translocation

Question 1

what is translocation

Answer 1

process used to transport dissolved products in the phloem, from an area of the plant which creates the substances in excess to areas where they are needed

Question 2

what is a source

Answer 2

region of plant where sucrose is loaded into phloem e.g. by photosynthesis

Question 3

what is a sink

Answer 3

region of plant where sucrose is removed from phloem
sugar used in either respiration or starch

Question 4

three examples of substances translocated in the phloem

Answer 4

amino acids
mRNA
viruses

Question 5

features of sieve tube elements

Answer 5

separated by thin cells
sieve plates: 2 cell walls of adjacent elements
large pores: open and have no barrier to fluid movement
cell wall of cellulose
companion cell attached adjacent

Question 6

main difference between phloem and xylem vessels

Answer 6

xylem vessels are dead, phloem living

Question 7

features of companion cells

Answer 7

adjacent to sieve tube elements
metabolically active: actively loads sucrose into sieve tube

Question 8

mass flow hypothesis

Answer 8

bulk movement from high hydrostatic pressure- source to low hydrostatic pressure at sink

Question 9

2 steps in mechanism for translocation

Answer 9

moving sucrose into sieve tube elements from source
mass flow of sucrose via sieve tube elements to sink

Question 10

process of moving sucrose into sieve tube elements from photosynthetic tissue

Answer 10

sucrose is made from products of photosynthesis (glucose)
diffuses down concentration gradient via facilitated diffusion from photosynthetic cells to companion cells
H+ ions actively transported from companion cells in spaces within cell walls via ATP
H+ ions diffuse down concentration gradient through carrier proteins into sieve tube elements
sucrose molecules are co transported with the hydrogen ions using co transport proteins, via facilitated diffusion

Question 11

process of mass flow of sucrose via sieve tube elements

Answer 11

sucrose actively loaded into phloem at source
lowers water potential of sieve tubes
xylem has a higher water potential, so water moves from xylem to sieve tubes via osmosis, creating a high hydrostatic pressure within sieve tubes
at respiring cells (sink) sucrose is either used up during respiration or converted to starch for storage
these cells thus have low sucrose content and sucrose is actively transported into them from sieve cells, decreasing water potential
thus water also moves into these respiring cells via osmosis from the sieve tubes
hydrostatic pressure at the sink is decreased
HP gradient formed- high at source, low at sink
sucrose moves in mass flow down this gradient in sieve tubes

Question 12

support of mass flow hypothesis

Answer 12

there is pressure within sieve tubes- shown by sap released as cut
sucrose conc higher in leaves-source- than roots-sink-
downward phloem flow happens in daylight, stops at night when no light
companion cells have many mitochondria, readily producing ATP

Question 13

against mass flow hypothesis

Answer 13

unclear function of sieve plates: seem to hinder mass flow
not all solutes move at same speed but they should in mass flow
sucrose is delivered at same rate to all regions, instead of going quickly to places which need it most

Question 14

sources of evidence for translocation

Answer 14

radioactive tracers
ringing experiments

Question 15

how are radioactive tracers used as evidence for translocation

Answer 15

plant supplied with radioactive carbon and allowed to photosynthesise, with sucrose containing this
as sucrose is moved to other parts of plant, it can be detected with autoradiography
sample exposed to radioactive film and any radioactive areas produce a dark shadow, showing where sucrose is transported

Question 16

how are ringing experiments used as evidence of translocation

Answer 16

woody stems have an outer protective layer of bark. the phloem is inside this bark. inside the phloem layer is xylem
a section of the outer layers are removed around the woody stem whilst attached to plant
after some time, the region of stem above the missing ring of tissue swells
xylem is left intact to ensure water/mineral salts can still be transported via transpiration
liquid in the swollen region is rich in sugars and organic substances
sucrose cannot be transported past this point in the ring
some non-photosynthetic tissues in region below ring are seen to wither and die, whilst those above the ring continue to grow

Question 17

a plant has its phloem tissues removed in a ring around the stem, what happens to the tissues above and below the ring and why

Answer 17

sugars accumulate above the ring causing swelling, while tissues below the ring may die due to lack of glucose for respiration- this shows phloem is responsible for translocation

Question 18

why does translocation require energy even though substances are moving in solution

Answer 18

because active transport is needed to load sucrose into the phloem at the source, against the concentration gradient

Question 19

in an experiment, aphids feed on phloem and their stylets are left in place after removal. what does analysis of the sap show

Answer 19

it contains sugars like sucrose, showing that phloem sap is rich in assimilates being transported through the plant

Question 20

how can a radioactive tracer be used to show the movement of sugars in translocation

Answer 20

the labelled carbon is incorporated into glucose during photosynthesis. the movement of labelled sugars can be tracked through autoradiography, showing translocation through the phloem

Question 21

why do companion cells have many mitochondria

Answer 21

to provide ATP for active transport of sucrose into sieve tube elements during phloem loading

Question 22

how would cooling a plants stem affect translocation

Answer 22

it would slow down enzyme activity and ATP production, reducing active transport of sugars into the phloem and slowing translocation

Question 23

explain how companion cells are adapted for their role in the translocation of sucrose in phloem tissue

Answer 23

companion cells are closely associated with sieve tube elements and are essential for loading and unloading sucrose during translocation. they have a high density of mitochondria to provide ATP for active transport of hydrogen ions out of the cell. this allows sucrose to be co transported back in via co transporter proteins. the membrane between the companion cells and sieve tube elements allow the transfer of sucrose into the phloem. additionally, they have large surface areas and many ribosomes to produce the proteins needed for membrane transport and phloem function. their active role is maintaining the pressure flow to ensure efficient translocation of assimilates from source to sink

Question 24

radioactive isotope to trace translocation
describe the experimental steps and explain what results would support pressure flow hypothesis

Answer 24

the researcher allows a plant to photosynthesise in an environment containing C14 C02. the plant incorporates the carbon into glucose, which is converted into sucrose and loaded into the phloem. after a period, the plant is frozen and sectioned. autoradiography is used- photographic film placed over the sections shows black areas where the C14 is concentrated. if the pressure flow hypothesis is correct, the labelled sucrose should appear in the phloem tissue and move from source to sink overtime.

Question 25

describe how sucrose is actively loaded into the phloem and how this generates pressure for mass flow

Answer 25

sucrose is actively transported from mesophyll cells into companion cells using co transport. first, hydrogen ions are pumped out of companion cells into the surrounding tissues using ATP. this creates a concentration gradient. hydrogen ions then move back into the companion cells via co transporter proteins, carrying sucrose with them against its concentration gradient. the sucrose then moves into the sieve tube element via membrane. this reduces water potential in the phloem, causing water to enter by osmosis from the xylem. the resulting hydrostatic pressure pushes the phloem sap toward areas of lower pressure (sinks) where sucrose is unloaded and water potential increases, allowing water to exit. the pressure difference drives the mass flow of assimilates throughout the plant

Question 26

A student cuts a stem and observes that sap oozes out of the phloem under pressure. How does this support the pressure flow hypothesis?

Answer 26

The pressure flow hypothesis states that translocation is driven by a pressure gradient from source to sink. The observation that sap oozes out when the phloem is cut shows that there is a positive pressure inside the phloem vessels. This pressure is created by the active loading of sucrose at the source, which draws in water by osmosis, increasing turgor pressure. The movement of sap upon cutting confirms that the phloem is under pressure, supporting the idea of mass flow driven by hydrostatic pressure gradients.