translocation

Question 1

what does the phloem transport

Answer 1

transports organic molecules for eg sucrose
the solutes are loaded in the leaves and taken to other parts where they are used for energy requiring processes such as growth

Question 2

describe the structure of the phloem

Answer 2

made up of living cells which allow them to produce atp for sucrose transport
sieve tube elements - living cells which no nucleus and just a few organelles so less resistance to flow
their end walls have perforations in them called sieve plates
companion cells- very active cells next to sieve tubes connected by plasmodesmata . they have lots of mitochondria to provide energy to the sieve tubes for active movement of sucrose

Question 3

explain mass flow hypothesis

Answer 3

1. Source produces glucose (by photosynthesis)
   which is then converted to sucrose.
2. Sucrose solution is actively transported into the phloem (using ATP from companion cells)
3. Water potential in the phloem decreases. Water moves in from xylem by osmosis - this increases the volume in the phloem.
4. Hydrostatic pressure builds up, forcing the sucrose solution along the phloem by mass flow to respiring cells or storage organ (from a high to a low pressure).
5. At the sink sucrose moves from the phloem into the sink cell by active transport. This lowers the sinks water potential so water moves down a gradient into the sink.
6. Water re-enters the xylem by osmosis.
   Sucrose is used in respiration or stored.

Question 4

what is needed for the mass hypothesis to be true

Answer 4

downward unidirectional flow
hiigh to low pressure
sucrose moves from source to sink
process is active

Question 5

ringing experiment

Answer 5

bark containing the phloem can be removed from stems to prevent movement of organic substances
a bulge then forms above ring
the fluid has a higher conc of sugar than blow
swelling is caused by build up of sugar solution . it cannot flow any further because sugar solution is transported in phloem which has been removed
as the ring is happening on one side of the bark it shows that flow ie happening in one direction

Question 6

use of aphids

Answer 6

aphids perice the phloem at the top and bottom of stem
then remove the aphid just leaving the mouth part. sap flows out faster at the top indicating a pressure gradient
this shows there is a high pressure at the top . greatest at the leaf which supports mass flow hypothesis

Question 7

autoradiography

Answer 7

can be used to track organic substances
use co2(14) which is converted into sugars and detect using a photographic film
explanation- the radioactive carbon in carbon dioxide is incorporated into sucrose . this occurs at the leaf (source) . from here the sucrose is transported towards the roots which occurs in the sink.
this provides movement from source to sink

Question 8

metabolic inhibitors

Answer 8

they stop ATP production by respiration or cooling to reduce the rate of respiration
if you put these in the phloem then translocation stops which shows that ATP is needed for the loading of organic molecules into the phloem by active transport

Question 9

what evidence contradicts mass flow hypothesis

Answer 9

Different organic substances travel at different rates in the phloem (e.g. amino acids travel more slowly than sucrose). If it was just mass flow all molecules would be forced to flow at the same rate as the fluid is pushed from a higher to lower pressure.
Different substances move in opposite directions in the same sieve tube - showing bidirectional flow, something not possible with just mass flow (higher to lower pressure)
• Sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.