transport in plants. plant transport systems forces acting on transport
TRANSCRIPT
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 phloemXylem Phloem
Cells living/dead Dead Living
Cell walls:
Thickness
Material
Permeability
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
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 regionsUsually hot and dry eg Australian & African deserts & savannahCan be cold & dry eg icecaps & tundra
Plant adaptations - arid
Problems faced• Water availability low• Humidity low• Temperature high• Light high• Air movement high
Solutions includeIncreased roots – either deep
or wide and shallowWater storage (roots, leaves
or stems) eg cacti, boabsReduced leavesReduced stomata in leavesProtection for leaves –
cuticle, thick epidermis, curling, sunken pits, hairs to guard stomata
Closing stomata in hot conditions
Arid region adaptationsWater storageReduced leaves
Water storage -succulent leaves
Water storage - trunkReduced leaves – losesleaves in summerLong deep roots
Reduced leavesWide shallow root system
Coastal environmentsHot, 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 includeWide shallow root
systemsRapid growthCan cope with burialRolled leaves, sunken
stomata, reduced stomata and/or hairs
Succulent leavesSalt secretion in leaves
RainforestLow 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 includeLarge leaves to trap
lightDeep veins to carry
water away from plantUsually many stoma
and thin epidermisLarge air spaces within
leaves
Aquatic environments
Plenty of water, problems with water logging/lack of airMarine 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 includeStomata 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 leafThick cuticle and epidermis
Rolled leafSunken pitsHairs to protect stoma
Reduced stomaSunken pitsThick cuticle and epidermis
Leaf adaptations 5
What adaptations can be seen in these that allow them to survive in aquatic conditions?