Xylem and phloem Flashcards
What is the structure of xylem?
Hollow and elongated Parenchyma Arranged end to end Called an element because it's dead Tracheids Xylem vessels Lignin Sclerenchyma fibres
What are the functions of xylem?
Transport water/dissolved substances
Hollow/elongated - reduce flow resistance
Parenchyma - support
Lignin - support/waterproof vessel
Sclerenchyma - support
Pits - support natural movement of water/allow water to leave
What are the 4 types of lignin?
Annular (rings)
Spiral
Pitted (holes)
Reticulate (dashed holes)
Describe tracheids
In all plants, long and thin, tapered end walls, thickened with lignin, has pits, when mature it is dead and hollow.
What is the structure of phloem?
Hollow and elongated Parenchyma Arranged end to end Sieve tube elements Companion cells
What is the structure of sieve tube elements?
Elongated, joined end to end, long tubes, living, thin cytoplasm wall/peripheral, no nuclei, few organelles
Why are companion cells needed?
They contain all the organelles needed for the the sieve tube element to survive.
Contain lots of mitochondria to provide ATP.
What is the function of phloem?
Transport organic material, such as sugars, in solution
Hollow and elongated - reduce flow resistance
Parenchyma - support
Thin cytoplasm wall/peripheral and few organelles to reduce resistance to flow.
Mitochondria - provide ATP needed for active transport
Where does loading occur in translocation?
At the source (eg leaves)
What are sources areas of?
Photosynthesis
Assimilates are moved from here
Where does unloading occur in translocation?
At the sink (eg roots, flowers, fruits)
What are sinks areas of?
Respiration
Where the assimilates are moved to
What 3 processes happen at the source?
Active transport
Facilitated diffusion
Osmosis
What are all the steps of translocation at the source?
H+ ions are removed by active transport (that’s why there is so many mitochondria)
Now there is a low concentration of H+ ions
So it cotransports H+ ions using a symport protein, taking sucrose with it.
High concentration of sucrose in companion cell means sucrose diffuses into sieve tube element
This lowers the water potential so water moves into the sieve tube element by osmosis
What are all the steps of translocation at the sink?
Sucrose would diffuse into the companion cells at the sink until equilibrium was reached however, companion cells contain invertase which hydrolyses the sucrose into glucose and fructose, maintaining the concentration gradient so sucrose continues moving into the companion cells.
Which 2 processes occur at the sink?
Diffusion and osmosis
What is the evidence for translocation?
The rate of mass flow is 10,000X faster than if diffusion alone was responsible.
Phloem sap has a relatively high pH (pH8) because H+ ions are lost so it becomes more alkali.
There is an electrical difference across the membrane of the companion cell because H+ ions are charged so the voltage changes when they move.
You can see with a microscope that there are lots of mitochondria in companion cells.
What 3 experiments are there for translocation?
Radioactive tracers
Ringing
Aphid styletectomy
Explain the radioactive tracers experiment
Give 1 leaf a source of radioactive carbon dioxide.
This radioactivity shows up on an X ray so you can see where the carbon goes.
Shows it goes from leaf to stem to sink.
Explain the ringing experiment
A ring of bark/phloem tissue is removed.
Sugars being transported down the phloem accumulate as starch above the ring.
Explain the Aphid styletectomy experiment
Cut stylet of feeding aphid.
Aphid is anaesthetised with CO2 and is removed from the plant.
Pure phloem sap runs from the stylet.
Analysis of the sap shows it contains mainly sucrose with some amino acids.
What shape are sieve tube elements?
Hexagonal
What are assimilates?
The result of combining and converting biochemicals for use by a plant
