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Transport in plants
Plant transport systems
Forces acting on transport
Forces in the leaves
• Transpiration – water lost from leaves
• Evapo-transpiration – water lost from
leaves as water evaporates
• Transpiration stream – water is pulled
up the stem as water is lost from the
leaves in transpiration
Forces in the stems
• Adhesion – water molecules stick to the
sides of the walls in the xylem
• Cohesion – water molecules stick to each
other
• Capillarity/capillary action – water is drawn
upwards through thin tubes
Forces in the roots
• Active transport – salts are actively
absorbed, increasing the osmotic
pressure within the roots
• Osmosis – water is pulled in due to
the concentration gradients
Translocation
Movement of sugar
• Translocation – movement of sugar – sugar is
actively transported from leaf to phloem (source)
and from phloem to roots (sink), thus setting up
a concentration gradient from leaf to roots
• Diffusion – sugar will diffuse downwards
because of this concentration gradient
• Osmosis – water will be pulled out of the xylem
near the leaves, and move downwards, then
return to the xylem near the roots, due to the
concentration gradient.
Plant transport systems
Roles of
• Leaves – carry out photosynthesis and
transpiration
• Stomata – allow water and gases to enter and
leave the leaf. Opening is controlled by guard
cells
• Xylem – transports water and salts upwards
• Phloem – transports sugars, mostly downward
• Roots – draw in water and salts
• Root hairs – increase surface area
Leaves and stomata 1
Leaves and stomata 2
• Leaves are responsible for photosynthesis
and exchange of gases and water
• Gases and water enter and leave through the
stomata
• Epidermis provides protection
• Palisade cells carry out photosynthesis
• The spongy mesophyll layer allows storage of
air and water vapour
• Vascular bundles contain xylem and phloem for
the transport of water, salts and sugars
Control of stomal opening
Control of stomal opening 2
• Stomatal opening is controlled by turgor
pressure in the guard cells
• This is controlled by pumping salts into the cells,
thus bringing in more water (opening stoma) or
pumping salts out of the cells, thus forcing water
to leave (closing stoma)
• Turgor pressure increases when water availability
is high
• Turgor pressure decreases when water availability
is low
Structures in the stem
Xylem and phloem
Xylem
Phloem
Dead
Living
Thick
Lignin
Impermeable
Thin
Cellulose
Permeable
Cross walls
None
Sieve plates
Cytoplasm
None
Yes
Function
Carries water & salts
Carries sugars
Direction of flow
Upwards
Down and up
Special features
Fibres
Companion cells
Cells living/dead
Cell walls:
Thickness
Material
Permeability
Roots and water transport
Image from Purves et al., Life: The Science of Biology, 4th Edition,
by Sinauer Associates and WH Freeman
Root hairs and water transport
Water moves in by osmosis
• Osmotic pressure in root
hair cell is higher than in
soil
• This can be maintained by
active transport of salts into
the root hairs
• Root hairs increase the
surface area available
Measuring water loss
• This can be done with a
device called a potometer
• The rate of transpiration is
shown by movement of a
bubble of air through the
tubing
• Key features include – air
tight seal between plant and
tubing, narrow tubing, intact
stem (cut under water so it
will draw up water), air
bubble, scale
Water loss in plants
Water is lost when the stomata open to allow gas exchange eg for
photosynthesis
Factors causing an increase in water loss
• Temperature
• Increased air movement
• Low water availability
• Increase in light intensity
Factors causing a decrease in water loss
• Humidity
• High water availability
If the rate of photosynthesis increases then water loss is likely to
increase
If the plant tries to reduce water loss, the rate of photosynthesis may
also decrease
Arid regions
Usually hot and dry eg Australian & African deserts & savannah
Can be cold & dry eg icecaps & tundra
Plant adaptations - arid
Problems faced
• Water availability low
• Humidity low
• Temperature high
• Light high
• Air movement high
Solutions include
Increased roots – either
deep or wide and shallow
Water storage (roots, leaves
or stems) eg cacti, boabs
Reduced leaves
Reduced stomata in leaves
Protection for leaves –
cuticle, thick epidermis,
curling, sunken pits, hairs
to guard stomata
Closing stomata in hot
conditions
Arid
region
adaptations
Water storage
Reduced leaves
Reduced leaves
Wide shallow root system
Water storage succulent leaves
Water storage - trunk
Reduced leaves – loses
leaves in summer
Long deep roots
Coastal environments
Hot, dry, saline, moving sand which can cover plants
Plant adaptations - coastal
Problems faced
• Water availability low
• Humidity low
• Temperature high
• Light high
• Air movement high –
lots of sand and salt
Solutions include
Wide shallow root
systems
Rapid growth
Can cope with burial
Rolled leaves, sunken
stomata, reduced
stomata and/or hairs
Succulent leaves
Salt secretion in leaves
Rainforest
Low light, high humidity
Plant adaptations - wet
Problems faced
• Water availability high
• Humidity high
• Temperature varies high (tropical) to
medium (temperate)
• Light can vary – high
in the canopy, low at
ground level
• Air movement usually
low
Solutions include
Large leaves to trap
light
Deep veins to carry
water away from plant
Usually many stoma
and thin epidermis
Large air spaces within
leaves
Aquatic environments
Plenty of water, problems with water logging/lack of air
Marine and estuarine plants must cope with high salinity
Plant adaptations - aquatic
Problems faced
• Water availability high
• Humidity usually high
• Temperature varies
• Light usually high (may
vary if plant deeper
under water)
• Air low
Solutions include
Stomata on surfaces of
leaves (eg water
lilies)
Large air spaces for
buoyancy and gas
storage
Aerial roots (eg
mangroves)
Salt secretion in leaves
(mangroves)
Leaf adaptations 1
Leaf adaptations 2
Leaf adaptations 3
Look at the stomata in these leaves.
Which of these is most likely to be adapted to arid
conditions?
This one- it has fewer stomata
Leaf adaptations 4
What adaptations can be seen in these that
allow them to survive arid conditions?
Rolled leaf
Thick cuticle and epidermis
Rolled leaf
Sunken pits
Hairs to protect stoma
Reduced stoma
Sunken pits
Thick cuticle and epidermis
Leaf adaptations 5
What adaptations can be seen in these that
allow them to survive in aquatic conditions?