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.
