Translocation

Question 1

What are the products of photosynthesis?

Answer 1

triose phosphate -> glucose

Question 2

What is triose phosphate used for?

Answer 2

To build nucleic acids
Amino acids
Sugars
Lipids

Question 3

Where are nucleic acids etc needed?

Answer 3

The shoots/roots/fruits of the plants

Question 4

What is the source of a plant?

Answer 4

the leaf

Where its made- high concentration

Question 5

What is the sink of a plant?

Answer 5

roots, shoots, flower/fruits
Where its used up- low concentration

Question 6

What is translocation?

Answer 6

the movement of solutes e.g. sugars like sucrose and amino acids to where they are required in the plant
Requires ATP
Moves solute from sources to sinks

Question 7

How do enzymes maintain a concentration gradient from the source to the sinks?

Answer 7

By changing the solutes at the sink e.g. by breaking them down
Ensures there is always a lower conc at the sink than the source

Question 8

What is a sieve tube?

Answer 8

Living cells that forms the tube for transporting solutes
They are a long cell
Have a thin layer of cytoplasm
Minimal obstructions to allow the flow of cell sap

Question 9

Phloem is on the outside or inside?

Answer 9

Outside
Xylem = inside

Question 10

Does the sieve tube have a nucleus?

Answer 10

No nucleus and has only few organelles

Question 11

Ratio of companion cells to sieve tubes?

Answer 11

There is a companion cells for each sieve tube element

Question 12

What do companion cells do?

Answer 12

Carry out living functions for sieve cells e.g. providing the energy needed for the active transport of solutes

Question 13

Do companion cells have mitochondria?

Answer 13

Yes- lots of mitochondria
Good ATP supply for active transport

Question 14

Loading sucrose into the phloem is done how?

Answer 14

By co-transport with hydrogen ions

Question 15

Co-transport of sucrose into the phloem?

Answer 15

Companion cell provides a high concentration of sucrose by active transport
H+ ions are actively transported from the companion cell cytoplasm into the cell wall using ATP
H+ ions move back into the cytoplasm- H+ ions attach to co-transport protein. Co- transport of H+ ions and sucrose from the cell wall into the companion cell cytoplasm. Sucrose moves against the concentration gradient.
High sucrose conc in companion cell. Sucrose diffuses through plasmodesmata into sieve tube.

Question 16

Mass flow?

Answer 16

high conc of sucrose as sugars are actively being transported into the phloem by companion cells- decreases the water potential of the sieve tubes- water enters the tubes by osmosis (from the xylem). This creates a high hydrostatic pressure inside the sieve tubes at the source end of the phloem.

At the sink end, there is a low sucrose conc as solutes are removed from the phloem to be used up- this increases the water potential inside the sieve tubes so water leaves the tubes by osmosis. This lowers the pressure inside the sieve tubes.

There is a pressure gradient from the source end to the sink end. Sap flows from high to low pressure towards the sink. When they reach the sink, the solutes will be used (e.g. in respiration) or stored (e.g. as starch)

Question 17

Mass flow FOR- active process? Mitochondria in companion cells?

Answer 17

Companion cells possess many mitochondria

Question 18

Mass flow: FOR (conc of sucrose into sources than sinks)?

Answer 18

Conc of sucrose is higher in the sources than in the sinks

Question 19

Mass flow- FOR: metabolic inhibitors?

Answer 19

Metabolic inhibitors (poison) and lack of O2 is put in the phloem- translocation stops- evidence that active transport is involved

Question 20

Mass flow- FOR: aphids?

Answer 20

Aphids pierce the phloem- allowing the sap to flow out. The sap flows out quicker nearer the leaves than further down the stem- evidence that there is a pressure gradient

Question 21

Mass flow: AGAINST: translocation and temperature?

Answer 21

The rate of translocation is affected by temperature

Question 22

Mass flow: AGAINST: sugar travelling to sinks?

Answer 22

Sugar travels to many different sinks- not just the one with the highest water potential (as the model would suggest)

Question 23

Mass flow: AGAINST- sieve plates?

Answer 23

The 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

Question 24

Radioactive tracers?

Answer 24

Using 14C will be pumped into a container which completely surrounds the leaf
The radioactive C is then incorporated into organic substances produced by the leaf- which will then be moved around the plant by translocation
Movement of these substances can be tracked by autoradiography- the plant is killed and is then placed on photographic film- the radioactive substance is present where the film turns black
Shows the translocation of substances from source to sink