Chapter 36 Guided Reading – Flashcards
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Plants reduce self-shading by undergoing self-pruning, where nonproductive leaves undergo programmed cell death.
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2. Competition for light, water, and nutrients is intense among the land plants. Plant success is generally related to photosynthesis, so evolution has resulted in many structural adaptations for efficiently acquiring light from the sun and CO2 from the air. As you read this section, focus on how this is accomplished. Let's look first at adaptations to increase light capture. How do plants reduce selfshading?
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When there is a high leaf area index, the lower leaves respire more than they photosynthesize; this triggers the nonproductive leaves to undergo programmed cell death and they are eventually shed in the process called self-pruning.
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3. What triggers self pruning?
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Grass, Keeps light rays parallel to the leaf surfaces, so no leaf receives too much light and light penetrates more deeply to the lower leaves
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Vertical leaf orientation
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Understory plants in tropical rain forest capture light more effectively in low-light conditions
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Horizontal leaf orientation
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The term mycorrhizae refers to the mutualistic associations between roots and fungi. Mycorrhizae are a critical step in the successful colonization of land by vascular plants, especially given the poorly developed soils available at the time.
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5. The evolution of mycorrhizae was a critical step in the successful colonization of land by plants. What are they, and what is their role in resource acquisition?
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In the symplastic route, water and solutes move along the continuum of cytosol. After entering one cell, substances can move from cell to cell via plasmodesmata.
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6. Water and solutes move through plant tissues in several ways, including between and around cell walls, and from cell to cell. Communication between plant cells is accomplished because the cytosol of adjacent cells is continuous. Describe and explain how this is accomplished by the plasmodesmata.
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The diffusion of a substance across a biological membrane from a region of high concentration to a region of low concentration
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A. What is passive transport?
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Membrane traffic that involves pumping a solute across a membrane against its gradient. This type of transport requires work and the cell must expend energy (ATP is required).
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B. What is active transport?
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Transport proteins span the membrane; they allow hydrophilic substances to avoid contact with the lipid bilayer of a cell membrane as they move.
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C. What are transport proteins?
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Membrane potential is the difference in electrical charge (voltage) across a cell's plasma membrane due to the differential distribution of ions. Membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances. It is established mainly through the pumping of H+ by proton pumps.
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E. Membrane Potential
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During cotransport, plant cells use the energy in the H+ gradient and membrane potential to drive the active transport of many different solutes. For instance, cotransport with H+ is responsible for absorption of neutral solutes, such as the sugar sucrose, by phloem cells and other plant cells. An H+/sucrose cotransporter couples movement of sucrose against its concentration gradient with movement of H+ down its electrochemical gradient.
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F. Cotransport
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The diffusion of water from a region of high concentration to a region of low concentration across a semipermeable membrane
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G. Chemiosmosis
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Water potential is the physical property that predicts the direction in which water will flow. It includes the effects of solute concentration and physical pressure. The equation for water potential is ? = ?s + ?p, where ? is water potential, ?s is solute potential, and ?p is the pressure potential.
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H. Water Potential
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By definition, what is the ?s of pure water? 0 Adding solutes has a negative effect on water potential. When solutes are added, they bind water molecules. As a result, there are fewer free water molecules, reducing the capacity of the water to move and do work. The solute potential of a solution is therefore always negative.
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b. Solute Potential
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Pressure potential is the physical pressure on a solution. Unlike solute potential, pressure potential can be positive or negative relative to atmospheric pressure.
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d. Pressure Potential
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When a flaccid cell is placed in pure water, water moves into the cell via osmosis because the solutes inside the cell create a lower water potential. The contents of the cell begin to swell and press the plasma membrane against the cell wall. The partially elastic wall, exerting turgor pressure, confines the pressurized protoplast. When this pressure is enough to offset the tendency for water to enter because of the solutes in the cell, then the solute potential equals the pressure potential, causing the water potential to equal zero.
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a. Flaccid
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Imagine that a cell is flaccid (limp) as a result of losing water. The cell has a pressure potential of zero. Suppose this cell is bathed in a solution of higher solute concentration than the cell itself. Because the external solution has the lower water potential, water diffuses out of the cell. The cell's protoplast undergoes plasmolysis—that is, it shrinks and pulls away from the cell wall.
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a. Plasmolyze
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Aquaporins are the transport proteins that facilitate the transport of water molecules across membranes.
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b. Aquaporins
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Bulk flow is the movement of liquid in response to a pressure gradient. The bulk flow of material always occurs from higher to lower pressure. Unlike osmosis, bulk flow is independent of solute concentration.
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9. What is bulk flow? Does it depend on solute concentration?
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Diffusion, active transport, and bulk flow act in concert to transport resources throughout the whole plant. Although diffusion is an effective transport mechanism over the spatial scales typically found at the cellular level, bulk flow, or the movement of liquid in response to a pressure gradient, is used for long-distance transport throughout the plant. And although sugars in the phloem are transported these long distances via bulk flow due to a pressure difference, the active transport of sugar at the cellular level maintains this pressure difference.
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10. Summarize the three processes that act together to transport resources through the whole plant.
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The endodermis, the innermost layer of cells in the root cortex, functions as a last checkpoint for the selective passage of minerals from the cortex into the vascular cylinder. Located in the transverse and radial walls of each endodermal cell, the Casparian strip serves as an impervious belt of waxy material called suberin. Because of the Casparian strip, water and minerals cannot cross the endodermis and enter the vascular cylinder via the apoplast. Instead, water and minerals that are passively moving through the apoplast must cross the selectively permeable plasma membrane of an endodermal cell before they can enter the vascular cylinder.
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11. Which structure controls the movement of water and minerals into the xylem? How are its cells modified to achieve this function?
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Many root cells have root hairs that dramatically increase the absorptive capacity of roots by increasing the membrane surface area.
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12. How is the surface area for absorption in roots greatly increased?
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Once inside the root cells, water can pass from cell to cell via diffusion. Water is also transported by way of the symplastic route, as well as the transmembrane route.
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13. What are the roles of the apoplast and the symplast, and how does the Casparian strip relate to these structures?
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Transpiration is the loss of water vapor from leaves through the stomata.
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15. Describe the process of transpiration in your own words and how it affects the transportation of minerals and water.
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Root pressure is the pressure exerted in the roots of plants as the result of osmosis, causing exudation from cut stems and guttation of water from leaves.
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16. There are two mechanisms that pull water up through the plant, from roots to leaves. Explain root pressure. Note that it is a minor mechanism for the upward movement of water.
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According to the cohesion-tension hypothesis, transpiration provides the pull for the ascent of xylem sap, and the cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots. Hence, xylem sap is normally under negative pressure or tension. Because transpiration is a "pulling" process, our exploration of the rise of xylem sap by the cohesion-tension mechanism begins not with the roots but with the leaves, where the driving force for transpirational pull begins.
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17. What is the cohesion-tension hypothesis?
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Hydrogen bonding, including cohesion between water molecules and adhesion between water molecules and xylem tubes, forms an unbroken chain of water molecules extending from leaves to the soil. The force driving the ascent of xylem sap is a gradient of water potential (?). For bulk flow over long distance, the ? gradient is due mainly to a gradient of the pressure potential (?p). Transpiration results in lower water potential at the leaf end where the stomata are found compared with higher water potential at the root end.
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18. The second mechanism that pulls water up through the plant involves transpiration, adhesion, and cohesion. Refer to Figure 36.11 in your text. Note that water is moving from a region of high water potential to a region of lower water potential. The arrow on the left side of the figure shows this gradient. Write an essay to explain the movement of water from the roots to the leaves. Include each of these terms in your essay, and label them on the figure; root hairs, lower water potential, higher water potential, hydrogen bonding, adhesion, cohesion, xylem tubes, and stoma. Spend time with this figure and its explanation. It is an essential concept! See page 789 of your text for the labeled figure.
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advantage: Enhances light absorption for photosynthesis. disadvantage: Increases water loss by way of the stomata.
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19. Leaves generally have large surface areas and high surface-to-volume ratios. Give an advantage and disadvantage of these traits.
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The waxy cuticle limits water loss through the remaining surface of the leaf. Each stoma is flanked by a pair of guard cells. Guard cells control the diameter of the stoma by changing shape, thereby widening or narrowing the gap between the guard cell pair. Under the same environmental conditions, the amount of water lost by a leaf depends largely on the number of stomata and the average size of their pores.
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20. What are stomata and how do they help regulate the rate of transpiration?
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Light stimulates guard cells to accumulate K+ and become turgid. This response is triggered by illumination of blue-light receptors in the plasma membrane of guard cells. Activation of these receptors stimulates the activity of proton pumps in the plasma membrane of the guard cells, in turn promoting absorption of K Stomata open in response to the depletion of CO2 within the leaf's air spaces as a result of photosynthesis. As CO2 concentrations decrease during the day, the stomata progressively open if sufficient water is supplied to the leaf. The internal "clock" ensures that stomata continue their daily rhythm of opening and closing. This rhythm occurs even if the plant is kept in the dark.
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21. What are some factors that could possible stimulate the opening and closing of stomata?
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Highly reduced leaves resist excessive water loss; photosynthesis is mainly carried out in the stem. Fleshy stems store water for use during long dry periods. Some xerophytes have roots that are more than 20 m long, allowing them to acquire moisture at or near the water table. Crassulacean acid metabolism (CAM) is a specialized form of photosynthesis found in succulents of the family Crassulaceae and several other families. Because the leaves of CAM plants take in CO2, the stomata can remain closed during the day, when evaporative stresses are greater.
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22. What are some evolutionary adaptations that allow xerophytes to survive in arid climates?
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The transport of products of photosynthesis
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23. What is translocation?
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A sugar source is a plant organ that is a net producer of sugar by photosynthesis or by breakdown of starch
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a. Sugar source
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A sugar sink is an organ that is a net consumer or depository of sugar. Growing roots, buds, stems, and fruits are sugar sinks. Although expanding leaves are sugar sinks, mature leaves, if well illuminated, are sugar sources.
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b. Sugar sink
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1. Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sieve-tube elements. This causes the tube to take up water by osmosis. 2. This uptake of water generates a positive pressure that forces the sap to flow along the tube. 3. The pressure is relieved by the unloading of sugar and the consequent loss of water at the sink. 4. In leaf-to-root translocation, xylem recycles water from sink to source
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25. Label the following diagram, describing pressure flow in a sieve tube. See page 795 of your text for the labeled figure.
