mass transport in plants

Question 1

transpiration

Answer 1

the loss of water via evaporation from the stoma in the leaves

Question 2

transpiration stream

Answer 2

the movement of water up the plant from roots to leaves

Question 3

purpose of dead cells with no end walls and a hollow lumen in the xylem

Answer 3

water moves in a continuous column using cohesion tension theory

Question 4

purpose of lignified walls which form rings and a thick strengthened cellulose wall in the xylem

Answer 4

allows pressure to be built up and lignin is waterproof and kills the xylem cells

Question 5

purpose of having pits in the walls of the xylem

Answer 5

allows lateral movement of water

Question 6

cohesion tension theory

Answer 6

-water molecules have dipoles which can cause an attraction between them called cohesion
-there are also forces of attraction between the polar water molecules and polar groups in other groups (adhesion)

Question 7

adhesion

Answer 7

water sticks to the wall of the xylem and moves up

Question 8

movement of water through the xylem

Answer 8

-water moves from top of xylem into leaf
-next water molecule is pulled up due to cohesion
-this continues up the xylem, each water molecule pulls the next one up
-this causes a gap at the bottom of the xylem
-water from the root moves into the xylem
water entering the leaf pulls the column of water up the xylem, water is under tension

Question 9

tree trunk diameter during the day

Answer 9

-more transpiration
-more tension in the xylem
-pulls walls of xylem inwards
-causes trunk diameter to shrink

Question 10

tree trunk diameter during the night

Answer 10

-less transpiration
-less tension
-walls aren’t pulled inwards
-causes trunk diameter to increase

Question 11

what happens if damage causes air to enter

Answer 11

-the continuous column of water is broken so water molecules can’t stick together
-can no longer draw up water

Question 12

why when a vessel is broken does water not leak out but air is drawn in?

Answer 12

air is drawn in which is consistent with it being under tension

Question 13

how does temperature affect transpiration?

Answer 13

if temp increases water molecules have more kinetic energy so are more likely to evaporate

Question 14

how does humidity affect transpiration?

Answer 14

if humidity decreases the water potential will increases as there is less water outside, leading to more transpiration

Question 15

how does windspeed affect transpiration?

Answer 15

if windspeed increases it takes water vapour away from the leaf, increasing the water potential gradient

Question 16

how does light intensity affect transpiration?

Answer 16

if it is light, the stomata will be open so water is able to be released

Question 17

translocation

Answer 17

-the movement of organic molecules and some mineral ions up and down the phloems sieve tube elements from source to sink
-sugars transported as sucrose

Question 18

source

Answer 18

where organic material is made (leaves)

Question 19

sink

Answer 19

where organic material is used (rest of the plant)

Question 20

the phloem

Answer 20

-alive
-sieve tube elements
-companion cells

Question 21

mass flow hypothesis- phase 1

Answer 21

-sucrose actively moved into sieve tube element
-water follows via osmosis

Question 22

mass flow hypothesis- phase 2

Answer 22

-movement from an area of high hydrostatic pressure to an area of low hydrostatic pressure, up or down
-pressure causes movement from source to sink

Question 23

mass flow hypothesis- phase 3

Answer 23

-sucrose is actively moved out of the sieve tube element
-water follows by osmosis
-hydrostatic pressure decreases

Question 24

phase 1 evidence- aphids

Answer 24

-attach to stem using stylet
-either cut aphid from the stylet or collect excreted fluid

Question 25

phase 2 evidence- ringing experiments

Answer 25

-phloem is interrupted -sugar accumulates above ring -swelling in tree -only in woody stems as the phloem is on the outer ring

Question 26

phase 3 evidence- tracer experiments

Answer 26

-leaf grown in 14CO2 isotope atmosphere -14C incorporated into sugar made by photosynthesis -sugars containing 14C detected in sinks

Question 27

other evidence for mass flow

Answer 27

-bleeding trees -companion cells have lots of mitochondria -metabolic poisons stop translocation

Question 28

evidence against mass flow

Answer 28

-why are there sieve plates, would impede mass flow -not all solutes move at the same speed -sucrose delivered at the same rate to all regions rather than faster to the lowest concentrations