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Biology 102 Week 5 Plant Structure, Growth, and Development Transport in Vascular Plants PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: No two Plants Are Alike • To some people, the fanwort is an intrusive weed, but to others it is an attractive aquarium plant • This plant exhibits plasticity, the ability to alter itself in response to its environment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In addition to plasticity, plant species have by natural selection accumulated characteristics of morphology that vary little within the species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 35.1: The plant body has a hierarchy of organs, tissues, and cells • Plants, like multicellular animals, have organs composed of different tissues, which are in turn are composed of cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Three Basic Plant Organs: Roots, Stems, and Leaves • Basic morphology of vascular plants reflects their evolution as organisms that draw nutrients from below-ground and above-ground Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Three basic organs evolved: roots, stems, and leaves • They are organized into a root system and a shoot system Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-2 Reproductive shoot (flower) Terminal bud Node Internode Terminal bud Vegetable shoot Leaf Shoot system Blade Petiole Axillary bud Stem Taproot Lateral roots Root system Roots • Functions of roots: – Anchoring the plant – Absorbing minerals and water – Often storing organic nutrients Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In most plants, absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Many plants have modified roots Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-4a Prop roots. LE 35-4b Storage roots. LE 35-4c “Strangling” aerial roots. LE 35-4d Buttress roots. LE 35-4e Pneumatophores. Stems • A stem is an organ consisting of – An alternating system of nodes, the points at which leaves are attached – Internodes, the stem segments between nodes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An axillary bud is a structure that has the potential to form a lateral shoot, or branch • A terminal bud is located near the shoot tip and causes elongation of a young shoot Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Many plants have modified stems Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-5a Stolons. LE 35-5b Storage leaves Stem Roots Bulbs. LE 35-5c Tubers. LE 35-5d Rhizomes. Node Rhizome Root Leaves • The leaf is the main photosynthetic organ of most vascular plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Leaves generally consist of – A flattened blade and a stalk – The petiole, which joins the leaf to a node of the stem Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leaves • Most monocots have parallel veins • Most eudicots have branching veins Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In classifying angiosperms, taxonomists may use leaf morphology as a criterion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-6a Simple leaf Petiole Axillary bud LE 35-6b Leaflet Compound leaf Petiole Axillary bud LE 35-6c Doubly compound leaf Leaflet Petiole Axillary bud • Some plant species have evolved modified leaves that serve various functions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-7a Tendrils. LE 35-7b Spines. LE 35-7c Storage leaves. LE 35-7d Bracts. LE 35-7e Reproductive leaves. The Three Tissue Systems: Dermal, Vascular, and Ground • Each plant organ has dermal, vascular, and ground tissues Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-8 Dermal tissue Ground tissue Vascular tissue • In nonwoody plants, the dermal tissue system consists of the epidermis • In woody plants, protective tissues called periderm replace the epidermis in older regions of stems and roots Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The vascular tissue system carries out longdistance transport of materials between roots and shoots • The two vascular tissues are xylem and phloem Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Xylem conveys water and dissolved minerals upward from roots into the shoots • Phloem transports organic nutrients from where they are made to where they are needed Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The vascular tissue of a stem or root is collectively called the stele • In angiosperms the stele of the root is a solid central vascular cylinder • The stele of stems and leaves is divided into vascular bundles, strands of xylem and phloem Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Tissues that are neither dermal nor vascular are the ground tissue system • Ground tissue includes cells specialized for storage, photosynthesis, and support Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Common Types of Plant Cells • Like any multicellular organism, a plant is characterized by cellular differentiation, the specialization of cells in structure and function Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some major types of plant cells: – Parenchyma – Collenchyma – Sclerenchyma – Water-conducting cells of the xylem – Sugar-conducting cells of the phloem Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-9 WATER-CONDUCTING CELLS OF THE XYLEM PARENCHYMA CELLS Vessel Parenchyma cells in Elodea leaf, with chloroplasts (LM) Tracheids 100 µm 60 µm Pits COLLENCHYMA CELLS 80 µm Cortical parenchyma cells Tracheids and vessels (colorized SEM) Vessel element Vessel elements with perforated end walls Tracheids SUGAR-CONDUCTING CELLS OF THE PHLOEM Collenchyma cells (in cortex of Sambucus, elderberry; cell walls stained red) (LM) Sieve-tube members: longitudinal view (LM) SCLERENCHYMA CELLS 5 µm Companion cell Sclereid cells in pear (LM) Sieve-tube member Plasmodesma 25 µm Sieve plate Cell wall Nucleus Cytoplasm Companion cell 30 µm 15 µm Fiber cells (transverse section from ash tree) (LM) Sieve-tube members: longitudinal view Sieve plate with pores (LM) Concept 35.2: Meristems generate cells for new organs • Apical meristems are located at the tips of roots and in the buds of shoots • Apical meristems elongate shoots and roots, a process called primary growth Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Lateral meristems add thickness to woody plants, a process called secondary growth • There are two lateral meristems: the vascular cambium and the cork cambium • The vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem • The cork cambium replaces the epidermis with periderm, which is thicker and tougher Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-10 Primary growth in stems Shoot apical meristems (in buds) Epidermis Cortex Primary phloem Primary xylem Vascular cambium Lateral meristems Cork cambium Pith Secondary growth in stems Periderm Cork cambium Pith Cortex Primary phloem Primary xylem Root apical meristems Secondary xylem Secondary phloem Vascular cambium • In woody plants, primary and secondary growth occur simultaneously but in different locations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-11 Terminal bud Bud scale Axillary buds Leaf scar This year’s growth (one year old) Node Stem Internode One-year-old side branch formed from axillary bud near shoot apex Leaf scar Last year’s growth (two years old) Scars left by terminal bud scales of previous winters Growth of two years ago (three years old) Leaf scar Concept 35.3: Primary growth lengthens roots and shoots • Primary growth produces the primary plant body, the parts of the root and shoot systems produced by apical meristems Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Primary Growth of Roots • The root tip is covered by a root cap, which protects the apical meristem as the root pushes through soil • Growth occurs just behind the root tip, in three zones of cells: – Zone of cell division – Zone of elongation – Zone of maturation Video: Root Growth in a Radish Seed (time lapse) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-12 Cortex Vascular cylinder Epidermis Key Root hair Dermal Zone of maturation Ground Vascular Zone of elongation Apical meristem Root cap 100 µm Zone of cell division • The primary growth of roots produces the epidermis, ground tissue, and vascular tissue • In most roots, the stele is a vascular cylinder • The ground tissue fills the cortex, the region between the vascular cylinder and epidermis • The innermost layer of the cortex is called the endodermis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-13 Epidermis Cortex Vascular cylinder Endodermis Pericycle Core of parenchyma cells Xylem 100 µm Phloem 100 µm Transverse section of a typical root. In the roots of typical gymnosperms and eudicots, as well as some monocots, the stele is a vascular cylinder consisting of a lobed core of xylem with phloem between the lobes. Endodermis Pericycle Transverse section of a root with parenchyma in the center. The stele of many monocot roots is a vascular cylinder with a core of parenchyma surrounded by a ring of alternating xylem and phloem. Key Dermal Ground Vascular Xylem Phloem 50 µm • Lateral roots arise from within the pericycle, the outermost cell layer in the vascular cylinder Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-14 100 µm Emerging lateral root Cortex Vascular cylinder Epidermis Lateral root Primary Growth of Shoots • A shoot apical meristem is a dome-shaped mass of dividing cells at the tip of the terminal bud • It gives rise to a repetition of internodes and leafbearing nodes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-15 Apical meristem Leaf primordia Developing vascular strand Axillary bud meristems 0.25 mm Tissue Organization of Stems • In gymnosperms and most eudicots, the vascular tissue consists of vascular bundles that are arranged in a ring Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-16 Phloem Xylem Sclerenchyma (fiber cells) Ground tissue Ground tissue connecting pith to cortex Pith Epidermis Key Cortex Epidermis Vascular bundles Dermal Vascular bundles Ground 1 mm A eudicot (sunflower) stem. Vascular bundles form a ring. Ground tissue toward the inside is called pith, and ground tissue toward the outside is called cortex. (LM of transverse section) Vascular 1 mm A monocot (maize) stem. Vascular bundles are scattered throughout the ground tissue. In such an arrangement, ground tissue is not partitioned into pith and cortex. (LM of transverse section) Tissue Organization of Leaves • The epidermis in leaves is interrupted by stomata, which allow CO2 exchange between the air and the photosynthetic cells in a leaf • The ground tissue in a leaf is sandwiched between the upper and lower epidermis • The vascular tissue of each leaf is continuous with the vascular tissue of the stem Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-17 Key to labels Guard cells Dermal Stomatal pore Ground Vascular Cuticle Sclerenchyma fibers Epidermal cells 50 µm Surface view of a spiderwort (Tradescantia) leaf (LM) Stoma Upper epidermis Palisade mesophyll Bundlesheath cell Spongy mesophyll Lower epidermis Guard cells Cuticle Vein Xylem Phloem Cutaway drawing of leaf tissues Guard cells Vein Air spaces Guard cells 100 µm Transverse section of a lilac (Syringa) leaf (LM) Concept 35.4: Secondary growth adds girth to stems and roots in woody plants • Secondary growth occurs in stems and roots of woody plants but rarely in leaves • The secondary plant body consists of the tissues produced by the vascular cambium and cork cambium Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-18a Primary and secondary growth in a two-year-old stem Epidermis Cortex Primary phloem Vascular cambium Primary xylem Pith Pith Primary xylem Vascular cambium Primary phloem Cortex Epidermis Phloem ray Xylem ray Primary xylem Secondary xylem Vascular cambium Secondary phloem Primary phloem First cork cambium Cork Periderm (mainly cork cambia and cork) Primary phloem Secondary phloem Vascular cambium Secondary xylem Primary xylem Pith Secondary xylem (two years of production) Vascular cambium Secondary phloem Bark Most recent cork cambium Cork Layers of periderm LE 35-18b Secondary phloem Vascular cambium Secondary xylem Cork cambium Late wood Early wood Periderm Cork Transverse section of a three-yearold Tilia (linden) stem (LM) Xylem ray Bark 0.5 mm 0.5 mm The Vascular Cambium and Secondary Vascular Tissue • The vascular cambium is a cylinder of meristematic cells one cell thick • It develops from undifferentiated cells and parenchyma cells that regain the capacity of divide Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In transverse section, the vascular cambium appears as a ring, with regions of dividing cells called fusiform initials and ray initials • The initials increase the vascular cambium’s circumference and add secondary xylem to the inside and secondary phloem to the outside Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-19 Vascular cambium Types of cell division Accumulation of secondary growth • As a tree or woody shrub ages, the older layers of secondary xylem, the heartwood, no longer transport water and minerals • The outer layers, known as sapwood, still transport materials through the xylem Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-20 Growth ring Vascular ray Heartwood Secondary xylem Sapwood Vascular cambium Secondary phloem Bark Layers of periderm Cork Cambia and the Production of Periderm • The cork cambium gives rise to the secondary plant body’s protective covering, or periderm • Periderm consists of the cork cambium plus the layers of cork cells it produces • Bark consists of all the tissues external to the vascular cambium, including secondary phloem and periderm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 35.5: Growth, morphogenesis, and differentiation produce the plant body • The three developmental processes of growth, morphogenesis, and cellular differentiation act in concert to transform the fertilized egg into a plant Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Molecular Biology: Revolutionizing the Study of Plants • New techniques and model systems are catalyzing explosive progress in our understanding of plants • Arabidopsis is the first plant to have its entire genome sequenced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-21 Cell organization and biogenesis (1.7%) DNA metabolism (1.8%) Carbohydrate metabolism (2.4%) Unknown (36.6%) Signal transduction (2.6%) Protein biosynthesis (2.7%) Electron transport (3%) Protein modification (3.7%) Protein metabolism (5.7%) Transcription (6.1%) Other biological processes (18.6%) Other metabolism (6.6%) Transport (8.5%) Growth: Cell Division and Cell Expansion • By increasing cell number, cell division in meristems increases the potential for growth • Cell expansion accounts for the actual increase in plant size Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Plane and Symmetry of Cell Division • The plane (direction) and symmetry of cell division are immensely important in determining plant form • If the planes of division are parallel to the plane of the first division, a single file of cells is produced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-22a Division in same plane Single file of cells forms Plane of cell division Division in three planes Cube forms Nucleus Cell divisions in the same plane produce a single file of cells, whereas cell divisions in three planes give rise to a cube. • If the planes of division vary randomly, asymmetrical cell division occurs Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-22b Developing guard cells Asymmetrical cell division Unspecialized epidermal cell Unspecialized Guard cell epidermal cell “mother cell” Unspecialized epidermal cell An asymmetrical cell division precedes the development of epidermal guard cells, the cells that border stomata (see Figure 35.17). • The plane in which a cell divides is determined during late interphase • Microtubules become concentrated into a ring called the preprophase band Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-23 Preprophase bands of microtubules Nuclei Cell plates 10 µm Orientation of Cell Expansion • Plant cells rarely expand equally in all directions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Orientation of the cytoskeleton affects the direction of cell elongation by controlling orientation of cellulose microfibrils within the cell wall Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-24 Cellulose microfibrils Vacuoles Nucleus 5 µm Microtubules and Plant Growth • Studies of fass mutants of Arabidopsis have confirmed the importance of cytoplasmic microtubules in cell division and expansion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-25 fass seeding Wild-type seeding Mass fass mutant Morphogenesis and Pattern Formation • Pattern formation is the development of specific structures in specific locations • It is determined by positional information in the form of signals indicating to each cell its location • Polarity is one type of positional information • In the gnom mutant of Arabidopsis, the establishment of polarity is defective Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Morphogenesis in plants, as in other multicellular organisms, is often controlled by homeotic genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Expression and Control of Cellular Differentiation • In cellular differentiation, cells of a developing organism synthesize different proteins and diverge in structure and function even though they have a common genome • Cellular differentiation to a large extent depends on positional information and is affected by homeotic genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-28 Cortical cells 20 µm Location and a Cell’s Developmental Fate • A cell’s position in a developing organ determines its pathway of differentiation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Shifts in Development: Phase Changes • Plants pass through developmental phases, called phase changes, developing from a juvenile phase to an adult phase • The most obvious morphological changes typically occur in leaf size and shape Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-29 Leaves produced by adult phase of apical meristem Leaves produced by juvenile phase of apical meristem Genetic Control of Flowering • Flower formation involves a phase change from vegetative growth to reproductive growth • It is triggered by a combination of environmental cues and internal signals • Transition from vegetative growth to flowering is associated with the switching-on of floral meristem identity genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Plant biologists have identified several organ identity genes that regulate the development of floral pattern Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-30 Pe Ca St Se Pe Se Normal Arabidopsis flower. Arabidopsis normally has four whorls of flower parts: sepals (Se), petals (Pe), stamens (St), and carpels (Ca). Pe Pe Se Abnormal Arabidopsis flower. This flower has an extra set of petals in place of stamens and an internal flower where normal plants have carpels. • The ABC model of flower formation identifies how floral organ identity genes direct the formation of the four types of floral organs Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-31a Sepals Petals Stamens A B Carpels C B+C A+B gene gene activity activity A gene activity A schematic diagram of the ABC hypothesis C gene activity • An understanding of mutants of the organ identity genes depicts how this model accounts for floral phenotypes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 35-31b Active genes: Whorls: Carpel Stamen Petal Sepal Wild type Mutant lacking A Side view of organ identity mutant flowers Mutant lacking B Mutant lacking C Chapter 36 Transport in Vascular Plants PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Pathways for Survival • For vascular plants, the evolutionary journey onto land involved differentiation into roots and shoots • Vascular tissue transports nutrients in a plant; such transport may occur over long distances Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 36.1: Physical forces drive the transport of materials in plants over a range of distances • Transport in vascular plants occurs on three scales: – Transport of water and solutes by individual cells, such as root hairs – Short-distance transport of substances from cell to cell at the levels of tissues and organs – Long-distance transport within xylem and phloem at the level of the whole plant Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A variety of physical processes are involved in the different types of transport Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-2_4 CO2 O2 Light H2O Sugar O2 H2O Minerals CO2 Selective Permeability of Membranes: A Review • The selective permeability of the plasma membrane controls movement of solutes into and out of the cell • Specific transport proteins enable plant cells to maintain an internal environment different from their surroundings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Central Role of Proton Pumps • Proton pumps in plant cells create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work • They contribute to a voltage known as a membrane potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-3 CYTOPLASM EXTRACELLULAR FLUID ATP Proton pump generates membrane potential and gradient. • Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-4a CYTOPLASM EXTRACELLULAR FLUID Cations ( , for example) are driven into the cell by the membrane potential. Transport protein Membrane potential and cation uptake • In the mechanism called cotransport, a transport protein couples the passage of one solute to the passage of another Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-4b Cell accumulates anions ( , for example) by coupling their transport to; the inward diffusion of through a cotransporter. Cotransport of anions • The “coattail” effect of cotransport is also responsible for the uptake of the sugar sucrose by plant cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-4c Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. Cotransport of a neutral solute Effects of Differences in Water Potential • To survive, plants must balance water uptake and loss • Osmosis determines the net uptake or water loss by a cell is affected by solute concentration and pressure Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Water potential is a measurement that combines the effects of solute concentration and pressure • Water potential determines the direction of movement of water • Water flows from regions of higher water potential to regions of lower water potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How Solutes and Pressure Affect Water Potential • Both pressure and solute concentration affect water potential • The solute potential of a solution is proportional to the number of dissolved molecules • Pressure potential is the physical pressure on a solution Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Quantitative Analysis of Water Potential • The addition of solutes reduces water potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-5a Addition of solutes 0.1 M solution Pure water H2O = 0 MPa P = 0 S = –0.23 P = –0.23 MPa • Physical pressure increases water potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-5b Applying physical pressure H2O = 0 MPa P = 0 S = –0.23 P = –0 MPa LE 36-5c Applying physical pressure H2O = 0 MPa P = 0.30 S = –0.23 P = –0.07 MPa • Negative pressure decreases water potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-5d Negative pressure H2O P = –0.30 S = –0.23 P = –0.30 MPa P = 0.30 S = –0.23 P = –0.23 MPa • Water potential affects uptake and loss of water by plant cells • If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and become plasmolyzed Video: Plasmolysis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-6 Plasmolyzed cell at osmotic equilibrium 0.4 M sucrose solution: P = 0 S = –0.9 P = –0.9 MPa P = 0 S = –0.9 P = –0.9 MPa conditions: cellular > environmental Initial flaccid cell: P = 0 S = –0.7 P = –0.7 MPa Distilled water: P = 0 S = 0 P = 0 MPa Turgid cell at osmotic equilibrium udings P = 0.7 S = –0.7 P = 0 MPa • If the same flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid Video: Turgid Elodea Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Turgor loss in plants causes wilting, which can be reversed when the plant is watered Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aquaporin Proteins and Water Transport • Aquaporins are transport proteins in the cell membrane that allow the passage of water • Aquaporins do not affect water potential Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Three Major Compartments of Vacuolated Plant Cells • Transport is also regulated by the compartmental structure of plant cells • The plasma membrane directly controls the traffic of molecules into and out of the protoplast • The plasma membrane is a barrier between two major compartments, the cell wall and the cytosol Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The third major compartment in most mature plant cells is the vacuole, a large organelle that occupies as much as 90% or more of the protoplast’s volume • The vacuolar membrane regulates transport between the cytosol and the vacuole Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-8a Cell wall Cytosol Vacuole Plasmodesma Key Symplast Apoplast Vacuolar membrane (tonoplast) Plasma membrane Cell compartments • In most plant tissues, the cell walls and cytosol are continuous from cell to cell • The cytoplasmic continuum is called the symplast • The apoplast is the continuum of cell walls and extracellular spaces Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-8b Key Symplast Apoplast Transmembrane route Apoplast Symplast Symplastic route Apoplastic route Transport routes between cells Functions of the Symplast and Apoplast in Transport • Water and minerals can travel through a plant by three routes: – Transmembrane route: out of one cell, across a cell wall, and into another cell – Symplastic route: via the continuum of cytosol – Apoplastic route: via the the cell walls and extracellular spaces Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bulk Flow in Long-Distance Transport • In bulk flow, movement of fluid in the xylem and phloem is driven by pressure differences at opposite ends of the xylem vessels and sieve tubes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 36.2: Roots absorb water and minerals from the soil • Water and mineral salts from the soil enter the plant through the epidermis of roots and ultimately flow to the shoot system Animation: Transport in Roots Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-9 Casparian strip Pathway along apoplast Endodermal cell Pathway through symplast Casparian strip Plasma membrane Apoplastic route Vessels (xylem) Symplastic route Root hair Epidermis Endodermis Vascular cylinder Cortex The Roles of Root Hairs, Mycorrhizae, and Cortical Cells • Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and root hairs are located • Root hairs account for much of the surface area of roots Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Most plants form mutually beneficial relationships with fungi, which facilitate absorption of water and minerals from the soil • Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hyphae Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-10 2.5 mm • After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Endodermis: A Selective Sentry • The endodermis is the innermost layer of cells in the root cortex • It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into the vascular tissue Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Water can cross the cortex via the symplast or apoplast • The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 36.3: Water and minerals ascend from roots to shoots through the xylem • Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant • The transpired water must be replaced by water transported up from the roots Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Factors Affecting the Ascent of Xylem Sap • Xylem sap rises to heights of more than 100 m in the tallest plants • Is the sap pushed upward from the roots, or is it pulled upward by the leaves? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pushing Xylem Sap: Root Pressure • At night, when transpiration is very low, root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential • Water flows in from the root cortex, generating root pressure Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Root pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous eudicots Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pulling Xylem Sap: The Transpiration-Cohesion Tension Mechanism • Water is pulled upward by negative pressure in the xylem Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transpirational Pull • Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata • Transpiration produces negative pressure (tension) in the leaf, which exerts a pulling force on water in the xylem, pulling water into the leaf Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-12 Y = –0.15 MPa Y = –10.00 MPa Cell wall Air-water interface Airspace Low rate of transpiration Cuticle Upper epidermis High rate of transpiration Cytoplasm Evaporation Mesophyll Airspace Air space Cell wall Evaporation Water film Lower epidermis Cuticle CO2 O2 CO2 Xylem O2 Stoma Vacuole Cohesion and Adhesion in the Ascent of Xylem Sap • The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution • Transpirational pull is facilitated by cohesion and adhesion Animation: Transpiration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-13 Xylem sap Outside air = –100.0 MPa Mesophyll cells Stoma Leaf (air spaces) = –7.0 MPa Water molecule Transpiration Atmosphere Trunk xylem = –0.8 Mpa Water potential gradient Leaf (cell walls) = –1.0 MPa Xylem cells Adhesion Cell wall Cohesion, Cohesion and by adhesion in hydrogen the xylem bonding Water molecule Root hair Root xylem = –0.6 MPa Soil = –0.3 MPa Soil particle Water Water uptake from soil Xylem Sap Ascent by Bulk Flow: A Review • The movement of xylem sap against gravity is maintained by the transpiration-cohesion-tension mechanism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 36.4: Stomata help regulate the rate of transpiration • Leaves generally have broad surface areas and high surface-to-volume ratios • These characteristics increase photosynthesis and increase water loss through stomata Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-14 20 µm Effects of Transpiration on Wilting and Leaf Temperature • Plants lose a large amount of water by transpiration • If the lost water is not replaced by absorption through the roots, the plant will lose water and wilt Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stomata: Major Pathways for Water Loss • About 90% of the water a plant loses escapes through stomata • Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-15a Cells turgid/Stoma open Cells flaccid/Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Changes in guard cell shape and stomatal opening and closing (surface view) • Changes in turgor pressure that open and close stomata result primarily from the reversible uptake and loss of potassium ions by the guard cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-15b Cells turgid/Stoma open H 2O Cells flaccid/Stoma closed H 2O H 2O H 2O K+ H 2O H 2O H 2O H 2O H 2O Role of potassium in stomatal opening and closing H 2O Xerophyte Adaptations That Reduce Transpiration • Xerophytes are plants adapted to arid climates • They have leaf modifications that reduce the rate of transpiration • Their stomata are concentrated on the lower leaf surface, often in depressions that provide shelter from dry wind Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-16 Cuticle Upper epidermal tissue Lower epidermal Trichomes Stomata tissue (“hairs”) 100 µm Concept 36.5: Organic nutrients are translocated through the phloem • Translocation is the transport of organic nutrients in a plant Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Movement from Sugar Sources to Sugar Sinks • Phloem sap is an aqueous solution that is mostly sucrose • It travels from a sugar source to a sugar sink • A sugar source is an organ that is a net producer of sugar, such as mature leaves • A sugar sink is an organ that is a net consumer or storer of sugar, such as a tuber or bulb Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Sugar must be loaded into sieve-tube members before being exposed to sinks • In many plant species, sugar moves by symplastic and apoplastic pathways Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-17 Key Apoplast Symplast Companion (transfer) cell Mesophyll cell Cell walls (apoplast) Sieve-tube member High H+ concentration Cotransporter Proton pump Plasma membrane Plasmodesmata Sucrose Mesophyll cell Bundlesheath cell Phloem parenchyma cell Low H+ concentration • In many plants, phloem loading requires active transport • Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pressure Flow: The Mechanism of Translocation in Angiosperms • In studying angiosperms, researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure Animation: Translocation of Phloem Sap in Summer Animation: Translocation of Phloem Sap in Spring Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-18 Sieve tube (phloem) Vessel (xylem) H2O Source cell (leaf) Sucrose H2O Sink cell (storage root) Sucrose H2O • The pressure flow hypothesis explains why phloem sap always flows from source to sink • Experiments have built a strong case for pressure flow as the mechanism of translocation in angiosperms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 36-19 25 µm Sievetube member Sap droplet Aphid feeding Stylet Stylet in sieve-tube member (LM) Sap droplet Severed stylet exuding sap