1). If you could connect and active xylem vessel from a shoot to an active phloem sieve-tube member from a leaf using a "micropipe," which way would the solution flow between the two?
a). The solution would flow from xylem to phloem.
b). The solution would flow from phloem to xylem.
c). The solution would flow back and forth from one to another.
d). The solution would not flow between the two.
2). 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). All of these could be the result of keeping stomata closed.
d). None of these would be the result of keeping stomata closed.
3). If a cell with a solute potential of – 0.2 MPa and a pressure potential of 0.4 MPa is placed in a chamber filled with pure water that is pressurized with 0.5 MPa, what will happen?
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.
4). You are a molecule of water traveling through the plant. Which of the following processes would not provide a driving force for you to move at either a cellular level or over longer distances through the plant?
a). mass flow
e). All the above are driving forces for water movement.
5). The movement of water in the xylem relies upon the
a). ability of water molecules to hydrogen-bond with each other.
b). active transport.
c). evaporation of water from the leaf surface.
d). Both a and b are correct.
e). Both a and c are correct.
6). You place a piece of potato weighing 0.3 gram with a water potential of 1 MPa in a beaker of Pepsi. After 10 minutes, you remove the potato piece, and it now weighs 0.25 gram. You conclude that
a). Pepsi Cola has a water potential greater than 1 MPa.
b). Pepsi Cola has a water potential of 0 MPa.
c). Pepsi Cola has a water potential less than 1 MPa.
d). Pepsi Cola does not have turgor pressure, and so you cannot conclude anything about its water potential.
7). Sucrose enters a phloem sieve-tube cell because of
b). water potential.
c). active transport.
d). a process regulated by auxin.
8). Blowing water up through a drinking straw is most like
c). mass flow in xylem.
d). mass flow in phloem.
9). 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.
10). The Casparian strip is analogous to
a). caulking to waterproof a seam in the bathtub.
b). axle grease to lubricate a wheel.
c). a condom to prevent fertilization.
d). masking tape to hold things together.
Test Your Visual Understanding
1). Which of the pairs of guard cells in the picture has more water inside?
Answer: The cell on the left actually has more water in its guard cells. When water moves into the guard cells, they expand. The inner wall is more rigid and cannot expand. Thus it gets pulled back leaving an opening in the center. The picture on the right shows guard cells that have lost turgor pressure. The opening in the center is covered as the cells become flaccid.
Apply Your Knowledge
1). Design a simple working model of the major transport systems in plants using commonly available equipment (e.g., vacuum cleaners, bicycle pumps, a garden hose). Be sure to discriminate between xylem transport mechanisms and phloem transport mechanisms.
Answer: Xylem transport: Place one end of a garden hose into a garbage can of water and extend the hose upwards. You would need some type of support to keep the hose vertical. Attach a vacuum cleaner (preferably a shop vac that can handle water) to the top of the hose and turn on the vacuum cleaner. The water molecules will be held together by their cohesive properties and the vacuum will provide the transpiration pull that evaporation of water through stomata normally provides in a plant.
Phloem: Seal one end of a garden hose and make tiny holes in the hose at that end. Fill the hose with water, place the sealed end (with its tiny holes) in a garbage can, and raise the other end vertically. You will need some type of support to keep the hose vertical. Take a bicycle pump and raise the handle to fill the pump with air. Now find a way to attach the bicycle pump to the top of the hose so that no water can leak between the pump and that end of the hose. Press the handle of the pump down so that you use the air to force water down in the hose. This should force droplets of water out of the small hole you have made at the other end of the hose into the garbage can. You have created a pressure flow model of phloem transport. In a real plant, sucrose would be actively transported into the phloem near a leaf where photosynthesis occurred. Water would then move by osmosis into the phloem and force the fluid to move away from the leaf, just like the bicycle pump would force water down your garden hose. Note: Since phloem transport is bidirectional, you could turn your model upside down and still have it represent phloem transport.
2). If you could turn a plant upside down without affecting the function of the major organs (roots, shoots, and leaves), would transport of water in xylem move upwards toward the roots, or would it still move toward the leaves? Would transport of photosynthate in phloem change as a result of this inversion?
Answer: Water would still move towards the leaves in the xylem of your inverted plant. Roots don't have stomates for water to move through and there is not a mechanism for water to move from the ground into leaves and then exit through the roots via transpiration. Transpiration would still move water out through the leaves, but this would stop as the roots dried out.
Transport in the phloem would not be altered. Photosynthate would still move from the source to the sink as described by the pressure flow model for phloem transport.
3). 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.