Students will recognize that plants remove nutrients from the soil, explain the roles of diffusion and active transport in moving nutrients from the soil to the plant, and relate the root and vascular systems of the plant to the human circulatory system. Grades 6-8

active transport: the movement of substance across a biological membrane against its concentration gradient; from a less-concentrated area to a more-concentrated area

adenosine triphosphate (ATP): a compound that has three phosphate groups and is used by cells to store energy

concentration gradient: a difference in the concentration of certain molecules over a distance

diffusion: the movement of a substance down its concentration gradient from a more-concentrated area to a less-concentrated area

lignin: a non-carbohydrate polymer that binds cellulose fibers together. It adds strength and stiffness to plant cell walls

phloem: a portion of the vascular system in plants, consisting of living cells arranged into tubes that transport sugar and other organic nutrients throughout the plant

xylem: the specialized cells of plants that transport water and nutrients from the roots to the leaves

Plants use their root systems for structural support, stability and nourishment. If you have ever seen a tree toppled by high winds, you have some idea of why trees are so stable. Even so, much of the root system remains in the soil, hidden from view. The primary function of the root system is to absorb water and nutrients from the soil. To do this, the root system is ever changing over the course of the plant’s life, capable of growing year-round, provided that conditions for growth are met and there isn’t competition from the plant’s top system. Roots also may serve as storage organs for starch or sugars. Carrots, beets, radishes, turnips, and potatoes are examples of storage roots.

The growth of roots is similar to the growth of shoots. However, there are important differences. In general, the more extensive a root system is, the more water and nutrients it can absorb. If you examine a root using a magnifying glass, you will see a large number of delicate, white root hairs growing out from the surface of the root (see Figure 2). This network of root hairs greatly increases the surface area of the root available to contact and absorb water. A single rye plant 60 centimeters tall is estimated to have a root system with a total length of 480 kilometers. Its surface area is more than 600 square meters—twice that of a tennis court!

The tip of an actively growing root is called the root cap (see Figure 3). The root cap produces a slimy secretion called mucilage that helps lubricate the root as it pushes its way through the soil. Just behind the root cap is the zone of active cell division, and behind it is a zone of cell elongation. The cells of the elongation zone grow by taking in water and swelling. The root cells contain salt and sugars. Because the root cells contain more solutes than the water in the soil, water flows into the cells by diffusion. This causes the cells to elongate, forcing the root deeper into the soil. Behind the elongation zone is the zone of cell differentiation. The cells in this area give rise to the cells of the vascular system, which transport water up the stem and sugars down from the leaves.

Roots may stop growing during the winter, not because they have become dormant like the buds at the top of the plant, but rather because the temperature is too cool to support growth. In order for roots to grow, they must have adequate moisture and temperature. Many people are under the misconception that roots grow in search of water. This isn’t the case. Roots can only grow where the conditions are suitable for growth. This means that roots grow where water is already present.

Water is absorbed by the root hairs and brings along with it any chemicals, including nutrients that are dissolved in it. Most nutrients are present in higher concentration in the root hairs as compared with the soil water. Active transport is used to move the nutrients deeper into the root system until they reach cells of the vascular system. The importance of active transport can be demonstrated by exposing plants to a chemical that interferes with cellular respiration. Without a supply of ATP (the cell’s energy molecule) produced through respiration, the rate of nutrient movement slows greatly.

The Plant Vascular System

Although plants don’t have a circulatory system like humans, they still must transport material from one part of the organism to another. The plant stem contains a vascular system that connects the leaves to the roots. The plant’s vascular system is composed of xylem tissue that transports water from the roots to the rest of the plant and phloem tissue that transports sugars produced in the leaves to the non photosynthetic parts of the plant (see Figure 4). The xylem is composed of dead cells that form long, empty tubes. Some tubes are wide, and others are narrow. The cell walls within the tubes are either missing or contain a series of holes that permits the passage of water. The cells that gave rise to the xylem lay down thick cell walls that contain a polymer called lignin. Lignin lends strength to the xylem and prevents it from collapsing under pressure. 

The capability of xylem tissue is truly amazing. In the case of the tallest trees, water must be transported from the roots up, over 100 meters and against gravity, to the leaves. Water is thought to move through the xylem by a process known as cohesion-tension. According to this view, water can be pulled upward provided that the diameter of the tube is sufficiently small and that the column of water is continuous, that is, without air bubbles. A further requirement is that the tube be made of a material to which water molecules will adhere. Within each xylem tube, the water molecules are attracted to adjacent water molecules, forming an unbroken chain. The plant loses water through evaporation from its leaves by a process called transpiration. As water is lost, a negative pressure or tension is created that pulls water up from the xylem. Transpiration is the process that drives the transport of water from the roots up through the stems to the leaves.

While water is moving up the plant, sugars and amino acids must move from the leaves downward to the non photosynthetic parts of the plant. Phloem tissue is composed of tubes made from living cells called sieve cells. Holes at the ends of their cell walls form sieve plates. The cytoplasm of one sieve cell connects with the cytoplasm of adjacent sieve cells through these holes, forming a continuous cell-to-cell sieve tube. As the sieve cells mature, they lose their nuclei and other organelles. Beside each sieve cell is a smaller companion cell that has a nucleus. The companion cells are thought to regulate the activity of the sieve cells.

Experiments have demonstrated that this movement occurs at a rate that is thousands of times faster than could be achieved by diffusion. Sugars are thought to move through the phloem by a process called pressure flow. According to this view, water and dissolved sugars flow through sieve tubes from areas of higher pressure to ones of lower pressure. Sugars made in the leaves are transported into the phloem by active transport. The high concentration of sugar causes water to flow into the phloem cells, increasing what is called the turgor pressure within the cell. This high turgor pressure forces the sugar-water solution into the adjacent phloem cell, increasing its turgor pressure. This process repeats,moving from cell to cell until the solution reaches a cell where it will be used. Once at its destination, the sugar is removed from the phloem by active transport. Water, too, is removed from the phloem cell, regenerating the lower turgor pressure needed to keep the flow moving.

1. Ask your students to compare soil (in a field or garden) in the spring, before a crop is planted and in the fall, after a crop is harvested. Will the levels of soil nutrients be more, less, or the same after plants grow and a crop is harvested?
2. Use guided questions to draw on prior knowledge. What is the function of a plant's roots? How are water and nutrients transported from the soil to the roots and eventually the rest of the plant?
3. After completing this lesson, students will be able to
   • recognize that plants remove nutrients from the soil,
   • explain the roles of diffusion and active transport in moving nutrients from the soil to the plant, and
   • relate the root and vascular systems of the plant to the human circulatory system.

Preparation:

Activity 1: Seedling preparation. In Step 4, students are asked to observe the root systems of young seedlings. For this activity, any type of seeds may be used so long as the roots have grown about 1 or 2 cm. Pinto bean seeds are easy to obtain and work well. To germinate the seeds, place several seeds in a row along one side of a paper towel as shown in Figure 3.2. Carefully roll up the paper towel from bottom to top. Place the rolled paper towel into a glass of water so that the seeds are at the top and out of the water glass (Figure 3.2). Water will wick up through the paper towel and keep the seeds moist. Prepare enough seedlings so that each group of 4 students will have a seed to observe. Assume that just 1/2 of the seeds you prepare will germinate. Set the glasses of seedlings in a location where they will not be disturbed. The seeds will need approximately 5 to 6 days for the roots to grow enough for observation. During the germination period, be careful to replace any water that is lost through evaporation.

Activity 2: Celery demonstration. Use a sharp knife to cut celery stalks into pieces approximately 2 inches long. Make sure that the cut surfaces are flat and will allow the celery to rest upright when placed into the paper cups.

Teacher note: The celery demonstration described in this section is designed to quickly transport water through the xylem. For a more impressive demonstration, you can have carnations take up colored water and see the edges of the petals take on the color of the dye. While more dramatic than the celery demonstration, it takes much longer to see the effect; about 2 to 3 hours, as compared with 15 to 20 minutes for the celery demonstration.

Optional Activity 2: Carnation demonstration.

1. Obtain a white carnation and cut the stem diagonally so that the stem is about 6 inches long.
2. Add about 2 inches of water to a paper cup.
3. Add 6 drops of food coloring (blue works well) to the water and mix.
4. Place the carnation into the colored water.
5. Within 2 hours, small colored areas will appear at the edges of the petals.

Activity 1: From Soil to Roots

Teacher note: Diffusion and active transport both play roles in moving nutrients from the soil into the plant roots. It is not necessary that middle school students be familiar with the details of these processes. The important things for students to know are

• in diffusion, molecules flow down their concentration gradient (meaning from an area of higher concentration to an area of lower concentration); and
• in active transport, energy is used to move molecules against their concentration gradient (meaning from an area of lower concentration to an area of higher concentration).

1. Remind students that air spaces in soil become filled with water and that many nutrients needed by plants are dissolved or suspended in the water. Ask, “How does the plant obtain nutrients from this water?”
   • Students’ responses will vary. If necessary, guide the discussion to mention the plant’s root system.
2. Display a transparency of Master 3.1, What Do You Know about Roots? Cover the transparency with a piece of paper. Reveal the first statement and ask the students to indicate by a show of hands whether they agree or disagree with the statement.
   • This discussion is designed to help you assess the students’ prior knowledge of the topic. If necessary, review for the class the essential features of diffusion and active transport.
     • Diffusion
       • Molecules move randomly due to their kinetic energy.
       • This movement causes molecules to intermingle.
       • The net movement of molecules is from an area of higher concentration to one of lower concentration. The net movement of molecules stops when the concentration of the molecules is the same everywhere.
       • The movement of the molecules comes from their kinetic energy and doesn’t need additional energy (unlike active transport).
     • Active Transport
       • Active transport is a process used by cells to move molecules from an area of lower concentration to one of higher concentration.
       • It requires energy.
       • If your students already have been introduced to the energy molecule ATP, you may mention it as the source of energy for active transport.
3. Continue revealing the rest of the statements, one at a time, and asking students whether they agree or disagree with the statements.
   • After students vote on each statement, ask for 1 or 2 volunteers to explain why they voted as they did. At this time, do not correct wrong answers. The students will come back to these statements later in the lesson. Answers are found and revealed to students in Step 15.
4. Explain that they will now investigate the mechanism by which roots obtain nutrients from the soil. Divide the students into groups of 4. Pass out to each group a young seedling (taken from the paper towel germination) and a hand lens.
   • This activity refers to the way that most plants obtain their nutrients through the root system. Plants that carry out nitrogen fixation in their roots are a special case and are not dealt with here.
5. Instruct the students to take a minute to observe the seedling’s root system with the hand lens and write down their observations on a piece of paper.
   • The root hairs are white and very fine. Provide a dark background against which the root hairs are more easily visible.
6. After the students have recorded their observations, ask for volunteers to describe what they saw.
   • Students will report seeing one large root emerging from the seed. They also will describe fine white hairs growing out from the root.
7. Remind students of the first statement from Master 3.1, What Do You Know about Roots? : “Plant roots have tiny hairs that absorb water.” Ask,“Why do you think that plants have so many root hairs?”
   • Student responses will vary. Guide the discussion to bring out that more root hairs mean more surface area with which to contact water and nutrients in the soil.
8. Ask students, “How do nutrients in the soil water get into the root hairs?”
   • Students’ responses will vary. At this time, accept all answers.
9. Explain that students will now investigate the process by which water enters the root hairs. Keep the class in their groups. Pass out to each group 1 copy of Master 3.2, Moving Water and Nutrients into Roots. 
10. Ask students to read over the procedure on the handout. Explain that the cup represents the root hair, the larger container represents the water in the soil, and the food coloring represents the nutrients dissolved in the water.
11. After students have completed their investigations, reconvene the class and ask for volunteers to explain what happened when the holes were poked through the cup.
    • Students will report that the colored water slowly entered the cup.
12. Ask students:
    • “Why did the colored water enter the cup?"
      • Students’ responses will vary. Guide the discussion to bring out the fact that although the concentration of water was the same on both sides of the cup, the concentration of the food coloring was higher outside the cup compared with inside the cup.
    • “What is the process called where a substance moves from an area of higher concentration to an area of lower concentration?”
      • The process of diffusion was summarized in Step 2. Students should recall that diffusion involves a net movement of a substance from an area of higher concentration to one of lower concentration.
    • “Where does the energy come from to drive this process?”
      • Students should recall from the discussion in Step 2 that the process is driven by the kinetic energy of the molecules in solution.
13. Display a transparency of Master 3.3, Experiments with Roots. Cover the transparency with a piece of paper. Reveal the first experiment and read it aloud. Ask the students what this data tells them about how nutrients move from the soil into the roots.
    • Since the concentrations of some essential elements move from an area of low concentration to one of higher concentration, this suggests that energy was required for the movement and the process involved was active transport.
14. Reveal the second experiment, read it aloud, and discuss its meaning.
    • Students should recognize that a chemical stops energy from being produced. They should reason that the lack of energy will cause the process of active transport to stop. Without active transport, those essential elements that were concentrated in roots hairs during experiment 1 would now be present in lower amounts. If not mentioned by a student, bring out the fact that the transport of other essential elements that move by diffusion would be unaffected.
15. Conclude the activity by once again displaying a transparency of Master 3.1,What Do You Know about Roots? As before, ask students to indicate by a show of hands whether they agree or disagree with each statement. Ask for volunteers to explain why they changed their minds about their answers. Students should be able to respond to the statements about roots as follows:
    • Answers to Master 3.1, What Do You Know about Roots?:
      1. Plant roots have tiny hairs that absorb water. (True)
         • Students were able to observe root hairs using the hand lens. A larger root system contacts and absorbs more water than a smaller one.
      2. Plants roots use energy to pump water into the plant. (False)
         • As shown in the demonstration,water entering the root hairs does so by the passive process of diffusion. When root hairs contact the water, the water flows from a higher concentration in the soil toward a lower concentration in the root cells.
      3. Nutrients enter root cells through the process of diffusion. (True)
         • Water enters the root system by diffusion and takes dissolved nutrients with it.
      4. Nutrients enter root cells through the process of active transport. (True)
         • As shown by the experiments described in Master 3.3, Experiments with Roots, some nutrients are moved by active transport.
      5. Plant roots grow until they find water. (False)
         • Students may believe that roots can sense and grow toward water. This is a misconception. Roots can only grow where water is already present. As the surface of the soil dries out, roots near the surface may die while roots further down are in contact with water and can grow still deeper.

Assessment: Pass out to each student 1 copy of Master 3.1, What Do You Know about Roots? Instruct students to write on their copies of Master 3.1 why each statement is true or false. Students should include specific evidence from the lesson that supports their conclusions.

Activity 2: From Roots to the Plant

1. Explain that getting nutrients into the plant roots is an important first step. Ask students, “How does water, and the nutrients it contains, get from the roots to the rest of the plant?”
   • Some students may recognize that plants have a vascular system.
2. Explain that students are going to investigate how water moves from the roots to the rest of the plant. As before, divide the class into groups of 4 students.
3. Pass out to each student 1 copy of Master 3.4, The Plant Vascular System and 1 copy of Master 3.5, Getting Water and Nutrients to the Plant. Instruct students to examine the plant vascular system as shown in Master 3.4, The Plant Vascular System, and conduct the celery demonstration as described in Master 3.5, Getting Water and Nutrients to the Plant. Give students about 15 to 20 minutes to complete their tasks.
4. Reconvene the class and ask for volunteers to report their predictions about the movement of the food coloring in the celery. Ask them if their predictions were correct or incorrect.
   • Students’ predictions will vary. They should report that the food coloring was transported up the celery stalk and was visible as a series of colored dots along the top of the stalk. Explain that the movement of water took place through the plant’s xylem system.
5. Conclude the lesson by reminding students that photosynthesis produces sugars in the leaves. Ask them how the sugars, needed for energy, reach the lower parts of the plant.
   • Students should recall from Master 3.4, The Plant Vascular System that phloem tissue is used to transport sugars downward from the leaves. You can point out that in the case of the celery stalk, the xylem and phloem tissues lie next to each other in structures called vascular bundles.

After conducting these activities, review and summarize the following key concepts:

• Roots absorb water and nutrients through the process of diffusion.
• Water and nutrients move throughout the plant through the vascular system. The xylem transports water and nutrients up the plant.

• Nutrients for Life Foundation
• BSCS-Biological Science Curriculum Study
• Reviewed by Smithsonian Institution

• #SoilScience Reader
• Living Soil Film
• Nutrients for Life eLessons
• Soil Health Education Resources
• Soil Life
• Soil Science Society of America
• SoilWeb Interactive Map
• The Great American Dust Bowl
• Topsoil Tour
• Web Soil Survey