translocation Flashcards
(26 cards)
what is translocation
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
what is a source
region of plant where sucrose is loaded into phloem e.g. by photosynthesis
what is a sink
region of plant where sucrose is removed from phloem
sugar used in either respiration or starch
three examples of substances translocated in the phloem
amino acids
mRNA
viruses
features of sieve tube elements
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
main difference between phloem and xylem vessels
xylem vessels are dead, phloem living
features of companion cells
adjacent to sieve tube elements
metabolically active: actively loads sucrose into sieve tube
mass flow hypothesis
bulk movement from high hydrostatic pressure- source to low hydrostatic pressure at sink
2 steps in mechanism for translocation
moving sucrose into sieve tube elements from source
mass flow of sucrose via sieve tube elements to sink
process of moving sucrose into sieve tube elements from photosynthetic tissue
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
process of mass flow of sucrose via sieve tube elements
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
support of mass flow hypothesis
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
against mass flow hypothesis
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
sources of evidence for translocation
radioactive tracers
ringing experiments
how are radioactive tracers used as evidence for translocation
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
how are ringing experiments used as evidence of translocation
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
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
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
why does translocation require energy even though substances are moving in solution
because active transport is needed to load sucrose into the phloem at the source, against the concentration gradient
in an experiment, aphids feed on phloem and their stylets are left in place after removal. what does analysis of the sap show
it contains sugars like sucrose, showing that phloem sap is rich in assimilates being transported through the plant
how can a radioactive tracer be used to show the movement of sugars in translocation
the labelled carbon is incorporated into glucose during photosynthesis. the movement of labelled sugars can be tracked through autoradiography, showing translocation through the phloem
why do companion cells have many mitochondria
to provide ATP for active transport of sucrose into sieve tube elements during phloem loading
how would cooling a plants stem affect translocation
it would slow down enzyme activity and ATP production, reducing active transport of sugars into the phloem and slowing translocation
explain how companion cells are adapted for their role in the translocation of sucrose in phloem tissue
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
radioactive isotope to trace translocation
describe the experimental steps and explain what results would support pressure flow hypothesis
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.
