3.4.9- Phloem Flashcards
What structure is found at either end of the sieve tube elements in the phloem?
Sieve plates
What is the role of sieve plates?
Allow sap movement
Describe how a high pressure in produced in leaves (3)
- Water potential becomes lower/ more negative (as sugar enters phloem)
- Water enter phloem by osmosis
- Increased volume of water causes increased pressure
During their experiment the rate of photosynthesis of their plants remained constant?
Why is this important?
- Rate of photosynthesis related to the rate of sucrose production
- Rate of translocation higher when sucrose conc is higher
Some translocation occurs in the spaces in cell walls
What info supports this
- Rate of translocation does not fall to 0
2. But sucrose no longer able to enter cytoplasm of phloem cells
x is a control. Explain how the measurement obtained from this is used by the scientist (2)
- Used to compare effect of other treatments as a baseline
- Shows/ meausers effect of substance x
or
accounts for effect of substancrs made naturally
The mass flow hypothesis is used to explain the movement of substances through phloem
evaluate whether the info from this investigation supports this hypothesis (4)
In support of mass flow hypothesis
1. (F shows) phloem is involved;
2. (G shows) respiration / active transport is
involved (in flow / movement);
3. Because 4 C / cooling reduces / slows / stops
flow / movement;
4. The agar block is the source;
5. Roots are the sink;
Against the mass flow hypothesis
6. No bulge above ringing (in F);
7. No (role for) osmosis / hydrostatic pressure /
water movement;
8. Movement could be due to gravity;
9. Roots still grow without (intact/functioning)
phloem;
10. No leaves / sugars / photosynthesis to act
as a source;
what can you conclude from treatments X and Y about root growth?
- Substance x is not needed for some root growth
- X moves though plant
- E shows x increases root growth
Describe the process of a ringing experiment and how it can provide evidence for the mass flow theory of translocation in plants ? (5)
Bark is removed in a ring (from tree trunk) containing the phloen (but not the xylem)
Solutes cannot move up or down (due to removal of phloem)
Bulge forms above ring
Fluid above the ring has more solutes than below
Shows that solutes are moving down
Describe the Mass flow hypothesis (11)
At the source there is a high conc of solute (e.g. Sucrose)
Solute actievly transported into sieve tube element from companion cell
Lowers WP
Water moves in by osmosis from companion cell and xylem
Creating high hydrostatic pressure in the phloem
The sink uses the solute
Creating low conc of solute
Increases wp
water moves out by osmosis
Decreases hydrostatic pressure in the phloem
solutes therefore moved from source to sink
Some plants produce a compound that inhibits alpha amylase in insects
Suggest how this compound helps to defend the plant against insects(3)
Amylase enzymes are needed to hydrolyze starch into smaller pieces
So they can be digested
Plant will become less valuble food source so will stop feeding on it
Explain how water potential inside the sieve tubes changes between sites 2 and 3 (4)
sugars from the leaves are loaded into the sieve tubes near site 2
meaning the water potentila is lower at this site
the water potential increases as the phloem sap moves towards site 3
becasue the solute sugars are being removed from sieve tubes to be used by cells
The volume of sap collected at site 2(near leaves ) was greater than site 1 (lower) suggest why
(4)
The stem at site 2 is nearer to leaves than site 1
thereofe more phloem sap at site 2 is going to have sugars
this causes more water to move into sieve tubes from the xylem and companion cells by osmosis
results in pressure of the phloem sap at site 2 higher which forces a greater volume out
