Transport in Vascular Plants – Purves et al., Chapter 35 Autotrophs, like plants, need inorganic raw materials to build organic molecules. These inorganic materials include CO2 that is used in photosynthesis, H O that is used in photosynthesis and that 2 provides the pressure to hold non-woody plants upright, and a range of mineral nutrients. Non-vascular plants (mosses, etc.) rely on diffusion to move things, but this is too slow for larger plants. The evolution of the vascular system allowed terrestrial plants to grow large in size. In these lectures, we will learn about the vascular systems of plants and how they function to circulate water, mineral nutrients, and organic solutes throughout the plant. We will also learn about how plants mediate gas exchange with the environment as they take in CO2 from the air and release water vapor and O2 in exchange. I. Three main types of transport at tissue level: A. Transpiration = movement of water from soil to leaf and, thence, to atmosphere. 1. Mineral nutrients from the soil move to the shoot via transpiration stream. 2. Occurs in xylem cells in vascular bundles. B. Gas exchange = controlled movement of CO2 into plant and water vapor and O2 out of plant. 1. Occurs at stomata in epidermis of leaves (and sometimes stems). C. Translocation = movement of sugars and other organic molecules throughout plant. 1. Occurs in phloem cells in vascular bundles. II. Transpiration A. Xylem 1. mostly dead cells (just cell walls remain) 2. functions: a. pipes for movement of water and mineral nutrients to shoot b. support plant i) secondary xylem = wood [Fig. 34.19-21] -- has lignin in cell walls to add strength and rigidity 3. Conducting cells: [Fig. 34.9, 34.10] a. tracheids = long thin cells with overlapping, tapering ends i. Pits in walls allow water to move from one cell to the next through cell walls. b. vessels (Angiosperms) = cells are connected end-to-end and lack end walls, so they form a hollow pipe. ii. Diameter of vessels is usually greater than that of tracheids. 4. Xylem, and the water stream within it, is continuous from roots to leaves. B. Transpiration begins when water diffuses into the root. 1. Water diffuses down a gradient of water potential = potential energy of water in solution [Fig 35.1]. a. water potential = Ψw = Ψp + Ψs b. solute potential = Ψs = presence of solutes lowers energy of water; therefore, Ψs < 0. c. pressure potential = Ψp = positive pressure increases energy; tension decreases energy; therefore, Ψp may be > 0, < 0, or = 0 (no pressure). d. cells lacking walls (like animal cells) cannot sustain pressure, so Ψp = 0, and we only consider Ψs (effects of solutes). e. Water always diffuses from a region of high water potential to a region of low water potential. f. Water diffuses into roots when Ψw of roots < Ψw of soil solution. This usually occurs when the solute concentration of the root cells is higher than that of the soil. (If too much water builds up in roots, Ψp increases, and water may stop diffusing into roots.) 2. Water diffuses into roots near root tips, where many root hairs extend from the epidermis [Fig 34.16]. a. Root hairs increase the surface area of the root, so more water (and minerals) can diffuse in. b. Mycorrhizal fungi, in mutualistic symbiosis with plant, live in roots of almost all plants in nature. i) extend outward into soil to provide surface for uptake of water and minerals for plant. ii) get sugars from plant for energy. 3. Water must diffuse to center of root to reach vascular bundle with xylem to move water to shoot. a. Xylem Ψw < Root Ψw, so water diffuses down water potential gradient. 4. Two pathways of movement [Fig. 35.3]: a. apoplastic = in cell walls i) area outside plasma membrane = apoplast b. symplastic = within cells i) area contained by plasma membranes = symplast ii) symplast is continuous throughout most of plant because thin strands of cytoplasm connect neighboring cells via holes in cell walls called plasmodesmata. [Fig. 34.7] 5. Apoplastic pathway is blocked at endodermis by the Casparian Strip. [Fig. 35.4] a. endodermis = layer of cells surrounding vascular bundle. b. Casparian Strip = layer of waxy suberin embedded in radial cell walls of endodermis. ii) suberin prevents movement of water through walls into vasc. bundle. iii) water and minerals can only move into v.b. by crossing plasma membrane to get into symplastic pathway. iv) p.m. is selectively permeable, so plant can control uptake of many substances and prevent many from going beyond root apoplast. C. Cohesion-Tension Theory explains water movement up xylem from root to shoot. 1. Water is polar molecule, so stick tightly to other water molecules = cohesion. i) It also sticks to polar molecules that form cell walls = adhesion. 2. Water diffuses down water potential gradient. 3. Water potential of atmosphere depends on humidity and is almost always lower than the water potential of water in leaves. 4. This water potential gradient causes water to evaporate rapidly from leaves and diffuse out into the air. i) slowed over most of plant by cuticle ii) stomata are pores on leaf surface that allow water vapor to escape when CO2 is let in for photosynthesis. 5. As water evaporates from leaf cells and goes out via stomata, more w ater molecules are pulled out of xylem by cohesion. i) as water molecules are pulled out of xylem in leaf, more water molecules are pulled up xylem from roots by cohesion. 6. Water movement to shoot is, therefore, caused by pull from above. Water in xylem is under tension (negative pressure) most of the time. i) diameter of trees shrink because of tension when they are transpiring rapidly. 7. Cavitation occurs when water is under too much tension. Column of water breaks, and water can no longer move up through that cell (like vapor lock in gas line). i) water can move around cavitated cells in tracheids, but not in vessels. ii) cavitation may occur when plants are under drought or when xylem water freezes. iii) occurs more often in wide vessels, rather than tracheids. iv) limits freeze- and drought-tolerance of angiosperm trees, while conifers (only have tracheids) are more tolerant. III. Gas Exchange A. Water is the greatest limiting resource to plant growth on land. 1. Cuticle slows down evaporation from surface, but prevents entry of CO2 needed for photosynthesis. 2. Stomata = pores on plant (usu. leaf) surface that allow CO2 in for Ps, while minimizing water loss to air. B. Stoma = pore surrounded by two guard cells that move to regulate the size of the opening into the leaf. [Fig. 35.9] 1. a. Guard cells swell --> opening of stomata b. Guard cells shrink --> closing of stomata 2. Movement of guard cells into or away from pore is due to differential thickening of their cell walls. 3. Swelling and shrinking of guard cells is caused by water movement into or out of guard cells. a. Water diffuses into guard cells when Ψw of guard cells is lower than that of surrounding cells, due to the movement of potassium ions into the g.c. (osmosis of water following potassium) b. Potassium is pumped into the guard cells in response to various environmental signals, such as sunrise, that will lead to opening of the stomata. c. K+ moves into g.c.; g.c. Ψw falls; water diffuses into g.c.; pressure builds; guard cells swell; stoma opens. d. Reverse sequence of events leads to closing of the stoma: c. K+ moves out of g.c., followed by water; pressure drops and cells relax back into the opening, closing the pore. 4. Environmental triggers for stomata: a. light --> opening need CO2 for photosynthesis in light b. low CO2 in leaf --> opening for photosynthesis c. low RH --> closing too much water loss d. low leaf Ψw --> closing overrides all other signals! This is a measure of the water status of the plant; when Ψw is too low, plant is under water stress, so must close stomata to conserve water. IV. Translocation A. Phloem 1. Cells are living, phloem contents are contained within symplast. 2. Functions to transport organic solutes throughout plant. 3. Conducting cells [Fig. 34.11]: a. sieve tube members = large-diameter cells, connected end-to-end (like xylem vessels). i) end walls, sieve plates, have large openings for movement of symplast. ii) lack vacuole, nucleus, most organelles iii) found in Angiosperms b. companion cells contain nucleus and mitochondria for sieve tube members. 4. Phloem also has some fiber cells (for structural support) and parenchyma cells that have not yet developed into one of the other types. B. Translocation 1. Movement of solutes in phloem. a. 90% sugar by dry weight b. remainder is amino acids, nucleotides, some hormones, potassium ions 2. Movement from source to sink. a. Source = region where solutes enter phloem. may be: i) photosynthesizing leaf ii) roots, tubers, bulbs, etc., that are breaking down stored starch and lipids for new growth (usually in spring) iii) seed that is germinating b. sink = region where solutes leave phloem and are used for metabolism (respiration) or are stored (usually as starch). may be: i) roots, tubers, bulbs, etc. ii) developing seeds iii) flowers iv) young, developing, leaves v) fruits 3. Movement occurs by bulk flow, under positive pressure. a. Münch Pressure-Flow Hypothesis [Fig. 35.14] b. at source: i) sugar is loaded into sieve tubes (energy is required to move sucrose into phloem against its concentration gradient) [Fig. 35.15]. ii) increased solute concentration lowers Ψw. iii) water diffuses into sieve tube down Ψw gradient (osmosis). iv) water pressure builds up in sieve tube. v) solution in sieve tube starts flowing under pressure. c. at sink: i) sugar is unloaded from sieve tubes. ii) water diffuses out, following sugar. iii) pressure in sieve tubes drops at sink as water moves out into the tissue. d. high pressure at source and low pressure at sink maintain pressure gradient to keep phloem contents flowing. e. Pressure is high, approx. 150 times higher than human blood pressure. f. Rate of flow may be 1.5 m per hour; each sieve tube member empties and refills every 2 seconds. g. P-proteins (slime proteins ) act like platelets in phloem to block cuts and prevent phloem contents from oozing out. h. Aphids = bugs that feed on phloem by inserting stylet into sieve tubes and getting force- fed. Excess sugar comes out anus as honeydew [Fig. 35.13].
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