Topic 9.2

Question 1

What is the phloem?

Answer 1

a vascular tissue that transports sugar from the source to the sink.

Question 2

What does the phloem consist of?

Answer 2

living cells called sieve tubes, each cell in the sieve tube is called a sieve tube element

Question 3

What do sieve tube elements lack?

Answer 3

cytoplasm, mitochondria and other organelles.

Question 4

We can tell that sieve tube elements need ‘life support’ from …

Answer 4

companion cells

Question 5

Where can the companion cells be found?

Answer 5

next to each sieve tube element

Question 6

what connects the sieve elements and companion cells?

Answer 6

plasmodesmata

Question 7

What are the sieve tube elements filled with?

Answer 7

phloem sap

Question 8

what is a sieve plate?

Answer 8

walls that have pores, which help connect sieve elements.

Question 9

Generally, describe the structure and the function of the phloem.

Answer 9

reduced organelles in sieve elements, companion cells, plasmodesmata, sieve plate, cell membrane.

Question 10

What is a source?

Answer 10

a part of the of the plant that exports sugar to other parts of the plant (sinks).

Question 11

What are sinks?

Answer 11

part of the plant that cannot produce sugars, thus the source transports it to the sink

Question 12

What is translocation?

Answer 12

the process of transporting sugars from the source to the sink.

Question 13

Discuss the process of translocation.

Answer 13

1) Sugar is made by the source (in photosynthesis or released from storage organs)
2) Sucrose is actively transported into the companion cells.
3) Sucrose diffuses through the plasmodesmata and into the sieve tube elements.
4) Solute concentration increases in the sieve tube.
5) water (from the xylem) will go into the sieve tube via osmosis.
6) There will be high hydrostatic pressure at the source.
7) which will cause water and dissolved sugar to be pushed towards source.
8) when the dissolved water and sugar reaches the sink, the sugar will be unloaded at the companion cells (actively or passively).
9) the solute concentration will decrease at the sink.
10) causing the water to go back to the xylem (via osmosis).

Question 14

What are aphids?

Answer 14

small sap sucking insects.

Question 15

How do aphids get to the phloem sap?

Answer 15

they use their long stylets (mouth parts that are sharp) to pierce the phloem tube without triggering defensive responses in the plant.

Question 16

if I were to cut the aphids stylet while it was feeding on the phloem sap, what would happen?

Answer 16

the aphid will die, and the phloem sap will continue to flow out (because of high hydrostatic pressure in the sieve tubes)

Question 17

Discuss the aphid stylets and radioactive CO2 experiment.

Answer 17

1) plants that have 1 leaf are trapped with radioactive CO2.
2) Leaves will use the radioactive CO2 to perform photosynthesis, thus also producing radioactive sucrose.
3) sucrose will be transported to different parts of the plant.
4) The aphides will feed on the phloem sap at different locations along the stem.
5) Stylet is cut off in order to collect plant sap, to see if there is radioactive sucrose.

Question 18

What is the equation for the rate of translocation

Answer 18

Distance travelled / time