Transport in Plants

Why do plants need transport systems?

• high metabolic demands

• large size

• small SA:V ratio

what is the vascular system?

a series of transport vessels running through the stem, roots, and leaves

structure of xylem tissue

• made of dead cells

• xylem vessels which are hollow and fused end-to-end

• parenchyma which stores food and contains tannin deposits

• lignin

function of xylem tissue

transports water and mineral ions from roots to shoots and leaves

function of phloem tissue

transports sugars and assimilates up and down the plant

structure of phloem tissue

• end to end cells

• sieve plates

• companion cells

• fibres and sclereids

symplast pathway

• water moves into plasmodesmata by osmosis to the endodermis

• slow transport

apoplast pathway

• water moves into cell walls and intercellular spaces to the endodermis

• rapid transport

journey from the endodermis to the xylem tissue?

• water from the apoplast way is forced into cytoplasm joining the symplast pathway

• this means water must pass through the selectively permeable membrane, excluding any potentially toxic solutes

• endodermal cells pump mineral ions into xylem by active transport which causes the water potential to be lower

• so water moves into xylem by osmosis down the water potential gradient (root pressure)

• this gives water a push up the xylem

evidence for active transport in root pressure?

• when cyanide is applied, root pressure disappears

• root pressure increases with a rise in temperature and decreases with a fall in temperature

• oxygen levels affect it

• guttation

transpiration

the loss of water vapour from the leaves and stems of plants

transpiration stream

when water vapour moves into external air through the stomata along a diffusion gradient

process of the transpiration stream

• water molecules evaporate from the surface of mesophyll cells into air spaces in the leaf and move out of stomata into surrounding air

• this lowers the water potential of the cell

• so water moves into the cell from an adjacent cell by osmosis via the symplast and apoplast pathways from the xylem

• capillary action allows for the transpiration pull

transpiration pull

when water is drawn up the xylem in a continuous stream to replace the water lost by evaporation

evidence for the cohesion-tension theory?

• changes in diameter of trees

• when a xylem vessel is broken, air is drawn in to the xylem rather than water leaking out

factors affecting transpiration

• light

• humidity

• temperature

• air movement

• soil-water availability

translocation

plant transports organic compounds in the phloem from sources to sinks

examples of sources?

• leaves

• stems

• tubers

• tap roots

• germinating seeds

examples of sinks

• roots

• meristems

• developing seeds

• fruits

• storage organs

apoplast route of phloem loading

• hydrogen ions are actively pumped out of the companion cells into surrounding tissue using ATP

• these then return to the companion cell down a concentration gradient via a co-transport protein alongside sucrose

• sucrose concentration in the companion cells increase

• therefore water also moves in by osmosis leading to turgor pressure

• water then moves into the tubes of the sieve elements up and down to sinks

phloem unloading

• sucrose is unloaded from the phloem to surrounding cells

• this increases the water potential in the phloem

• so water moves into the cells by osmosis

evidence for translocation

• adaptations of companion cells

• translocation stops when mitochondria of companion cells are poisoned

• flow of sugars in phloem is faster that it would be by diffusion alone

xerophytes

plants in dry habitats that have evolved to enable them to live and reproduce in places where water availability is very low

adaptations of xerophytes

• thick waxy cuticle

• sunken stomata

• hairy leaves

• curled eaves

• succulent

• root adaptations

hydrophytes

plants in flooded habitat that have evolved to enable them to live and reproduce in places where water availability is abundant

adaptations of hydrophytes

• thin waxy cuticle

• stomata always open

• wide, flat leaves

• small roots

• air sacs

• aerenchyma