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Green Plants as Organisms

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					Green Plants as Organisms
                      Contents

   Photosynthesis
   Photosynthesis vs. Respiration
   Plant Hormones
   Commercial Uses of Plant Hormones
   Transport and Water Relations
                  Photosynthesis
   Photosynthesis is the chemical change which happens in
    the leaves of green plants. It is the first step towards
    making food. During this reaction, carbon dioxide and
    water are converted into glucose and oxygen. The
    reaction requires energy in the form of sunlight, and
    chlorophyll must also be present.
   Plants are producers – they produce their own food
   Plants produce their food via photosynthesis
                     Photosynthesis
   Occurs in the leaf

 Needs:
Chlorophyll – not used up
CO2 (from the air)
Water (from the soil)
Sunlight energy (any light except green light) – not used up

 Produces:
Glucose
Oxygen (waste product)

                         chlorophyll + light energy
     carbon dioxide + water                   glucose + oxygen
          6CO2      + 6H2O                    C6H12O6 + 6O2
      Factors Affecting Photosynthesis
Three factors limit photosynthesis from going any faster:

1.   Light level
    Without enough light a plant cannot photosynthesise very fast, even if
     there is plenty of water and carbon dioxide. Increasing the light intensity
     will make photosynthesis faster.

     2. Carbon dioxide level
     Photosynthesis can be limited by the level of carbon dioxide. Even if
     there is plenty of light a plant cannot photosynthesise if it has run out of
     carbon dioxide.

     3. Temperature.
    If it gets too cold the rate of photosynthesis will slow right down. If it
     gets too hot then plants cease to be able to photosynthesise.
            Night-time vs. Daytime

   Daytime:
    Photosynthesis & Respiration




   Night-time: (no sunlight therefore no photosynthesis)
    Only respiration
                     Plant Hormones
   Stem  always grows towards light
                                             controlled by auxins
   Roots  always grow away from light


   Auxins are produced in the growing tips of shoots and roots
   Auxins respond to light, gravity and moisture
   Auxins cause some cells to elongate or grow at a faster rate


   Tropism = plant’s response to light, gravity or moisture
   Positive tropism = response towards stimulus
   Negative tropism = response away from stimulus
                      Plant Hormones
 Geotropism = plant’s response to gravity
 Positive geotropism = roots grow towards direction of gravity
 Negative geotropism = shoots grow away from direction of gravity



 Hydrotropism = plant’s response to water
 Positive hydrotropism = shoots always follow direction of water



   Phototropism = plant’s response to light
   Light destroys auxins
   Auxins = plant hormones controlling its growth
   Auxins on sunlit side of plant are destroyed,
   growth slows…
    Growth of plant on shaded side continues
     Commercial Uses of Plant Hormones

1)    Growing cuttings: cuttings are dipped into powder
      containing growth hormones. Plants are cloned quickly and
      cheaply

2)    Producing fruit without seeds: Growth hormones sprayed on
      unpollinated flowers  grow fruit without fertilisation

3)    Ripening fruit: Plant hormone ethene ripens fruit. Sprayed
      onto unripened fruit during transportation to supermarkets

4)    Increasing size of fruit: e.g. grapes
           Transport & Water Relations
   Plants require water for growth, temperature regulation and
    to hold themselves up

   Transpiration is the loss of water from the leaves by
    evaporation

 Transpiration system:
Water absorbed into roots through root hair cells
Water carried to leaves through xylem vessels
Water evaporates from underside of leaves
Water escapes through holes called stomata
As water evaporates, more is sucked up xylem

   Stomata (tiny holes under a leaf allowing it to breathe) open
    and close to control water loss
             Xylem and Phloem Vessels
 Xylem are vein like tissues that transport water and minerals up a plant
 Phloem are vein like structures through which food is transported around a plant

                            Xylem                          Phloem
     Made of               Dead cells                     Living cells
Cell wall thickness          Thick                            Thin
Cell wall material       Lignin (rigid)                    Cellulose
   Permeability          Impermeable                      Permeable
   Cytoplasm?                 None                    Cytoplasm lining
   Transports…        Water & minerals                        Food
   Carried to…               Leaves          Growing parts & storage organs
Direction of flow          Upwards                      Up and down
Tissue also has…             Fibres                    Companion cells
             Water Provides Support
   Green plants rely on cell turgor which is the
    stiffness given to cell walls to hold themselves up
   Cell walls become turgid with water

   In woody plants, they do not need cell turgor, but
    instead rely on the support of the strong,
    impermeable xylem vessels
                          Summary
   Photosynthesis: carbon dioxide + water  glucose + oxygen
   Limiting factors: temperature, CO2, light intensity
   Daytime: respiration & photosynthesis; night-time: respiration

   Plant hormone, auxin, responds to light, gravity and moisture
   Light destroys auxin
   Auxin speeds up plant growth

   Water travels around the plant via the transpiration system
   Xylem and phloem vessels transport water, minerals and food
   Water is needed in green plants for structure and support