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VCE Biology Unit 2 Area of Study 01 .._1_


									   VCE Biology Unit 2
    Area of Study 01
Adaptations of Organisms
             Chapter 14
Plant tropisms and hormonal control
    Chapter 14.1 Environmental cues
Plants must survive in the immediate
• They depend on the immediate environment
  for materials and energy
• They are relatively tolerant of environmental
  changes which they cannot escape
    Chapter 14.1 Environmental cues
Plant growth and reproduction are synchronised
with seasonal changes and climate conditions
• Environment provides cues for many stages of
  plant growth.
• Flowering, ripening of fruit, seed germination
  – All which must occur in the ‘most ideal’ conditions
• Also respond to light and temperature
    Chapter 14.1 Environmental cues
Communication between cells in different parts of
plant are required for direction and timing of
• Other plant responses are also co-ordinated
• Plants do not have a nervous system
• Internal conditions are controlled by hormones
    Chapter 14.1 Environmental cues
Hormonal systems
• A hormone is a chemical produced by cells in one
  part of an organism, transported throughout,
  accepted by specific receptor sites and affect
  specific cells
• Hormones act as intercellular messengers and
  regulate cell functions
     Chapter 14.1 Environmental cues
Hormonal systems (continued)
• Target cells possess specific receptors
• Involved in metabolic functions, rates of chemical
  reactions, transport of substances across cell membranes,
  secretion and cell growth
• Alter specific biochemical reactions
   – Production of enzymes
   – Change in membrane transport
   – Switching on of off of specific genes
    Chapter 14.1 Environmental cues
Hormonal systems (continued)
• Exert their affects directly by passing through cell
  membrane into cell or
• Indirectly by interacting with a receptor on the
  outside of the cell membrane
• Hormone-receptor associations trigger particular
  biochemical events
• Hormones are affective in low concentrations
     Chapter 14.1 Environmental cues
• A particular stimulus will only affect a specific group of
  hormone-secreting cells. E.g. a sudden shock to
  humans causes the release of adrenaline
• Hormones are transported throughout the organism,
  but only the cells with specific receptors are able to
  respond to the hormone. This specificity is not only
  restricted to specific organs, but specific
  cells/structures within organs.
    Chapter 14.1 Environmental cues
Specificity (continued)
• Flower plants have fewer hormones than animals
  and they tend to affect most cells.
• Hormone producing cells in plants are not
  organised into glands.
• The cells receiving the environmental cue
  (stimulus) produce the required hormone.
  – E.g. a growing shoot tip of a plant receiving light.
    Chapter 14.1 Environmental cues
Specificity (continued)
•Plant response is specific.
  –E.g. growing tip of wheat, lettuce or spinach
  receives increasing daylight (i.e. after winter solstice)
  will be stimulated to produce flowers.
    Chapter 14.1 Environmental cues
• Hormonal response generally slower than
• Effects last longer
• Affects cells widely distributed
     Chapter 14.1 Environmental cues
Speed (continued)
• Plant hormones slower in response than animals
• Transported through phloem and xylem
• Transported between cells
• Transported through air to other plants
• Transported through soil (inhibit root growth from
  neighbouring plants)
    Chapter 14.1 Environmental cues
Speed (continued)
• Movement of a plant hormone requires the
  expenditure of energy (ATP)
• Active transport is 10× faster than diffusion
• Still may be as slow as 1 cm/hour (1 cmhour-1)
    Chapter 14.1 Environmental cues
Hormones and responses of flowering plants
• Orientate the growth of roots, stems and
• Timing of flowering, fruit ripening and seed
    Chapter 14.1 Environmental cues
Hormones and responses of flowering plants
• Response triggered by environmental factor
• Tropism
  – When the response is directed towards the
    environmental factor, this is called “Tropism”
  – Positive towards
  – Negative away
      Chapter 14.1 Environmental cues
Hormones and responses of flowering plants
• Hormones in plants are responsible for
  –   Phototropism – growth in response to light
  –   Geotropism – growth in response to gravity
  –   Apical dominance – inhibition of lateral branches
  –   Ripening of fruit – conversion of starches to sugars
  –   Abscission – shedding of leaves and flowers
    Chapter 14.1 Environmental cues
• Phototropism – reaching for light
• Growing plants bend and grow towards light
• Stimulated by chemical – auxin (“to grow”)
• Chemical diffuses downwards from growing
  tip (meristem)
    Chapter 14.1 Environmental cues
Auxins (continued)
• Produced continuously in meristem
• Diffuses through layers of cells
• Light interacts with receptors that control
  membrane permeability to auxin
• Auxin moves laterally away from light
     Chapter 14.1 Environmental cues
Auxins (continued)
• Higher concentration of auxin on dark side of tip
• Those cells grow faster (elongation not cell division)
• Softens cell walls allowing elongation under turgor
• Cells outside growing area lack auxin receptors
• Auxins used as herbicides – excess auxin causes growth
  to the detriment of the plant (cells rupture, etc.)
Phototropism Experiment
Auxin causes elongation of cells on shaded side.
    Chapter 14.1 Environmental cues
Geototropism – responding to gravity
• Auxins involved in negative geotropism (growing
  away from gravity)
• Plant laid on side in dark will bend and grow
  away from gravity
• Auxin concentration on lower side of lateral
Negative Geotropism
     Chapter 14.1 Environmental cues
Apical dominance – one main stem
• Auxin produced in the apical tip of a plant
• Diffuses down stem
• Inhibits lateral bud development
• Results in taller plants with less side branches
• Gardeners cut out apical tips to promote bushiness of
  cultivated plant
• Bushfires burn off the apical tips of eucalypts in the crown
Apical dominance
    Chapter 14.1 Environmental cues
• Promote cell elongation
• Promote growth for whole plant
• Promote cell division (mitosis and cytokinesis)
  – Flowering
  – Fruit enlargement
  – Seed germination
     Chapter 14.1 Environmental cues
• Synthesised in flowers, developing fruits, seeds,
  growing buds and elongating stems
• Important for triggering cereal grain germination
  – Produced in embryo
  – Moves to target cells in the endosperm
  – Induces the formation of enzymes that digest the
    endosperm to produce malt
      Used in the brewing of malt whisky
    Chapter 14.1 Environmental cues
• In the presence of auxins, cytokinins stimulates
  cell division and cell differentiation
• Ratio of auxins to cytokinins determines path of
  differentiation of new cells
  – Stems and leaves develop when more cytokinins than
  – Roots develop when less cytokinins than auxins
    Chapter 14.1 Environmental cues
Abscisic acid
• Opposite action to auxins
• Involved in overall regulation of plant
• Little is known about them
    Chapter 14.1 Environmental cues
Abscisic acid
• Assist plants to tolerate or avoid adverse
  – E.g. drought, salinity, low temperature
  – Promotes leaf drop, bud and seed dormancy and
    increased frost resistance
    Chapter 14.1 Environmental cues
Abscisic acid (ABA)
• Synthesised mainly in chloroplasts
• Dropping of ripe fruit, unfertilised flowers and
  falling of leaves in deciduous trees known as
• Disintegration of special layer of cells at base of
  organ being dropped
    Chapter 14.1 Environmental cues
Abscisic acid (ABA)
• Development of seed dormancy and
  vernalisation requires ABA
• Appears to act on gene expression in nucleus
    Chapter 14.1 Environmental cues
Abscisic acid (ABA)
• Short term effects – stomatal movement
  – Plant loses too much water, ABA stimulates guard
    cells to close
• [ABA] increases under stressful conditions
    Chapter 14.1 Environmental cues
Ethylene (Ethene) (C2H4)
• Released by ripening fruit (apples, pears,
  citrus, banana and avocados)
• Stimulates fruit to ripen
• Ripening of fleshy fruit shows colour changes
  and softening of flesh
    Chapter 14.1 Environmental cues
Ethylene (Ethene) (C2H4)
• Wild plants use fruit to attract animals to eat
  them and disperse the seeds
• Ethylene increases rate of respiration and other
  processes of fruit ripening (e.g. breaking down
  starch and oils into sugars).
• Triggered by auxin and abscisic acid
                Summary of plant hormones
Hormone         Where produced              Effective site                Action                      Visible effect
auxins          Shoot tip (meristem)        Growing region of shoot       Cells elongate under        Tip bends towards light, apical
                                                                          turgor pressure             dominance, used as herbicides,
                                                                                                      stimulate cuttings to root

gibberellin     Fruits, seeds, growing      Roots, shoots, seeds, also    Growth of cells             Shoot elongation, germination of
                buds, elongating stems      found in fungi                                            seeds, flowering fruit

cytokinins      Roots and developing        Branch and leaf buds, moves   Stimulates cell division,   Growth of lateral branches, used
                fruits                      through phloem and xylem      cell elongation and         to promote life of vegetables in
                                                                          tissue differentiation      storage

Abscisic acid   chloroplasts                Gene expression in nuclei     Growth inhibition           Seed dormancy, vernalisation,

ethylene        Ripening fruits, flowers,   Cellular metabolism           Fruit ripening and leaf     Fruit ripening, leaf and fruit drop,
                seeds, leaves and roots                                   drop                        used commercially to ripen
                                                                                                      bananas and pineapples

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