• Animals are monophyletic; share three traits:
2. Heterotrophs—obtain food from other organisms
3. Move under own power at some point in their life cycle
• All animals except sponges also have:
1. Neurons (nerve cells) that transmit electrical signals to other cells
2. Muscle cells that can change the shape of the body by contracting.
• Only multicellular heterotrophs on tree of life that ingest their food!
• Between 30-35 major animal phyla are recognized.
o Each phylum has synapomorphies that identify it as a monophyletic group
o Phyla are characterized by fundamental aspects of morphology and development that changed as animals diversified
o Animals are incredibly diverse, particularly in morphology.
Estimate 10-50 animal species living today
Analyzing Comparative Morphology
• Animals are moving eating machines!!! - but how?
o Origin and early evolution of animals was "based on" four aspects of the fundamental body plan of animals:
1. Number of embryonic tissue layers
2. Type of body symmetry and degree of cephalization (formation of a head region)
3. Presence or absence of a fluid-filled body cavity
4. How earliest events of embryo development proceed
Origin and Diversification of Tissues
• All animals have tissues (tightly integrated structural and functional units of cells
• Number of tissue layers in embryo varies (except sponges)
o Diploblasts: embryos have two types of germ layers:
1. ectoderm ("outside skin")
2. endoderm ("inside skin")
o Triploblasts: embryos have three germ layers:
Includes all animals except 3 phyla
3. mesoderm ("middle skin")
Origin and Diversification of Tissues
• Germ layers develop into distinct adult tissues and organs.
1. Ectoderm gives rise to skin and nervous system
2. Endoderm gives rise to lining of digestive tract
3. Mesoderm gives rise to circulatory system, muscle, and
internal structures such as bone and most organs
a. evolution of mesoderm important because gave rise to first complex muscle tissue used in movement!!!
Nervous Systems and Body Symmetry
• All animals (except sponges) have neurons
o cnidarians and ctenophores have nerve cells organized into a nerve net
o all other animals have a central nervous system (CNS)
some neurons are clustered into tracts or cords throughout body
some neurons are clustered in masses called ganglia
• Organisms with nerve nets tend to have radial symmetry
o have at least two planes of symmetry
• Organisms with a CNS tend to have bilateral symmetry
o single plane of symmetry and long, narrow bodies
Relationship Between Symmetry and the Nervous System
• Radially symmetric organisms: equally likely to encounter their environment in any direction
o diffuse nerve net can receive and send signals efficiently
• Bilaterally symmetric organisms: tend to encounter their environment at one end
o advantageous to have many neurons concentrated at that end
nerve tracts carry information from there down length of the body
o allowed for development of head region (cephalization) where structures for feeding, sensing the environment, and processing information are concentrated
Symmetry and Cephalization
• All triploblastic animals have bilateral symmetry except
Echinodermata (sea stars, sea urchins...)
• Why is bilateral symmetry so prevalent in animals?
o Locating and capturing food is particularly efficient when
movement is directed by a distinctive head region and
powered by rest of the body
§ mesoderm (muscle) + bilateral symmetry...allowed for
rapid directed movement and hunting!
Evolution of a Body Cavity
• Coelom: enclosed, fluid-filled body cavity
o possessed by all (except a small handful of) triploblasts
o lined on both sides with cells from mesoderm
o organs bathed in fluid instead of embedded in solid tissue
allows for growth and independent movement of organs
creates container for efficient circulation of O2 and nutrients
Evolution of a Body Cavity
• Coelom also acts as
o allows soft-bodied
animals to move even
without fins or limbs -
able to move efficiently
in search of food
Protostome and Deuterostome Development Patterns
• All coelomates are bilaterally symmetric (except for adult echinoderms)
• Bilateria: bilaterally symmetric with 3 embryonic tissue layers (triploblasts); divided into 2 groups:
o Significance of two groups unclear - end result is the same!
1. Protostomes: mouth develops before anus, and blocks of mesoderm hollow out to form the coelom
Includes arthropods, mollusks, and segmented worms.
2. Deuterostomes: anus develops before mouth, and pockets of mesoderm pinch off to form the coelom
Includes chordates and echinoderms
The Tube-within-a-Tube Design
• 99+% animal species today = bilaterally symmetric triploblasts with coeloms that follow protostome or deuterostome pattern of development!
o animal body plan = "tube-within-a-tube design"
outer tube forms body wall; inner tube forms gut
easy to see in worms - but the body plan of animals with limbs is the same - but the tubes are mounted on limbs
o Evolved by natural selection -further diversification was triggered by evolution of novel structures for moving and capturing food!
Evaluating Molecular Phylogenies (Based on DNA Sequences)
• Choanoflagellates (group of protists) - closest living relatives of animals
o Not animals because not multicellular
o Quite similar to sponges (sister group to all other animals)
Themes in Diversification of Animals
• Within each animal phylum, basic features of body plan do not vary from species to species.
o diversification of species within each lineage was mostly triggered by evolution of innovative methods for sensing environment, feeding, and moving!
Sensing the Environment
• Key aspect of cephalization is concentration of sensory organs in the head region
o tremendous diversity of sensory abilities and structures
o most animals have touch, balance, smell, taste, and hearing
o most animals can detect light
o many animals have well-developed sense of sight
• As animals diversified, wide array of more specialized sensory abilities evolved!
o Magnetism: detect and use magnetic fields for navigation
o Electric fields: some aquatic predators detect electrical activity in muscles of passing prey
o Barometric pressure: some birds can avoid storms by sensing changes in air pressure.
Feeding: How Animals Eat
• Animals from same lineage can have same basic body architecture but feed in extremely different ways, because:
1. Their mouthparts vary, and
2. The structure of an animal's mouthparts correlates closely with its method of feeding.
• Four general types of animal feeding tactics:
1. Suspension feeders.
2. Deposit feeders.
3. Fluid feeders.
4. Mass feeders.
• capture food by filtering out particles suspended in water or air
• employ a variety of structures to trap suspended particles and bring them to their mouths
• commonly aquatic - and many are sessile
• digest organic matter that has been deposited in the soil or on the seafloor
• usually have simple mouthparts and their body shape is wormlike.
• suck or mop up liquids like nectar, plant sap, blood, or fruit juice
• often have mouthparts that allow them to pierce a structure to withdraw fluids
• take chunks of food into their mouths
• structure of the mouthparts correlates with the type of food pieces eaten
Feeding: What Animals Eat
• Three general sources of food for animals:
1. Plants or algae
2. Other animals
• Herbivores: animals that feed on plants or algae
• Carnivores: animals that feed on other animals
• Detritivores: animals that feed on dead organic matter
• Omnivores: animals that feed on both plants and animals
Feeding: How Animals Find Food
• Predators: kill for food using variety of mouthparts and hunting strategies - generally larger than their prey and kill them quickly
• Parasites: usually much smaller than their victims and often harvest nutrients without causing death
o Endoparasites: live inside their hosts
o Ectoparasites: live outside their hosts
• Movement has three functions in adult animals:
1. Finding food
2. Finding mates
3. Escaping from predators
• The ways in which animals move are highly variable: swim, fly, crawl, slither, walk, run...
• Limbs: made highly controlled, rapid movement possible!
o Hypothesis: all animal appendages have some genetic homology and were derived from appendages present in common ancestor in early history of Bilateria that later diverged through natural selection
Concepts to Remember
• Animals are multicellular, heterotrophic eukaryotes that move under their own power and ingest their food.
• Fundamental changes in morphology and development occurred as animals diversified.
• Evolutionary diversification was based on innovative ways of sensing the environment, feeding, and moving.
• BE ABLE TO: apply concepts of evolution to diversification of animals!
• Protostomes include some of the most familiar and abundant animals on Earth
o Ex.) arthropods: insects, spiders, and crustaceans
o Ex.) Mollusks: snails, clams, octopuses, and squids
o Most animals are protostomes
o include some of the most important model organisms
fruit fly (Drosophila melanogaster)
roundworm (Caenorhabditis elegans)
An Overview of Protostome Evolution
• Two major groups of bilaterally symmetric,
triploblastic, coelomate animals: protostomes and deuterostomes
• Phylogenetic studies support hypothesis that protostomes are a monophyletic group (developmental sequence arose just once)
o monophyletic groups within protostomes:
Lophotrochozoa and Ecdysozoa
Lophotrochozoans vs. Ecdysozoans
o Include mollusks, annelids, and flatworms
o Grow continuously and incrementally - like other animals
o No common features for ALL members (no synapomorphies)
Although are common features that are found in some...
o Grow by molting—shedding of soft cuticle or hard exoskeleton
o Cuticle and exoskeleton protect animals from predators
o Most prominent of seven ecdysozoan phyla are: roundworms (Nematoda) and the arthropods (Arthropoda)
Themes in Diversification of Protostomes
• Protostomes diverged into 22 different phyla recognized by distinctive body plans or specialized mouthparts
o diversification was triggered by evolutionary innovations in body plan, feeding, moving, and reproducing
• Triploblastic bilaterians with similar embryonic development:
o Most protostome phyla have wormlike bodies with a basic tube-within-a-tube design.
Outer tube is ectoderm-derived skin.
Inner tube is endoderm-derived gut.
Between two tubes are mesoderm-derived muscles and organs.
Body Plans Vary among Phyla
• In wormlike phyla, coelom is well developed and functions as a hydrostatic skeleton (for movement)
• Coelom is absent in flatworms, and drastically reduced in Arthropoda and Mollusca
o Other structures fulfill functions of a
coelom in arthropods and mollusks
Arthropod: hemocoel: (reduced coelom) spacious body cavity w/ space for internal organs and fluid circulation; also can function as hydrostatic skeleton
Mollusks: visceral mass: (reduced coelom) space for organs and circulation of fluids; also can function as hydrostatic skeleton
Evidence for Multiple Water-to-Land Transitions
• Ability to live in terrestrial environments evolved
independently in arthropods, mollusks, roundworms, and annelids
o Evidence based on phylogenetic analyses support hypothesis that ancestors of terrestrial lineages in each major subgroup of protostomes were aquatic!
Adaptation to Terrestrial Environments
• Transition opened up new habitats and resources to explore and led to continued diversification!
• Water to land transition was easier for animals than plants
o protostomes already had hydrostatic skeletons, exoskeletons and other adaptations for support and locomotion, which happened to work on land as well as in water
• New adaptations involved ways to:
o Exchange gases
o Avoid drying out
• Terrestrial protostomes have evolved many solutions to these challenges.
Concepts to Remember
• Most animals are protostomes
o monophyletic group divided into two major subgroups:
Lophotrochozoa and the Ecdysozoa
• Body plans vary: ex.) Mollusca and Arthropoda
• Multiple water to land transitions
o triggered the diversification of protostomes
• Include the largest-bodied and some of the most morphologically complex of all animals.
• Deuterostomes contain four phyla:
• Echinodermata—includes sea stars and sea urchins
• Chordata—includes vertebrates (animals with backbones)
• All share important features of embryonic development, but have widely variable morphology and behavior.
• Echinoderms ("spiny-skins") - named for spines or spikes observed in many species
• All are marine animals.
• About 7000 species of echinoderm have been cataloged thus far.
• Phylum is defined by several adaptations.
Echinoderm Body Plan (Synapomorphies-Monophyletic Group)
• Larvae are bilaterally symmetric
o adults have radial symmetry
• endoskeleton (hard protective/supportive structure inside body)
• Defined by water vascular system: series of branching, fluidfilled tubes and chambers that forms hydrostatic skeleton
o tube feet - elongated, fluid-filled appendages
Podia - sections of tube feet that project outside the body and are involved in motion along a substrate
• Defined by presence of four morphological features (at SOME stage in their lifetime):
1. Openings into the throat called pharyngeal gill slits
o important for suspension feeding
2. A dorsal hollow nerve cord that runs the length of the body, comprised of projections from neurons
o electrical signals are carried - (coordinate muscle movement)
3. A supportive but flexible rod, called the notochord, that runs the length of the body
o stiffens the muscular tail
4. A muscular post-anal tail.
.• Cephalochordates (lancelets or amphioxus): small, mobile suspension feeders that resemble fish.
• Urochordates (tunicates or sea
squirts): pharyngeal gill slits in both larvae and adults, but the notochord, dorsal hollow nerve cord, and tail occur only in larvae
• Vertebrates: dorsal hollow nerve cord is elaborated into spinal cord; pharyngeal pouches develop into gills in aquatic species, but not in terrestrial species (vestigial trait)
• Monophyletic group distinguished by two synapomorphies:
o Vertebrae: column of cartilaginous or bony structures that protects the spinal cord
o Cranium: bony, cartilaginous, or fibrous case that encloses and protects the brain
• Coordinated movements of vertebrates are possible in part because they have large brains -divided into three distinct regions (KEY INNOVATION):
1. Forebrain: housing the sense of smell
2. Midbrain: associated with vision
3. Hindbrain: responsible for balance and hearing
Key Innovations in Vertebrate Lineage
• Fossil record documents series of key innovations as lineage diversified:
1. Bony exoskeleton - first fossils with bone!
2. Jaws - new ways to eat (now armed and dangerous!)
3. Bony endoskeleton - supported movement (different from earlier cartilaginous endoskeleton)
4. Limbs capable of moving on land
5. Amniotic egg - membranes that protect and nourish young (allow for larger and better developed young)
The Tetrapod Limb
• Lungfish is closest-living relative to tetrapods
o Some species have fleshy fins supported by bones and are capable of walking short distances.
o Fossils provide strong links between ancestors of today's lungfish and earliest land-dwelling vertebrates!
• Hypothesis: mutation and natural selection gradually transformed fins into limbs as first tetrapods became more and more dependent on terrestrial habitats
o supported by fossil record and molecular genetic evidence
Concepts to Remember
• Variety of innovations and diversity developed throughout evolution:
o Echinoderms: radially symmetric as adults; water vascular system.
o Chordata includes vertebrates with several key innovations
Innovations led to continued diversification
Detailed example = evolution of limbs
• Unlike many animals, plants continue to grow and develop throughout life (whether live 2 weeks or thousands of years)
• Most plant cells retain ability to de-differentiate and redifferentiate!
• Arabidopsis thaliana (small flowering plant) frequently used as model organism
o relatively easy to grow
o produces large numbers of offspring
o completes its entire life cycle in six weeks
o example of a flowering plant (most species rich and abundant group of plants)
Life Cycle of a Flowering Plant
• Gametogenesis: gamete formation
• Fertilization: sperm and egg combine in womb-like ovule inside protective female reproductive structure of flower
• Embryogenesis: continued development continues inside ovule
o In many plants embryogenesis ends with maturation of ovule into a seed
embryo can remain in this nongrowing state for months, years or even centuries!
Life Cycle of a Flowering Plant
• Germination: seed resumes growth to form a seedling
o occurs when conditions are favorable
• Organogenesis: seedling develops into adult plant with vegetative (nonreproductive) organs
o Vegetative organs: leaves, roots, and stems
• Later...cells in stem are converted to reproductive structures... producing flowers
o Gametogenesis occurs in flowers...starting the cycle again
• One of most dramatic differences between plant and animal development
o Plants - sperm and egg cells are produced from haploid cells via mitosis
o Animals - sperm and egg cells are produced from diploid cells by meiosis
Sperm Formation in Flowering Plants
1. Diploid cells in sporophyte undergo meiosis to form haploid cells
2. Haploid cells undergo mitosis to form pollen grain (gametophyte)
3. One haploid cell (microspore) within pollen grain undergoes mitosis to produce two sperm cells
Egg Formation in Flowering Plants
1. Diploid cells in sporophyte divide by meiosis, producing four daughter cells
2. Only one cell survives (other three undergo apoptosis)
3. Surviving cell (megaspore) divides by mitosis to produce embryo sac (tiny, multicellular structure = gametophyte)
4. Inside embryo sac, one haploid cell differentiates into an egg (also note central maternal cell with 2 nuclei)
• Female reproductive structure = carpel
o Ovule (contains egg) is at the bottom of the carpel
o Stigma is the top of the carpel
• Pollen grains (male gametophyte) are carried by wind, water, or animal to a mature flower, where pollination occurs
o Pollination: pollen grain surface proteins interact with stigma surface proteins
Interactions are specific, preventing cross-species
fertilization and, often, self-fertilization
similar to bindin-fertilizin interaction
• Following successful pollination, pollen tube begins to grow and extend downward to the egg cells.
o growth guided by signals released from egg at base of carpel
• When the pollen tube reaches the base of the carpel...
1. Two sperm cells move down the pollen tube, through the ovule wall and into the embryo sac
a. One sperm nucleus fuses with egg to form diploid zygote
b. Other sperm nucleus fuses with maternal cell to form a triploid (3n) cell = double fertilization
• Triploid cell divides repeatedly - forms endosperm
o similar to yolk in animal eggs
o stores nutrients inside the seed for:
early seedling growth
• In flowering plants, embryogenesis takes place inside ovule as seed matures
o produces tiny, simplified plant
• After fertilization, the zygote divides asymmetrically
o produces a large basal (base) cell and small apical (tip) cell
The basal cell gives rise to a column of cells = the suspensor (anchors the embryo as it develops)
The apical cell gives rise to the mature embryo
• Asymmetries in the basal and apical cells help establish apicalbasal axis (top and bottom) of the plant.
More Plant Embryogenesis
• Radial axis (inside and outside) of the plant is established next - when the embryo is in its globular stage (ball of cells)
• Once the apical-basal and radial axes are established, the vegetative organs (leaves, roots and stems) begin to take shape
o initial leaves = cotyledons
connected to root by stem-like structure = hypocotyl
cotyledons + hypocotyl = shoot (will become above ground portion of the plant body)
shoot system function = photosynthesis and reproduction
o root forms below ground portion
function = water & nutrient gathering
Even More Plant Embryogenesis
• Groups of cells called the shoot apical meristem (SAM) and root apical meristem (RAM) form next
o Meristem: undifferentiated cells that divide repeatedly, with some daughter cells becoming specialized cells
Meristematic tissues produce cells that can differentiate into adult structures throughout the plant's life!!!
• Plant cells have stiff cell walls and cannot migrate (unlike animal cells)
o Plant embryonic structures take shape because cell divisions occur in precise orientations
resulting cells exhibit differential growth (some grow larger than others)
• In addition to establishing the two body axes, early development in Arabidopsis produces three embryonic tissues:
o Epidermis: outer protective covering
o Ground tissue: mass of cells inside the epidermal layer
may later differentiate into specialized cells for
photosynthesis, food storage, and other functions
o Vascular tissue: in the center of the plant
will differentiate into specialized cells that transport food and water between the root and shoot
• Plants cannot move to different location when environment is unsuitable
o adjust to changing environmental conditions through continuous growth and development of roots, stems, and leaves
can change direction of growth to face proper environmental conditions
constant adjustment possible because of meristems that are located at tips of shoots and roots
Meristems Allow Continuous Growth and Development
• Once embryonic development is complete, further plant body development is driven by meristems.
• Shoot apical meristems (SAM) exist at tips of shoots
• Root apical meristems (RAM) found at tips of roots
o SAM and RAM allow plant to grow in any direction, both above and below ground
cells can later differentiate into any type of plant tissue
• Within each meristem, rate and direction of cell growth are dictated by cell-cell signals produced in response to environmental cues
• Unlike animals, plants do not have germ cells set aside early in development
o Flowering and gametogenesis occur when shoot apical meristem converts from vegetative development to reproductive development (flowers)
Floral Meristem and the Flower
• Floral meristem: modified shoot apical meristem that produces reproductive organ-containing flowers
o Conversion of SAM into floral meristem occurs in response to environmental cues (shorter nights, warmer temperatures)
• Floral meristem produces four whorls of organs (each is a modified leaf):
3. Stamens - male reproductive organ
4. Carpels - female reproductive organ
What Genes and Proteins Set up Body Axes?
• Studies done in the 1990s
• Goal: identify genes involved in establishing the body axes
o Arabidopsis mutants with misshapen bodies were studied
o Researchers focused on development of apical-basal axis
o Found several "bizarre-looking" mutants
apical mutants: lacked cotyledons (first leaves)
central mutants: lacked hypocotyl (embryonic stem)
basal mutants: lacked both hypocotyls and roots
Discovery of Involved Gene
• Found gene, MONOPTEROS, critical in setting up apical-basal axis
o responsible for mutants lacking hypocotyls and roots
o DNA sequence suggested that protein product binds DNA
suggests it is a transcription factor
• MONOPTEROS gene codes for MONOPTEROS protein, a transcription factor
o gene is "turned on" by auxin
auxin is a master regulator (similar to bicoid)
• Auxin: cell-to-cell signal molecule (protein hormone)
o produced in shoot apical meristem - transported toward basal parts of embryo
o concentration of auxin along apical-basal axis of plant forms a concentration gradient
provides positional information
o part of regulatory cascade that triggers production of MONOPTEROS and other regulatory transcription factors - setting up the apical-basal axis.
Combinations of Gene Products Control Flower Development
• Several types of mutant flowering plants are homeotic mutants
o Homeotic flower mutant: one floral organ replaced by another
homeotic genes identify each "segment's" structural role
homeotic gene products (transcription factors) trigger development of appropriate structures at given segments
ABC Model:: hypothesis for genetic control of flower development
• Three basic ideas underlie ABC model:
1. Each of 3 genes involved is expressed in 2 adjacent whorls
2. Total of 4 different combinations of gene products occur
3. Each of four combinations of gene products triggers development of a different floral organ
• Four hypotheses (later proven to be true):
1. A protein alone causes cells to form sepals.
2. Combination of A and B proteins sets up formation of petals.
3. B and C combined specify stamens.
4. C protein alone designates cells as precursors of carpels.
Concepts to Remember
• In sharp contrast to animals, plants:
o develop continuously
o do not commit cells to gamete production until late in development
o produce gametes by mitosis in haploid cells
• Embryogenesis results in the formation of the major body axes and three types of embryonic tissue.
o Auxin - master regulator of regulatory cascade
• Key regulatory transcription factors (homeotic gene products) control the identity of floral organs
• Plants stay in one place!
o Extend roots and shoots to harvest diffuse resources
o Make their own food through photosynthesis
• The structure of a plant's body is dynamic!
o most plants exhibit indeterminate growth (grow throughout their lives)
Plant Form: Themes with Many Variations
• Photosynthesis...plants need large amounts of light and carbon dioxide and a small amount of water as an electron source
• Need water to fill cells to maintain normal volume and pressure
• Must obtain nutrients to synthesize macromolecules needed to build/run cells - most elements exist as ions dissolved in water.
Plant Form: Themes with Many Variations
• Root system: below ground portion that anchors the plant and takes in water and nutrients from the soil
• Shoot system: above ground portion that harvests light and carbon dioxide from the atmosphere to produce sugars.
• Both systems...
o grow throughout life of plant - allows for growth, competition with other plants, and acquisition of resources
o connected by vascular tissue - facilitates transport between them
Surface Area / Volume Relationship
• Root and shoot systems both function in absorption (of light or key ions and molecules)
o Absorption takes place across a surface, but cells that use the absorbed light and molecules also occupy a volume
plant body more efficient as absorbance-and-synthesis machine when has large surface area relative to volume
The Root System
• Many have vertical section (taproot) and numerous roots that run more or less horizontally (lateral roots)
• Anchors plant; absorbs water and ions and conducts them to the shoot; stores material produced in shoot for later use
• Many plants devote great deal of energy and resources to roots
• Most root systems contain same general structures, but root systems observed in different species are diverse on 3 levels
o Morphological diversity
o Phenotypic plasticity
o Modified roots
Morphological Diversity in Root Systems
• Ex) morphological diversity is seen in prairies or grasslands:
o Root systems of plants can be different - even when living next to each other!
• Hypothesis: natural selection has favored structures that minimize competition for water and nutrients
• Phenotypic plasticity: form can change in response to environmental conditions
o Even genetically identical individuals will have very different root systems if growing in different environments
o Important because plants grow throughout their lives
Roots grow into soil where resources are abundant
Roots do not grow (or die) where resources are lacking
• Roots can be modified for functions other than anchoring and absorbing water and ions from the soil.
o Adventitious roots develop from the shoot system instead of the root system
Prop roots: help brace and stabilize plants
o Pneumatophores: specialized lateral roots in mangroves that function in gas exchange.
The Shoot System
• Consists of 1+ stems (vertical above ground structures)
• Each stem consists of:
o Nodes: points where leaves are attached
site of axillary buds which may develop into
reproductive structures or a branch (lateral extension of the shoot system)
o Internodes: segments between nodes
o Leaves: appendages that project from the stem laterally
usually function as photosynthetic organs
o Apical bud: located at tip of each stem or branch - where growth occurs that extends length of stem or branch
may develop into reproductive structures
The Shoot System
• Essentially a repeating series of nodes, internodes, leaves, and buds
o As plants grow, number of nodes, internodes, and leaves increases
o Once a part of the shoot system forms, it does not increase much in size over time.
Instead, plants grow by adding more parts.
• As with roots, shoot diversity can be analyzed on three levels:
o morphological diversity
o phenotypic plasticity
o modified shoots with specialized functions
• Size and shape can vary greatly among species.
• Morphological variation in the shoot system allows:
o plants of different species to harvest light at different locations, minimizing competition
o plants to thrive in a wide array of habitats
• Size and shape of shoot system can vary dramatically based on variation in growing conditions (temperature, exposure to wind, and availability of water, nutrients, and light)
• Because shoot system continues to grow through life of plant, it can respond to changes in environmental conditions!
o Ex.) Shoot system grows in directions that maximize its chances of capturing light.
• Not all stems grow vertically; not all stems acquire carbon dioxide and photons - modified stems are common.
o Stems of cacti store water.
o Stolons are stems that run over the soil surface.
o Rhizomes are stems that grow underground horizontally.
o Tubers are rhizomes modified to store carbohydrates.
o Thorns are stems that protect the plant from herbivores.
• In most plant species, vast majority of photosynthesis occurs in leaves.
o Relatively large surface area available for absorbing photons
• External anatomy of a typical leaf consists of:
o Blade: expanded portion
o Petiole: stalk
• Leaves usually have an easily recognizable blade, but blades may vary in size and shape
o Compound leaves: blades divided into a series of leaflets - or doubly compound (leaflets are again divided)
• Needle-like leaves: smaller surface area, which reduces the likelihood of water loss through transpiration
• Arrangement of leaves on a stem can also vary.
• For example, leaves can be:
o Paired opposite each other on the stem.
o Arranged in a whorl.
o Arranged to alternate on either side of the stem.
o Compact arrangement where internodes are extremely short
—leading to rosette growth form
• Leaves don't grow continuously, but exhibit phenotypic plasticity
• Oak tree leaves vary depending on amount of sun exposed to
o Sun leaves: relatively small surface area, which reduces water loss in areas of the body where light is abundant
o Shade leaves: relatively large and broad, providing a high surface area that maximizes absorption of rare photons
• Some leaves have functions other than photosynthesis:
o Cactus spines: modified leaves that protect stem
o Onion bulbs: specialized for storing nutrients
o Succulents (ex. aloe vera): thick leaves store water
o Tendrils: modified leaflets/leaves that enable vines to climb
o Poinsettias: bright red leaves attract pollinators
o Carnivorous: some leaves function to trap insects
• Plants grow continuously due to meristems (undifferentiated cells; continuously divide and produce new cells)
o Apical meristems - located at tip of each root and shoot
o cells divide, enlarge, and differentiate - allowing root and shoot tips to extend plant body outward to explore new space = primary growth
o give rise to three distinct populations of cells (primary meristematic cells):
protoderm, ground meristem, and procambium
give rise to three major plant tissue systems (epidermis, ground tissue, vascular tissue)
(tissue: group of cells that function as unit)
Apical Meristems Produce the Primary Plant Body
• Protoderm: gives rise to dermal tissue system (epidermis) - single layer of cells that covers and protects the plant body
• Ground meristem: gives rise to ground tissue system - bulk of plant body; responsible for photosynthesis and storage
• Procambium: gives rise to vascular tissue system - provides support; transports water, nutrients and sugar
Primary Root System Organization
• Root cap: group of cells that protects root apical meristem
o regularly loses cells; replenished by root apical
o senses gravity and determines direction of growth
o secretes mucigel (slimy substance; lubricates root tip as moves through soil)
Primary Root System Organization
• Three distinct populations of cells behind root cap:
o Zone of cellular division: contains apical meristems and primary meristematic cells
o Zone of cellular elongation: comprised of cells that are actively increasing in length
o Zone of cellular maturation: older cells complete differentiation into dermal, vascular, and ground tissues
o epidermal cells produce root hairs (sites of water and nutrient absorption) and lateral roots begin to grow
Dermal and Ground Tissue Systems
• Dermal Tissue System:
o Interface between individual and external environment
o Primary function is to protect plant body from water loss, disease-causing agents, and herbivores
• Ground Tissue System:
o Location of most photosynthesis and carbohydrate storage
Vascular Tissue System
• Functions in support and long-distance transport of water and dissolved nutrients
• Moves products made and stored in ground tissue
• Comprised of two tissues:
o Xylem: conducts water and dissolved ions from root system to shoot system
o Phloem: conducts sugar, amino acids, chemical signals, and other substances throughout plant body
• All xylem cells are dead at maturity - not filled with cytoplasm; instead are filled with fluids that they conduct
o found in all vascular plants
o long, slender cells with tapered ends
o sides and ends have pits that lack secondary cell wall
o water moves between tracheids through the pits
• Vessel elements
o found in angiosperms and a few other species
o shorter and wider than tracheids
o pits and perforations (openings that lack both primary and secondary cell walls)
o conduct water more efficiently than tracheids
• Contains two types of specialized parenchyma cells:
o Sieve-tube members
long, thin cells that lack nuclei, chloroplasts, and most other major organelles
responsible for transporting sugars and other nutrients
connected to neighboring companion cells by many plasmodesmata
o Companion cells
not conducting cells
assist with loading and unloading carbohydrates and other nutrients from solution inside sieve-tube members
Concepts to Remember
• Plant body grows and changes throughout life
o root system anchors plant and absorbs water and key ions
o shoot system absorbs carbon dioxide and sunlight
• Variation in body size and shape allows different species to harvest water, light, and other resources in unique ways.
• Primary growth:
o cells located at tips of root and shoot divide and enlarge
o lengthens roots and shoots
o gives rise to three primary tissue systems that are specialized for protection, food production and storage, and transport
• Vascular System: Xylem and Phloem
• Q : How do plants transport water against force of gravity?!?!?
• A: It's all about water loss!
o Water loss is inevitable consequence of plant's need to obtain CO2 and release O2 (due to photosynthesis)
o Evaporation from leaves cools the plant and can actually be beneficial under some conditions.
o If lost water not replaced, plant cells dry out and die.
Water Potential and Water Movement
• Transpiration: water loss via evaporation from aerial parts of plant, which occurs when:
o Stomata are open AND
o Air surrounding leaves is drier than air inside leaves
• Plants replace water lost from leaves with water absorbed by roots (moves passively from roots to leaves - no expenditure of ATP)
o movement occurs because of differences in water potential
What Is Water Potential?
• Water potential: potential energy of water in a particular environment compared with potential energy of pure water at atmospheric pressure and room temperature (under these conditions, pure water has a water potential of 0)
• Water potential is symbolized by the Greek letter Ψ (psi).
• Differences in water potential determine direction water moves.
o Water always flows from areas of high water potential to areas of lower water potential.
What Factors Affect Water Potential?
1. Solute Potential
• Solution: homogenous mixture of a liquid (often water) and several substances (solutes)
o Isotonic Solutions: solute concentrations are the same
o Hypotonic Solution: solution has lower concentration of solutes that reference cell
o Hypertonic Solution: solution has higher concentration of solutes than reference cell
o Osmosis: movement of water across a membrane in response to differences in water potential
The Role of Solute Potential
• Solute potential or osmotic potential (ΨS): tendency of water to move in response to differences in solute concentration
o defined by solutions solute concentration relative to pure water
Inverse relationship...if water contains a high
concentration of solutes...it has a low solute potential compared with pure waterter.
The Role of Pressure Potential
• Wall pressure: force exerted by relatively rigid cell wall as it resists expansion of cell volume in response to influx of water
o plant cell swells in response to incoming water, and plasma membrane pushes against cell wall in response
o as water enters cell, pressure inside cell (turgor pressure) increases until wall pressure is induced
• Turgor pressure counteracts osmotic movement of water
• Pressure potential (ΨP): physical pressure on water and tendency to of that water to move in response to pressure
o Inside a cell, pressure potential consists of turgor pressure.
Calculating Water Potential
• Water potential: stored energy that makes water move in response to combined effects of pressure potential and solute potential
o Ignoring effects of gravity, water potential is:
ψ = ψp + ψs
o measured in units called megapascals (MPa)
• Water movement due to water potential is a combination of the following tendencies:
o water moves from high solute potential to low solute potential
o water moves from areas of high pressure potential to low pressure potential
Assigning Units of Pressure and Signs
• Solute potentials are always negative because are measured relative to solute potential of pure water, which is 0 Mpa
o Recall inverse relationship - high solute concentration indicates low solute potential (negative number)
• There are always some solutes inside a plant cell, so...
o water inside cell always has lower solute potential than pure water...so pure water (higher solute potential) will tend to move into the cell
o ψ = ψp + ψs
Water moves with high pressure (positive #) and high solute potential (larger (less negative) number)
Water Movement in Absence of Pressure
• Pressure potential from turgor pressure is positive inside cells
o Increases potential energy of water inside by exerting pressure on water, making it more likely to move out of cell
• Pressure potential can also be negative (pulling force/vacuum)
• If no pressure on water, it will move from higher solute potential (less solutes) to lower solute potential (more solutes).
Solute Potential and Pressure Potential Interact
• Solute potential and pressure potential can cancel each other out.
• When positive turgor pressure plus the cells negative solute potential equals 0 MPa (water potential of pure water), then the system reaches equilibrium!
o no additional net movement of water
Water Potentials in Soil and Plants
• Water in leaf, roots, and entire plant has pressure potential and solute potential, just as water inside a cell does
• Air and soil also have water potentials.
o Water potential in soil tends to be high (low in solutes) relative to plant roots (high in solutes)
Exceptions) Salty soil (high solutes); dry soil (low water sucked into soil-negative pressure similar to vacuum)
Water Potential in Air
• In atmosphere, water exists as vapor with ψs = 0 MPa
o Pressure exerted can be low or high:
In dry air, few water molecules present exert low pressure
In warm air, water molecules move farther apart and exert lower pressure.
Warm, dry air has a very low water potential.
Water potential of air is lower than inside leaf (unless rainy or foggy)!
• Water potential high in soil/roots, but low in leaves/atmosphere
o Sets up water-potential gradient between roots and shoots
Moving up plant, water moves down water potential gradient existing between soil, plant, and atmosphere
replaces water lost to transpiration
How Does Water Move from Roots to Shoots?
• Biologists have tested three major hypotheses for how water can be transported in a plant:
1. Root pressure: pressure potential develops in roots and drives water up against force of gravity
2. Capillary action: water is drawn up the cells of the xylem
3. Cohesion-tension: force generated in leaves pulls water up from roots through the xylem
Movement of Water and Solutes into Root
• Roots have several different tissue layers (from the outside in)
o Water must move inward toward the xylem
Movement of Water and Solutes into Root
• Travel through root cortex occurs via one of three pathways:
o Transmembrane route: flow through aquaporin proteins and direct diffusion across plasma membranes
o Apoplastic pathway: outside plasma membrane through porous cell walls and spaces that exist between cells
o Symplastic pathway: continuous connection through cells that exists via plasmodesmata
Water Movement via Root Pressure
• Root pressure is generated at night, after stomata close.
o When ions accumulate in xylem of roots, enough water may enter xylem via osmosis to force water and ions up the xylem
• In low-growing plants, enough water can move to force water droplets out of leaves (guttation)
• Research shows root pressure alone cannot push water all the way up a tall tree.
Water Movement via Capillary Action
• Capillarity: water movement up narrow tube (xylem) occuring in response to 3 forces: surface tension, adhesion, & cohesion.
o Surface tension: downward pull that exists on water molecules at air-water interface
water molecules at surface can form hydrogen bonds only with water molecules below them - bonds pull topmost layer of water molecules downward
o Adhesion: attraction of unlike molecules (water hydrogen bonds with glass or another cell)
o Cohesion: mutual attraction among like molecules
• Interaction between cohesion and surface tension forms concave boundary layer (meniscus)
Water Movement via Capillary Action
• Capillarity is a result of:
o Surface tension creates downward pull at the surface
o Adhesion creates upward pull at water container surface
o Cohesion transmits both forces to water below
• However, amount of water that can be transported by capillarity is limited!
o Capillarity is resisted by gravity and by surface tension.
The Cohesion-Tension Theory
• Leading hypothesis to explain water movement in vascular plants!
• Cohesion-tension theory of water movement: water is pulled up to tops of trees along water potential gradient, via forces generated by transpiration at leaf surfaces
o Key concept: negative force or pull (tension) generated at airwater interface is transmitted through water outside of leaf cells, to water in xylem, to water in the vascular tissue of roots, and finally to water in the soil!
• Continuous transmission of pulling force is possible because water is present throughout the plant and all water molecules hydrogen bond to one another (cohesion).
• Plant does not expend energy to create the pulling force—this force is generated by energy from the Sun which drives evaporation (transpiration) - water transport is solar powered.
Creating a Water-Potential Gradient
• In xylem, water movement is driven entirely by differences in pressure potential
• The pulling force due to transpiration lowers the pressure potential of water in leaves (negative pressure - like sucking from a straw)
• Although the tension at each meniscus is small, there are millions of menisci in the entire plant.
o Tension has remarkable cumulative effect - creates steep enough water-potential gradient between leaves and roots to overcome force of gravity and pull water up long distances.
• Translocation: movement of sugars through plant from sources to sinks
o Source: tissue where sugar enters phloem
o Sink: tissue where sugar exits phloem
sugar concentrations are high in sources and low in sinks
The Anatomy of Phloem
• Sieve-tube members are connected end to end by perforated structures (sieve plates)
o creates direct connection between cytoplasms of adjacent cells
• Sieve-tube members and companion cells are alive at maturity
o sieve-tube members lack nuclei and many major organelles
Companion cells are "support staff" for sieve tube members
• Phloem sap flows through phloem
The Pressure-Flow Hypothesis
• Pressure-flow hypothesis: steep pressure potential gradient in phloem
o movement along water-potential gradient created by changes in pressure potential
o water in phloem sap moves down pressure gradient, and sugar is carried along by bulk flow
large differences between turgor pressure in phloem near source and sink tissues generate the necessary force
creating differences in turgor pressure may require ATP
o Transpiration does not provide driving force to move phloem sap.
The Pressure-Flow Hypothesis
• Phloem loading: sucrose moved by active transport from source cells through companion cells to sieve-tube members
o result = phloem sap near source has high sucrose concentration (low solute and water potential)
causes water to move from nearby xylem (higher water potential) into the sieve-tube members
• Phloem unloading: companion cells remove sucrose from sieve-tube members into sink cells
o result = phloem sap has lower sucrose concentration (high solute and water potential)
causes water to move back into the xylem
The Pressure-Flow Hypothesis
• Net result of phloem loading and unloading is high turgor pressure near source (where water entered from xylem) and low turgor pressure near sink (where water exited)
o pressure drives water (and phloem sap) from source to sink
• One-way flow of sucrose and a continuous loop of water movement, with water being supplied to and from the xylem
Phloem Loading and Unloading
• Must establish high pressure potential in sieve-tube members near source cells
o enough sugar must be transported into phloem sap to raise solute concentration of sieve-tube members (and cause water to flow in from the xylem)
• Loading is often active (requires ATP and membrane transport system)
o Extremely high sucrose concentrations in source cells can result in loading via passive diffusion.
o Sugar must sometimes be unloaded against concentration gradient at sinks, requiring ATP
How Are Sugars Concentrated in Phloem at Sources?
• Sucrose is actively transported from source cells into companion cells, and enters companion cells with protons (cotransport).
o involves a proton pump (H+-ATPase) and a cotransporter (protons and sucrose enter the cell together)
proton pump hydrolyzes ATP to move hydrogen ions to the exterior of the cell - resulting high concentration of H+ establishes gradient that allows transport of sucrose into the cell against its concentration gradient
Concepts to Remember
• Water moves from high water potential to low water potential.
o Waters potential energy is a combination of:
tendency to move in response to differences in solute concentration
pressure exerted on it
• Transpiration is the driving force for water flow: water moves from soil and roots to leaves along a water-potential gradient
o driven by the Sun
o creates a negative pressure (tension) that pulls water up
• In phloem, sugars are transported from sources to sinks -
o moves due to a pressure gradient that favors movement of water and sucrose to sinks
• Plants cannot live on sugar alone. They also need to synthesize all their own macromolecules (nucleic acids, proteins...)
• Therefore, plants need other elements in addition to those available from carbon dioxide and water.
• Soil provides most of these nutrients.
When Key Nutrients Are in Short Supply
• N, P, K, and Mg are mobile elements.
o When supply is limited they are transferred to newer leaves
o Their scarcity is reflected in the deterioration of older leaves
• Iron and calcium are immobile nutrients
o They remain tied up in older leaves
o Their scarcity is reflected in newer leaves
• Weathering: forces applied by rain, running water, and wind begin process of breaking down solid rock into soil
o Gravel, sand, silt, or clay break off from solid rock and are first ingredients in soil
• Organisms occupy rock fragments - add dead cells and tissues
o this organic matter is called humus.
• Mature soils are complex mixture of organic and inorganic components
o soil-dwelling organisms include fungi and animals, along with vast numbers of bacteria, archaea, and microscopic protists
• Texture: proportions of different-sized particles present in soil
o affects ability of roots to penetrate the soil
o affects ability of soil to hold water
• Texture and water content dictate availability of oxygen
• Best soils are loams: contain roughly equal amounts of sand, silt, and clay along with a high proportion of humus
Importance of Soil Conservation
• Soil erosion: soil is carried away from a site by wind or water
• Sustainable agriculture: techniques that maintain long-term soil quality and productivity
o Plant rows of trees as windbreaks
o Minimize plowing and tilling needed to control weeds
o Plant crops in strips that follow the contour of hillsides
What Factors Affect Nutrient Availability?
• Nutrients required for plant growth occur in soil as ions (charged particles)
o Anion: negatively charged particle
usually dissolve in soil water, because interact with water molecules via hydrogen bonding
available to plants for absorption, but easily washed out of the soil by rain
leaching: loss of nutrients via washing
o Cation: positively charged particle
dissolve in soil water - but not immediately available
interact with negative charges on soil (organic matter and clay)
The Role of Soil pH
• Soil pH can influence the availability of essential elements
o acidic (low pH; high [H+]) or alkaline (high pH; low [H+])
• Cation exchange: occurs when ...
1. protons or other cations in soil water bind to negative charges on soil particles...causing
2. bound cations such as Mg2+ or Ca2+ to be released...making
3. cations available to nearby plant roots
• Nutrient uptake occurs in the zone of maturation
• Root hairs increase surface area available for nutrient and water absorption.
• Root hairs create zone of nutrient depletion in soil immediately surrounding them; continued root growth vital to plant health
Mechanisms of Nutrient Uptake
• Plant cell walls - permeable to ions, small molecules, and even large molecules
• Plasma membrane - highly selective - protein channels/carriers allow only specific ions to cross the plasma membrane
Establishing a Proton Gradient
• Proton pumps (H+-ATPases):
o located in plasma membranes of root-hairs and other epidermal cells concentration gradient.
o activity leads to excess of protons on exterior of plasma membrane relative to interior
difference in proton concentration on either side of plasma membrane results in strong concentration gradient
favors movement of protons into epidermal cell
outside epidermal membrane becomes positively charged relative to inside; separation of charge across membrane
Using Proton Gradient to Import Cations
• Electrochemical gradient: combination of concentration and electrical gradients
o Electrical gradient established by proton pumps in membranes of root hair cells favors entry of cations
strong enough to overcome concentration gradient, which opposes their entry
Using a Proton Gradient to Import Anions
• Anions enter root hairs via cotransporters
o so much energy released when proton enters cell along its electrochemical gradient that anions can be cotransported against their electrochemical gradients!
• Summary: electrochemical gradient set up by proton pumps makes it possible for plant roots to:
o absorb key cations via ion channels
o absorb anions via symporters
• Most fungi are mutualists
o live in association with other organisms - but benefit hosts
o roots of virtually every land plant colonized by mutualistic fungi
In exchange for sugars synthesized by plant, fungi provide water and key nutrients (without which host plants grow more slowly or even starve)
Mechanisms of Ion Exclusion
• Many of the metal ions found in soils are poisonous to plants
o even essential nutrients can become toxic if present in excessive quantities
o Plants exclude detrimental ions by passive or active exclusion
• Passive exclusion of ions occurs at Casparian strip and root hairs
o Some ions are excluded due to lack of necessary types of membrane proteins
Active Exclusion by Antiporters
• Plant cells actively remove toxic substances from cytosol and store them in vacuole, where they cannot poison enzymes
• H+-ATPases in membrane move protons into vacuole
o creates electrochemical gradient that favors movement of H+ out of vacuole
antiporter uses gradient to conduct protons out of vacuole, along electrochemical gradient, and brings Na+ ions into vacuole—against concentration gradient.
• Plants and other eukaryotes cannot use atmospheric nitrogen gas (N2)
• Some bacteria are able to absorb N2 from the atmosphere and convert it to ammonia, nitrates, and nitrites (nitrogen fixation)
• In many cases, nitrogen-fixing bacteria take up residence inside plant root cells.
o Ex) members of the bacterial genus Rhizobium associate with plants in the pea family (legumes)
root cells of legumes form distinctive structures (nodules) where nitrogen-fixing rhizobia are found
Rhizobia provide plant with ammonia in return for sugar and protection
Concepts to Remember
• In addition to needing carbon dioxide and water, plants require an array of essential nutrients to support growth
o nutrients available as ions dissolved in soil water and are taken up by roots (ex. cation exchange)
• Nutrient absorption and exclusion occurs via specialized proteins in plasma membranes of root cells
o involves active transport (proton pumps)
o most plants also obtain nitrogen or phosphorus from fungi associated with their roots
o some species of plant have specialized methods of obtaining nutrients, including associations with nitrogen-fixing bacteria
• Plants receive and respond to wide variety of environmental stimuli, including light, gravity, pressure, and attacks from enemies.
• Plants have sophisticated systems for:
o collecting information about their environments
respond to maximize chances of surviving, thriving, and producing offspring!
Information Processing in Plants
• Plants monitor aspects of environment that affect ability to stay alive and reproduce.
• Plants gather, process, and respond to information they monitor in a series of three steps:
1. Receptor cell receives an external signal.
2. Receptor cell sends signal to cells in another part of plant
3. Responder cells receive signal and change their activity appropriately.
How Cells Receive and Transduce External Signals
• Signals from environment are received by protein specialized for function...
o Receptor proteins change shape in response to stimulus!
Result = Signal Transduction: causes information to change form from external to intracellular signal
Once information has been transduced to intracellular form, it travels down signal transduction pathway.
How Cells Receive and Transduce External Signals
• Two basic types of signal transduction pathways:
1. Phosphorylation cascades: triggered when receptor protein's shape leads to transfer of phosphate group from ATP to receptor or nearby protein
2. Second messengers: produced when hormone binding results in release of intracellular signal (usually Ca2+ in plants) from storage areas
• Signal transduction in a receptor cell often results in release of hormone that carries information to responder cells.
How Are Cell-Cell Signals Transmitted?
• Because plants perceive wide variety of stimuli, they have wide array of hormones.
o Each hormone is specific for a given receptor and function!
• Plant cells routinely receive information from several different hormones at same time
o common for different types of hormones to interact with each other and modulate the cells response
How Do Cells Respond to Cell-Cell Signals?
• Hormones can elicit response only if cell has appropriate receptor.
• Hormones are tiny molecules in tiny concentrations, but signal is amplified during signal transduction and produce big results!
• When cells respond to hormone, change in activity helps plant cope with environmental change sensed by receptor cell.
How Do Cells Respond to Cell-Cell Signals?
• Hormone binding can result in:
o Activation of membrane transport proteins - producing change in membranes electrical potential or cell walls pH
o Changes in gene expression - resulting in new suites of proteins or RNAs in the cell
Blue Light: The Phototropic Response
• Plants sense and respond to specific, narrow range of wavelengths in visible spectrum
• Phototropism: directed movement by organism toward light
• Charles Darwin and son Francis experimented with coleoptiles (tissue protecting developing shoots and embryonic leaves)
o exhibited phototropic response to blue light but showed no response in its absence
plants move toward blue light because it drives photosynthesis
Phototropins as Blue-Light Receptors
• Biologists knew that blue-light receptor must be pigment (molecule that absorbs certain wavelengths of light), but took decades to find it!
o PHOT1: pigment; blue-light receptor that phosphorylates itself in response to blue light
Once receptor protein is phosphorylated, a phototropic response is initiated
Auxin as Phototropic Hormone
• Sensory and response cells are not the same in phototropic response
• Experiment (Darwins): convincing evidence that blue light is sensed at tip and is transmitted to lower cells
o seedling with tip removed or covered did not bend toward light
o covering lower cells did not stop plant from bending
Auxin as Phototropic Hormone
• Darwins hypothesized that substance (hormone) produced at tip acts as signal that is transported to area of bending
o Later experiments by Peter Boysen-Jensen and Fritz Went support the hormone hypothesis for phototropism.
Went later named hormone (first plant hormone ever discovered) auxin.
Auxin promotes cell elongation (growth) in the shoot.
• Cholodny and Went independently proposed that phototropism results from asymmetric distribution of auxin.
o Hypothesis contends that:
1. Auxin produced in tips is shunted from one side of tip to other in response to light
2. Auxin is transported straight down one side of the shoot
3. Asymmetric distribution causes cells on shaded side to elongate more than cells on illuminated side
The Cholodny-Went Hypothesis
• Later experiments by Winslow Briggs supported Cholodny- Went Hypothesis
o found that asymmetric distribution of auxin results from lateral redistribution
• Summary: Cholodny, Went, and Briggs explained how auxin leads to asymmetric cell elongation, and thus the bending response (phototropism)
• Auxin receptors are located in plasma membrane of stem and leaf cells
• Researchers proposed that once receptors have bound to auxin...
1. signal transduction cascade follows
2. Increases number of proton pumps (H+ ATPases) in plasma membrane
3. triggers cell elongation and leads to phototropism
• Acid-growth hypothesis for cell elongation:
o proton pumps drive protons out of cell against
electrochemical gradient...lowering pH of the cell wall
o cell expansion follows
• Two things have to happen for a plant cell to get larger:
1. Cell wall must expand to create larger volume
2. Water has to enter cell to increase volume and generate turgor pressure on cell wall
• Lowering pH of cell wall beyond 4.5 activates expansins
o increase cell length by releasing molecular links connecting cellulose microfibrils
• Electrochemical gradient (created by H+ pumps) causes positively charged ions such as potassium (K+), as well as sugars (co-transport), to enter the cell.
• Water follows by osmosis, causing turgor of cell to increase
Pathogens and Herbivores: The Defense Responses
• Defenses that poison herbivores are effective, but are expensive to produce (ATP and materials invested)
o Plants produce defenses or increase existing defenses only in direct response to attacks by pathogens or herbivores!
• Induced defenses: responses to attacks induced by presence of a threat
o Must be able to sense the attack
How Do Plants Sense and Respond to Pathogens?
• Hypersensitive response (HR): defense mounted by plant in response to virus, bacterium, or fungus invasion
o causes rapid and localized death of cells surrounding site of infection...starving the pathogen!
o But how do cells sense the infection!?!?
• Resistance (R) genes: disease-resistance genes in plants
• Avirulence (avr) genes: genes associated with virulence in pathogens
• Gene-for-gene hypothesis: predicts that...
o R gene products act as receptors
o avr gene products act as ligands
receptors bind ligands...initiating hypersensitive response
If R allele in host and avr allele in pathogen do not correspond, no HR occurs and plant succumbs to disease!
• Research has confirmed that R products and avr products do bind to each other.
• R genes are highly polymorphic
o duplicated R genes mutate over time..
generates new alleles - allow individuals to recognize and respond to novel avr products and pathogens
Plants that are heterozygous for each of many R genes should sense wide variety of disease-causing agents and respond by triggering HR!
The Hypersensitive Response (HR)
• HR triggers reactions that:
o help reinforce cell walls, and
o kill host cells at the point of infection
o lead to production of antibacterial and antifungal compounds
• Summary: HR leads to:
o walling-off infected area (reinforced cell walls)
o direct killing of pathogens by antibacterial/antifungal compounds
o starvation of pathogens as nearby cells commit suicide
• Summary: HR leads to:
o walling-off infected area (reinforced cell walls)
o direct killing of pathogens by antibacterial/antifungal
o starvation of pathogens as nearby cells commit suicide
Alarm Hormone Extends the HR
• Interaction between R and avr gene products also leads to release of hormone that initiates systemic acquired resistance (SAR)
o SAR: slower, widespread set of events triggered after HR
acts globally as well as locally
results in expression of large suite of genes (pathogenesisrelated (PR) genes)
Alarm Hormone Extends the HR
• Observations support hypothesis that methyl salicylate (MeSA) is involved in SAR (but scientists not sure if its THE hormone since its short lived):
o MeSA levels increase dramatically after tissues are infected by pathogen
o Phloem sap leaving infected sites has elevated levels of MeSA
o Treatments that reduce MeSA reduce or abolish SAR
o Adding MeSA to lower leaves of tobacco plants leads to SAR in upper, untreated leaves
Concepts to Remember
• Plants are selective about information they process - only respond if affects survival or reproduction.
o Plants perceive wide variety of environmental stimuli that affect ability to grow and reproduce.
• Sensory cells receive a stimulus, transduce the signal and respond by producing hormones that carry information to target cells elsewhere in body
• Hormones responsible for regulating how plants grow and respond to changes in environmental conditions
o Phototropic response (auxin)
o Hypersensitivity Response and Systemic Acquired Resistance
• Every structure and process of plants exists to maximize the chance that the individual will produce offspring.
o Flower: reproductive structure that produces gametes, attracts gametes from other individuals, nourishes embryos,
and develops seeds and fruits
Seed: consists of embryo and nutrient stores surrounded by protective coat
Fruit: develops from flower's seed-producing organ and contains seeds
• Asexual reproduction: does not involve fertilization; results in production of genetically identical copies of parent plant
o advantage - very efficient
o disadvantage - genetically similar populations are more likely to succumb to diseases
Apomixis: occurs when mature seeds form without fertilization occurring, resulting in seeds that are genetically identical to parent (ex. dandelions)
When Does Flowering Occur?
• Flower formation begins when:
1. apical meristem stops making energy-harvesting stems and leaves
2. apical meristem becomes floral meristem able to produce modified leaves that comprise flowers
• Flowering can be stimulated by external cues, internal cues, or both.
• External environmental cues may involve length of day and night, arrival of seasonal rains, or other mechanisms
• Plants also respond to internal cues, such as favorable nutritional status or hormones
The General Structure of the Flower
• Four basic organs that are essentially modified leaves:
1. Sepals: leaf-like structures; outermost part of the flower
2. Petals: brightly colored; advertise flower to pollinators
3. Stamens: reproductive structures that produce male gametophytes (pollen grains), which in turn produce sperm
4. Carpels: produce female gametophytes, which in turn produce eggs
• Four organs are attached to compressed portion of stem called the receptacle
• All four organs are not necessarily present in all flowers.
• Coloration, size, and shape of these four components are fabulously diverse
Production of Female Gametophytes
• Typical angiosperm ovary contains one or more ovules
o Each ovule contains diploid megasporocyte inside megasporangium
divides by meiosis to produce four haploid megaspores
surviving megaspore divides by mitosis to produce multicellular, haploid gametophyte (embryo sac)
haploid nuclei segregate to different positions in embryo sac - cell walls form around them
large central cell contains two "polar nuclei"
egg cell is located at one end
Production of Male Gametophytes
• Inside anther (part of stamen), microsporangia contain diploid microsporocytes
o divide by meiosis to produce four haploid microspores
each microspores divides mitotically to form pollen grain (haploid, immature male gametophyte with two nuclei)
Immature pollen grain: small generative cell enclosed within a larger tube cell
Mature pollen grain: generative cell produces two sperm via mitosis; wall of pollen grain develops tough outer coat that protects it once released into environment
Pollination and Fertilization
• Pollination: transfer of pollen grains from an anther to a stigma
• Fertilization: sperm and egg unite to form diploid zygote
o Self-fertilization (selfing): sperm and egg from same individual combine to produce offspring
o Outcrossing: much more common that selfing; sperm and eggs from different individuals combine
Costs/Benefits of Selfing vs. Outcrossing
o advantage - virtually assures successful pollination
o disadvantage - offspring are usually less genetically diverse than outcrossed offspring; may suffer inbreeding depression
many plants have ways to prevent selfing
o advantage - results in more genetically diverse offspring that may be much more successful at warding off attacks from pathogens
o disadvantage - riskier in terms of likelihood of pollination
• Fertilization in angiosperms involves four steps:
1. Pollen grain lands on stigma and resumes growth
2. Pollen tube grows toward the ovary
o generative cell divides to form two sperm
3. Pollen tube reaches base of embryo sac and enters interior
4. In angiosperms double fertilization occurs
o nuclei of egg and one sperm unite to form zygote
o other sperm nucleus fuses with two polar nuclei, forming a triploid (3n) cell = endosperm
stores nutrients that will be needed by the embryo
• Fertilization triggers development of young sporophyte
o In angiosperms, first stage in sporophyte's life is maturation of the seed
As seed matures:
embryo and endosperm develop inside ovule and become surrounded by a covering (seed coat)
ovary around ovule develops into fruit (encloses and protects the seed (or seeds, if single ovary contains multiple ovules))
Seeds contain stored nutrients that increase offspring success in crowded habitats.
Seedlings can survive on stored nutrients until well enough established to absorb water from soil and feed themselves via photosynthesis
Fruit Development & Seed Dispersal
• Fertilization initiates fruit development as well as seed and embryo development.
• Three basic fruit types:
o Simple fruits: ex.) apricot; develop from single flower that contains single carpel or several carpels fused together
o Aggregate fruits: ex.) raspberry; develop from single flower, but one that contains many separate carpels
o Multiple fruits: ex.) pineapple; develop from many flowers and thus many carpels
Fruit Development and Seed Dispersal
• As fruit matures, walls of ovary thicken to form pericarp that surrounds the seeds.
• Fruits protect seeds from damage and seed predators, and aid in seed dispersal.
o Most dry fruits (nuts) are dispersed by wind, animals, or mechanical action, or simply fall to the ground
o Fleshy fruits are commonly dispersed by animals
What is the association between body symmetry type and nervous system type?
Bilaterally symmetric animals have a central nervous system
Species that have embryos inside the female's body during development, followed by birth to live young are said to be _____.
Protostomes are different from deuterostomes because they _____.
have two blocks of solid mesoderm that split to form the coelom (This is a protostome trait.)
How does the interaction between pollen and stigma in flowering plants differ from the interaction between sperm and egg in sea urchins?
Sperm and egg cells fuse, pollen and stigma cells do not.
How does gametogenesis occur in Arabidopsis and other flowering plants?
Gametes are produced by mitosis of haploid cells produced by meiosis.
Which of these terms is NOT correctly paired with its definition?
endosperm: diploid nutritive tissue of seeds
The ABC model explains how three genes (A, B, and C) can specify the identities of four floral organs. The pattern of gene expression that makes this possible is _____.
Each gene is expressed in two adjacent whorls, so a total of four different combinations of gene products can occur.
In flowering plants, fertilization is called "double fertilization" because _____.
the two sperm cells each fuse to a different maternal cell
What leaf form would you expect to find in a hot wind-blown desert?
needle-like leaves to avoid water loss
Both root system and shoot system in plants are involved in common functions as well as distinct functions. Which of the following defines a function common to both roots and shoots?
harvesting resources from the environment
What is meant by phenotypic plasticity in roots and shoots?
ability to change form depending on the environmental conditions
Water transport is the primary function of which cell type?
Tracheids (the major component of xylem tissue and responsible for movement of water)
An herbicide that targets the shoot apical meristem will _____.
cause the tips of the shoots to stop growing (The protoderm cells give rise to the newly developing shoot tissue)
What defines the process known as "transpiration" in plants?
Water is lost into the environment through stomata.
Water potential (the potential energy of water) at a given location is determined by _____.
the pressure on water and amount of solutes dissolved in it
Which of the following has the highest water potential value on average?
Soil (the least negative water potential due to having the most water availability, although drought conditions will heavily alter this value)
When water moves from soil into the vascular tissues inside the root, it can take three possible pathways. Of these, the apoplastic path is _____.
along cell walls and air spaces
Some plants growing in arid climates have small leaves and thick waxy cuticles on the upper epidermis. This is an adaptation for _____.
decreasing the water loss from leaves
According to the pressure-flow hypothesis, which direction are sucrose and water flowing when roots are sources and leaves are sinks?
Water and sucrose flow up the stem.
According to the pressure flow hypothesis, what mechanism causes the movement of phloem sap from sources to sink tissues?
pressure potential difference between source and sink
Loading of sugars to the phloem sieve tube elements at the source tissues happens by _____.
active and passive transport across the plasma membrane
The ability of plant roots to penetrate through soil is determined by which of the following?
The presence of protons in the soil as a result of moderate soil acidity helps nutrient availability by _____.
promoting cation exchange
How do proton pumps impact potassium uptake?
They decrease positive charge within the cell to attract potassium ions (Removal of H+ ions creates a favorable electrical gradient for the import of K+ ions.)
Root hair cells absorb nutrients from soil against concentration gradient. To achieve this, root hair cells use _____.
Could a tiny concentration of a contaminant that mimics a hormone affect a plant significantly? Why or why not?
Yes. The contaminant could trigger a phosphorylation cascade. (Tiny amounts of hormone (or contaminants that mimic them) can trigger a large system-wide response.)
When a signal arrives at the cell, it gets transduced. What is meant by the term "signal transduction"?
transmission of signal inside the cell in another form
The Cholodny-Went hypothesis has been proposed to explain how plants bend toward a directional light source. What does this hypothesis suggest?
Asymmetric auxin distribution causes differential cell elongation that causes bending.
Systemic acquired resistance (SAR) in a plant in response to a pathogen attack means _____.
inducing resistance in other parts of the plant by sending a message about infection
Which of the following is developed from the male spore?
pollen (developed from the male microspore)
Which of the following is a trait that both a megasporocyte and a microsporocyte share?
Both are 2n and divide to form gametophytes. (Both divide via meiosis to form haploid gametophytes.)
The _____ proteins on pollen and stigma surfaces prevent
rose pollen from fertilizing daisy ovules (and vice versa).
Unlike animals, plants can adjust their growth and development continuously in response to changing environmental conditions. Which plant-specific feature is responsible for this developmental plasticity?
Plants contain meristem.
During embryogenesis in Arabidopsis, what is the role of the plant hormone auxin?
It provides positional information along the apical-basal axis.
Which of the following is most accurate regarding flower development?
It is controlled by the combinations of three different gene
products: A, B and C.
Which of the following are true for both root and shoot systems?
Morphological Diversity, Phenotypic Plasticity and Modified
Which of the following are involved in water transport from
roots to shoots?
A. Surface tension of water in leaves
B. Cohesion among water molecules in the xylem
D. A difference in pressure potential between roots and shoots
E. All of the above
How does turgor pressure develop in the phloem near a source?
Water flows from the xylem (high water potential) to the phloem
(low water potential).
Cation exchange is a process _____.
in which protons replace positive ions bound to soil particles
making them available for plants