3.3.4.2 Mass Transport in Plants

xylem

transports mineral ions and water up the plant from the roots to leaves

structure of xylem

-very long, tube-like structure formed from dead cells

-no end walls between cells = creates series of continuous tubes to allow water to flow easily

cohesion-tension theory

1) water is lost from leaf due to transpiration

2) this lowers the water potential of the leaf cells

3) water is pulled up the xylem creating tension

4) so water molecules ‘stick together’ due to hydrogen bonds which is cohesion

5) this forms a continuous water column

6) adhesion of water molecules to walls of xylem

transpiration + FACTORS that affect this

the evaporation of water from a plant’s surface

-light intensity, temperature, humidity, wind

potometer practical

-measures rate of water uptake

-the leafy shoot is cut underwater = prevents air from entering + loss of water and helps to maintain the continuous flow of water

-joints are sealed with waterproof jelly = stops water leaking out and air entering

-leaves must be kept dry = the moisture can affect the rate of transpiration

phloem

transports organic substances (sugars and amino acids) up and down the plant = bidirectional

structure of phloem

-living tissue cells

-sieve tube elements

-companion cells

translocation

the movement of solutes/assimilates

-the higher the concentration of sucrose, the higher the rate of translocation

-requires energy

-moves solutes from source to sink

-source = where solutes are produced

-sink = where solutes are used up

mass flow hypothesis 1) loading from the source

-in source, sucrose/sugars are actively transported from companion cells into the sieve cells of phloem

-this lowers the water potential of sieve cells so water enters phloem from xylem by osmosis so this generates pressure inside

-ATP hydrolysis is involved for the energy needed

mass flow hypothesis 2) pressure flow from source to sink

-the water entering increases the pressure in sieve cells which causes mass movement towards sink

-in sink, solutes are removed to be used up which increases water potential in sieve cells so water leaves by osmosis

-this lowers the pressure in sieve cells

mass flow hypothesis 3) unloading into the sink

-this creates a pressure gradient and solutes are pushed down the gradient towards sink

-when solutes reach the sink, the sugars are used or converted in roots for respiration as storage

evidence of mass flow 1) ringing experiments

-the bark of the tree contains the phloem

-this is removed in a ring so the tissues above missing ring swell due to the accumulation of sugars so tissue below begins to die

-so there is a downward flow of sucrose

evidence of mass flow 2) tracer experiments

-radioactive isotopes can be used to trace the movement of sugars

e.g. plant is grown in radioactive CO2 then a thin cross-section of plant stem is obtained and placed on x-ray film, using autoradiography the parts where the film goes dark is where the sugars have been exposed to radiation