Notes to Instructors
Chapter 36 Transport in Vascular Plants
What is the focus of this activity?
In this activity, students will examine the basic structure and function of transport systems
What is this particular activity designed to do?
Activity 36.1 How are water and food transported in plants?
This activity is designed to help students review and understand
• how water and specific dissolved minerals can enter and be transported in xylem,
What misconceptions or difficulties can this activity reveal?
Activity 36.1
To understand transport in plants, students must have a good understanding of the
properties of water and of osmotic relationships (Chapter 3) and the properties of cell
Many students don’t understand how water in the xylem can move laterally into the
phloem. They may assume that xylem vessels and tracheids are laterally impermeable.
Figure 35.10, Water-Conducting Cells of the Xylem, in Campbell Biology, 9th edition,
demonstrates that both tracheids and xylem vessels contain pits in their lateral walls.
These pits allow for the lateral movement of water.
246 Notes to Instructors
students cut a different-sized hole (from pin holes to up to 1/8 inch across) in the side of
one of their straws. Now have them test how much suction they need to exert to pull water
up both straws. Students will find that it is still possible (though harder) to pull water up
the straw with the hole. Next, to simulate the environment of the xylem inside a plant,
have students wrap the cut straw with wet paper toweling (analogous to the water-filled
cell walls of a plant). They will discover that the amount of energy they need to exert to
Answers
Activity 36.1 How are water and food transported in plants?
Using the cross sections of the plants you developed in Activity 35.1 you identified the
location of water-conducting xylem in roots, stems, and leaves. In this activity, you will
use playdough, cutout pieces of paper, chalk, or some other material to create a model of
how that transport occurs. Be sure to include all the terms and concepts in the list.
Use your model to demonstrate how both water and K+ions are transported from the soil
into the xylem of the plant and to the leaves of the plant.
Terms
Activity 36.1 247
endodermis
channels
symplast
Use your model to answer the questions.
1. How do water’s properties of adhesion and cohesion help maintain the flow of water
in the xylem of a plant?
Water molecules cohere to one another and adhere to the sides of the xylem vessels
2. a. If water flows from a region of more positive (higher) water potential to a region
of more negative (lower) water potential, how does the water potential in the root
compare to that in the soil outside the root?
b. How does the water potential in the air compare to that in the leaf of a plant
undergoing transpiration?
3. A student uses an AUBtube for a series of experiments. Sides A and B of the
tube are separated by a membrane that is permeable to water but not to sugar or
starch. What results would you expect under the experimental conditions given
below? Explain your answers in terms of osmotic potential, water potential, and the
equation
= P+ S
(Hint: Solute pressure is always negative and a 0.1 Msolution of any substance has
S= ⫺0.23. Therefore, a 0.2 Msolution would have S= ⫺0.23 ⫻2 = ⫺0.46.)
Experiment a: A solution of 10 g of sucrose in 1,000 g of water (the molecular weight
of sucrose is 342) is added to side A. An equal volume of pure water is added to side B.
What will happen to the concentrations of water and sugar in the two sides over time?
Explain.
A solution with 10 g of sucrose per 1,000 g of water is equivalent to 10 g of sucrose
water potential.
248 Activity 36.1
Experiment b: A solution of 10 g of soluble starch in 1,000 g of water is added to side A.
Assume the molecular weight of soluble starch is about 63,000. An equal volume of pure
water is added to side B. What will happen to the concentrations of water and starch in
the two sides over time. Explain how this compares with the results in Experiment a.
A solution with 10 g of starch per 1,000 g of water is equivalent to 10 g of starch divided
Because the of pure water (on the other side of the U tube) is zero, water will move
from this region of higher water potential to the side with the starch, which has a lower
water potential. This movement will be much slower, however. In fact, it will be
0.069/0.0004 = 172.5 times slower.
4. Fertilizer generally contains nitrogen and phosphorus compounds required by plants.
The nitrogen is often in the form of nitrates, and the phosphorus is in the form of
phosphates. Based on what you know about chemistry and water potential, why would
overfertilizing lead to the death of plants?
Adding nitrates and phosphates to the soil will decrease the water potential in the soil.
5. a. One of the most common ways of killing a plant is overwatering. Why does
overwatering kill a plant?
Roots, like all other living tissues of the plant, require oxygen for cellular
b. If overwatering kills plants, why can you sprout roots from cuttings of stems
in water?
Oxygen from the atmosphere can diffuse into the water in a jar or vase. As a
6. Xylem cells are dead when functional. Why must phloem cells be alive when
functional?
Phloem cells must be capable of loading sugars against a concentration gradient. To do
Activity 36.1 249
7.
250 Activity 36.1
8. Refer to your diagram of the cross-sectional structure of a typical angiosperm leaf
from Activity 35.1. Explain how this structure (that is, the type and placement of
cells, and so on) is correlated with the activities of the leaf as they relate to
photosynthesis, water conservation, and food and water transport.
When the stomata are open in the leaf, carbon dioxide can enter and oxygen and
water vapor can exit. It is this water loss through the stomata that generates the
What forces bring about: How are these forces generated in the plant?
a. xylem conduction?
Transpirational loss of water at the leaf
b. phloem conduction?
This force is generated by the difference in
36.1 Test Your Understanding
1. Scientists have measured the circumference of trees at 2 A.M. and at 2 P.M.
If they collect measurements when the ground has adequate moisture and the days are
sunny and dry, they find that the circumference (and therefore the diameter) of the tree
trunk is smaller at 2 P.M. than at 2 A.M. From your knowledge of the mechanisms of water
transport, suggest the reasons for this decrease in circumference.
On bright sunny days, the transpirational pull generated by evaporation from the leaves
2. Outline an experiment that would allow you to determine
a. how fast a substance is carried in the xylem,
b. in what direction the substance flows in the xylem, and
c. what percentages of solutes are in the xylem at various distances away from the
leaves or roots.
There are a number of different ways to design an experiment. Xylem is dead when
functional. As a result, you can insert taps and/or measuring devices into it. For an
3. When researchers have tried to tap into phloem cells during experiments, they find
that the disrupted phloem immediately stops functioning. However, aphids can
pierce through plant tissues with their mouthparts and locate individual phloem
cells. Once inside a phloem cell, the aphids are essentially force-fed phloem sap. If
the aphid body is removed from the mouthparts, phloem sap will continue to flow
and can be collected (as honeydew).
Activity 36.1 251
A
10 cm
Soil
surface
You place two aphids on a plant you are using for radioactive tracer experiments.
One is located 5 cm above leaf A and the other is located 5 cm below leaf A. Once
the aphids begin feeding, you remove the aphid bodies, allowing you to sample
phloem sap from the mouthparts remaining in the plant. You then cover leaf A with a
plastic bag and inject 14CO2(radioactive carbon dioxide) into the plastic bag
continuously for 10 minutes, during which time leaf A continues to conduct
photosynthesis.
You start collecting honeydew from the two aphids’ mouthparts from the time the
14CO2is first injected into the bag and every 30 seconds thereafter for a period of
one hour.
If you analyze the phloem samples for 14C, where would you expect to find it?
Explain your reasoning.
The 14CO2would be used by the cells in the leaf to make sugars in photosynthesis. These
4. On a separate sheet of paper, outline an experiment that would allow you to
determine:
a. how fast a substance is carried in the phloem,
b. in what direction the substance flows in the phloem, and
c. what percentages of solutes are in the phloem at various distances away from the
leaves or roots.
Different methods could be used. One is to sample phloem sap at different distances
from the leaves and determine the concentration of sugar present. If you added a
252 Activity 36.1