Chapter 34

1. Plant cells swell when they take up water. The cell wall resists this swelling, causing an increase in pressure within the cell. This results in more rigid forms of the plant organs and also reduces the tendency of the plant cells to take in more water.

2. Aquaporins are proteins that form membrane channels, allowing the passage of water via osmosis. They occur in the cell membrane and vacuolar membrane. Their presence and numbers affect the rate of water movement across the membrane, from a region of higher water potential to a region of lower water potential. One would expect increased expression of aquaporins in the vacuolar membrane as the cell expands, allowing increased uptake of water and turgor pressure.

3. The apoplast lies outside the cell membrane and consists of the cell wall and intercellular spaces. The symplast is the cytoplasm of cells that can be considered as a continuous compartment if cells are connected by plasmodesmata. Water moves rapidly through the apoplast and more slowly through the symplast.

1. A tree was cut near its base, and the upper part of the cut stem was placed in a poisonous solution. The poison rose in the tree, killing cells as it went. This indicated that the fluid could rise without the need for the root.

2. 1. The difference in water potential between the soil and the leaf (1.7 MPa) is enough to overcome gravity and draw water to the top of the tree.

   2. If the soil water potential decreased to –1.0 MPa, it would be more negative than inside the root cells and water would leave the roots (and enter the soil).

   3. If all the stomata closed, the leaf water potential would not be as negative. This in turn would make the xylem water potential less negative, and so on down to the roots. This would make the difference between the leaf water potential and the root water potential insufficient for water to flow from the roots to the leaves (toward a more negative water potential).

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3. Transpiration evaporates water from cell walls in the leaves. This must occur because it begins the process. The increase in negative pressure potential resulting from transpiration draws more water into the cell walls and begins to exert tension (the second part of the process) on the entire water column within the xylem. Cohesion draws water molecules together and makes them “stick.” It prevents water in the column from breaking, thus losing tension and failing to rise.

4. The great difference between water pressure potential in the soil and the air is sufficient to pull water upward through the xylem, as water evaporates into the air. This is evidence for transpiration. The continuous column of water moving upward through the xylem is evidence for cohesion. Cut stems show high negative pressure potentials, showing that the xylem is under considerable tension.

1. In sunlight, an H⁺ pump in the cell membrane of guard cells pumps H⁺ out of the cells. This sets up an electrochemical gradient across the guard cell membrane and K⁺ enters the cells. This in turn sets up an osmotic gradient and water enters the guard cells, causing them to become turgid and a gap to appear between adjacent guard cells. This gap is an open stoma. In darkness, the proton pump becomes less active, K⁺ diffuses passively out of the cell, and the stoma closes.

2. On a hot, dry day, mesophyll cells lose water and this results in stomatal closure. Stomata remain open on cooler, humid days.

1. A source is an organ that produces more carbohydrates that it requires. Sources can be photosynthetic (e.g., leaves) or storage organs with starch (e.g., seeds). A sink is an organ that does not produce enough carbohydrates for its own use and must import them from a source. Examples of sinks include roots and flowers.

2. Source cells load sucrose into sieve tubes of the phloem, decreasing their water potential. Water from adjacent xylem vessels enters the sieve tubes. This results in pressure inside the tubes, so the sap flows. At the sink, sucrose is removed, so the water potential inside the tubes increases. Water leaves to enter the xylem. The combination of these two events causes pressure flow.

1. There was increased water-use efficiency (biomass accumulation divided by transpiration) in plants overexpressing HARDY because there was decreased transpiration (line 2) and increased photosynthesis–carbon fixation (line 3).

2. Use a t-test for paired samples.

1. The increase in cell volume indicated that TIP may increase water permeability of the cell membrane and is therefore an aquaporin. The unrelated mRNA condition showed no increased cell volume, so the volume increase was not just due to any mRNA. The uninjected cells were controls to show that there was no natural increase when cells were placed in an environment that had increased water potential relative to the cell interior. This confirms low osmotic permeability of the frog oocytes.

2. The oocytes burst because there was a continuing uptake of water due to the plant aquaporin. No, plant cells injected with TIP mRNA would not have burst. In plant cells, the cell wall exerts turgor pressure potential that stops the cell from bursting.

3. The table shows direct measurements of increased osmotic water permeability in oocytes injected with TIP mRNA. This indicates that the increase in size shown in the figure, due to the presence of TIP in the oocyte membrane, was due to increased water flow, and in this case water flowed into the cell because the oocytes were in an environment of higher water potential (hypotonic).

4. In the apoplastic pathway, the PIPs would be most prominent in the stele and in the endodermis, where the Casparian strip prevents apoplastic movement of water. In the symplastic pathway, the PIPs would also be found in epidermal and cortical cells.

5. Use a specific stain that can visualize only PIP. Elongating cells would have increased staining in their cell membranes for PIP and in their tonoplasts for TIP.