translocation

Question 1

assimilates

Answer 1

substances made by the plant using substances absorbed from the environment

Question 2

sink

Answer 2

a part of the plant that removes assimilates from the phloem sieve tubes e.g the roots receive sugars and store them as starch

Question 3

source

Answer 3

a part of the plant that loads assimilates into the phloem sieve tubes

Question 4

translocation

Answer 4

the movement of organic compounds or assimilates, like sugars, from where they are produced (sources) to where they are needed or stored (sinks)

Question 5

where does translocation occur

Answer 5

occurs in the phloem

Question 6

what are the assimilates of a plant

Answer 6

sugars and amino acids

Question 7

what are sieve tubes

Answer 7

sites of sugar production

Question 8

where are the sugars used up

Answer 8

the sink e.g. root and shoot tips

Question 9

what type of process is translocation

Answer 9

translocation is an active process that requires energy to take place and substances can be transported up and down the plant

Question 10

what is made during photosynthesis

Answer 10

glucose

Question 11

which assimilate is transported around the plant

Answer 11

sucrose

Question 12

how much sucrose is contained in cell sap

Answer 12

0.5%

Question 13

how much sap can be in phloem

Answer 13

20-30%

Question 14

what are the main sources of assimilates

Answer 14

• green leaves and green stems
• storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period
• food stores in seeds when they germinate

Question 15

what are the main sinks in a plant

Answer 15

• roots that are growing or actively absorbing mineral ions
• actively dividing meristems
• developing seeds, fruits, storage organs

Question 16

what happens to the glucose thats produced in photosynthesis

Answer 16

leaves are the source where glucose is produced in photosynthesis and converted to less reactive sucrose
in the sink such as the growing shoot sucrose is converted into glucose and used in respiration. in the roots (another sink) the sucrose is stored as insoluble starch

Question 17

briefly describe translocation

Answer 17

1) sugar is loaded into the phloem
2) water moves into the phloem increasing the pressure
3) contents of phloem move from the source to the sink
4) sucrose leaves the phloem - water follows due to decreased water potential
5) water returns to the transpiration stream in the xylem

Question 18

why is sucrose transported instead of glucose

Answer 18

sucrose isnt used in metabolism so its less likely to be metabolised during the transport process. it also doesnt affect the water potential

Question 19

do substances move by diffusion in the phloem

Answer 19

it doesnt just occur by diffusion as the transport in phloem occurs too fast. its a combination of transport across membranes, osmosis, and pressure

Question 20

what is cotransport

Answer 20

use of hydrogen ions to transport sucrose

Question 21

what is another name for cotransport

Answer 21

secondary active transport

Question 22

what happens during cotransport

Answer 22

it results from the active transport of the hydrogen ions out of the cell and moves the sucrose against its concentration gradient.

Question 23

what happens as the concentration of sucrose in the companion cell increases

Answer 23

it can diffuse through the plasmodesmata into the sieve tubes

Question 24

what are the adaptations of companion cells

Answer 24

companion cells have many infoldings in their cell membranes to give an increased surface area for the active transport of sucrose into the cell cytoplasm. they also have many mitochondria to supply the ATP needed for the transport pumps

Question 25

how can sucrose move against the concentration gradient

Answer 25

phloem contains concentrated sucrose solution. the hydrolysis of ATP required to provide energy to move sucrose against the concentration gradient

Question 26

how is sucrose loaded into the sieve tube

Answer 26

sucrose is loaded into the sieve tube by an active process. this involves the use of energy as ATP in the companion cells. the energy is used to actively transport hydrogen ions out of the companion cells. this increases their concentration outside the cells and decreases their concentration inside the companion cell

Question 27

how does a plant load assimilates into the phloem by the symplast route

Answer 27

in some plants sugar moves from palisade cells into companion cells and sieve tube elements via plasmodesmata. this route is largely passive. sucrose ends up in sieve elements and water follows by osmosis. this creates a pressure of water that moves the sucrose through the phloem by mass flow

Question 28

how does plant load assimilates into the phloem by the apoplast route

Answer 28

energy is used to actively transport hydrogen ions out of the companion cells. as a result a concentration gradient is created. the hydrogen ions diffuse back into the companion cells through special cotransporter proteins. these proteins only allow the movement of hydrogen ions into the cell if they are accompanied by sucrose molecules

Question 29

how does active loading of sucrose in the sieve tube occur

Answer 29

hydrogen ions are actively pumped out of companion cells using energy from ATP. high concentration of hydrogen ions causes facilitated diffusion back into the companion cell. sucrose is carried with the hydrogen ions through cotransport proteins in the plasma membrane.

Question 30

what does increasing the concentration of sucrose in companion cells do

Answer 30

it causes the sucrose to diffuse through the plasmodesmate into the sieve tube element

Question 31

how does movement of sap occur in the phloem from the source to sink

Answer 31

1) sucrose is actively loaded into the sieve tube element and reduces the water potential 2) water follows from the xylem by osmosis increasing the hydrostatic pressure in the sieve tube element 3) water and sucrose moves down the sieve tube from higher hydrostatic pressure at source to lower hydrostatic pressure at the sink 4) sucrose is removed from the sieve tube by the surrounding cells and increases the water potential in the sieve tube 5) water moves out of the sieve tube by osmosis into the roots reducing hydrostatic pressure.

Question 32

what is mass flow

Answer 32

the process of moving water and solutes through plant tissues particularly in the phloem

Question 33

how does movement of sucrose occur

Answer 33

mass flow - a solution of sucrose, amino acids and other assimilates flows along the tube

Question 34

what is sap

Answer 34

a solution of sucrose and amino acids

Question 35

where does sap move

Answer 35

either up or down the plant as required. from source to sink

Question 36

why does mass flow occur

Answer 36

its caused by a difference in hydrostatic pressure between the two ends of the tube producing a pressure gradient. water enters the tube at the source increasing the pressure and leaves the tube at the sink reducing the pressure

Question 37

what happens when sucrose enters the sieve tube element

Answer 37

it makes the water potential inside the sieve tubes more negative (lower). as a result, water molecules move into the sieve tube element by osmosis from surrounding tissues. this increases the hydrostatic pressure in the sieve tube at the source

Question 38

what happens when water leaves the sieve tube at the sink

Answer 38

it reduces the hydrostatic pressure. this happens when the removal of sucrose from the sap makes the water potential less negative (higher) so water moves out

Question 39

where does sap flow

Answer 39

from high pressure to low pressure

Question 40

what direction does sap flow

Answer 40

could be in either direction depending on where sucrose is being produced and where its needed. sap could be flowing in opposite directions in different sieve tubes at the same time.

Question 41

what does it mean when sap in one tube is all moving in the same direction

Answer 41

mass flow

Question 42

what was the ringing experiment

Answer 42

an experiment called ringing done on a woody stem to investigate how sugars are transported in a plant

Question 43

what happened before ringing

Answer 43

(normal stem) the stem contains xylem and phloem. the phloem lies just beneath the bark

Question 44

what is ringing

Answer 44

a ring of tissue including the phloem and protective layer (bark) is cut away all the way around the stem. the xylem is left intact so water can still move up the plant

Question 45

what happens after ringing

Answer 45

above the ring - the stem swells because sugars accumulate there - they cant move past the cut to go down the plant below the ring - tissues eventually die because they don't get sugars (no energy/organic nutrients)

Question 46

what does the ringing experiment prove

Answer 46

sugars are transported in the phloem not the xylem when you cut away the phloem but leave the xylem, sugar movement stops below the ring. therefore translocation happens through the phloem

Question 47

give supporting evidence that shows that the phloem is under positive pressure

Answer 47

needle like mouthpart of an aphid is inserted into the phloem. when the body of the aphid is removed, sap continues to exude which shows the phloem is under positive pressure and transports organic substances - sap

Question 48

what can radioactive isotopes be used for

Answer 48

tracing precisely where different compounds are being transported from and to as well as measuring the rate of transport. it can be used to trace sugars, uins or even water

Question 49

how can the radioactivity be traced

Answer 49

using photographic films (an autoradiograph or a Gieger-Muller Tube)

Question 50

describe one radioactive tracer experiment

Answer 50

in a typical experiment, a plant is grown in the lab and one leaf is exposed for a short time to carbon dioxide containing the radioactive isotope, carbon-14 this carbon-14 can be taken up by photosynthesis and the carbon-14 is incorporated into glucose and then sucrose. the plant is then frozen in liquid nitrogen to kill and fix it quickly and placed onto photographic film in the dark.

Question 51

what does the radioactive tracer experiment show

Answer 51

that organic compounds (sugars) are transported downwards from the leafs to the roots.