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PLANT STRUCTURES & THEIR FUNCTIONS
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photosynthesis
endothermic reaction using light energy: Carbon dioxide + water → glucose + oxygen
6CO2+ 6H2O → C6H12O6+ 6O2
reactants
products
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photosynthesis limiting factors – interactions
Limiting factor: when factor prevents rate from increasing
Not limiting factor when increasing it doesn’t increase rate – different factor limiting rate
temperature effects enzyme activity
Too low: enzymes work slower
Too high: enzymes denature – 45˚C
light intensity
Light transfers energy needed in photosynthesis – more energy absorbed by chlorophyll
Rate & light intensity directly proportional up to certain point where light intensity doesn’t make difference: not limiting factor
carbon dioxide concentration
Provides more carbon for production of glucose
Increase CO2 concentration: increases rate until it’s no longer limiting factor
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Rate of photosynthesis:

directly proportional to light intensity
inversely proportional to distance from light source
inverse square law: Light intensity =
1
Distance2
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xylem & phloem: invascular bundles with cambium cells between them
xylem Takes water/minerals up
Lignin: thickens & strengthens walls so it won’t collapse
Dead cells (no internal structures): water can flow through easily as there is more space
Pits: holes in sides for water/minerals to move out
In transpiration stream

Phloem transports sucrose made in leaves to rest of plant: to store as starch (insoluble) / use its energy to function
Phloem vessel cells: columns of elongated living cells connected by cytoplasm with end walls
End walls have small pores (sieve plates) which allow dissolved sugars to diffuse
Companion cells connected to phloem vessel cells by pores: contain mitochondria providing energy (by respiration) for
translocation (active transport)
Both directions
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Translocation: transportation of sucrose around plant
Transports glucose in phloem vessels
Spring: sucrose transported from root to sinks in the leaf
Summer: sucrose transported from leaves to sinks in root
Pesticide chemicals also move up & down phloem through translocation
To test: test parts above and below to see if substance has translocated up/down through phloem
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Environmental factors on transpiration rate
light intensity
Light = faster rate – stomata open for photosynthesis so water vapour escapes
Dark: stomata closed – photosynthesis can’t happen in dark so no need to allow CO2 in
air flow
Windy = faster rate – steeper water concentration gradient
temperature
High = faster rate – water particles have more energy to evaporate/diffuse out
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Adaptations to extreme environments – Hot/dry
Small leaves or spines: reduces surface area for water loss

Curled leaves / hairs on leaf surface / spines / stomata sunken in pits – makes stomata lower than surface of leaf
Reduce air flow close to leaf – trap water vapour to reduce concentration gradient
thick waxy cuticle & fewer stomata: reduce water loss by evaporation
Thick stem: stores water
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commercial uses
auxins
Selective weedkillers: Developed from auxins that only affect broad-leaved plants
Disrupts their normal growth pattern: kills them – leaving grass/crops untouched
Rooting powder (contains auxins) added to soil with cutting – produces roots rapidly and grows as new plant
gibberellins
germination:

makes plants germinate at time of year they wouldn’t usually
used to make plants flower earlier than they would / under different conditions than they would
Produce seedless fruit from flowering plants that haven’t been pollinated by insects (needed for fruit & seed)
ethene: fruit ripening while fruit on its way to supermarket to be perfect on shelves



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