Instructions for printer-friendly copy.

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    Plants Surround Us

    Land Plants are incredibly diverse.

    • They comprise about 98% of earth's biomass.
    • They are the main source of our oxygen-rich atmosphere.
    • They fix carbon and nitrogen into forms other organisms can use.
    • They are the first link in terrestrial food chains.
    • They provide habitat and food for all types of organisms

    Without plants, life on earth as we know it would not exist.

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(click on pic for source)

    Why Care About Plants?

    Humans rely on plants to provide

    • food
    • fabrics
    • medicines
    • spices
    • food stabilizers

    • biofuels
    • perfumes and dyes
    • Starbucks
    • the list goes on.

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(click on pic for source)

    Plants are the Foundations of Ecosystems

    Climate and soil conditions determine the flora.

    Flora, in turn, influences the fauna.

    The greater the botanical diversity, the greater the animal diversity.

    Plant/animal interactions characterize the earth's biomes.


    (click on pic for larger view)

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(modified from original by Ruth Evangeline Timme; click for source)

    Viridiplantae: Green Plants

    Viridiplantae (Green Plants) includes all plants,
    from several taxa of green algae to all land plants.

    The cladogram here shows the current understanding
    of green plant phylogenetic relationships.

    Green plants have notable synapomorphies that
    distinguish them from photosynthetic protists.

    And of course, each of the taxa within Viridiplantae
    has its own synapomophies that set it apart
    from the other green plants.

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(click on pic for source)


(click on pic for source)

    The First Plants: Pond Scum

    The first land plants were green algae, affectionately known as "pond scum".
    They appeared on wet shores during the Cambrian, 500-600 million years ago.

    These ancestral algae

    • had few, if any, specialized cells
    • lacked true tissues
    • gave rise to small, amphibious descendants that
      • had primordial tissues
      • lacked vascular tissue

    The descendants of these amphibious plants are the non-vascular plants:

    • liverworts
    • hornworts
    • mosses

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(Illustration by Francoise Gantet, 1997)

    The First Vascular Plants: Just Stems

    Over the next 100 million years, non-vascular ancestors
    gave rise to vascular plants with
    • thicker cell walls
    • erect stems
    • true tissues, including vascular tissue

    A true plant organ is defined by the presence of vascular tissue.

    Plants with stems appear in the fossil record ~440 mya (Silurian),
    but molecular data suggest that they were already established ~400mya (Ordovician).

    The earliest vascular plants were little more than thin, herbaceous stems.
    But these early stems gave rise to the fully vascularized true organs:
    stem, root and leaf.

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    The Green Plants: Kingdom Viridiplantae

    Viridiplantae (Green Plants) includes all plants,
    from several taxa of green algae to all land plants.

    Green plants have synapomorphies that separate them
    from photosynthetic protists.

    • diverse cell types
    • chlorophyll b
    • unique form of starch

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(click on pick for larger image)

(By LadyofHats, Wikimedia Commons)

    Structures Unique to Plant Cells

    Plant cell architecture reflects the natural selection
    that shaped our photosynthetic benefactors.

    Features unique to plant cells include

    • rigid, layered cell wall (cellulose, hemicellulose, lignin)
    • plasmodesmata (intercellular contact/communication)
    • large, central vacuole(s) (structural support, storage)
    • plastids (various types & functions)

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    Plant Cells are Photosynthesis Factories

    Different plant cell types each participate in specific functions.
    • infrastructure
    • flexible strength
    • compressional (rigid) strength
    • photosynthesis
    • transport of water and minerals
    • transport of organic nutrients
    • reproduction
    • defense

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I. Biological Molecules Unique to Plants

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    Viridiplantae Synapomorphy:
    Photopigments

    Plants' emerald green color is conferred by photosynthetic pigments:
    • chlorophyll a
    • chlorophyll b
    • various carotenoids (yellow, orange, red)

    The role of these pigments is to capture photons,
    wave/particles of light energy.

    The pigments transfer this electrical energy
    to biological machinery that converts that energy
    into the chemical bonds of carbohydrates.

    Photosynthesis!

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    Fun Fact: Carotenoids and Animal Vision

    Orange, red, and yellow fruits and vegetables are rich in
    carotenoid pigments, notably β-carotene.

    • Carotenoids serve as light-capturing pigments in photosynthesis.
    • Animals cannot manufacture Vitamin A (retinol) de novo
    • However, they can break down β-carotene to yield retinol's precursor.
    • Retinol serves as the light-capture component in rhodopsins,
      the visual pigments in animal photoreceptors.

    Evolution as opportunism:

      Animals could not have evolved visual systems (as we know them today)
      without eating the light-capturing molecules made by plants.

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    Viridiplantae Synapomorphy:
    Plant Starch

    Plants store their energy as a unique form of starch.
    • Starch is a polymer of the simple sugar glucose
    • It is stored as tiny granules inside amyloplasts in the cell.
    • A granule consists of alternating (1) semi-crystalline
      and (2) squishy layers of...
        • branching amylopectin (~75-80%)
        • linear amylose (~ 20-25%)
    • Its (α glycosidic) bonds are readily broken by animal digestive enzymes.
    • These vital molecules are at the base of every ecological food web.

    Starch has myriad uses,
    in both modified and unmodified forms.

    It is the most important carbohydrate
    in the animal diet.


    (click on pic for source)

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(click on pic for source)

    Viridiplantae Synapomorphy:
    Cellulose

    Cellulose is the most abundant organic molecule on earth.

    • It is a linear, unbranched polymer of glucose.
    • Its (β glycosidic) bonds can't be lysed by animal digestive enzymes.
    • Herbivorous animals require microbial symbionts to digest cellulose.
    • It is crystalline and strong.
    • It is greyish-white in color.
    • It is arranged in fibers in the plant cell wall.
    • This provides flexible strength to the cell wall.

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    Viridiplantae Synapomorphy:
    Lignins

    Lignins are the second most abundant of all organic molecules.
    Lignins...

    • are complex organic polymers with variable functional groups.
    • fortify land plant cell walls (and those of some green algae)
    • are particularly abundant in woody plants
    • provide rigid, compressional strength to cell walls
    • are nearly indestructible
    • vary in composition among plant species
    • are waterproof, and so help direct water through xylem tubes
    • are reddish-brown in color
    • have significant anti-microbial properties

    Lignins may originally have evolved to serve as microbe inhibitors.
    Only later were they conscripted for structural uses.

    Toxic aromatic compounds released into the air by wood and other biomass burning and into rivers from paper mills contain lignins and their chemical byproducts.

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    Viridiplantae Synapomorphy:
    Sporopollenin

    Sporopollenin
    • is first seen, evolutionarily, in Charophyte cell walls.
    • is a major component of land plant spore and pollen outer walls.
    • is one of the most chemically inert biological polymers known.
    • is a common component of soils where pollen and spores
      have dispersed and decayed.

    It just doesn't go away!

    Fossil spores and pollen are well preserved because of
    the inert nature of the sporopollenin in their outer coats.

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II. Cellular Structures Unique to Plants

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    Viridiplantae Synapomorphy:
    Cell Wall

    The cell wall surrounds the plasma membrane. It provides
    • tensile strength (resistance to breaking under tension)
    • protection against mechanical stress
    • some degree of filtration

    The cell wall helps the cell maintain its shape
    despite osmotic changes.

    • A normal supply of water keeps the central vacuole full.
    • Turgor pressure is the force of the cell's liquid contents against the cell wall.
    • Under drought conditions, the vacuole shrinks, the cell becomes flaccid.
    • The cell wall helps retain infrastructure until water is restored and the cells become turgid again.

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    Cell Wall Composition

    Plant cell walls are primarily made of cellulose
    • Cellulose fibers are bundled into microfibrils.
    • Microfibrils are bundled into macrofibrils.
    • These form a network throughout the cell wall.

    Cell walls also contain
    • hemicellulose
      • a branched heteropolymer of various sugars
      • amorphous, conferring little structural strength
      • water soluble at high pH (don't bleach your kitchen sponge!)
      • not digestible by animals, but by some microbes

    • pectin
      • a heteropolymer of various sugars
      • gelatinous, sticky (we use it to thicken preserves)
      • water soluble at high pH (don't bleach your kitchen sponge!)
      • not digestible by animals, but by some microbes
      • sometimes marketed as a remedy for high LDL ("bad" cholesterol)
      • However, it can interfere with absorption of
        • β carotene
        • digoxin (heart medication)

        • lovastatin (cholesterol-lowering drug)
        • tetracycline antibiotics

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(click on pic for source)

    The Cell Wall is Layered

    Cell wall composition varies among species, among tissues,
    and even with a plant's developmental stage.

      The cell wall can consist of up to three layers.
      • primary cell wall - thin, flexible first wall formed by a growing cell
        • cellulose

        • hemicellulose
        • pectin

      • secondary cell wall- thick layer inside primary wall (some cells)
        • cellulose
        • hemicellulose

        • lignin (not all cells)
        • waxy suberin and cutin (not all cells)

      • middle lamella - gelatinous layer between adjacent cell walls.
        • composed primarily of pectins
        • acts like a "glue" to adhere adjacent cells

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(click on pic for source)

    Viridiplantae Synapomorphy:
    Plasmodesmata

    In order for a multicellular organisms to function,
    its cells must constantly interact and share information.

    Plant cells, enclosed in tough cell walls, solve this problem
    with plasmodesmata, narrow threads of
    cytoplasm that pass through pores in the cell walls of adjacent cells.

    Plasmodesmata (singular plasmodesma) facilitate
    communication and transport between plant cells.

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(click on pic for source)

    Viridiplantae Synapomorphy:
    Central Vacuole

    A typical plant cell contains a large, single central vacuole.
    It can occupy 80% or more of the cell's internal volume.

    The vacuole

    • stores water
    • maintains cellular turgor pressure
    • shrinks and grows with water availability

    The water-filled vacuole pushes the cytoplasm
    and its contents--notably the chloroplasts--outwards.

    The vacuole positions the chloroplasts closer to incoming light,
    making more energy available for photosynthesis.

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(By Mariana Ruiz Villarreal LadyofHats, Wikimedia Commons)

    Viridiplantae Synapomorphy:
    Plastids

    Plant cells employ a variety of organelles collectively called plastids

    • An undifferentiated proplastid can develop into a(n)...
      • etioplast - chloroplast not yet exposed to light
        • chloroplast - site of photosynthesis
        • chromoplast - pigment synthesis and storage

      • leucoplast - fatty acid and amino acid synthesis
        • amyloplast - starch synthesis and storage
          • statolith - allows plant to detect gravity
        • elaioplast - lipid storage (usually as oil)
        • proteinoplast - protein storage and modification

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    The VIP (Very Important Plastid):
    The Chloroplast

    All plants have similar chloroplasts
    • enclosed by a double membrane
    • photopigments are embedded in internal thylakoid membranes
    • thylakoids are arranged in stacks called grana
    • grana are connected by a membrane channel system
    • colorless, aqueous stroma forms the fluid matrix around the grana

    The chloroplast is the site of photosynthesis, the biochemical reaction
    that allows life on earth (as we know it) to exist.

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(By Bewareofdog, Wikimedia Commons)

    Embryophyta: The Land Plants

    All embryophytes share a derived character that gives them their name:

    The plant embryo develops inside the multicellular
    sex organ (archegonium) of its female parent.

    But that is not their only synapomorphy.

    In order for plants to colonize land, they needed

    • support against gravity
    • structures for atmospheric gas exchange
    • mechanisms and structures to draw water and nutrients from soil
    • mechanisms and structures to prevent desiccation

    The earliest land plants were amphibious, tied to water by
    • sperm that require water to swim to the female
    • a lack of water conservation mechanisms

    But even the earliest embryophyte had ways to survive on land.

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    Viridiplantae Synapomorphy:
    Alternation of Generations

    Plants have more steps to their life cycle than animals do.
    • A diploid sporophyte produces spores via meiosis.
    • Each spore grows into a haploid gametophyte.
    • The gametophytes produce gametes via mitosis.
    • Sperm and egg join to form a diploid zygote.
    • The zygote grows (via mitosis) into a diploid sporophyte.
    • Repeat forever.

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    Embryophyte Synapomorphy:
    Heteromorphic Alternation of Generations

    Although some algae also undergo alternation of generations,
    it is isomorphic: sporophyte and gametophyte
    are physically indistinguishable.

    In land plants, the sporophyte and gametophyte look very different
    from one another. Alternation of generations is heteromorphic.

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    Embryophyte Synapomorphy:
    Multicellular Sex Organs

    Unlike green algae, land plants have multicellular sex organs.
    These appear only in the gametophyte.

    The sex organs are generically known as gametangia
    (Greek: angion, "box").

    The female gametangium
    • is known as the archegonium
    • produces ova via mitosis
    • is somewhat analogous to the animal ovary

    The male gametangium
    • is known as the antheridium
    • produces sperm via mitosis
    • is somewhat analogous to the animal testis

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    Embryophyte Synapomorphy:
    Heterogamy (= Anisogamy)

    Land plants are heterogamous and oogamous.
    • male and female gametes are physically distinguishable
    • the ovum is large, nutrient-filled, and sedentary
    • the sperm is small and motile

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    Embryophyte Synapomorphy:
    Protected Embryo

    The fertilized zyote is not released to the environment,
    as it is in most green algae.

    Instead, it is retained inside the female archegonium,
    where it grows and develops into the sporophyte.

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    Embryophyte Synapomorphy:
    Meristem

    Plants exhibit indeterminate growth:
    they produce new tissues throughout their lifetime.

    The source of new tissues is meristem:

    • embryonic and totipotent
    • can differentiate into any other type of cell

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    Embryophyte Synapomorphy:
    True Tissues

    Most green algae consist of relatively similar cells,
    each of which performs its own functions.

    A tissue is an aggregation of cells coordinated
    to perform a particular function or set of functions.

    • Simple tissue is composed of one type of cell.
      • parenchyma
      • collenchyma
      • sclerenchyma

    • Complex tissue is composed of more than one type of cell.
      • epidermis
      • phloem
      • xylem

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    Embryophyte Synapomorphy:
    True Tissue

    • dermal tissue
      • forms epidermis in herbaceous plants
      • forms bark and associated structures in woody plants

    • ground tissue
      • is incorporated into plant infrastructure
        • parenchyma - thin-walled cells alive at maturity
        • collenchyma - thick-walled cells alive at maturity
        • sclerenchyma - thick-walled cells dead at maturity

    • vascular tissue
      • performs water and nutrient transport
      • xylem
        • transports water and inorganic solutes
        • flow direction: roots to stems to leaves
        • transport cells dead and hollow at maturity
      • phloem
        • transports aqueous products of photosynthesis, organic solutes
        • flow direction: multidirectional, can change direction
        • transport cells alive at maturity

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    Embryophyte Synapomorphy:
    Waxy Cuticle

    Epidermal cells produce an acellular layer of wax
    that covers all plant surfaces.

    Its function is to prevent desiccation.

    The primitive condition, exhibited by non-vascular plants: thin cuticle
    that affords little protection, keeping them amphibious.

    More derived plants have a thicker cuticle.

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    Embryophyte Synapomorphy:
    Stomates

    If you're covered in wax, you need some breathing holes.

    Stomates are gas exchange pores in the leaf surface.

    Because plants perform both photosynthesis and respiration,
    both O2 and CO2 come and go through the stomates.

    Two curved guard cells flank the opening.
    In higher plants, these open and close the stomate
    via internal turgor pressure changes.

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    Embryophyte Synapomorphy:
    Secondary Metabolites

    An organism produces primary metabolites necessary for normal growth, development, or reproduction.

    A secondary metabolite is an organic compound not directly involved in an organism's normal growth, development, or reproduction of the organism.

    Secondary metabolites are derived from primary metabolites

    • alkaloids
    • amines
    • cyanogenic glycosides
    • aromatic scent compounds

    • terpenes
    • steroids
    • flavonoids
    • non-protein
      amino acids
    • lectins
    • phenolic compounds
    • tannins
    • waxes

    • ...which may be components of nectars, fragrances,
      toxins, herbivore deterrents, etc.

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