Plant Transport

Question 1

Structure of Xylem

Answer 1

• Two types of cells -> tracheids and vessel elements
• Dead, containing no cytoplasm
• Cells have secondary cell wall made out of lignin and cellulose microfibrils

Question 2

Features of Tracheids

Answer 2

• Long, slender
• Tapered ends
• Lignified secondary cell wall -> main function of structural support
• Pits in cell wall and ends

Question 3

Features of Vessel Elements

Answer 3

• Short and wide
• Pits in cell wall
• Perforated ends
• Transports most water
• Also has lignified secondary cell wall

Question 4

Function of Xylem

Answer 4

• Transports water and minerals and ions dissolved in water
• By simple diffusion/osmosis
• From roots to transpiring leaves

Question 5

Structure of Phloem

Answer 5

• Two types of cells -> sieve-tube elements and companion cells
• Alive at maturity, no 2nd cell wall

Question 6

Features of sieve-tube elements

Answer 6

• Long
• Lack cellular content to conduct nutrients effectively
• Perforated ends called ‘sieve plates’
• Conducts all nutrients

Question 7

Features of companion cells

Answer 7

• Provides material & energy to sieve-tube
• Contains cellular content -> does not conduct nutrients

Question 8

Function of Phloem

Answer 8

• Transports water dissolved solutes (eg. sucrose)
• By active transport
• From sources (mature leaves) to sinks (growing leaves or fruits, storage roots)

Question 9

Similarities of Xylem and Phloem

Answer 9

• Both transport substances throughout the plant
• Both transport substances dissolved in water

Question 10

What is a vascular bundle?

Answer 10

• Consists of xylem, phloem and cambium
• Surrounded by bundle sheath cells

Question 11

What are the differences in Xylem and Phloem in stems/leaves/roots?

Answer 11

Xylem -> Larger, on the inside/top
Phloem -> Smaller, on the outside/bottom

Question 12

Routes of water movement into root

Answer 12

1. Symplastic Route
2. Transmembrane Route
3. Apoplastic Route

Question 13

What is the symplastic route?

Answer 13

Moves along cell cytosol through plasmodesma channels

Question 14

What is the transmembrane route?

Answer 14

Water moves through water channels in the cell walls

Question 15

What is the apoplastic route?

Answer 15

• Water moves along cell walls
• Stopped by Casparian strip at endodermis of vascular bundle -> joins other paths

Question 16

How does root pressure cause movement of water?

Answer 16

1. Root hair cells absorb ions from soil
2. Ions pumped into xylem by active transport -> lowers water potential of xylem
3. Water moves from higher water potential root cells into xylem via osmosis

Question 17

How does capillary action cause water movement?

Answer 17

1. Works through three forces -> adhesion, cohesion and surface tension
2. Adhesion -> attraction between water and cell walls of xylem
3. Cohesion -> attraction between water molecules
4. Surface tension -> force betwen water molecules at an air surface

Question 18

What is transpiration?

Answer 18

Loss of water as water vapour from aerial parts of plant:
1. Water from xylem moves through spongy mesophyll cells -> evaporates as water vapour into air spaces
2. Diffuses from air spaces through stomata to surroundings
3. Loss of water vapour drives greater evaporation

Question 19

What is transpiration pull?

Answer 19

1. Evaporation of water in leaves -> creates tension trasmitted from roots to leaves
2. Tension pulls water from roots into xylem and up xylem into leaves

Question 20

How are sugars transported?

Answer 20

• Movement of sugars by bulk flow from source to sink
• Source -> sugar enters phloem
• Sink -> sugar exits phloem
• Sources and sink may vary during seasons

Question 21

What is the bulk flow at source?

Answer 21

1. Surcrose from source pumped into phloem by mainly active transport or simple diffusion
2. High solute concentration in phloem sap -> lower water potential
3. Water moves from xylem into phloem -> increases pressure

Question 22

What is bulk flow at sink

Answer 22

1. Sucrose from phloem moves into sink cells by active transport/simple diffusion
2. Low solute concentration -> high water potential
3. Water moves from phloem into xylem -> decreases pressure

Question 23

How does translocation occur?

Answer 23

Difference in pressure at source at sink drives sucrose in phloem from source to sink by bulk flow