3.3 TRANSPORT IN PLANTS

Question 1

Explain why plants need transport systems (3)

Answer 1

• There is a long distance from external surface to cells
• There is too small of a surface area:volume ratio
• Diffusion would not be fast enough

Question 2

State what two vessels make up the vascular bundle

Answer 2

• Xylem
• Phloem

Question 3

Describe structure of the phloem (2)

Answer 3

• Perforated sieve tube elements that are living, no nucleus and minimal organelles to reduce resistance to sugar flow
• Companion cells with mitochondria to provide ATP for active transport of sugars

Question 4

Describe the structure of the xylem (2)

Answer 4

• Continuous, dead, hallow collumns
• Strengthened by lignin to make it waterproof

Question 5

Define transpiration in terms of water vapour potential

Answer 5

• Evapouration of water through the stomata
• From an area of higher water vapour potential (inside stomata) to an area of lower water vapour potential, down a water vapour potential gradient

Question 6

State 3 importances of transpiration

Answer 6

• Transports useful mineral ions from soil up the xylem
• Maintains plant turgidity
• Provides water for growth/photosynthesis

Question 7

State the 5 factors affecting rate of transpiration

Answer 7

1) Temperature
2) Light intensity
3) Humidity
4) Air movement
5) Water availability

Question 8

Explain how higher temperature affects transpiration rates

Answer 8

• Increases transpiration rate
• Because more kinetic energy so increased rate of evapouration and higher water vapour potential inside leaf

Question 9

Explain how higher light intensity affects transpiration rates

Answer 9

• Increases transpiration rate
• Results in stomata being more open so gaseous exchange rates increase

Question 10

Explain how higher relative humidity affects transpiration rates

Answer 10

• Decreases transpiration rate
• Reduces the water vapour potential gradient

Question 11

Explain how higher wind movement affects transpiration rates

Answer 11

• Increases transpiration rate
• Higher water vapour potential gradient

Question 12

Explain how low water availability affects transpiration rates

Answer 12

• Stops transpiration
• Stomata closes as water lost cannot be replaced if there is none in soil

Question 13

When using a bubble potometer, why is the root of the plant cut underwater and at an angle?

Answer 13

• To prevent air bubbles entering xylem
• To provide a large surface area in contact with water

Question 14

When using a bubble potometer, what is the rate of transpiration formula?

Answer 14

Question 15

State and describe the two pathways water can take from root to xylem

Answer 15

1)Symplast pathway (through cytoplasm of cells)
2)Apoplast pathway (through cell walls of cells)

Question 16

Explain how water moves through the symplast pathway

Answer 16

• Each successive cells cytoplasm has a lower water potential so water can move through osmosis

Question 17

Explain how water moves through the apoplast pathway

Answer 17

• Cohesion allows molecules of water to move in a continuous stream to ensure mass flow
• Until water reaches the endodermal casparian strip which blocks apoplast pathway so forced to take symplast pathway
• fastest path as little resistance to water in cell walls

Question 18

Define casparian strip

Answer 18

• Impermeable barrier on endodermis blocking the apoplast pathway

Question 19

Explain the effect of the casparian strip on water moving to xylem vessel

Answer 19

• Impermeable
• Forces apoplast pathway through plasma cell surface membranes instead of cell walls
• Increases root pressure
• Controls dissolved mineral ions that enter

Question 20

State and explain 3 things that allow water to mass flow up the xylem

Answer 20

• Root pressure thanks to casparian strip forces water into xylem
• Cohesion creating transpiration stream via hydrogen bonds
• Adhesion which narrows xylem walls so more water molecules can adhere to move against gravity

Question 21

Define xerophyte with two examples

Answer 21

• Cacti, maram grass
• Plants adapted to dry conditions by maximising water conservation

Question 22

Define hydrophyte with an example

Answer 22

• Water lily
• Plants adpated to freshwater by maximising gaseous exchange

Question 23

State and explain three adaptations of xerophytes

Answer 23

1) Leaves are rolled up to trap transpired water thus increasing humidity to reduce transpiration rate
2) Thick waxy cuticle on upper epidermis of leaf to reduce evapouration
3) Stomata on lower epidermis of leaf so protected by air space

Question 24

State and explain three adaptations of hydrophytes

Answer 24

1) Thin, flat leaf with airspace to keep afloat so can absorb sunlight
2) Stomata on upper epidermis of leaf to allow gaseous exchange
3) Stem is short with many airspaces to allow rapid diffusion of oxygen for aerobic respiration

Question 25

Define plasmodesmata

Answer 25

• the gaps in the cell walls that connects two cells

Question 26

State the source and sink in translocation

Answer 26

• Source is site of sucrose production so the photosynthesising cell in leafs
• Sink is site of sucrose use so the respiring cell in roots

Question 27

Define translocation

Answer 27

• the movement of soluble organic substances in the phloem due to the difference in hydrostatic pressure between the source and sink end of the sieve tube element (the hydrostatic pressure gradient)

Question 28

Explain the steps of translocation -phloem loading

Answer 28

• Glucose converted to sucrose in photosynthesising source cell
• High concentration of sucrose diffuses into companion cells via facilitated diffusion
• Apoplast pathway
• Proton pump in companion cell membrane active transports H+ ions from companion cell cytoplasm to cell wall
• H+ concentration gradient produced (higher in cell wall, lower in cytoplasm)
• Protein co-transporter moves H+ ions and sucrose back across to companion cell cytoplasm
• Sucrose into sieve tube element of phloem through plasmodesmata
• Water potential of phloem sieve tube element decreases as sucrose enters
• Water enters phloem from xylem via osmosis
• Hydrostatic pressure of phloem sieve tube element increases
• Solution is forced towards sink cells

Question 29

Explain the mass flow hypothesis

Answer 29

• Sucrose produced in photosynthesising source cell lowers water potential of source cell
• Water moves into source cell by osmosis
• Hydrostatic pressure increases in source cell
• Sink cell uses up sucrose for respiration so increases water potential for sink cell
• Water leaves the sink cell by osmosis
• Hydrostatic pressure decreases in source cell
• Hydrostatic pressure gradient created between source and sink (higher HP in source, lower HP in sink) forces solution to sink

Question 30

Explain the steps of translocation -phloem unloading

Answer 30

• Sucrose diffuses into sink cells from phloem sieve tube
• Sucrose is converted back to glucose (for respiration) or stored as insoluble starch (for storage)
• Loss of sucrose into sink cells increases water potential of phloem sieve tube
• Water leaves phloem to xylem via osmosis
• Hydrostatic pressure of phloem decreases