concentration of water molecules inside the root hair cell than there is outside the root hair cell.
Incorrect answer.
There is a net movement of water molecules from the soil surrounding the root hair cell, into the root hair cell, down a water concentration/water potential gradient from a solution with a high concentration of water molecules/high water potential to a solution with a lower concentration of water molecules/lower water potential, through a partially permeable membrane.
b) There is a higher concentration of water molecules outside the root hair cell than there is inside the root hair cell.
Correct answer.
There is a net movement of water molecules from the soil surrounding the root hair cell, into the root hair cell, down a water concentration/water potential gradient from a solution with a high concentration of water molecules/high water potential to a solution with a lower concentration of water molecules/lower water potential, through a partially permeable membrane.
c) Water molecules outside the root hair cell are moving about at a faster rate than they are inside the cell.
Incorrect answer.
Water molecules in the soil and inside the root hair cell are likely to be moving about at the same speed. The higher concentration of water molecules outside the root hair cell makes it likely that a large number of water molecules in the soil will collide with the outside of the root hair cell and enter the cell by osmosis. The lower concentration of water molecules/lower water potential inside the root hair cell makes it likely that fewer water molecules inside the cell will collide with the cell membrane and leave the root hair cell by osmosis. This overall difference leads to a net movement of water into the root hair cell from the soil, by osmosis.
d) Water molecules move about randomly all the time.
Correct answer.
The higher concentration of water molecules outside the root hair cell makes it likely that a large number of randomly moving water molecules in the soil will collide with the outside of the root hair cell and enter the cell by osmosis. The lower concentration of water molecules/lower water potential inside the root hair cell makes it likely that fewer randomly moving water molecules inside the cell will collide with the cell membrane and leave the root hair cell by osmosis. This overall difference leads to a net movement of water into the root hair cell from the soil, by osmosis.
2. Water molecules move across the root from root hair cell towards the Xylem vessels: (pick two)
a) Along a water concentration/water potential gradient.
Incorrect answer
Water molecules move across the root from a cell with a higher concentration of water molecules/higher water potential in its cytoplasm and vacuole to a cell that contains a lower concentration of water molecules/lower water potential, by osmosis. Water molecules therefore move down the water concentration/water potential gradient that exists across the root tissues from the epidermis, where the root hair cells are found, to the Xylem vessels.
b) Down a water concentration/water potential gradient.
Correct answer
Water molecules move across the root from a cell with a higher concentration of water molecules/higher water potential in its cytoplasm and vacuole to a cell that contains a lower concentration of water molecules/lower water potential, by osmosis. Water molecules therefore move down the water concentration/water potential gradient that exists across the root tissues from the epidermis, where the root hair cells are found, to the Xylem vessels.
c) Up a water concentration/water potential gradient.
Incorrect answer
Water molecules move across the root from a cell with a higher concentration of water molecules/higher water potential in its cytoplasm and vacuole to a cell that contains a lower concentration of water molecules/lower water potential, by osmosis. Water molecules therefore move down the water concentration/water potential gradient that exists across the root tissues from the epidermis, where the root hair cells are found, to the Xylem vessels.
d) from cell to cell via the cytoplasm, the vacuoles and the cell walls..
Correct answer
Water molecules move across the root, down the water concentration/water potential gradient that exists across the root tissues from the epidermis, where the root hair cells are found, to the Xylem vessels. The water moves across the root from cell to cell by osmosis through the partially permeable cell membrane via the cytoplasm or the vacuole, or through the freely permeable cell wall.
3. The process that moves water up the stem of the plant in the Xylem vessels is called:
a) Evaporation
Incorrect answer.
Evaporation of water from the leaves of the plant helps to set up the pressure gradient that draws water up the stem of the plant in the Xylem vessels in the transpiration stream.
b) Cohesion
Incorrect answer.
Water molecules cohere to one another because they are held together by hydrogen bonds. When water evaporates from the leaves, this cohesion, together with adhesion of the water molecules to the walls of the xylem vessels, helps to create tension which draws the water up the xylem from the bottom to the top of the plant in the transpiration stream.
c) Transpiration
Correct answer.
Water is pulled up the xylem in the transpiration stream. from the bottom to the top of the plant. The water is drawn up by the tension created by evaporating water in the leaves, and cohesion between water molecules.
d) Translocation
Incorrect answer.
Translocation is the transport of dissolved substances, mainly sugars, from a source to a sink, via the sieve tubes in the phloem. Water is pulled up the xylem in the transpiration stream. from the bottom to the top of the plant. The water is drawn up by the tension created by evaporating water in the leaves, and cohesion between water molecules.
4. Which of the following are not functions of xylem vessels: (pick two)
a) To provide support for the leaf.
Correct answer.
This is a function of xylem vessels. The xylem tissue provides a supporting structure for the leaf to help prevent the leaf from drooping.
b) To carry water from the roots of the plant to cells to maintain their turgor pressure.
Correct answer.
This is a function of Xylem vessels. The cells of the leaf need to be kept turgid, using water from the xylem to maintain turgor pressure. A failure to maintain this turgor pressure results in wilting.
c) To move sugars around the plant.
Incorrect answer.
This is not a function of Xylem vessels. Xylem vessels transport water and ions about the plant. Sugars are translocated around the plant in the sieve tubes of the phloem.
d) To increase the surface area for the absorption of water from the soil.
Incorrect answer.
This is not a function of Xylem vessels. Xylem vessels transport water and ions about the plant. The root hair cells increase the surface area for the absorption of water from the soil.
5. Guard cells help to regulate the loss of water from the leaves through the stomata by: (pick two)
a) Closing the stomata at high light intensities.
Incorrect answer.
Most plants will open their stomata at high light intensities. Carbon dioxide for photosynthesis will diffuse into the leaf via the stomata and into the photosynthesising palisade cells, down a concentration gradient. There are some exceptions: some xerophytic plants that are adapted to live in very dry, hot environments, close their stomata during the hottest part of the day, reducing water loss.
b) Closing the stomata when the plant experiences a shortage of water.
Correct answer.
If the guard cells become flaccid, they change shape and the stoma between the two guard cells will be closed. If the plant experiences a shortage of water, this response will reduce further water loss but it will also reduce the exchange of gases necessary for photosynthesis and respiration.
c) Being more abundant on the upper surface of the leaf.
Incorrect answer.
Stomata (and hence guard cells) are usually more numerous on the lower surface of the leaf. The lower leaf surface is usually shaded and therefore cooler, reducing the rate of water loss through evaporation from the stomata. The exchange of carbon and dioxide can still take place between the plant and the atmosphere via the stomata, but water loss from the plant’s leaves is reduced.
d) Being more abundant on the lower surface of the leaf.
Correct answer.
Stomata (and hence guard cells) are usually more numerous on the lower surface of the leaf. The lower leaf surface is usually shaded and therefore cooler, reducing the rate of water loss through evaporation from the stomata. The exchange of carbon and dioxide can still take place between the plant and the atmosphere via the stomata, but water loss from the plant’s leaves is reduced.
Model Investigations
It can be a challenge for students to use ideas from physics and chemistry to explain biological concepts, and so here we explore two activities which model water transport in plants to provide opportunities for students to do this.
Giant Redwoods can grow to 100m tall and they have to be able to move the water that they absorb through their roots up through the tree to the very top. How do they do it?
Take a look at the following activities:
Can You Beat the Giant Redwood? and the Ins and Outs of Water.
A Simple Potometer
First-hand, practical experience of water transport in plants will provide your students with meaningful and memorable learning opportunities. The following video shows a practical investigation which allows simple potometers to be set up by individuals or small groups and used to investigate transpiration and the factors that affect it.
In this video you will find out how to set up and use a simple potometer. As a safety precaution, it is suggested that a technician should set up the potometers in advance, although students will enjoy inserting the assembled bungs into the glass jars in bowls of water at the start of their investigation.
This simple piece of apparatus allows students to work individually or in pairs to measure the rate of water loss (transpiration) in a plant shoot within a one hour lesson. This low-cost potometer gives your students opportunities to investigate first-hand the effects of different environmental factors on the rate of transpiration.
Instructions for the use of this potometer in A-level set practicals and additional information about the equipment can be found on the potometer resource page.
What questions would you ask your students to check their understanding during this practical?
More Practical Investigations
Take a look at these further practical activities on the SAPS website that will provide your students with first-hand experience of some of the specialized plant cells and tissues involved in water transport:
These investigations are all quick and easy to carry out. How might they help your students to engage with the topic of transport of water in plants?
Conclusion
Reliable, affordable resources that all the students in a class can have access to, such as the simple potometer or drinking straw xylem model, can enable each class member to have the same experiences and opportunities to investigate water transport in plants. They can discuss their investigations, share their results and prompt one-another’s understanding through questioning each other and answering each other’s questions. The inclusion of models, animations and practical investigations into a teaching and learning programme can ensure that each pupils’ journey through the biology curriculum is meaningful, memorable and inspiring.
This article was written using reworked content from the course Teaching Biology: Inspiring Students with Plant Science codeveloped by SAPS and STEM Learning.
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