3.4.2.2: Phloem & translocation Flashcards
What is the function of the phloem
transports sucrose and amino acids
From where to where do the phloem transport sucrose and amino acids?
from source - site of photosynthesis (leaf) - to all other parts of the plant where they will be used or stored - sinks
Do phloem contain lignin, how do you know?
no as they are living
describe the structure of phloem tissue
companion cell, cell wall, cell membrane, cytoplasm, vacuole, nucleus, sieve tube cell, end wall
describe the structure of the sieve tube element in phloem
1 - elongated cells and are stacked on top of each other to form a series of tubes
2 - perforated end walls - sieve plates - to allow cytoplasmic connections
3 - no nucleus and no organelles
what is the function of the sieve tube element in phloem tissue
transport of sucrose and amino acids
describe the structure of companion cells in phloem
1 - directly adjacent to every sieve tube element
2 - large nucleus and have all their organelles
3 - plasmodesmata in cell wall allows movement of substances between companion cell and sieve tube element
what is the function of the companion cells in phloem tissue
1 - synthesise ATP and proteins to sieve tube elements as they have no organelles and so no ATP or proteins for energy
2 - Loading and unloading of sucrose from sieve tube element
what is the function of the fibre in phloem tissue
provide mechanical support for tissue
what is the function of the parenchyma cells in phloem tissue
packing cell between vessel elements
state the name of the main organic molecule transported in the phloem
sucrose
state the meaning of translocation
the long distance transport of organic substances around plants
give an example of a source in a plant in terms of translocation
leaves - where sucrose is produced
give an example of a sink in a plant in terms of translocation
buds - sucrose using parts of the plant
what is the most widely accepted mechanism of translocation
Münch’s mass flow hypothesis
what are 2 suggestions that are made from Münch’s mass flow hypothesis
1 - a passive flow of sucrose from source to sink
2 - moving down a hydrostatic pressure gradient
what are 2 criticisms made about Münch’s mass flow hypothesis
1 - sucrose and amino acids have been observed to translocate at different rates in the same tissue whereas in Mmfh they’d move all together
2 - sucrose and amino acids have been observed to translocate in different directions in same tissue whereas in Mmfh this would not be possible
Give three points of evidence that translocation is not passive
1 - companion cells contain high number of mitochondria (and so ATP)
2 - phloem tissue has high oxygen consumption (aerobic respiration for ATP synthesis)
3 - Rs inhibitors added to phloem tissue results in reduced rate of translocation
Explain the process of mass flow in plants using Münch’s hypothesis
1 - active transport of sucrose into phloem vessels by companion cells
2 - water moves from xylem into phloem down a water potential gradient, therefore creating hydrostatic pressure in the phloem
3 - sucrose moves down a hydrostatic pressure gradient as there is lower hydrostatic pressure at the bottom of phloem
4 - Sucrose moves passively via facilitated diffusion into sink cell, and so hydrostatic pressure is less and water potential is higher in phloem in this area than the xylem (as there is a lower conc. of sucrose)
5 - active transport of mineral ions and osmosis of water up the xylem vessel
6 - water moves down a water potential gradient from phloem to xylem
7 - water moves up xylem vessel via cohesion-tension theory
explain the process of sucrose loading into phloem vessels by companion cells
1 - Hydrogen ions are actively pumped out of companion cells by ATP into source tissue via a carrier protein creating a hydrogen-ion gradient
2 - Hydrogen ions diffuse back into companion cells by facilitated diffusion down the concentration gradient through a cotransport protein (symport), also taking sucrose in with it (indirect active transport as sucrose is not moving via its own concentration gradient rather against its own gradient)
3 - sucrose builds up in the companion cell creating a sucrose concentration gradient
4 - Sucrose diffuses passively into sieve tube element down the gradient
Is the process of loading sucrose into phloem tissue by the companion cells passive or active? how do you know?
whole process is active as ATP is used
Name three experiments that show that translocation is bidirectional in plants
Aphids
Ringing
Tracer (Radioactive)
explain how using aphids proves that translocation is bidirectional
when using stylets above and below the source, the concentration of sucrose in the phloem is similar meaning that translocation is bidirectional as sucrose is being transported upwards and also downwards
Explain why aphids are used and not a syringe and needle to take out the sap out of the phloem
aphids produce an enzyme which stops the coagulation of sap as the sap dries quickly, if you were to use a syringe and needle, the sap would dry up and get stuck.
How are aphids used to see if translocation is bidirectional
aphids are are put on different parts of the stem, they have specialised mouth parts called stylets which can pierce the stem. When the aphids are eating from the stem, they are killed off so their body falls off but their stylets remain in the stem which can be used as sampling tubes
How would you carry out an experiment to see if translocation is bidirectional by using the ringing method
remove ring of bark and phloem and leave for a week
What happens when you remove a ring of bark and phloem and leave the plant for a week and what does this mean?
bulging occurs above the cut which means sap moves downwards as there is no phloem, the sap pools as it has nowhere to go.
Underneath the cut there is reduced growth for the stem as it has a lack of sucrose - lack of ATP - lack of growth.
BUT the stem underneath the cut doesn’t die, it is still alive which means that sap also moves upwards and hence proving that translocation is bidirectional.
explain how you would carry out a tracer experiment to see if translocation is bidirectional
1 - expose 1 leaf to radioactive CO2 (14CO2)
2 - 14C is taken up by plant during photosynthesis
3 - After varying exposure times, plants are frozen in liquid Nitrogen - kills the plants and stops any reactions from occuring so that all molecules stay in the same place
4 - An autoradiograph is taken to show the location of any compounds containing 14C
explain how a tracer experiment proves that translocation is bidirectional
the autoradiograph shows that the 14C travels upwards and also downwards meaning translocation is bidirectional
Other than proving that translocation is bidirectional, what is another use of radioactive tracers in plants
to calculate the rate of translocation - how long it takes for the tracer to travel from the source to the sinks in the plant
explain (regarding mass flow) how you can conclude that phloem sap is under pressure
if a phloem sieve tube is punctured, then phloem sap oozes out
explain (regarding mass flow) how you can conclude that osmosis occurs into and out of sieve tube elements
sucrose concentration at source is higher than at sink so there is water potential gradient for osmosis to occur
explain (regarding mass flow) how you can conclude that production of sucrose, via photosynthesis, is required for translocation
if a plant virus is applied to well illuminated leaves, the virus moves downwards in the phloem towards roots and in the dark they are not transported
Explain how the observation that sucrose travels faster than amino acids contradicts mass flow
substances in the sieve tubes should travel at the same rate according to Münch’s mass flow hypothesis as they all travel via the same process and are all connected to the same source - leaves
Explain how the observation that different substances move in opposite directions in the same sieve tube element contradicts mass flow
substances should move in one direction down the hydrostatic pressure gradient from soucre to sink according to the mass flow hypothesis
