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PLANT BIOLOGY (transport in the Xylem of plants (root uptake (The…

- - - - Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata
      - New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)
      - Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves (transpiration stream
    - - As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by the level of photosynthesis
      - Hence, transpiration is an inevitable consequence of gas exchange in the leaf
      - This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
      - This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)
      - The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls
    - - The amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata
      - Guard cells flank the stomata and can occlude the opening by becoming increasingly flaccid in response to cellular signals
      - When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
      - Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
      - A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening
      - Transpiration rates will be higher when stomatal pores are open than when they are closed
        
        Stomatal pores are responsible for gas exchange in the leaf and hence levels of photosynthesis will affect transpiration
        
        Other factors that will affect transpiration rates include humidity, temperature, light intensity and wind
  - - - Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
  - - - cohesion
        
        Cohesion is the force of attraction between two particles of the same substance (e.g. between two water molecules)
        
        Water molecules are polar and can form a type of intermolecular association called a hydrogen bond
        
        This cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream
      - adhesion
        
        Adhesion is the force of attraction between two particles of different substances (e.g. water molecule and xylem wall)
        
        The xylem wall is also polar and hence can form intermolecular associations with water molecules
        
        As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension
      - structured of the Xylem
        
        The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant
        
        It is a tube composed of dead cells that are hollow (no protoplasm) to allow for the free movement of wate
        
        Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only
        
        The cell wall contains numerous pores (called pits), which enables water to be transferred between cells
        
        Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension
      - xylem properties
        
        conducts water and minerals (movement in one way only)
        
        composed of tracheids (all plants) and vessel elements (angiosperms)
        
        walls composed of dead cells and are pitted (allows water exchange)
        
        walls impregnated with lignin (spiral or annular arrangement)
        
        water movement requires both cohesion and adhesion
        
        Xylems can be composed of tracheids (all vascular plants) and vessel elements (certain vascular plants only)
        
        Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer
        
        In vessel elements, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer
        
        All xylem vessels are reinforced by lignin, which may be deposited in different ways:
        
        In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other
        
        In spiral vessels, the lignin is present in the form of a helix or coil
  - - - Some plants have a fibrous, highly branching root system which increases the surface area available for absorption
      - Other plants have a main tap root with lateral branches, which can penetrate the soil to access deeper reservoirs of water
    - - Materials absorbed by the root epidermis diffuse across the cortex towards a central stele, where the xylem is located
      - The stele is surrounded by an endodermis layer that is impermeable to the passive flow of water and ions (Casparian strip)
      - Water and minerals are pumped across this barrier by specialised cells, allowing the rate of uptake to be controlled
    - - Fertile soil typically contains negatively charged clay particles to which positively charged mineral ions (cations) may attach
        
        Minerals that need to be taken up from the soil include Mg2+ (for chlorophyll), nitrates (for amino acids), Na+, K+ and PO43–
      - Mineral ions may passively diffuse into the roots, but will more commonly be actively uploaded by indirect active transport
        
        Root cells contain proton pumps that actively expel H+ ions (stored in the vacuole of root cells) into the surrounding soil
        
        The H+ ions displace the positively charged mineral ions from the clay, allowing them to diffuse into the root along a gradient
        
        Negatively charged mineral ions (anions) may bind to the H+ ions and be reabsorbed along with the proton
    - - Water will follow the mineral ions into the root via osmosis – moving towards the region with a higher solute concentration
        
        The rate of water uptake will be regulated by specialised water channels (aquaporins) on the root cell membrane
      - Once inside the root, water will move towards the xylem either via the cytoplasm (symplastic) or via the cell wall (apoplastic)
        
        In the symplastic pathway, water moves continuously through the cytoplasm of cells (connected via plasmodesmata)
        
        In the apoplastic pathway, water cannot cross the Casparian strip and is transferred to the cytoplasm of the endodermis
  - - - Vessel elements should be drawn as a continuous tube (tracheids will consist of interlinking tapered cells)
      - The remnants of the fused end wall can be represented as indents (these forms perforated end plates)
      - The xylem wall should contain gaps (pits), which enable the exchange of water molecules
      - Lignin can be represented by either a spiral (coiled) or annular (rings) arrangement
  - - - Xerophytes will have high rates of transpiration due to the high temperatures and low humidity of desert environments
      - Halophytes will lose water as the high intake of salt from the surrounding soils will draw water from plant tissue via osmosis
    - - Xerophytes are plants that can tolerate dry conditions (such as deserts) due to the presence of a number of adaptations:
        
        Reduced leaves
        
        reducing the total number and size of leaves will reduce the surface area available for water loss
        
        Rolled leaves
        
        rolling up leaves reduces the exposure of stomata to the air and hence reduces evaporative water loss
        
        Thick, waxy cuticle
        
        having leaves covered by a thickened cuticle prevents water loss from the leaf surface
        
        Stomata in pits
        
        having stomata in pits, surrounded by hairs, traps water vapour and hence reduces transpiration
        
        Low growth
        
        low growing plants are less exposed to wind and more likely to be shaded, reducing water loss
        
        CAM physiology
        
        plants with CAM physiology open their stomata at night, reducing water loss via evaporation
    - - Halophytes are plants that can tolerate salty conditions (such as marshlands) due to the presence of a number of adaptations
        
        Cellular sequestration
        
        halophytes can sequester toxic ions and salts within the cell wall or vacuoles
        
        Tissue partitioning
- - - - Fertilisation
      - Seed dispersal
      - Pollination
        
        Cross-pollination
        
        mutualistic relationship between pollinators and flower
  - - - enlargement of the shoot apical meristem
      - influence by abiotic factors (seasonal)
      - most common trigger is by day/night length (photoperiodism)
    - - http://ib.bioninja.com.au/higher-level/topic-9-plant-biology/untitled-3/flower-structure.html
  - - - Inactive form (Pr) converted to active form when absorbs red light (660 nm)
      - active form (Pfr) break down to inactive form when absorbs far red light (725 nm)
      - active form will gradually revert to the inactive form in the absence of light (darkness reversion)
    - - Pfr inhibit flowering hence needed low level of Pfr
      - Horticulturalists
        
        by covering the plant with an opaque black cloth for ~12 hours a day to promote flowering
    - - Pfr activates flowering hence needed high level of Pfr
      - Horticulturalists
        
        by exposing the plant to a light source during the night to trigger flowering
  - - - Testa – an outer seed coat that protects the embryonic plant
      - Micropyle – a small pore in the outer covering of the seed, that allows for the passage of water
      - Cotyledon – contains the food stores for the seed and forms the embryonic leaves
      - Plumule – the embryonic shoot (also called the epicotyl)
      - Radicle – the embryonic root
  - - - basically the journey of seeds to become an adult
    - - oxygen
      - water
      - suitable temperature for optimal function of enzymes
      - suitable pH
    - - the rate of seed growth over a set period of time
- - - - Aphids can be used to collect sap at various sites along a plant's length and thus provide a measure of phloem transport rates
      - A plant is grown within a lab with the leaves sealed within a glass chamber containing radioactively-labelled carbon dioxide
      - The leaves will convert the CO2 into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem
      - Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap
      - Once feeding has commenced, the aphid stylet is severed and sap continues to flow from the plant at the selected positions
      - The sap is then analysed for the presence of radioactively-labelled sugars
      - The rate of phloem transport (translocation rate) can be calculated based on the time taken for the radioisotope to be detected at different positions along the plant’s length
    - - The rate of phloem transport will principally be determined by the concentration of dissolved sugars in the phloem
        The concentration of dissolved sugars in the phloem sap will be affected by
        
        The rate of photosynthesis (which is affected by light intensity, CO2 concentration, temperature, etc.)
        
        The rate of cellular respiration (this may be affected by any factor which physically stresses the plant)
        
        The rate of transpiration (this will potentially determine how much water enters the phloem)
        
        The diameter of the sieve tubes (will affect the hydrostatic pressure and may differ between plant species)
    - - a group of insects, belonging to the order Hemiptera, which feed primarily on sap extracted from phloem
      - Aphids possess a protruding mouthpiece (called a stylet), which pierces the plant’s sieve tube to allow sap to be extracted
      - The penetration of the stylet into the sieve tube is aided by digestive enzymes that soften the intervening tissue layers
      - If the stylet is severed, sap will continue to flow from the plant due to the hydrostatic pressure within the sieve tube
  - - - long and narrow cells that are connected to form the sieve tube
      - connected by sieve plates at their transverse ends, which are porous to enable flow between cells
      - no nuclei
        
        to reduce numbers of organelles to maximise space for the translocation of materials
      - thick and rigid cell walls
        
        to withstand the hydrostatic pressures which facilitate flow
    - - provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
        
        Possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
        
        Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
        
        Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube
      - Sieve elements are unable to sustain independent metabolic activity without the support of a companion cell
        
        This is because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)
        
        Plasmodesmata exist between sieve elements and companion cells in relatively large numbers
        
        These connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites
    - - In monocotyledons, the stele is large and vessels will form a radiating circle around the central pith
      - Xylem vessels will be located more internally and phloem vessels will be located more externally
      - In dicotyledons, the stele is very small and the xylem is located centrally with the phloem surrounding it
      - Xylem vessels may form a cross-like shape (‘X’ for xylem), while the phloem is situated in the surrounding gaps
    - - In monocotyledons, the vascular bundles are found in a scattered arrangement throughout the stem
      - Phloem vessels will be positioned externally (towards outside of stem) – remember: phloem = outside
      - In dicotyledons, the vascular bundles are arranged in a circle around the centre of the stem (pith)
      - Phloem and xylem vessels will be separated by the cambium (xylem on inside ; phloem on outside)
    - - transports water and food (movement is in two ways)
      - composed of sieve element cells which connect to form a tube
      - connecting sieve cells share a highly perforated sieve plate
      - supported by companion cells that help with loading/unloading
      - movement of sap is mediated by hydrostatic pressure fro xylem
  - - - Materials can pass into the sieve tube via interconnecting plasmodesmata (symplastic loading)
      - Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading
    - - Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)
      - The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient
      - Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement
      - This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source
  - - - Translocation
        The movement of organic compounds from sources to sinks
        
        sources
        
        where the organic compounds are synthesised
        
        leaves (photosynthesis occur)
        
        sinks
        
        where the compounds are delivered to for use or storage
        
        roots
        
        fruits
        
        seeds
      - organic compounds are transported from sources to sinks via a vascular tube system called the phloem
  - - - High concentrations of solutes in the phloem at the source lead to water uptake by osmosis
      - Incompressibility of water allows transport along hydrostatic pressure gradients
      - Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks
    - - The active transport of solutes (such as sucrose) into the phloem by companion cells makes the sap solution hypertonic
      - This causes water to be drawn from the xylem via osmosis (water moves towards higher solute concentrations)
      - Due to the incompressibility of water, this build up of water in the phloem causes the hydrostatic pressure to increase
      - This increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure (mass flow)
      - Hence, the phloem transports solutes away from the source (and consequently towards the sink
    - - The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)
      - This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)
      - Consequently, water is drawn out of the phloem and back into the xylem by osmosis
      - This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source
      - Hence, phloem sap will always move from the source towards the sink
      - When organic molecules are transported into the sink, they are either metabolised or stored within the tonoplast of vacuoles
- - - - Apical meristems
        
        occur at shoot and root tips
        
        responsible for primary growth (plant lengthening)
      - Lateral meristems
        
        occur at the cambium
        
        responsible for secondary growth (plant widening / thickening)
  - - - Growth occur in Nodes with the remaining meristem tissue forming an inactive axillary bud
    - - place
        
        released from the shoot apex
      - functions
        
        growth of the stem
        
        formation of new nodes
      - Example : auxin
        
        Function
        
        promotes growth in the shoot apex via cell elongation and division
        
        Apical dominance
  - - - set up concentration gradients within tissues
    - - stimulates cell elongation thus high concentrations of auxin promote growth
      - the steps
        
        Auxin activates a proton pump in the plasma membrane which causes the secretion of H+ ions into the cell wall
        
        pH fall, causes cellulose fibres within the cell wall to loosen (by breaking the bonds between them)
        
        Additionally, auxin upregulates expression of expansins, which similarly increases the elasticity of the cell wall
        
        With the cell wall now more flexible, an influx of water (to be stored in the vacuole) causes the cell to increase in size
    - - inhibits cell elongation thus high concentrations of auxin limit growth
    - - Definition
        
        the growth or turning movement of an plant in response to a directional external stimulus
      - Types
        
        Phototropism
        
        response to a unidirectional light source
        
        light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant
        
        shoots ( Positive phototropism )
        
        roots ( negative phototropism )
        
        Geotropism
        
        response to gravitational forces
        
        auxin will accumulate on the lower side of the plant in response to the force of gravity
        
        hydrotopism
        
        responding to a water gradient
        
        thigmotropism
        
        responding to a tactile stimulus
  - - - technique used to produce large numbers of identical plants (clones)
      - cultured in the laboratory (in vitro)
    - - Select and sterilised the undifferentiated shoot apex
      - That tissue sample (explant) grown on a sterile nutrient agar gel
      - Treated explant with with growth hormones (auxin) to stimulate shoot/root development
      - the growing shoots will continuously divided and separated
      - until fully developed, the cloned plant can be transferred to soil
    - - rapid bulking
      - virus-free strains
      - Propagation of rare species