3.4.2.2: Phloem & translocation

Question 1

What is the function of the phloem

Answer 1

transports sucrose and amino acids

Question 2

From where to where do the phloem transport sucrose and amino acids?

Answer 2

from source - site of photosynthesis (leaf) - to all other parts of the plant where they will be used or stored - sinks

Question 3

Do phloem contain lignin, how do you know?

Answer 3

no as they are living

Question 4

describe the structure of phloem tissue

Answer 4

companion cell, cell wall, cell membrane, cytoplasm, vacuole, nucleus, sieve tube cell, end wall

Question 5

describe the structure of the sieve tube element in phloem

Answer 5

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

Question 6

what is the function of the sieve tube element in phloem tissue

Answer 6

transport of sucrose and amino acids

Question 7

describe the structure of companion cells in phloem

Answer 7

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

Question 8

what is the function of the companion cells in phloem tissue

Answer 8

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

Question 9

what is the function of the fibre in phloem tissue

Answer 9

provide mechanical support for tissue

Question 10

what is the function of the parenchyma cells in phloem tissue

Answer 10

packing cell between vessel elements

Question 11

state the name of the main organic molecule transported in the phloem

Answer 11

sucrose

Question 12

state the meaning of translocation

Answer 12

the long distance transport of organic substances around plants

Question 13

give an example of a source in a plant in terms of translocation

Answer 13

leaves - where sucrose is produced

Question 14

give an example of a sink in a plant in terms of translocation

Answer 14

buds - sucrose using parts of the plant

Question 15

what is the most widely accepted mechanism of translocation

Answer 15

Münch’s mass flow hypothesis

Question 16

what are 2 suggestions that are made from Münch’s mass flow hypothesis

Answer 16

1 - a passive flow of sucrose from source to sink
2 - moving down a hydrostatic pressure gradient

Question 17

what are 2 criticisms made about Münch’s mass flow hypothesis

Answer 17

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

Question 18

Give three points of evidence that translocation is not passive

Answer 18

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

Question 19

Explain the process of mass flow in plants using Münch’s hypothesis

Answer 19

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

Question 20

explain the process of sucrose loading into phloem vessels by companion cells

Answer 20

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

Question 21

Is the process of loading sucrose into phloem tissue by the companion cells passive or active? how do you know?

Answer 21

whole process is active as ATP is used

Question 22

Name three experiments that show that translocation is bidirectional in plants

Answer 22

Aphids
Ringing
Tracer (Radioactive)

Question 23

explain how using aphids proves that translocation is bidirectional

Answer 23

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

Question 24

Explain why aphids are used and not a syringe and needle to take out the sap out of the phloem

Answer 24

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.

Question 25

How are aphids used to see if translocation is bidirectional

Answer 25

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

Question 26

How would you carry out an experiment to see if translocation is bidirectional by using the ringing method

Answer 26

remove ring of bark and phloem and leave for a week

Question 27

What happens when you remove a ring of bark and phloem and leave the plant for a week and what does this mean?

Answer 27

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.

Question 28

explain how you would carry out a tracer experiment to see if translocation is bidirectional

Answer 28

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

Question 29

explain how a tracer experiment proves that translocation is bidirectional

Answer 29

the autoradiograph shows that the 14C travels upwards and also downwards meaning translocation is bidirectional

Question 30

Other than proving that translocation is bidirectional, what is another use of radioactive tracers in plants

Answer 30

to calculate the rate of translocation - how long it takes for the tracer to travel from the source to the sinks in the plant

Question 31

explain (regarding mass flow) how you can conclude that phloem sap is under pressure

Answer 31

if a phloem sieve tube is punctured, then phloem sap oozes out

Question 32

explain (regarding mass flow) how you can conclude that osmosis occurs into and out of sieve tube elements

Answer 32

sucrose concentration at source is higher than at sink so there is water potential gradient for osmosis to occur

Question 33

explain (regarding mass flow) how you can conclude that production of sucrose, via photosynthesis, is required for translocation

Answer 33

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

Question 34

Explain how the observation that sucrose travels faster than amino acids contradicts mass flow

Answer 34

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

Question 35

Explain how the observation that different substances move in opposite directions in the same sieve tube element contradicts mass flow

Answer 35

substances should move in one direction down the hydrostatic pressure gradient from soucre to sink according to the mass flow hypothesis