Study Questions 3 PLANT TRANSPORT
Plant Transport
1. How do mineral nutrients get into the vascular tissue of roots? How does water get into the vascular tissue of roots? What is the function of the endodermis? How does the structure of the endodermis allow it to perform its function?
Mineral nutrients are actively transported into the vascular tissue of roots at the endodermis. Water follows the minerals by osmosis as long as the solute concentration inside the root is higher than the solute concentration in the soil outside. The endodermis forces all substances through membrane before entering the vascular tissue. It can force membrane selectivity because the cell walls on the top, bottom, and radial walls are coated with a hydrophobic waxy layer called the Casparian strip that prevents water from passing along the cell walls at the endodermis layer.
2. What tissue transports water from roots to leaves? How does water get to the top of a plant? How does the structure of the water-transport tissue allow it to perform its function? Why can the water-transport tissue be dead and still perform its function?
Xylem transports water from roots to leaves. It is moved because evaporation from the stomates creates tension in the xylem, which pulls water up the stem. This pull can be transmitted from the top of the plant to the roots because of cohesion of water. Xylem is specialized to perform this function by having hollow, dead cells with no end walls (creating a narrow tube up the plant), by having spiral or ring-like thickenings to keep the cells from collapsing under tension, and by having lignin in the cell walls, also to strengthen the walls against collapse. Because xylem is under tension, rather than under pressure, there is no tendency for xylem contents to move out of the xylem. Therefore, there is no need for a membrane to keep the xylem contents in. And being dead, there is no cytoplasm to interfere with transport.
3. Why do plants have stomates? By what mechanism are stomates opened and closed? How does the structure of guard cells allow them to perform their function? What environmental conditions will increase/decrease transpiration?
Plants have stomata to allow gas exchange. Plants must allow CO2, a raw material for photosynthesis, in and O2, a waste product of photosynthesis, out. Stomates open when turgor pressure is created by osmosis of water into the cells following active transport of potassium ions into the guard cells. The unevenly thickened walls of the guard cells forces them to bow out when under pressure. The two guard cells bow out away from each other, creating a hole, the stomate, between them. Any environmental condition that increases evaporation (e.g., low humidity, warmer temperatures) will increase transpiration (and vice versa), but in addition, any environmental condition that will cause the plant to open its stomates more fully (e.g., high light intensity, low CO2 levels) will also increase transpiration.
4. What tissue transports sugar (and water) in plants? How is sugar transported from leaves to other parts of a plant? How does the structure of the sugar-transport tissue allow it to perform its function? Why must the sugar-transport tissue be living?
Sugar and water are transported in the phloem. Sugar water moves from areas of high sugar concentration (a source, typically leaves) to areas of low sugar concentration (a sink, such as fruits, growing stem tips, roots). This movement happens because the high sugar concentration developed by actively transporting sugar into the phloem causes water to follow by osmosis. The increased water causes pressure to build up and that pressure forces the sugar water to move to an area of lower pressure, typically where sugar is being actively transported out of the phloem for use. This flow from high pressure to low pressure is called bulk flow. Phloem is specialized to deliver sugar water by bulk flow in that the phloem sieve tubes are stacked end to end, they have large holes in the cross walls that facilitate movement from one cell to the next, and they have a membrane to keep the sugar inside the phloem cells. The living cell necessary to support the membrane, and the membrane is necessary to keep the sugar water inside the cells. Without the membrane, the sugar water would just leak out as soon as pressure was developed. The membrane prevents sugar from leaking out until it has reached a point where it is actively unloaded.
5. You've cut a plant stem and you notice a small amount of sap on the cut stump. From where did this liquid come? Why?
Xylem is under tension, and when it is cut, air is sucked in. Phloem is under pressure, and when it is cut, fluid leaks out. Therefore, the sap on the cut stump is probably from the phloem.
6. Plants don't have transport systems for waste products. Why?
Plants don't need a waste product transport system because they only take in what they need. They don't make much in the way of waste products (and what they do produce, they either break down metabolically and reuse, or they store it in their central vacuoles).
7. How are plant roots modified to perform their absorption function?
Plant roots need to absorb water and nutrients from the soil. Absorption requires a large surface area so that the roots can be in contact with as much soil as possible. This high surface area is accomplished by having lots of branches with small diameters, and also by having root hairs. In addition, mycorrhizal interactions extend the absorption capability of most roots by vastly increasing the surface area.
8. If you were to measure very carefully, you would find that the diameter of trees decreases slightly when conditions are warm and dry compared to when conditions are cool and moist. Why?
Trees are made mostly of xylem. Because warm dry conditions cause rapid transpiration, the loss of water puts the many xylem cells under tension. This tension decreases the diameter of each xylem tube by a tiny amount, but put all together, the decrease in diameter is measurable.
9. Although most plants open their stomates in the daytime and close them at night, some desert plants do just the opposite: they close their stomates in the daytime and open them at night. Why? How might these plants have to alter their photosynthetic machinery to accommodate this change in stomate function?
In the desert, it is very hot and dry in the day and plants would lose a lot more water to gain the CO2 they need for photosynthesis than if their stomates were open in the cooler night. However, this means that these plants acquire their CO2 when they aren't producing the energy to do anything with it. So they must store the CO2 in chemical form until morning, when the stomates close, but the light allows the generation of energy. The CO2 storage occurs in the form of Crassulacean Acid, which gives this photosynthetic system the name CAM, for Crassulacean Acid Metabolism.
10. Overwatering is one of the most common houseplant problems. Why is overwatering bad for plants?
Plant roots respire and need O2 that they get from the soil's large pores. Overwatering causes the large air pores to fill up with water and exclude the O2 that the roots need to survive. Without the O2, the roots can't respire and die. Without roots, the plant can't get water from the soil and the plant dies.
11. Fertilizers often come with the warning "Do not exceed recommended application rate." Shy would it be damaging to overfertilize a plant?
Although plants need mineral nutrients, these nutrients shouldn't be in higher concentration in the soil than they are in the plant. Water must enter the roots by osmosis, moving into the root because the root has a higher solute concentration than the soil outside. If too much fertilizer is added to the soil, the solute concentration in the soil is higher than in the plant root, water leaves the root, and the plant can't get any water into the stem.
12. Far more species of insects feed on phloem sap (the solution transported in phloem) than on xylem sap. What is one difference between phloem and xylem that could account for this pattern in insect feeding? Explain.
Phloem sap contains sugar; xylem sap contains minerals. Sugar is a much better food source than are minerals.
13. It has been noted that when phloem-feeding aphids attack a plant, more sugar is sent to the area with the aphids than before the aphids attacked. How can this observation be explained by the mechanism of phloem transport?
Because of the mechanism of bulk flow, sugar water is sent to any site where sugar is being removed from the phloem and reducing the pressure. Therefore, when aphids feed on phloem sap, it creates a lower-pressure sink than was there before, and more sap is moved to that point than before.