=ñjtl>Chapter Outline
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Chapter 35: Transport in Plants

Chapter Outline

Chapter 35: Transport in Plants <.íz"

35.0 Introduction

  1. Two Transport Processes Occur in Plants

    1. Transport Process Must Occur for Plants to Function

      1. Carbohydrates carried from leaves to other parts

      2. Water transported from roots to other plant parts fig 35.1

      3. Fluid transport via phloem and xylem

    2. Passive Forces Drive the Movement of Plant Liquids

      3. Not driven actively by a pump like in animals

    3. Passive forces rely on microscopically narrow transport tubes in plants

35.1 Evaporation of water from leaves draws water up the stem

  1. Overview of Water Movement Through Plants

    1. Water Must Travel Far to Reach Top of Trees fig 35.2

      1. Plants possess water-conducting xylem elements

      2. Water eoí{ls through roots, travels to xylem by osmosis

    2. The Puzzle: How Does Water Get to the Top of a Tree?

      1. Water in dish full of water will rise in narrow tube

        1. Narrower tube, higher water will rise fig 35.3

        2. Limit set by atmospheric pressure

        3. Water rises to 10.4 meters at sea level

      2. Weight of water pulls itself down in tube taller than 10.4 meters

          3. Capillarity in narrow tube occurs only if tube end is open, not true in plant

        1. Capillary action still only works to height of one meter

    3. The Solution: Evaporation Sucks It Up

      1. Renner suggested solution in 1911

      2. Model to examine plants water transport

        1. Plant is cylinder embedded in moist soil, has small pores at top

        2. Assume water held at top via capillary action (not!ílUe reason)

        3. Blow air across top of tube, water evaporates from pores

        4. Water level remains constant as more water drawn up through cylinder

      3. Passage of air across leaf evaporates water, pulls new water in

      4. Water can rise beyond point supported by atmospheric pressure (10.4 meters)

        1. Caused by tension resulting from evaporation at leaves

        2. Water molecules removed from leaves

          1. Tensile strength varies with diameter of tube

          2. Smaller diameter, greater tensile strength

      5. Deformed cells or freezing causes air bubbles to form in xylem

        1. Tracheids and vessel members connected by pits in walls

        2. Bubbles block openings of vessel members and tracheids

        3. Bubbles lack plasticity, unacõ{to pass through end pores, flow is blocked

        4. Bubbles remain in elements where they form

        5. Overall flow upward continues through parallel elements

    4. Water Potential

      1. Potential energy of water

        1. Pressure can prevent osmosis from occurring

        2. Solute or osmotic potential is amount to stop movement by osmosis

          1. Based on concentration gradients of solute and rörUent across membrane

          2. Water enters cell by osmosis

          3. Continues until solute potential offset by cell's resistance to expansion

      2. Pressure potential is a physical pressure

        1. Turgor pressure results as water enters cell vacuole

        2. Water through hose is another physical pressure

      3. Water potential = pressure potential + solute potential

        1. Represents tníH potential energy of water in plant

        2. Adjacent cells with different water potentials

        3. Water moves from cell with height potential to cell with lower potential

      4. Water moves along gradient in plant

        1. High water potential in soil

        2. Lower water potential in roots, stems, leaves and atmosphere

      5. Forces that result in upward movement of water in plants

        1. OslöjLc absorption at roots

        2. Negative pressure caused by evaporation

      6. Process defined as transpiration fig 35.4

35.2 Plants absorb water into their roots and lose it from their leaves

  1. The Absorption of Water by Roots

    1. Value of Root Hairs

      1. Most water enters through root hairs

        1. Root hairs always turgid due!íqsolute potential

        2. Transport of minerals requires expenditure of ATP energy

      2. Root hair plasma membranes contain various protein transport channels

        1. Proton pumps transport specific ions against huge concentration gradients

        2. Proton pumps are H+-ATPases

        3. Chemiosmosis: Flow of ions across membranes

          1. Compared to movement of molecules during osmosis

          2. ADP + Q¹nUoduce ATP, coupled to ion movement

        4. Ions, plant nutrients, get in roots and are transported to plant via xylem

    2. Water and Minerals Pass into Conducting Elements of Xylem fig 35.5

      1. Nonselectively follow along cell walls and spaces between cells

      2. Selectively go through plasma membrane and through protoplasm of adjacent cells

      3. Water and mineral passage stopped at Casparian strip

        1. Endodermal cells selectively control mineral movement

        2. Endodermis, cortex and epidermis control which ions reach xylem

      4. Transpiration helps water and dissolved ions enter root cells fig 35.6

        1. Transpiration may cease at night due to high relative humidity of air

        2. Negative pressure component of water potential becomes very small

        3. Active transport of ions still occurs in roots

          4. “"eI>Water passes inward via osmosis, called root pressure

        4. Active transport increases solute potential of roots

        5. Water moves into plant, up xylem without transpiration

      5. Phenomenon called root pressure

        1. Pressure strong enough to force water from cut ends of plant

        2. Strong pressure causes guttation, water is forced out of cells in veins fig 35.7

        3. Occurs only in very short plants

          4. =¶n

  2. Transpiration of Water from Leaves

    1. Transpiration

      1. Majority of water taken up by roots is lost to atmosphere

        1. Exit leaves through stomata in form of water vapor

        2. Water first passes into intercellular spaces between mesophyll

      2. Water in spaces renewed by flow from leaf veinlets

    2. Conflicting Requirements of Photosynthesis and Watdë>yetention

      1. Plant must have continual source of water to survive

        1. Must minimize water lost to atmosphere

        2. Must admit CO2

        3. Plant features evolved to balance these two situations

      2. Tension created in xylem during active transpiration

        1. Walls of vessels pulled close together

        2. Diameter of tree trunk may be less during the day than at night

        3. Negative water potential also occurs in columns of dead xylem cells

      3. Warming of leaves and small branches increases rate of evaporation

        1. Creates pull in veinlets at upper end of water column

        2. Pull extends downward to small branches, then tree trunk

        3. Creates force that pulls water in through roots

        4. Sun is ultimate source of potential energy

    3. The Regulation of Tsøp^piration Rate

      1. Control short term loss of water by closing stomata

        1. Must counter balance water loss with need for CO2

        2. Intercellular spaces must be moist for CO2 to enter cells

      2. CO2 must dissolve in water before it can enter plant cells

        1. Dissolves mainly in water on walls of intercellular spaces

        2. Walls kept moist by flow of water from roouê"li>

      3. Changing water pressure in guard cells regulates stomata fig 35.8a

        1. Guard cells are only epidermal cells with chloroplasts

        2. Distinctive curved shape, cell wall thicker next to stomatal opening

        3. Turgid cells have bowed shape, open stomata

      4. Loss of guard cell turgor causes uptake of K+ ions, ATP-powered ion transport channels

        1. Creates solute potentiam¹wA guard cells, water enters osmotically

        2. Cells accumulate water, are turgid, open stomata fig 35.8b

      5. Turgidity results from active ion pumping

        1. Requires expenditure of energy

        2. When active transport stops ions move out by diffusion

        3. With lower solute potential water leaves guard cell

        4. Guard cells become flaccid, close stomata

      6. Ions important to stomatal regulathöp primarily potassium

        1. Large numbers of K+ held in cells between guard cells

        2. Gradient of ion changes rapidly, stomata may open or close rapidly

        3. Photosynthesis in guard cells provide ATP

        4. Changes in gradient results in rapid stomata opening or closing

        5. Accompanied by passage of Cl- inward or H+ outward

      7. Plant wilts when water is scarce, guard cells lose!íkCgor, stomata close

        1. Guard cells may be turgid in morning with photosynthesis

        2. May lose turgor in evening independent of water availability

        3. When guard cells are turgid, stomata open, CO2 enters

        4. When guard cells are flaccid CO2 is excluded and water loss is retarded

      8. Abscissic acid effects passage of K+

        1. Allows K+ uö>@apidly pass out of guard cells, stomata close

        2. Hormone produced in leaves under water stress

        3. Binds to receptors on guard cell plasma membrane

        4. Application of abscissic acid may conserve water in plants

    4. Other Factors Regulating Transpiration

      1. Concentration of CO2

        1. Guard cells become flaccid with high concentrations

        2. Stomata close bebøk@e there is no need for more CO2

      2. Temperature: Stomata close when temperature is above 30° to 34° C

      3. In dark stoma will open if CO2 is low

      4. CAM photosynthesis allows plants to conserve water by taking in CO2 at night and fixing it during the day

      5. Seasonal dormancy regulates water loss

        1. Deciduous plants lose leaves dusðpS dry seasons, including winter

        2. Annual plants exist only in the form of seeds

      6. Leaf morphology regulates water loss

        1. Thick, hard leaves with few stomata are more resistant to drying

        2. Wooly trichomes trap humid layer of air near the leaf surface

        3. Stomata may be present in pits in the leaf surface

35.3 Nutrients move up the stem, carbohydrates down

  • Nutrient Movement

    1. Ion Transport Through the Xylem

      1. Movement of ions occurs through protoplasts of cells, not through walls

        1. Passage is active and carrier-mediated

        2. Specific ions maintained at levels different from those of the soil

      2. Lose capacity to absorb nutrients when deficient in oxygen

      3. Ions ultimately transported throughout plant=¶r_>

        1. Seasonal abundance of phosphorus, potassium, nitrogen, iron in xylem

        2. Essential nutrients may be translocated from parts to be shed

        3. Calcium cannot be translocated once it is deposited

    2. Carbohydrate Translocation through the Phloem

      1. Carbohydrates manufactured in leaves distributed through phloem

      2. Process called translocation

      3. Carbohydrates may be concentratdý>^n storage organs like tubers

        1. Generally stored in the form of starch

        2. May be converted to transportable forms like sucrose

      4. Translocation studied using aphids fig 35.9

        1. Sucrose comprises most of dry matter of phloem liquid

        2. Movement may be as rapid as 50 to 100 centimeters per hour

    3. Flow of Phloem Liquid

      1. Mass-flow hypothesis, amêqcalled pressure flow or bulk flow hypothesis

        1. Source is location of sucrose production

        2. Sink is region where sucrose is utilized

        3. Sources include areas of photosynthesis and food storage tissues

        4. Sinks are growing tips of roots and shoots, developing fruit

      2. Process called phloem loading fig 35.10

        1. Sucrose actively loaded into phloem of leaf veinlets

        2. Turgor pressure in sieve tubes increases

        3. Drives fluid through plant's sieve tube system

        4. At sink water leaves as carbohydrates are actively removed

        5. Turgor pressure drops

        6. Results in mass flow from high pressure at source to lower pressure at sink

      3. Water diffuses back into xylem, recycled or lost through transpiration

    =ÕWFlooding: Too Much of a Good Thing

    1. Plant Responses to Flooding

      1. Flooding depletes available oxygen in soil

        1. Blocks normal reactions in roots

        2. Affects transport of minerals and carbohydrates

      2. Result in abnormal growth patterns, plants may "drown"

        1. Changes in hormone levels

        2. Ethylene increases

        3. Giberillins and cytnòwUins decrease

      3. Flooding associated with moving water is less damaging

        1. Brings in new supplies of oxygen

        2. Standing water does not bring in oxygen

      4. Flooding during plant dormancy is less harmful than if actively growing

      5. Oxygen deprivation may cause physical changes in roots

        1. Halts water flow through plant

        2. Dries out leaves

        3. Stomata füpYrally close with such stress

        4. Closing stomata may maintain some leaf turgor

    2. Adapting to Life in Fresh Water

      1. Many plants have evolved to live in continuously wet places fig 35.11

      2. Common adaptation is formation of aerenchyma

        1. Loose parenchyma tissue with large air spaces

        2. Prominent in water lilies and other aquatic plants

      3. Oxygen transported frnô>\bove water level to below water level through passages

        1. Supplies oxygen to submerged parts of plant

        2. Allows oxidative respiration to occur

      4. Some plants for aerenchyma only when needed

        1. In corn, ethylene induces its formation

        2. Becomes abundant in flooded conditions

      5. Additional responses to flooding

        1. Forming larger lenticels to fachõwJate gas exchange

        2. Formation of additional adventitious roots

    3. Adapting to Life in Salt Water

      1. Example: Mangroves

      2. Must supply oxygen and control salt balance

      3. Salt excluded, actively secreted or diluted

        1. Mangroves have arching silt roots connected to spongy air roots

        2. Spongy air roots have lenticels on above water portions

        3. Oxygen enters rnöjL, transported to submerged roots

      4. Succulent leaves contain large amounts of water to dilute salt that reaches them

      5. Other plants secrete salt through specialized glands

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