Xylem and phloem

structure of xylem

long tube like structures formed from dead cells joined end to end with no end walls

what are the strong walls made of?

lignin to prevent tube collapsing

what does the xylem transport?

water and mineral ions in solution

method of water transport in plants

cohesion-tension theory

cohesion tension theory

• water evaporates from the leaves at the top of the xylem

• creates tension which pulls more water into the leaf

• water molecules are cohesive so when some are pulled into the leaf others follow, meaning whole column moves upwards - transpiration pull

• water then enters stem through the roots

cohesion tension theory ms

• Water lost from leaf because of transpiration / evaporation of water (molecules) / diffusion from mesophyll / leaf cells; OR Transpiration / evaporation / diffusion of water (molecules) through stomata / from leaves

• Lowers water potential of mesophyll / leaf cells

• Water pulled up xylem (creating tension)

• Water molecules cohere / ‘stick’ together by hydrogen bonds

• (forming continuous) water column

• Adhesion of water (molecules) to walls of xylem;

what is cohesion?

water molecules form hydrogen bonds between one another and hence tend to stick together

what is adhesion?

force between different substances eg water molecules and xylem - aids resisting gravity

what is the transpiration pull?

a column of water is therefore pulled up the xylem as a result of transpiration

evidence to support the cohesion tension theory

when a xylem vessel is broken water doesn’t leak out (it would if under pressure) but instead air is drawn in suggesting tension

what is transpiration?

the evaporation of water from a plants surface

how does water leave the leaves?

water evaporates from the moist cell walls and accumulates in spaces between cells in leaf, then when stomata open they move out by water potential gradient

factors affecting transpiration rate?

light intensity, temperature, humidity and wind

how does light intensity affect transpiration rate?

increases because stomata open when it gets light to let in CO2 for photosynthesis

how does temperature affect transpiration rate?

higher the faster because water molecules have more energy so they evaporate from cells faster increasing water potential gradient between inside and out of leaf

how does wind and humidity affect transpiration rates?

affect the water potential

how is transpiration measured?

potometers

potometers

what does phloem transport?

organic solutes which are dissolved substances (usually sugars like sucrose)

what is the structure of phloem?

cells arranged in tubes with sieve tube elements and companion cells

why are there companion cells?

sieve tube elements have no nucleus and few organelles so the companion cells carry out living functions for them eg providing energy needed for AT of solutes

what is translocation?

movement of solutes to where they’re needed in a plant

where does translocation move solutes (assimilates)?

from source where they are produced to sink

what do enzymes do in translocation?

maintain a concentration gradient from the source to sink by changing the solutes at the sink (eg breaking them down) so lower conc

mass flow hypothesis

• active transport loads solutes from companion cells into sieve tubes of phloem at source

• lowers water potential of sieve tubes so water enters tubes by osmosis

• creates high pressure at source end

• at sink end solutes removed from phloem increasing water potential in sieve tubes water also leaves by osmosis

• lowers pressure at sink end

• results in pressure gradient from source to sink end

mass flow hypothesis ms

• In source / leaf sugars actively transported into phloem

• By companion cells

• Lowers water potential of sieve cell / tube and water enters by osmosis

• Increase in pressure causes mass movement (towards sink / root)

• Sugars used / converted in root for respiration for storage.

supporting evidence of mass flow hypothesis

• if section of bark removed that contains phloem, bulge above with high conc of sugars because can’t move so downward flow of sugars

• aphids piece phloem and there is quicker fow out out leaves than stem showing pressure grad

• radioactive tracker can track organic substances

• metabolic inhibitor stops ATP production, translocation stops

objecting evidence for mass flow hypothesis

• sugar travels to many sinks not just one with highest water potential

• sieve plates would create a barrier to mass flow, lots of pressure would be needed for solutes to go through at reasonable rate

radioactive tracers

• CO2 containing radioactive isotope 14C used as radioactive tracer pumped into leaf

• radioactive carbon then incorporated into organic substances produced by leaf which then move by translocation

• tracked using autoradiography

• plant killed then placed on X-ray and if film goes black then radioactive present

ringing experiments

• removing outer bark layers in section containing phloem not xylem

• swells and sugars accumulate above, tissues below die

• suggests phloem responsible for transport of sugar

• correlation not causation?