1. Which of the following statements is inaccurate?
a. Water moves to areas of low water potential.
b. Xylem transports materials up the plant while phloem transports materials
down the plant.
c. Water movement in the xylem is largely due to the cohesive and adhesive
properties of water.
d. Water movement across membranes is often due to differences in solute
concentrations.
The correct answer is b—
A. Answer a is an incorrect. Water will move from areas of high to low water
potential.
The correct answer is b—Xylem transports materials up the plant while phloem
transports materials down the plant.
B. Answer b is incorrect. Phloem will transport photosynthate anywhere in the plant,
whether it is up to the growing stem, or down to the roots. Phloem does not
simply transfer materials down.
The correct answer is b—
C. Answer c is inaccurate. Water movement in the xylem completely depends on
cohesion and adhesion.
The correct answer is b—
D. Answer d is correct. Much of the movement of water from cell to cell (across
membranes) is driven by differences in solute concentrations.
2. Water movement from the soil, through the plant and into the atmosphere
requires—
a. decreasing water potential from soil to plant to atmosphere
b. increasing water potential from soil to plant to atmosphere
c. equal water potentials in soil, plant, and atmosphere
d. none of the above
The correct answer is a—decreasing water potential from soil to plant to atmosphere
A. Answer a is correct. Water moves toward areas of lower water potential. Thus,
water potential must continually decrease from soil, to plant, to atmosphere for
water to move through this continuum.
The correct answer is a—
B. Answer b is incorrect. It is incorrect for the reason stated in a. If water potential
increased from soil to plant to atmosphere, water would want to move from the
atmosphere, through the plant, and into the soil.
The correct answer is a—
C. Answer c is incorrect. Water moves toward areas of lower potential. If water
potentials were equal in soil, plant, and atmosphere, water would not move.
The correct answer is a—
D. Answer d is incorrect. Answer a is correct.
3. If you could override the control mechanisms that open stomata and force them to
remain closed, what would you expect to happen to the plant?
a. Sugar synthesis would likely slow down.
b. Water transport would likely slow down.
c. Both of these could be the result of keeping stomata closed.
d. Neither of these would be the result of keeping stomata closed.
The correct answer is c—
A. Answer a is correct. Closing the stomata would limit the uptake of carbon dioxide,
thereby limiting photosynthesis.
The correct answer is c—
B. Answer b is correct. Closing the stomata would largely stop evaporation of water
from the leaves, thus stopping movement of water in the xylem.
The correct answer is c—Both of these could be the result of keeping stomata closed.
C. Answer c is correct. Answers a and b are correct.
The correct answer is c—
D. Answer d is incorrect. Both a and b are correct.
4. What will happen if a cell with a solute potential of – 0.4 MPa and a pressure
potential of 0.2 MPa is placed in a chamber filled with pure water that is
pressurized with 0.5 MPa?
a. Water will flow out of the cell.
b. Water will flow into the cell.
c. The cell will be crushed.
d. The cell will explode.
The correct answer is b—
A. Answer a is incorrect. The water potential of the cell is –0.2 MPa (–0.4 MPa +
0.2 MPa = –0.2 MPa), and the water potential of the chamber is 0.5 MPa. Water
will move from high potential to low potential, in this case from the chamber to
the cell.
The correct answer is b—Water will flow into the cell.
B. Answer b is correct. It is correct for the reasoning provided in A.
The correct answer is b—
C. Answer c is incorrect. The difference in water potential is not huge, and because
the cell wall would help prevent the cell from being crushed.
The correct answer is b—
D. Answer d is incorrect. The difference in water potential is not huge, and because
the cell wall would help prevent the cell from exploding.
5. If you were able to remove the aquaporins from cell membranes, which of the
following would be the likely consequence?
a. Water would no longer move across membranes.
b. Plants would no longer be able to control the direction of water movement
across membranes.
c. The rate of movement of water across the membrane would greatly
decrease.
d. Cells would no longer stay turgid.
The correct answer is c—
A. Answer a is incorrect. Water would still move across the membrane.
The correct answer is c—
B. Answer b is incorrect. Aquaporins provide passive transport of water, thus they do
not provide the plant with means of controlling the direction of water movement.
The correct answer is c—The rate of movement of water across the membrane would
greatly decrease.
C. Answer c is correct. Aquaporins greatly speed up the transport of water across
membranes.
The correct answer is c—
D. Answer d is incorrect. Aquaporins do not directly affect cell turgidity.
6. A molecule of water is traveling through the plant. Which of the following
processes would not provide a driving force for it to move at either a cellular level
or over longer distances through the plant?
a. Osmosis
b. Diffusion
c. Transpiration
d. All the above are driving forces for water movement
The correct answer is d—
A. Answer a is correct. Osmosis accounts for the movement of water from the soil
into the roots, creating positive pressure in the roots.
The correct answer is d—
B. Answer b is correct. Diffusion is important as water diffuses out of the xylem of
leaves onto the surface of mesophyll in the leaves, thus acting as a means of
pulling water up the xylem.
The correct answer is d—
C. Answer c is correct. As transpiration occurs, that is the evaporation of water off of
the mesophyll cells in the leaves, water is continually pulled up the xylem to
replace the lost water of the mesophyll cells.
The correct answer is d—All of the above are driving forces for water movement
D. Answer d is correct. A, b and c are all correct.
7. Which of the following statements about water and mineral transport between cells
in roots is incorrect?
a. The apoplast pathway moves water and minerals through cell walls and in
the space between cells.
b. The symplast pathway moves materials water and minerals through
plasmodesmata.
c. The transmembrane route utilizes membrane transport across cells.
d. All of the above statements are correct.
The correct answer is d—
A. Answer a is a incorrect. This statement is correct about the apoplast pathway, but
this is not the only answer.
The correct answer is d—
B. Answer b is a incorrect. This statement is correct about the symplast pathway, but
this is not the only answer.
The correct answer is d—
C. Answer c is a incorrect. This statement is correct about the transmembrane route,
but this is not the only answer.
The correct answer is d—
D. Answer d is correct. All of the above statements are correct.
8. What would be the consequence of removing the Casparian strip?
a. Water and mineral nutrients would not be able to reach the xylem.
b. There would be less selectivity as to what passed into the xylem.
c. Water and mineral nutrients would be lost from the xylem back into the
soil.
d. Water and mineral nutrients would no longer be able to pass through the
cell walls of the endodermis.
The correct answer is b—
A. Answer a is incorrect. Water and mineral nutrients would probably have easier
access to the xylem with the removal of the Casparian strip.
The correct answer is b—There would be less selectivity as to what passed into the
xylem.
B. Answer b is correct. The Casparian strip forces transport across the membrane,
thus providing greater selectivity of what enters the xylem for transport.
The correct answer is b—
C. Answer c is incorrect. The solute concentration would be the driving force for
water uptake into the roots.
The correct answer is b—
D. Answer d is incorrect. The Casparian strip precludes materials from passing
through the cell walls of the endodermis.
9. The movement of water in the xylem relies on—
a. the ability of water molecules to hydrogen-bond with one another
b. active transport
c. evaporation of water from the leaf surface
d. both a and c are correct
The correct answer is d—
A. Answer a is correct. The column of water in the xylem is maintained in part by the
cohesive properties of water.
The correct answer is d—
B. Answer b is incorrect. Active transport is not required for water movement in the
xylem.
The correct answer is d—
C. Answer c is correct. Evaporation of water from the surface of the leaf provides the
negative pressure to draw water up the xylem.
The correct answer is d—both a and c are correct
D. Answer d is correct. Answers a and c are correct.
10. When stomata close at night—
a. water potential in the xylem no longer exists
b. water cannot exit the leaves
c. the roots cease to draw water from the soil
d. none of the above
The correct answer is d—
A. Answer a is incorrect. The roots continue to pull water in from the soil, thus
providing positive pressure in the root end of the xylem.
The correct answer is d—
B. Answer b is incorrect. Water could still exit the leaves through guttation.
The correct answer is d—
C. Answer c is incorrect. The roots will continue to draw water from the soil through
osmosis.
The correct answer is d—none of the above
D. Answer d is correct. A, b, and c are all incorrect.
11. If you wanted to force stomata to open, which of the following would work?
a. Treat the plant with abscisic acid.
b. Stimulate water movement into the guard cells.
c. Stimulate water movement out of the guard cells.
d. Force the dermal cells around the stomata to dehydrate, thereby pulling the
guard cells apart.
The correct answer is b—
A. Answer a is incorrect. Treating a plant with abscisic acid would cause the stomata
to close.
The correct answer is b—Stimulate water movement into the guard cells.
B. Answer b is correct. Movement of water into the guard cells is what causes them
to open.
The correct answer is b—
C. Answer c is incorrect. Removing water from the guard cells would cause the
stomata to close.
The correct answer is b—
D. Answer d is incorrect. The dermal cells surrounding the stomata do not control
stomatal opening and closing.
12. Blowing water up through a drinking straw is most like—
a. guttation
b. diffusion
c. bulk flow in xylem
d. osmosis
The correct answer is a—guttation
A. Answer a is correct. Guttation is the process by which water is pushed (blown) up
the xylem (straw) because of positive pressure, and the water exits the leaf
through guttation.
The correct answer is a—
B. Answer b is incorrect. Diffusion is not linked with pressure.
The correct answer is a—
C. Answer c is incorrect. Bulk flow in the xylem is primarily driven by transpiration.
The correct answer is a—
D. Answer d is incorrect. Osmosis does not depend on pressure differences, only
solute differences.
13. Sucrose enters a phloem sieve-tube cell because of—
a. osmosis
b. water potential
c. active transport
d. a process regulated by auxin
The correct answer is c—
A. Answer a is incorrect. Sucrose will not move across membranes through osmosis.
Osmosis is the movement of water across membranes.
The correct answer is c—
B. Answer b is incorrect. The movement of sucrose into the sieve tube is not
mediated by water potential.
The correct answer is c—active transport
C. Answer c is correct. The concentration of sucrose is higher in the sieve-tube cells,
thus requiring active transport against the concentration gradient.
The correct answer is c—
D. Answer d is incorrect. Auxin does not affect this process.
14. Which of the following statements accurately describes the mass flow hypothesis?
a. Carbohydrates move from a source to a sink.
b. The movement of carbohydrates into the phloem occurs passively,
whereas removal from the phloem is done actively.
c. Carbohydrates in the phloem near the leaves increases the density of the
phloem contents, thereby causing it to flow downward to the roots.
d. Answers a and b are both correct.
The correct answer is a—Carbohydrates move from a source to a sink.
A. Answer a is correct. The movement of carbohydrates follows the pattern from
source to sink, whether the sink is above or below the source.
The correct answer is a—
B. Answer b is incorrect. Loading and unloading requires active transport.
The correct answer is a—
C. Answer c is incorrect. The density of the phloem contents does not drive mass
flow.
The correct answer is a—
D. Answer d is incorrect. Answer b is incorrect.
Challenge Questions
1. Roots are highly specialized to acquire water from the environment, yet plants
that grow in wet, boggy environments have roots that are specialized to acquire
oxygen! Discuss these structural adaptations and why they are important for
survival of the plant.
Answer—Structural adaptations to very wet environments include pneumatophores that
are root outgrowths that grow above the surface of the water and allow roots to obtain
sufficient oxygen. Other plants have extra lenticels, fairly large openings on stems that
enhance the exchange of oxygen. Yet another adaptation occurs in the leaves with extra
air space in the mesophyll. This type of tissue is called aerenchyma and it allows the
plant to collect an unusually large amount of oxygen that can be transported down to the
roots.
Roots need both water and oxygen. For many plants there is not enough dissolved
oxygen in very wet environments for the root cells to undergo respiration. Without
cellular respiration there is not enough energy for cells to survive.
2. Cavitation presents considerable challenges to the function of xylem in plants.
Plants can cope with this because water can be rerouted around the blocked
passage. Some plants are capable of removing air bubbles by producing enough
positive pressure to cause the air bubble to dissolve back into the water. How
might plants do this?
Answer—When the stomata are open, the atmosphere acts as the force to draw water up
through the plant, thus producing a negative pressure. Remember that positive pressure
also develops in the xylem at the root end of the plant. If a plant closes its stomata and
increases the solute concentration, water can be drawn out of the soil and it might
produce enough positive pressure to dissolve the air bubble.