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?