Xylem and Phloem

Question 1

How are xylem vessels adapted to their function? (Cell type)

Answer 1

• Large cells, thick cell walls

- Form a column

Question 2

How are xylem vessels adapted to their function? (Production)

Answer 2

• Production of lignin

Question 3

What is the role of lignin?

Answer 3

• Impregnates cellulose wall, restricting solute and water entry
• Provides Stability

Question 4

What is autolysis?

Answer 4

Tonoplast breaks down. Organelles, cytoplasm, membrane are broken down by enzymes

Question 5

What happens to end cell walls in xylem?

Answer 5

• They are lost or highly perforated

Question 6

What two things give xylem strength?

Answer 6

• Cellulose microfibrils

- Lignin

Question 7

What is the casparian strip?

Answer 7

Cell wall material deposited around endodermal cells

Question 8

Why is the casparian strip different to cell walls?

Answer 8

• Made of suberin, and sometimes lignin

Question 9

What does the casparian strip do?

Answer 9

• Forces solutes and water to pass through plasma membrane via symplastic route to cross endodermic level
• Regulates water and mineral intake by roots

Question 10

Why is there low water potential at the bottom of the xylem?

Answer 10

• Loss of water by transpiration and the transpiration stream draw water up, causing low potential. Water diffuses in by osmosis

Question 11

How do root hair cells gain water?

Answer 11

• Large surface area for diffusion
• Soil water is dilute with large water potential
• Cytoplasm of root hairs have low water potential due to dissolved ions

Question 12

How does water move from root hair to xylem?

Answer 12

• Apoplast, diffusion between cell walls. Symplast, movement between cytoplasm

Question 13

Why is cytoplasm continuous?

Answer 13

Plasmodesmata, narrow fluid-filled channels maintain a continuous cytoplasm

Question 14

Where does water evaporate from?

Answer 14

Diffuses through the stomata in the leaves, then evaporated from the surface of cells lining the substomal cavity

Question 15

How does water mass flow work?

Answer 15

• Continuous column of water drawn up through column
• Capillary action due to surface tension, cohesion, adhesion
• Pulling force on water behind
• Energy comes from the sun

Question 16

What components is phloem made up of?

Answer 16

• Sieve elements
• Companion cells
• Parenchyma cells
• Fibres

Question 17

What is the role of phloem?

Answer 17

Transports sucrose and amino acids

Question 18

What is the source?

Answer 18

The cell that photosynthesyses

Question 19

How is sucrose transported into companion cells?

Answer 19

Active transport

Question 20

How does sucrose move from companion cells to sieve elements?

Answer 20

Diffusion

Question 21

What does the diffusion of sucrose into the sieve elements cause?

Answer 21

Reduction of water potential

Question 22

When there is low water potential, what occurs into the cells?

Answer 22

Osmosis

Question 23

What happens to the sieve element cell following osmosis?

Answer 23

• Becomes turgid, hard, high pressure

Question 24

What is a sink?

Answer 24

Region where sugars are respired or stored as starch

Question 25

What do sinks include?

Answer 25

Roots, tubas, actively growing buds etc

Question 26

What happens when there is a low concentration of sucrose in the sinks?

Answer 26

Sucrose passes from sieve tubes into sinks by diffusion

Question 27

Why is there lower water potential in phloem by sinks?

Answer 27

Low sucrose means high water concentration and diffuses out

Question 28

Why does sap move down the plant?

Answer 28

Higher pressure compared to low pressure means mass movement

Question 29

What is the organelle difference between xylem and phloem?

Answer 29

- Xylem is dead with no organelles - Phloem remains alive. Nucleus and most contents disintegrate, with a few organelles in a thin layer of cytoplasm. Membrane, cytoplasm, ER, mitochondira

Question 30

What is most of the space in a phloem sieve tube?

Answer 30

Liquid filled space called a lumen

Question 31

What do phloem sieve cell walls do to interact?

Answer 31

- Aligned holes, allowing transfer of material

Question 32

What is the section between two sieve plates?

Answer 32

Sieve tube element

Question 33

What do companion cells do?

Answer 33

Line the elements with more mitochondria and ribosomes, and produce and transfer materials required by sieve elements

Question 34

Why are xylem vessels narrow?

Answer 34

More contact with the water means more adhesion