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SBI4UZ: Topic 9 Plant Growth (TRANSPORT (PHLOEM TRANSPORT (Active…
SBI4UZ: Topic 9 Plant Growth
TRANSPORT
PHLOEM TRANSPORT
Active translocation: movement of organic compounds from sources to sinks via the phloem
Source: where organic compounds synthesized (photosynthetic tissues/leaves) **sugars transported as sucrose b/c soluble (plant sap)
Sink: where compounds delivered to for use or storage (roots, fruits, seeds)
Phloem structure: sieve tubes composed of sieve element cells and companion cells (and additional cells to fill spaces and provide support; movement of sap mediated by hydrostatic pressure from xylem
Sieve elements: long narrow cells connected to form sieve tube *unable to sustain metabolic activity without companion cells b/c no nuclei, plasmodesmata exist between them in large numbers (which connect cytoplasms and mediate exchange)
Connected by sieve plates at transverse ends (porous, enable flow)
No nuclei and reduced numbers of organelles to maximize space for translocation
Thick and rigid cell walls to withstand hydrostatic pressures
Companion cells: provide metabolic support and facilitate loading/unloading of materials
Infolding plasma membrane which increases SA:volume ratio
Mitochondria to fuel active transport
Transport proteins to move materials into/out of sieve tube
Differences in distribution and arrangement of roots and shoots
Roots
Monocotyledons: large stele, vessels form radiating circle around central pith
Xylem vessels located more internally
Phloem externally
Dicotyledons: small stele, xylem located centrally w/ phloem surrounding it
Xylem vessels form cross-like shape ('X' for xylem)
Phloem situated in the surrounding gaps
Stems
Monocotyledons: vascular bundles scattered throughout stem
Phloem positioned externally (phlOem = Outside)
Dicotyledons: arranged in circle around centre of stem (pith)
Phloem on outisde
Xylem on inside
Separated by cambium
Phloem loading (and unloading): organic compounds produced at source are actively loaded into phloem sieve tubes by companion cells
Materials can pass into sieve tube via plasmodesmata (symplastic loading)
Materials can be pumped across cell wall by membrane proteins (apoplastic loading)
Active transport requiring ATP
H+ actively transported out of phloem cells by proton pumps (hydrolysis of ATP)
Concentration of H+ increases, creating proton gradient
H+ ions passively diffuse back into phloem cell via co-transport protein (requiring sucrose movement)
Build-up of sucrose within phloem sieve tube for transport from source
Active transport of sucrose makes sap solution hypertonic
Causes water to be drawn from xylem via osmosis (water moves towards higher solute concentrations)
Buildup of water causes hydrostatic pressure to increase
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Translocation rate
Aphids extract sap through stylet, aided by digestive enzymes that soften tissue layers → if stylet severed, sap will continue to flow due to hydrostatic pressure
Aphids used to collect sap and provide measure of phloem transport rates
Positioned along plant's length, encouraged to feed on sap
Stylet severed, sap analysed for presence for radioactively-labelled sugars
Rate of translocation calculated based on time taken for radioisotope to be detected at different positions along plant's length
Plant grown within lab with leaves sealed in chamber containing radioactively-labelled CO2 → leaves convert CO2 into radioactively-labelled sugars transported by phloem
Factors
Rate of photosynthesis (light intensity, CO2 concentration, temperature, etc.)
Rate of cellular respiration
Rate of transpiration
Diameter of sieve tubes
XYLEM TRANSPORT
Transpiration: loss of water vapour from stems/leaves of plants (affected by level of photosynthesis)
Process
Water flows via the xylem along pressure gradient to replace lost water from leaves
Light energy converts water to vapour, evaporates via stomata (pores on underside of leaf that facilitate gas exchange necessary for photosynthesis)
New water absorbed by roots → difference in pressure between leaves and roots (low and high)
**Transpiration rates higher when stomata are open, measured by potometers
Temperature: increase in rate of vaporisation
Humidity: less vapour will diffuse from lea if more vapour is in the air
Light intensity: more stomata will open to facilitate photosynthetic gas exchange
Wind: remove water vapour from near the leaf, reducing humidity
Transpiration Stream: flow of water through xylem against gravity
Cohesion: attractive force b/t two particles of same substance
Water molecules are polar, form hydrogen bond
Adhesion: attractive force b/t two particles of diff. substance
Xylem wall polar, form intermolecular assocations w/ water molecules
Water molecules move up via capillary action → pull inward on xylem walls to generate tension
Evaporation: water lost from leaves when converted into vapour and diffuses from stomata
Process
Light energy absorbed by leaves converted into heat; evaporates water in spongy mesophyll
Vapour diffuses via stomata creating negative pressure gradient → creates tension force in cell walls to draw water from xylem
Water pulled from xylem under tension due to adhesive attraction b/t water and cell walls
Regulating Water Loss (regulated by opening and closing of stomata)
Guard cells flank stomata; can occlude opening by becoming increasingly flaccid in response to cell signals
Plant wilts from water stress → dehydrated mesophyll cells release plant hormone (ABA)
ABA triggers efflux of potassium from guard cells, decreasing water pressure
Loss of turgor makes stomata close, guard cells become flaccid and block the opening
Xylem Structure
Tube composed of hollow dead cells (no protoplasm) allowing for free movement
Movement entirely passive, occurs in one direction
Cell wall contains numerous pores (pits), enabling water to transfer between cells
Thickened cellulose, reinforced by lignin to provide strength as water transported under tension
Composed of tracheids (all vascular plants) and vessel elements (certain only)
Tracheids: tapered cells exchanging water via pits (slow transfer rate of water)
Vessel elements: end walls become fused to form continuous tube (faster rate)
Reinforced by lignin
Annular vessels: forms pattern of circular rings at equal distance
Spiral vessels: lignin in helix or coil
DRAWING FOR IB
Vessel elements drawn as continuous tube vs. interlinking tapered cells of tracheids
Remnants of fused end wall represented as indents
Wall should contain pits
Lignins either spiral or annular
Root Uptake: plants require max. surface area to optimise uptake of water and minerals
Structure
Fibrous, branching system to increase SA
Main tap root with lateral branches, pentrates soil to access deeper reservoirs
Root hairs: increases SA
Materials absorbed by root epidermis diffuse across cortext toward central stele (xylem located here)
Stele surrounded by endodermis layer that is impermeable to passive flow of water/ions
Uptake
Mineral: fertile soil contains negatively charged clay particles which cations may attach to (e.g. magnesium for chlorophyll, nirates for amino acids, phosphate, potassium, sodium, etc.)
May passively diffuse
Actively uploaded by indirect active transport
Root cells contain proton pumps that expel H+ (stored in vacuole)
H+ displace mineral cations allowing them to diffuse
Anions may bind to H+ and be reabsorbed along w/ proton
Water: osmosis/moving towards region w/ higher solute concentration regulated by aquaporins
Water moves towards xylem either via cytoplasm (symplastic) or via cell wall (apoplastic)
Symplastic: water moves continuously
Apoplastic: water cannot cross Casparian strip; transferred to cytoplasm of endodermis
Water conservation (xerophytes: plants that can tolerate dry conditions due to:)
Reduced leaves (less SA available for water loss)
Rolled leaves: reduces exposure of stomata to air
Thick, waxy cuticle: covers leaves to prevent water loss
Stomata in pits: surrounded by trichores (hairs), traps vapour and reduces transpiration
Low growth: less exposed to wind, more likely to be shaded
CAM physiology: open stomata at night
Fewer stomata
Extensive roots
XYLEM VS. PHLOEM
Moving materials
Xylem: transpiration
Phloem: active translocation
Transport
Xylem: water/minerals from roots to aerial parts of plant; unidirectional
Phloem: food/nutrients to storage organs and growing parts; bidirectional
Location/structure
Xylem: inner portion, vessel elements and tracheids
Phloem: outer portion, sieve tube elements and companion cells
Vessel wall structure
Xylem: fused cells, continuous tube for unimpeded flow
Phloem: cells connected at transverse ends to form porous sieve plates
Tissue
Xylem: dead at maturity, vessels hollow w/ no cell contents
Phloem: living tissue, sieve tube elements lack nuclei/have few organelles, require assistance
PLANT GROWTH/REPRODUCTION
GROWTH
Meristems: tissues consisting of undifferentiating cells capable of indeterminate gre growth (totipotent), can allow plants to regrow or form entirely new plants
Apical
Shoot and root tips
Responsible for primary growth (plant lengthening) through mitosis and cytokinesis (cell enlargement and division)
Occurs in nodes (in the stem) while remaining meristem tissue forms inactive axillary bud
Axillary/lateral buds can form new branching shoots w/ leaves and flowers
Controlled by plant hormones released from shoot apex (e.g. auxins which change pattern of gene expression)
Promotes growth via cell elongation and division; regulates growth
Apical dominance: prevents growth in lateral buds
Ensures plant will use energy to grow up towards the light to outcompete other plants
Distance b/t terminal and axillary bud increases, inhibition of axillary bud by auxin decreases
Different species show different levels of dominance
Auxin efflux pumps set up [] gradients to change distribution
Control direction of plant growth by determining which regions have high levels
Change position w/in membrane due to fluidity
Shoots vs. roots
Shoots
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Roots: inhibits elongation and high concentrations limit growth
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Tropisms: growth/turning movement of plant in response to directional external stimulus
Phototropism: growth in responseto unidirectional light source
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Geotropism/gravitropism: growth movement in response to gravity
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Hydrotropism: water gradient
Thigmotropism: tactile stimulus
Give rise to leaves/flowers through differentiation
Lateral
At cambium
Responsible for secondary growth (plant widening, thickening)
Give rise to bark
Micropropagation: used to produce large numbers of identical plants from selected stock plant in laboratory (in vitro) (b/c plants can reproduce asexually from meristems b/c undifferentiated cells) → plant tissue selected from stock and sterilised, grown on agar gel, explant treated with growth hormones, once root and shoot are developed, plant can be transferred to soil
1:10 ratio, where auxin<cytokinin: shooting medium; vice versa: root medium
Types of plant tissue
Dermal: "skin", tightly packed and offers protection
Ground: "bulk", metabolic and storage functions e.g. palisade mesophyll
Vascular: transport system includes xylem and phloem
REPRODUCTION
Sexual reproduction in flowering plants involves transfer of pollen
Pollination: transfer of pollen grains from anther to stigma (male to female); Some plants possess both male/female structures (monoecious), can self-pollinate
Cross-pollination: transferring pollen grains from one plant to ovule of different plant
Pollinators involved in mutualistic relationship → plant gains means of reproduction, animal gains source of nutrition (plants secrete nectar to attract pollinators such as birds/bats/insects)
Fertilisation: fusion of male gamete nuclei with female gamete nuclei to form zygote (male gamete stored in pollen grain, female gamete found in ovule)
Seed dispersal: fertilisation results in formation of seed and it moves away from parental plant → seed dispersal reduces competition for resources (through wind, water, fruits, animals, etc.)
Flowers: reproductive organs of angiospermophytes (flowering plants), develop from shot apex due to changes in gene expression affected by abiotic factors
Flowering plants come into bloom when suitable pollinator most abundant
Photoperiodism: response of plant to relatively lengths of light/darkness -- most common trigger for change in gene expression (FT gene): some plants bloom in long day conditions (summer) and other in short-day (autumn/winter)
Phytochromes: leaf pigments used by plant to detect periods of light/darkness
Active (PFr): broken down into inactive form when it absorbs far red light and in the absence of light; predominant during the day b/c sunlight contains more red light than moonlight
Capable of causing flowering (to spread pollen) by producing proteins that bind to receptors (slow break down of PFr at night)
Short-day plants: require night period to exceed critical length so that PFr inhibitor is broken down (phytochrome inhibits protein production/flowering)
Long-day require night to be less than critical length: phytochrome activates flowering
Inactive (PR): converted to active form when it absorbs red light (600nm) (more stable → unstable)
Horticulturalists can manipulate flowering by controlling light exposure
Exposing plant to light source at night can trigger flowering in long-day plants (e.g. carnations) by maintaining adequate quantities of PFr
Trigger flowering in short-day plants by covering plant with opaque cloth for 12 hours a day, e.g. chrysanthemum
Structures
Stamen: male part of flower
Anther: pollen-producing organ (pollen is male gamete)
Filament: slender stalk supporting anther, making it accessible to pollinators
Pistil/carpel: female part of flower
Stigma: sticky, receptive tip responsible for catching pollen
Style: tube-shaped connection b/t stigma and ovule (elevates stigma)
Ovule: structure containing female reproductive cells (develops into seed after fertilisation)
Petals: brightly colored modified leaves, attract pollinators
Sepal: outer covering protecting the flower when in bud
Peduncle: stalk of flower
Seed structure: ovule develops into seed (sometimes contained in fruit) when fertilisation occurs; dispersed from parental plant and then germinates, giving rise to new plant
Testa: outer seed coat
Micropyle: small pore in outer covering allowing for water
Cotyledon: contains food stores and forms embryonic leaves
Plumule: embryonic shoot
Radicle: embryonic root
Germination: process by which seed emerges from period of dormancy and begins to sprout
Requirements
All plants
Oxygen: for aerobic respiration (ATP synthesis)
Water: metabolically activate seed (triggers synthesis of gibberellin)
Temperature: for optimal enzyme function
pH: suitable soil pH in order to sprout for optimal enzyme function
Some
Fire: intense heat e.g. after bushfires
Freezing: intense cold following winter snows
Digestion: prior animal digestion to erode seed coat
Washing: covered with inhibitors
Scarification: seed coat weakened from physical damage
Monocots vs. dicots: angiospermophyta originally divided into two categories
Seed
M: one cotyledon
D: two cotyledon
Root
M: fibrous roots
D: tap roots
Vascular
M: scattered
D: ringed
Leaf
M: parallel veins
D: net-like veins
Flowers
M: multiples of 3
D: 4/5