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The need for transport systems in multicellular plants
To include an appreciation of size, metabolic rate and surface area to volume ration (SA:V)
Metabolic demands – internal and underground parts can’t photosynthesise so need oxygen and glucose transported to
them and waste products of cell metabolism removed
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Size – plants need very effective systems to move substances up and down from the tip to the roots
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They cannot rely on diffusion alone to supply their metabolic needs
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Dicotyledonous plants (dicots) makes seeds containing
two cotyledons that act as food stores for the developing
plant and form the first leaves when the seed
germinates
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Woody (arborescent) dicots
have hard lignified tissues and a long life cycle
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In herbaceous dicots the
main transport vessels are xylem and phloem arranged
together in vascular bundles in the leaves, stems and
roots
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It also helps to support the structure
of the leaf
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In the stem, the vascular bundles are around the edge to give
strength and support
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Xylem transports water and dissolved mineral ions up the stem towards the leaves and supports the plant
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Lignin strengthens and thickens the xylem wall keeping the vessel open and preventing
collapse, waterproofs the wall so the cells don’t die and improves adhesion of water molecules creating a continuous
column and limiting the lateral flow of water
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It contains
bordered pits which are small unlignified areas which allow water to pass into adjacent vessels, bypass blockage and
supply water to other tissues
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There are two other tissues associated with xylem in herbaceous dicots
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Tannin is a bitter, astringent-tasting chemical that protects plant
tissues from attack by herbivores
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Phloem transports assimilates such as sucrose and amino acids in both directions from source to sink
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The main transporting vessels are sieve tube elements
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As the large pores appear in the cell walls, the tonoplast (vacuole membrane), nucleus
and some other organelles break down
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Companion cells form with sieve tube elements and
maintain their nucleus and all their organelles
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There are many gaps in the cell walls between companion cells and sieve tube elements called plasmodesmata is the
continuous flow of cytoplasm between adjacent cells
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Phloem tissue also contains supporting tissues including fibres and sclereids, cells with extremely thick cell walls
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Transpiration is a consequence of gaseous
exchange
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Water is transported up the stem in the xylem and passes into the mesophyll cells of the leaf by osmosis
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From the air spaces in the leaf, water vapour diffuses out of the leaf through the
stomata down a concentration gradient
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Water is
drawn up from the xylem, from either the apoplast or symplast pathways, due to the tension caused by the cohesion and
adhesion of water molecules resulting in the transpiration pull and moved into the cell from an adjacent cell by osmosis
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Gaseous
exchange is needed for photosynthesis which is essential for the plant to gain energy by making sugars
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Higher temperatures during the day cause greater evaporation
through the cuticle
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Increasing light intensity increases the rate of transpiration
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Most stomata will close in the dark
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Increasing relative humidity decreases the rate of transpiration
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Very humid air reduces water vapour potential
between the inside of the leaf and the outside air
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Increasing temperature increases the rate of transpiration as it increases the kinetic energy of the water molecules which
increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf
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Overall this increases the diffusion gradient between the air inside and outside
the leaf
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Each leaf has a layer of still air
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However a long period of still air
will reduce transpiration
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Practical investigations to estimate transpiration rates
To include use of a potometer
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When setting up the potometer, the apparatus must be
checked that it is working correctly
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2-3cm of the shoot must
be cut off at the end, at an angle
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The apparatus must be air tight
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Water moves into the xylem down a water potential gradient
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Water is pulled
up in a continuous column by capillary action due to the combined effects of cohesion between water molecules and the
adhesion of water molecules to the xylem
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The transpiration stream is the movement of water up xylem vessels, from roots to leaves
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Cohesion is
when water molecules form hydrogen bonds with each other so tend to stick together
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Water is drawn
up the xylem in a continuous stream to replace the water lost be evaporation
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Adaptions of plants to the availability of water in their environment
To include xerophytes (cacti and marram grass) and hydrophytes (water lilies)
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Adaptions that reduce evaporation include:

 Hairy leaves like the spines in cacti trap water vapour
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 Stomata in sunken pits trap water vapour by reducing air movement
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 A thick waxy cuticle waterproofs the plant and is relatively impermeable minimising water loss
 Small leaves or needles reduce surface area
 Fewer stomata reduces the diffusion of water vapour
 Stomata closing during the day reduces diffusion of water vapour
 Most of the stomata being on lower surfaces means there’s less exposure to the sun keeping them cool and reducing
the diffusion of water vapour
 More densely packed spongy mesophyll reduces surface area for evaporation from mesophyll cell surfaces
Hydrophyte plants live in wet conditions (submerge in water or in surface or at edges of bodies of water) and are adapted
to increase water loss and prevent water-logging (air spaces are full of water, not air)
Adaptions that increase evaporation include:
 Very thin or no waxy cuticle doesn’t waterproof the cuticle so is relatively permeable to gases and provides a short
diffusion distance for water vapour to evaporate from
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This also increases the rate of transpiration
 Wide flat leaves increase surface area allowing the leaves to get as much light as possible
 Smaller roots mean water can diffuse directly into the stem and leaf tissue as there is less need for the uptake of water
by the roots
 Aerenchyma are specialised parenchyma tissue forming the leaves, roots and stem
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The mechanism of translocation
Details of active loading at the source and removal at the sink
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Assimilates are products of photosynthesis that are transported around a plant
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Sources are sites where assimilates are loaded into phloem
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Sinks are sites where assimilates are unloaded from the phloem
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A tissue may act as a sink or a source at different times
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Assimilates are loaded into the phloem by the symplast or apoplast route
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In the symplast route, assimilates from the source diffuse through the plasmodesmata from companion cells into sieve tube
elements
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This route is largely passive
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Hydrogen ions are
pumped out of companion cells into the surrounding tissue using ATP
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Hydrogen ions flow back into the companion cells by facilitated
diffusion using cotransport proteins
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Assimilates
diffuse through the plasmodesmata from companion cells into sieve tube elements
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Assimilates diffuse into surrounding cells or is converted into
another substance, for example glucose for respiration and starch for storage, so that a concentration gradient of
assimilate is maintained between the contents of the phloem and the surrounding cells
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Water moves into the surrounding cells
by osmosis
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