dissolved substances - solutes are sometimes called assimilates (these are substances that get incorporated into the plant tissue)
What is the function of the phloem
transports organic substances/solutes (mainly sucrose) in plants
What does organic mean
contains carbon
Phloem tissue consists of 2 types of living cells
sieve tube elements form the tube for transporting solutes
companion cells contain many mitochondria to make ATP through aerobic respiration for the active transport of solutes
when says: to allow continuous flow of organic substances also right to reduce resistance to flow of sucrose
How is the phloem adapted for its function
sieve tubes have no nucleus, little cytoplasm + few organelles = to allow continuous flow of organic substances (sucrose)/ to reduce resistance to flow of sucrose
end walls of sieve tubes are perforated with holes so it allows continuous flow of organic substances through sieve tube/ to reduce resistance to flow of sucrose
companion cells contain many organelles e. mitochondria for aerobic respiration to produce ATP for active transport of sucrose into sieve tube elements + many carrier proteins for active transport of sucrose
What is translocation
the movement of organic substances/solutes (e.g. sucrose and amino acids) in the plant from source to sink - its an energy requiring process that happens in the phloem
source = produce carbohydrates/assimilates (so they are in high concentration at the sink) like sucrose during photosynthesis e.g. leaves
sink = areas which need energy such as growing regions (roots) or storage organs (potatoes) - where assimilates are used or converted to a storage molecule (so low concentrations here)so a conc gradient from source to sink
More on translocation
occurs in both directions (e.g in winter moves down to roots to store sucrose as starch) there4 sucrose can move in both directions up and down the phloem
How are the products of photosynthesis transported around the plant
by the mass flow hypothesis
Why is sucrose transported through the phloem rather than glucose
after being synthesised during photosynthesis, glucose is converted to sucrose for transport as its more stable and less reactive than glucose
Mass flow hypothesis pt1
Mass flow hypothesis pt2
Mass flow hypothesispt3
the higher the conc of sucrose at the source, the higher the rate of translocation
Exam Q
Evidence for mass flow hypothesis pt1
Evidence for mass hypothesis pt2
pressure in the phloem can be investigated using aphids. their mouth parts can pierce the phloem allowing it to feed. scientists remove the aphids body leaving the mouth parts behind in the plant which allows sap to flow out. the phloem sap flows out quicker nearer the leaves than further down the stem
due to pressure gradient - higher pressure at leaves (source) and lower pressure at sink, due to movement of water into and out of phloem by osmosis
Evidence foe mass flow hypothesis pt3
14C can be used as radioactive tracer to track movement of organic substances in a plant
radioactive CO2 can be supplied to plant which is used for photosynthesis so its incorporated into sucrose at the source when cells photosynthesise + will then be moved around plant in translocation
radioactivity can be tracked using autoradiography (= plant is placed onto photographic film)
radioactivity observed more at leaves first then further away overtime as radioactive sucrose is actively transported into phloem + translocated towards sink via mass flow
More on autoradiography
Evidence against mass flow hypothesis
sugars move to all areas of the plant not just form source to sink with the highest water potential
the pressure gradient wouldn't be high enough for a sufficient rate of flow through sieve plates so sieve plates would create a barrier to mass flow