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Topic 9: PLANT BIOLOGY, Screenshot 2024-04-11 at 19.57.01, Screenshot 2024…

- - - - stomata are pores on the underside of the leaf which facilitate gas exchange in photosynthesis; stomata open to allow gaseous exchange/entry of CO2.
        when co2 enters the cell, h20 vapor will inevitably escape through the stomata.
      - Plants minimise water losses through the stomata using guard cells.
        guard cells control the rate of transpiration pull they are found in pairs around the side of the stroma
        
        when a plant has water stress it releases abcisic acid: this triggers the efflux of pottasium from guard cells:
        abscisic acid stimulates closing of stomata;
        
        transpiration rates will be higher when stomatal pores are open. the rate of photosyntehsis will affect the transpiration rate.
  - - - water is cohesive so can pulled up/so can be moved under tension in xylem;
      - water pulled out of xylem creates tension; transpiration pull results;
      - tension in leaf cell walls mantains the transpiration stream
      - when water evaporates through the stomata, adhesion causes water to be drawn from the cell wall to replace water lost to transpiration
      - the force of adhesion between water and leaf cell walls is strong enough to absorb water out of the xylem at low pressure. the low pressure generates a pulling force called the transpiration pull that pulls water upwards in the xylem against gravity.
      - Plants transport water from roots to leaves to replace those lost to transpiration
        
        water leaving through the stomata by transpiration is replaced by water from xylem vessels.
        
        water in the xylem climbs up the stem by the transpiration pull from adhesion and cohesion.
        
        water moves from the soil into the roots by osmosis.
    - - plants absorb minerals and water from the soil by the roots in osmosis and active transport:
        
        the solute concentration in root cells is greater than in the soil. using proton pumps, mineral ions are absorbed in active transport
      - water is absorbed by osmosis;. F. solute concentration inside the root is higher than in the soil;. G. due to active transport of ions into the root;
  - - - some xerophytes are ephemeral = they have short life cycles and rely on short periods when water is available (otherwise dormant)
        others are perennial = rely on a storage of water
      - most cacti are xerophytes.
        
        reduced leaf size to reduce surface area available for water loss
        
        thick waxy cuticle to prevent water loss.
        
        stomata are open at night for traspiration to occur more slowly
        
        folded leaves to reduce exposure of stomata to air
        
        low growth
    - - leaves reduced to spines
        
        water storage structures
        
        long/deep roots to search for water
        
        salt glands that actively excrete salt to stop it from building up
        
        reduced leaf surface area
- - - - sieve tubes are composed of columns of specialsed cells called sieve tube cells.
      - sieve tube cells are separated by perforated walls called sieve plates. sieve tube cells are also associated with companion cells.
      - 1) sometimes sinks turn into sources so the phloem transports nutrients and organic compounds in either direction.
        2) movement of organic compounds is an active process so energy is needed to generate pressures
      - the functions of the phloem include loading and unloading of carbohydrates at sources and sinks.
        
        phloem has sieve tubes with specialised columns called sieve tube cells. the sieve tube cells carry out active transport and associate with companion cells (these have many mitochondria to support the active transport of sucrose).
        
        plasmodesmata connects sieve tube cells and companion cells
    - - sucrose is transported in the phloem: it is a solute in phloem sap and it is used because it is not readily metabolised = it is a good transport form of carbohydrates that will not be metabolised during transport.
      - Phloem loading is the process by which organic compounds are loaded into the phloem sieve tubes by active transport at the source.
        The primary function of the companion cells in the phloem is providing energy for the transport of substances in the phloem.
        
        a concentration gradient of sucrose is established by active transport at the phloem.
        
        h+ ions are actively transported out of the companion cell using ATP as an energy source.
        
        build up of h+ ions flows down the concentration gradient and energy released through atp SYNTHASE is used to carry sucrose into the phloem through companion cells.
    - - In the apoplastic pahtways, water and organic compounds pass through the cell wall.
    - - The build up of sucrose and carbohydrates causes water to be drawn into companion cells by osmosis.
        
        the rigid cell wall and incompressibilty of water causes a buil-up/increase of pressure at the source.
        
        water will flow from an area of high pressure to an area of low pressure by osmosis (h-->l)
        
        at the sink sucrose is unloaded and used or stored
        
        loss of solute causes a reduction in osmotic pressure. water returns back to the transpiration stream in the xylem.
        
        so basically sucrose at source (loaded) --> build up of sucrose --> water enters companion cells --> comapinion cells undergo chemiomsis and produce atp --> atp used to translocate sucrose from the spruce to sink --> build up of pressure --> hydrostatic pressure gradient --> sucrose moves through xylem to sink
    - - phloem becomes hypertonic to xylem due to the active transport of sucrose into the phloem.
        
        water moves into the phloem by osmosis.
        
        The active transport of sucrose into the phloem by companion cells makes the sap solution hypertonic.
        
        UNLOADING of sucrose at the sink reduces the water potential/osmotic pressure inside sieve tube cells, causing water to enter by osmosis
  - - - State two ways xylem cells can be identified in cross sections of stem and root.
        
        1) Look for large hallow cells that are orientated
        2) Towards the center of the plant (or in the stem, towards the center of the vascular bundle).
      - Identify xylem given microscope images of stem and root.
      - Identify phloem within the vascular bundle of a stem and root.
      - monocotylledons = steel is large
        dicotyledons = steel is small (X shaped
- - - - auxin is a plant hormone;
      - produced by the tip of the stem/shoot tip;
      - causes transport of hydrogen ions from cytoplasm to cell wall;
      - decrease in pH / H pumping breaks bonds between cell wall fibres;
      - makes cell walls flexible/extensible/plastic/softens cell walls;
      - auxin makes cells enlarge/grow;
      - gene expression also altered by auxin to promote cell growth;
      - at high concentrations auxin inhibits the growth of LATERAL auxillary buds.
    - - the further distant a node is from the shoot apical meristem, the lower the concentration of auxin and the less likely that the auxillary bud will be inhibited.
        
        the greater the distance between a node and the shoot apical meristem, the lower the inhibition by auxin and the effect of apical dominance.
        
        cytokinis are hormones produced to increase the growth of auxillary buds.
- - - - changes in gene expression is triggered by night length, which affects the elongation of shoot apical meristems.
      - The switch to flowering is a response to the length of light and dark periods in many plants.
        
        long day plants flower in the summer when the nights are short
        
        short day plants flower in the autumn/winter when the nights are long
        
        a critical factor for flowering is the duration of night periods, detected by phytochromes.
        
        phytochromes are leaf pigments used to detect periods of darkness by the plant (phototropism is the plants response to dark periods)
        
        INACTIVE PHYTOCHROME (pr) is converted into active phytochrome when it absorbs red light
        ACTIVE PHYTOCHROME (pfr) is converted into inactive phytochrome by absorbing far red light
        in the darkness, the active form will gradually change to inactive form because inactive phytochrome is more stable.
        active phytochrome is present during the day as the sun has more red light.
        
        only the active form of phytochrome can cause flowering:
        1) long day plants flower when days are long so they require night period to be less than critical length (so pfr/active phyto activates flowering, while inactive inhibits)
        2) short day plants flower when nights are long so they require night period to exceed critical length (excess active phytochrome inhibits flowering)
        
        Explain how flowering is controlled in long-day and short-day plants.
        
        flowering affected by light;
        
        phytochrome;
        
        exists in two (interconvertible) forms/P and P ;
        
        P (red absorbing/660 nm) converted to P (far-red/730 nm absorbing) in red or day light;
        
        sunlight contains more red than far red light so P predominates during the day;
        
        gradual reversion of P to P occurs in darkness;
        
        P is active form / P is inactive form;
        
        in long-day plants, flowering induced by dark periods shorter than a critical length / occurs when day is longer than a critical length;
        
        enough P remains in long-day plants at end of short nights to stimulate flowering;
        
        P acts as promoter of flowering in long-day plants;
        
        short-day plants induced to flower by dark periods longer than a critical length/days shorter than a critical value;
        
        at end of long nights enough P has been converted to P to allow flowering to occur;
        
        P acts as inhibitor of flowering in short-day plants;
        
        Methods to induce short day plants (autumn) to flower out of season are:
        Horticulturalists can manipulate the flowering of short-day and long-day plants by controlling the exposure of light.
        The critical night length required for a flowering response must be uninterrupted in order to be effective.
        
        LDP = require less light exposure
        
        SDP = require more light exposure
        
        Short day plants are kept in the:
        Dark during daylight
        Given bursts of far red light
        Keeps P(fr) low so flowering isn’t inhibited
        Long day plants:
        Artificially lit during night
        Short bursts of light using red light can interrupt darkness and promote flowering
        It’s possible to expose plants to light for short periods to keep costs down but long enough to interrupt the dark period
  - - - Some plants have seeds that require specific variables in order for germination to take place such as fire or scarcification