Transport in plants

Structure of dicotyledon root

• Root hair

• Epidermis

• Cortex

• Endodermis

• Pericycle

• Xylem & Phloem

Apoplast pathway

• Quickest pathway

• Water moves in the cell walls

• Cellulose fibres in cell wall are separated by spaces through which the water moves

Symplast pathway

• Water moves through cytoplasm & plasmodesmata

• Continual pathway across root cortex

Vacuolar pathway

Water moves from vacuole to vacuole

Structure & function of endodermis

• Cell wall of endodermis contains suberin

• Suberin forms a distinctive band called Casparian strip

• Casparian strip is waterproof - prevents water moving further in apoplast & drives it into cytoplasm

Structure of a dicotyledon stem

• Epidermis

• Collenchyna

• Cortex

• Medulla

• Vascular bundle + fibres along the circumference

Transpiration

The loss of water vapour from leaves and shoots of plants

The transpiration stream

Continuous movement of water from roots to leaves via xylem due to cohesion between water molecules

Cohesion-tension theory

• Cohesion of water molecules

• Tension in water column

Capilliarity

• Movement of water up xylem by capillary action

• Only operates over short distances

Root pressure

• Osmotic movement of water in the xylem pushing it further up

• Movement of water down water potential gradient across root and into base of xylem

• Operates over short distances

Function of xylem

• Transports water and mineral ions

• Providing mechanical support

Structure of xylem

• Unidirectional

• Thick cell wall made of lignin & cellulose

• Pits - allow water up the plant in a twisted manner

• Tracheids - elongated cells with narrower lumen

How does temperature affect transpiration rate?

• Increase in temp = decrease in water potential of atmosphere

• Increases molecules KE = accelerating evaporation rate from mesophyll

• If stomata open = speeds up diffusion rate into atmosphere

• Higher temp = water diffuses away quicker = reducing water potential around leaf

How does humidity affect transpiration rate?

• Air inside leaf is saturated so humidity = 100%

• When stomata open, water vapour diffuses out leaf, down water potential gradient

• Transpiration in still air leads to accumulation of saturated air layer on leaf’s surface

• Water vapour diffusing away gradually, leaving concentric rings of decreasing humidity further from leaf

How does air speed affect transpiration rate?

• Movement of surrounding air blows away layer of humid air at leaf’s surface

• Water potential gradient between inside and outside leaf constantly increases

• Water vapour diffuses out of stomata more quickly

• Faster air movement = faster concentric water vapour shells get blown away = faster transpiration

How does light intensity affect transpiration rate?

• As light intensity increases, the stomata open wider, more water vapour diffuses out

• Increasing rate of transpiration

• Widest during middle of day, less wide in morning and evening, closed at night

Mesophyte

Land plants that grow in temperate regions

Adaptations of mesophytes

• Shed leaves before winter - reduces water loss via transpiration

• Close stomata at night - decrease water loss

• Produce dormant seeds - underground organs survive winter

Xerophyte

Plants which live in hot, dry regions

Adaptations of xerophytes

• Rolled leaves - reduces area of leaf exposed directly to air

• Sunken stomata - increases humidity in an air chamber above stomata, reducing diffusion gradient & therefore water loss

• Hairs - interlocking hairs trap water vapour, reducing water potential gradient & therefore water loss

• Thick cuticle - waterproof, reduces water loss by evaporation from epidermal tissue

• Dense spongy mesophyll - less air spaces, decreases evaporation & transpiration rate

Hydrophytes

Plants that grow partially or wholly submerged in water

Adaptations of hydrophytes

• Little to no cuticle - no need to conserve water

• Stomata on upper surface inwards - lower surface submerged in water

• Poorly developed xylem - no need to transport water

• Large air spaces - provide buoyancy & act as reservoirs of gas

Function of phloem

• Transports sucrose and amino acids

Translocation

Movement of sucrose and amino acids from source to sink bidirectionally

Structure of phloem

• Sieve tubes - no nucleus, vacuole, ribosome. Little cytoplasm

• Plasmodesmata

• Pores in sieve plate - allow movement of sucrose & amino acids bidirectionally

• Companion cells - large nucleus, 80s ribosomes, mitochondria. Provides ATP for sieve tubes

Ringing

• Remove phloem from stem

• Causing swelling above due to sucrose build up

• Reduced growth below ring due to no sucrose

Radioactive isotopes

• Expose leaf to CO2 containing radioactive C14

• Produces radioactive glucose in photosynthesis

• Becomes radioactive sucrose and is transported

• Track C14 with X-rays

Aphids

• Feed from phloem through a stylet

• Kill aphids and remove from plant, leaving stylet in place

• Samples can be collected from phloem

• Can be combined with radioactive isotopes method

Cytoplasmic streaming

• Strands of cytoplasm contract and relax in a rhythm

• Forces sucrose from source to sink

Mass flow hypothesis

• Sucrose transported into phloem, decreasing water potential

• Water leaves xylem & enters phloem down water potential gradient via osmosis

• Increase hydrostatic pressure

• Sucrose solution moves down pressure gradient

• Sucrose is transported into sink, increasing water potential in phloem

• Water returns to xylem down water potential gradient

Arguments against mass flow hypothesis

• Hydrostatic pressure isn’t as high as calculated

• Translocation needs lots of ATP - with MFH it’s unclear what ATP is for

• Substances move in opposite directions at the same time