Xylem and phloem

Question 1

What is the structure of xylem?

Answer 1

Hollow and elongated
Parenchyma
Arranged end to end
Called an element because it's dead
Tracheids
Xylem vessels
Lignin 
Sclerenchyma fibres

Question 2

What are the functions of xylem?

Answer 2

Transport water/dissolved substances
Hollow/elongated - reduce flow resistance
Parenchyma - support
Lignin - support/waterproof vessel
Sclerenchyma - support
Pits - support natural movement of water/allow water to leave

Question 3

What are the 4 types of lignin?

Answer 3

Annular (rings)
Spiral
Pitted (holes)
Reticulate (dashed holes)

Question 4

Describe tracheids

Answer 4

In all plants, long and thin, tapered end walls, thickened with lignin, has pits, when mature it is dead and hollow.

Question 5

What is the structure of phloem?

Answer 5

Hollow and elongated
Parenchyma 
Arranged end to end
Sieve tube elements
Companion cells

Question 6

What is the structure of sieve tube elements?

Answer 6

Elongated, joined end to end, long tubes, living, thin cytoplasm wall/peripheral, no nuclei, few organelles

Question 7

Why are companion cells needed?

Answer 7

They contain all the organelles needed for the the sieve tube element to survive.
Contain lots of mitochondria to provide ATP.

Question 8

What is the function of phloem?

Answer 8

Transport organic material, such as sugars, in solution
Hollow and elongated - reduce flow resistance
Parenchyma - support
Thin cytoplasm wall/peripheral and few organelles to reduce resistance to flow.
Mitochondria - provide ATP needed for active transport

Question 9

Where does loading occur in translocation?

Answer 9

At the source (eg leaves)

Question 10

What are sources areas of?

Answer 10

Photosynthesis

Assimilates are moved from here

Question 11

Where does unloading occur in translocation?

Answer 11

At the sink (eg roots, flowers, fruits)

Question 12

What are sinks areas of?

Answer 12

Respiration

Where the assimilates are moved to

Question 13

What 3 processes happen at the source?

Answer 13

Active transport
Facilitated diffusion
Osmosis

Question 14

What are all the steps of translocation at the source?

Answer 14

H+ ions are removed by active transport (that’s why there is so many mitochondria)
Now there is a low concentration of H+ ions
So it cotransports H+ ions using a symport protein, taking sucrose with it.
High concentration of sucrose in companion cell means sucrose diffuses into sieve tube element
This lowers the water potential so water moves into the sieve tube element by osmosis

Question 15

What are all the steps of translocation at the sink?

Answer 15

Sucrose would diffuse into the companion cells at the sink until equilibrium was reached however, companion cells contain invertase which hydrolyses the sucrose into glucose and fructose, maintaining the concentration gradient so sucrose continues moving into the companion cells.

Question 16

Which 2 processes occur at the sink?

Answer 16

Diffusion and osmosis

Question 17

What is the evidence for translocation?

Answer 17

The rate of mass flow is 10,000X faster than if diffusion alone was responsible.
Phloem sap has a relatively high pH (pH8) because H+ ions are lost so it becomes more alkali.
There is an electrical difference across the membrane of the companion cell because H+ ions are charged so the voltage changes when they move.
You can see with a microscope that there are lots of mitochondria in companion cells.

Question 18

What 3 experiments are there for translocation?

Answer 18

Radioactive tracers
Ringing
Aphid styletectomy

Question 19

Explain the radioactive tracers experiment

Answer 19

Give 1 leaf a source of radioactive carbon dioxide.
This radioactivity shows up on an X ray so you can see where the carbon goes.
Shows it goes from leaf to stem to sink.

Question 20

Explain the ringing experiment

Answer 20

A ring of bark/phloem tissue is removed.

Sugars being transported down the phloem accumulate as starch above the ring.

Question 21

Explain the Aphid styletectomy experiment

Answer 21

Cut stylet of feeding aphid.
Aphid is anaesthetised with CO2 and is removed from the plant.
Pure phloem sap runs from the stylet.
Analysis of the sap shows it contains mainly sucrose with some amino acids.

Question 22

What shape are sieve tube elements?

Answer 22

Hexagonal

Question 23

What are assimilates?

Answer 23

The result of combining and converting biochemicals for use by a plant