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Created by Julia Hsu Levy – Version 1.5 Chapter 27: Plants Plant characteristics: Eukaryote: nucleus and organelles Cellulose cell walls Chlorophyll Four main groups: bryophytes, pteridophytes, gymnosperms, and angiosperms. Plants may have evolved from Chara. Evidence includes that these protists have: cellulose rosettes to make cellulose cell walls peroxisomes flagellated sperm cell morphology What land plants evolved that did not exist in algae: apical meristems multicellular embryos alternation of generation life cycle in all Sporangia (developed 2n structure) that produce spores with cell walls Gametangia (developed 1n structure) that produce gametes Embryo developed in female tissues that provides nutrients Spores covered in sporopollenin (durable organic material) adaptations for acquiring, transporting, and conserving water (ex: cuticle, stomata, vascular tissue) adaptations for reducing the harmful effect of UV radiation adaptations for repelling terrestrial herbivores and resisting pathogens (ex: secondary compounds such as termpenes, tannins, phenolics, alkaoids) Alternation of generation life cycle: Generic plant sexual life cycle: 1. Sperm and egg fertilize to form zygote. 2. Zygote undergoes mitosis and becomes a multicellular 2N body (sporophyte). 3. Some of the 2N cells of the sporophyte undergo meiosis to create spores. 4. 1N spores germinate into a multicellular 1N body (gametophyte) which can be male or female. 5. The gametophyte produces gametes by mitosis. Sexual reproduction: process of creating offspring with DNA recombinations different from the parent 141 Created by Julia Hsu Levy – Version 1.5 Description 1. Nonvascular 2. Seedless vascular 3. Gymnosperms Evolutionary History of Plants Evolutionary Adaptation Example from Predecessors Terrestrial adaptations: Moss, hornwort, liverwort cutin Xylem and phloem roots, Ferns, lycophytes, stems, and leaves horsetails, whisk ferns Seed-bearing cones, pollen Gingko: ornamental, plant , sporophyte dominance only males because female seed coat decaying produces foul odor Cycads: resembles palms but nonflowering Gnetum: Welwitchia with strange meristematic growth, Ephedra natural speed 4. Angiosperms Flowers and seeds within fruits Conifers: cone-bearing plants such as pine, redwood, etc. All flowering dicot and monocot plant Significant events in plant history: o The flora and fauna of Earth changed dramatically during the formation of the supercontinent Pangaea in the Permian. o This likely led to major environmental changes, including drier and warmer continental interiors. o Many groups of organisms disappeared and others emerged as their successors. (Ex: Reptiles increased, amphibians decreased in diversity) o Seedless vascular plants that dominated in Carboniferous swamps were largely replaced by gymnosperms, which were more suited to the drier climate. 142 Created by Julia Hsu Levy – Version 1.5 Life cycle of typical moss: o Antheridia: male gametophyte o Archegonia: female gametophyte o Gametophyte dominant life cycle (majority of the plant structure is gametophyte) o Still requires water for reproduction o Rhizoids: root-like structures that anchor plants to the ground o Few cells thick because diffusion required for water transport o Ex: peat moss Life cycle of Pterophyta fern: first formed in “coal forests” of Carboniferous period o Sorus: sporangia (spore-bearing structure) found on the underside of the frond o Characteristic gametophyte stage that looks like a heart (thallus) o Sporophyte dominant life cycle (megaspores female, microspore male) o Requires some water for reproduction o Branching possible because of vascular tissue 143 Created by Julia Hsu Levy – Version 1.5 Life cycle of Gymnosperm pine: o Pine cones: sporangia o Sporophyte dominant o Wind pollinated o Seed: multicellular package of food and embryo, developed from ovule o Seeds exposed to elements (no fruit for protection or animal attraction) o Does NOT require water for fertilization o Male gametophyte: pollen grain with sperm cells inside o Importance of seed: to disperse offspring o Spores retained in parent whereas seeds are released o Integuments: layers of protective tissue around the female structures such as the ovule, later develops into the seed coat o Mostly nonflagellated sperm delivered via pollen tube from pollen grain to ovule o Male and female cones o Spores produced on scale-like sporophylls that are the scales of a cone (male and female) o Two sperm cells develop in the pollen grain that enters the ovule at the micropyle o Sperm + egg embryo o Mostly evergreen (keeps leaves all year) Life cycle of Angiosperm: o “flowering plants” o most diverse o Phylum Anthophyta o divided into monocots and dicots o sporophyte dominant o does not require water for fertilization o sperm cells in male gametophyte (pollen grain) o seeds may be animal (fleshy) or wind dispersed (dry) o DOUBLE FERTILIZATION endosperm (sperm+2 polar bodies) and embryo (sperm+egg) 144 Created by Julia Hsu Levy – Version 1.5 o Ovule undergoes meiosis to make 4 1N cells that go through mitosis to produce 8 1N cells (2 synergids, 2 polar bodies, 3 antipodal cells, 1 egg) o Produces fruits and flowers o Flower specialized for reproduction and for attracting animal pollinators Flower anatomy: o Sepals: base of the flower, modified leaves that enclose the flower before it opens o Petals: inside the ring of sepals, brightly colored in plant species that are pollinated by animals, lack bright coloration in wind-pollinated plant species o Stamens: the male reproductive organs, produce microspores in anther pollen grain (male gametophyte), filament + anther) o Carpals: female sporophylls that produce megaspores and their products, female gametophytes; stigma+style+ovary (with ovule inside) Flower structure: o Complete flowers have parts, whereas imcomplete flowers have 1 or more parts missing. o Perfect flowers have both male and female organs, whereas imperfect flowers have only male or female organs. Fruit: mature ovary, may be dry or fleshy; helps to disperse seeds o Dry fruits: dandelions and maples with propellers or lightness for wind dispersal, may be prickly (burrs) for animal dispersal o Fleshy fruits: eaten, seeds deposited unharmed with fertilizer Fruit types: o Simple fruits are derived from a single ovary such as oranges or cherries. o Aggregate fruit, such as a blackberry, results from a single flower with several carpals. o Multiple fruit, such as a pineapple, develops from an inflorescence, a tightly clustered group of flowers. 145 Created by Julia Hsu Levy – Version 1.5 * * * Typical Plant Cell * * * Protoplasm: everything excluding the cell wall 1. Plasma membrane: membrane that covers the surface of the protoplasm 2. Nucleus: permanent storage place for genetic information (DNA or RNA) 3. Central vacuole: single membrane (tonoplast) organelle that store water and necessary salts, can expand rapidly to force cells to elongate 4. Cytoplasm: all substances excluding cell wall, nucleus, vacuole 5. *Mitochondria: contains enzymes to carry out cellular respiration 6. Plastids: organelles that carry out many functions, including photosynthesis, storage, export of specialized lipid molecules, pigment storage Chloroplast: plastids when matured and exposed to light, green due to presence of chlorophyll Amyloplast: stores amylose (starch) Chromoplast: red, yellow, orange lipids accumulate to give certain fruits/flowers bright colors 7. Ribosome: particles used during protein synthesis (translation) Polysome: cluster of ribosomes, associated with mRNA, to form complexes 8. Endoplasmic reticulum: carries large molecules such as proteins when diffusion is not possible to transport the molecule around the cell rough ER: ribosomes attached to ER, proteins made pass through ER into the lumen to form vesicles smooth ER: lipid synthesis and membrane assembly 9. Dictyosomes (Golgi body, apparatus in animals): stack of thin vesicles that modifies materials that the cell will secrete, ER vesicles gather to one face of the dictyosome Forming face (cis): vesicles accumulate on this side Maturing face (trans): vesicles releasing modified components 10. Endomembrane system: continuation of membrane system in cell, exclusing mitochondrial and plastid membranes 11. Microbodies: 0.5 - 1.5 m diameter Peroxisomes and glyoxysomes: breaks down and builds up hydrogen peroxide (H2O2) in cell 12. Cytosol: most volume of cytoplasm (mostly water, enzymes, and chemicals) 146 Created by Julia Hsu Levy – Version 1.5 14. Microtubules: act as a cytoskeleton, made of two glocular proteins (alpha tubulin and beta tubulin) Centrioles: 9 sets of 3 short microtubules (9+3 arrangement) Cilia: short (2 m) microtubules, arranged in groups Flagella: longer microtubules, arranged singly or in sets of 2 or 4 Basal body: like centrioles, associated with flagella and cilia 15. Microfilmanets: made of flobular protein (actin) 16. Storage bodies: holds nutrient reserves in times of scarcity spherosomes: (lipid bodies): large oil droplets (ex: peanut oil, sunflower oil, etc) amyloplasts: stores amylose 17. Cell wall: provides structure and protection for all plant cells, except sperm cells middle lamella: adhesive layer found between two adjoining plant cells, made of pectin primary cell wall: thin cell wall secondary cell wall: protoplast deposits secondary wall between primary cell wall and plasma membrane, usually full of lignin to make wall stronger than hemicellulose lignin: sugar protein that is fungal , bacterial, chemical, and water resistant Cells Tissues Organs Systems Cells Tissues Organs Parenchyma cell: Dermal: covering, Roots: anchoring, thin primary cell wall may have waxy storage, and covering (epidermis- absorption Collenchyma cell: cuticle, endodermisunevenly thickened Casparian strip) Stems: support primary cell wall Ground: all Leaves: Sclerenchyma cell: undifferentiated photosynthesis, thickened cells (pith and transpirational pull secondary cell wall cortex) Systems Shoot: aerial for photosynthesis and reproduction Root: subterranean for water and mineral absorption and storage Meristematic: cell division (growth); occurs throughout lifetime (unlike animals with growth limited to juvenile stage) Vascular: conductive 147 Created by Julia Hsu Levy – Version 1.5 Vascular tissue: continuous throughout the plant, transport of materials between roots and shoots 1. Xylem: water and mineral transport by transpiration from root to shoot o Ex: tracheids (long and thin) and vessel elements (short and wide) o Functionally dead at maturity (lose protoplasm) o Perforated ends to allow for water conduction 2. Phloem: food transport by translocation (“source ot sink”) from photosynthetic cells to storage cells o Sucrose, other organic compounds, and some mineral ions move through tubes formed by chains of cells, sieve-tube members. o alive at functional maturity (lack the nucleus, ribosomes, and a distinct vacuole) o sieve plates: end walls with pores o companion cell: nonconducting nucleated companion cell, connected to the sieve-tube member, may assist the sieve-tube cell. Plant Organ Leaf Stem Root Flowers Comparing Monocots to Dicots Monocot Dicot Parallel veins Main vein with branched veins “Scattered” bundles Ring bundles Ring bundles within endodermis Centralized vascular core within endodermis (“X” xylem pattern) Fibrous roots Taproots 3x multiples 4x and 5x multiples 148 Created by Julia Hsu Levy – Version 1.5 * * * Stem * * * Twig: o o o o o node: points where leaves are attached internodes: points between nodes axillary bud: potential to form branch terminal bud: growth of young shoot Modified stems include stolon, tuber, bulb, rhizome. Apical dominance: The presence of a terminal bud is partly responsible for inhibiting the growth of axillary buds (auxin induced growth). o Evolutionary advantage: By concentrating resources on growing taller, apical dominance increases the plant’s exposure to light. In the absence of a terminal bud, the axillary buds break dominance and gives rise to a vegetative branch complete with its own terminal bud, leaves, and axillary buds (cytokinin induced growth). Dicot stems can form wood because of secondary growth in the vascular bundles. Monocot stems cannot because the bundles are “scattered.” 149 Created by Julia Hsu Levy – Version 1.5 * * * Leaf * * * Cuticle: waxy covering (cutin) to prevent against water loss Epidermis: covering Palisade parenchyma: primary photosynthetic cells Spongy parenchyma: air pockets to trap moisture, CO2, and other gases Stoma(ta): space between two guard cells, K+ ion pump regulated with water pressure Vascular bundle: surrounded by bundle sheath cells, xylem and phloem for conduction Leaf structure: blade + petiole (small stalk) that is attached to stem o Simple: undivided blade o Compound: divided blade o Modified leaves: tendrils, spines, needles, coloration * * * Roots * * * Function: water and mineral absorption root hairs: increase surface area, mycorrhizae association with fungus root cap: protects apical meristem Stele: ring within endodermis where vascular tissues form 150 Created by Julia Hsu Levy – Version 1.5 Zone of cell division: area of rapid mitosis Zone of elongation: cells growing in length Zone of maturation: roots hairs and specialized cells pericycle: outer layer of stele that gives rise (forms) lateral roots (root hairs) Specialized roots: adventitious above-ground roots for support (aka…prop or aerial roots) * * * Plant Growth and Development * * * Annual plants complete their life cycle - from germination through flowering and seed production to death - in a single year or less. Biennial plants span two years, which may require vernalization (cold period). Plants that live many years are perennials. Growth is the irreversible increase in mass that results from cell division and cell expansion. Mitosis Cell plate forms between two daughter cells Development is the sum of all the changes that progressively elaborate an organism’s body. 151 Created by Julia Hsu Levy – Version 1.5 Meristematic tissue: areas of indeterminate growth 1. Apical meristems: tips of roots and in the buds of shoots o primary growth: elongation o produces primary plant body (initial roots and shoots) o Ex: length 2. Lateral meristems: located within roots and shoots o secondary growth: thickening (girth) o Ex: bark, thicker roots, wood Ways to protect the meristems: root cap, bud scales, hypocotyls while dicots pushing out of ground, coleoptile while monocots are pushing out of the ground Secondary plant body: vascular cambium and cork cambium 152 Created by Julia Hsu Levy – Version 1.5 * * * Plant Transport * * * Transpiration: water transport from root to shoot Endodermis covered with Casparian strip: a belt of suberin, a waxy material that is impervious to water and dissolved minerals Ways that material gets into the root 1. Apoplast: cell walls until the endodermis 2. Symplast: through cell interior and between cells via plasmodesmata 153 Created by Julia Hsu Levy – Version 1.5 Guttation: exudation of water droplets from root pressure Transpiration involves forces Cohesion: water molecule attractions Adhesion: water molecule attraction to cell walls Tension force exerted from adhesion LOWERS water potential. Most amount of tension (negative pressure) in the leaf xylem, where water is pulled towards the air spaces in the spongy parenchyma. Cavitation: break in the water column lessened with adhesion to xylem cell walls Tracheids are longer, thinner cells with more cell wall. Plants that are very tall (ex: pine) have tracheid cells that reduce cavitation. Vessels are shorter, wider which transports more water but are more prone to cavitation. 154 Created by Julia Hsu Levy – Version 1.5 Changes in turgor pressure that open and close stomata result primarily from the reversible uptake and loss of potassium ions (K+) by guard cells. * * * Plant Nutrition * * * Root system: absorbs water (but 90% lost through transpiration) and minerals Shoot system: obtains CO2 incorporated into photosynthesis to make the bulk of the organic matter A variety of plants parasitize other plants to extract nutrients that supplement or even replace the production of organic molecules by photosynthesis by the parasitic plant. Ex: type of mistletoe which supplements its nutrition by using projections called haustoria to siphon xylem sap from the vascular tissue of the host tree. Plant would be unable to obtain adequate nutrition from roots without these two symbiotic relationships. 1. Mycorrhizae: increases absorptive surface area (sucks in more water) 2. Nitrogen-fixing bacteria: root nodules with pockets of bacteria to convert atmospheric nitrogen Bacteria “fix” atmospheric nitrogen to ammonium which is converted to NO2 (nitrite) and then nitrate (NO3). Plants use the nitrates which then consumers eat. Some of the nitrates are released back into the atmosphere as nitrogen gas. Genus: Rhizobium 15-20% of the plant body is dry matter (non-aqueous). 95% = organic 5% = inorganic (not carbonbased) Types of nutrients 1. Essential nutrient: required to complete life cycle (seed germination to reproduction) – K Hydroponics can determine essential nutrients. 2. Macronutrients: needed in large quantities – C H O N P S K Ca Mg 3. Micronutrients: needed in small quantities – Fe Cl Cu Mb Bo Ni Nutrient deficiencies are easily diagnosed from characteristic signs. Ex: lacking magnesium chlorosis 155 Created by Julia Hsu Levy – Version 1.5 Nitrogen fixation: All plants require nitrogen, which is in the amine-functional group of the amino acids that makes up proteins. The atmosphere is 80% nitrogen gas (N2) which plants cannot incorporate. Nitrogen fixing bacteria are required for plants to use nitrogen. N2 NH3 NO2- NO3- N2 Humans rely upon plants for nutrition. Plants bred to be “super” varieties have increased protein because plants naturally do not have much protein. Fixed nitrogen fertilizers are required in large quantities. Countries that most need the proteinaceous plants are least able to afford it. Nodules: legume’s root swellings called, composed of plant cells that contain nitrogenfixing bacteria (Rhizobium). Bacteriods: vesicles formed by the root cell to house Rhizobium The development of root nodules begins after bacteria enter the root through an infection thread. Crop rotation and nitrogen fixation Non-legume crop (ex: corn) planted Following year legume is planted to restore the concentration of fixed soil nitrogen. Legume crop is not harvested but plowed under to decompose as “green manure”. To ensure the formation of nodules, the legume seeds may be soaked in a culture of the correct Rhizobium bacteria or dusted with bacterial spores before sowing. Signal Transduction Pathway: how external signals are carried into the cell 1. Reception: “getting” signal 2. Transduction: transferring signal with secondary messengers 3. Response: reacting to signal Greening: phenomenon of plants developing chlorophyll in response to light Phytochrome: molecule with receptor abilities o Detects light o Activates Gprotein o Forces production of second messenger 156 Created by Julia Hsu Levy – Version 1.5 (usually Ca2+ or cGMP or cAMP) o Stimulates activity within the cell * * * Plant Hormones * * * Hormones are chemical substances that are produced in one part of the body and transported to another part to regulate development. Hormones are found in very low concentrations. Each hormone has more than one function. Angiosperm life cycle: Dormancy is the stage when development is suspended in the seed stage. Cells are not dividing. There are ways to break dormancy. o Mechanical abrasion o Fire: dries the seed , melts waxes in seed coat so water can be absorbed o Water immersion to leach out inhibiting chemicals (hormones) o Temperature: vernalization Germination is the stage when dormancy is broken and the seed begins to develop. . Flowers are timed to develop according to day-night length. Plants have the ability to detect the amount of darkness. They are photoperiodic. The mechanisms for light detection are yet unknown. Florigen, even though its not isolated, is believed to be a hormone that travels from the leaf to where the flowers will begin development. Short-day plants: long night plants (require longer than critical night length time for flowering) Long-day plants: short night plants (require less than critical night length time for flowering) Bursts of light alter phytochrome from Pf to Pfr no flowering but far red light converts Pfr Pf. 157 Created by Julia Hsu Levy – Version 1.5 Blue-light is found to be most effective at inducing plant responses. Phytochromes are responsible for light absorption. Senescence is the deterioration of plant tissues (cells) from aging. Perennial plants do not senesce but instead enter dormancy during winter months. Hormone Gibberellins Auxin Cytokinins Ethylene Abscisic acid Major plant hormones Function Made in embryo, young leaves, apical meristems Made in embryo, young leaves, apical meristems Made in roots Made in ripening fruit, senescing tissue, stem nodes Made in root cap, older leaves, stem 158 Created by Julia Hsu Levy – Version 1.5 159 Created by Julia Hsu Levy – Version 1.5 Auxin induced growth: “acid-growth hypothesis” Auxin transport across two cells deposts proton ions in the cell wall. The cell wall acidifies (drops pH) which weakens the cellulose cross-links. The wall is weaker so that when the cell fills with water, it elongates. Auxin and phototropic response: true in monocots Important: Hormones are produced in very low concentrations. The amplification of the second messenger is what triggers the significant responses. Hormone effects are relative to other hormone concentrations and rather than a specific concentration of that particular hormone. Ex: Having X amount of auxin will not produce a certain effect, but having X amount of auxin while there is Y amount of cytokinin present can induce a specific effect. Plant responses to environmental cues: 1. Phototropism: light 2. Gravitropism: gravity (may not be statolith starch granule settling because they are NOT in all plants but rather protoplasm pulling on membrane proteins) 3. Thigmotropism: touch 160