mass transport in plants

Question 1

describe the structure of the xylem

Answer 1

dead, hollow cells with no end walls → no organelles and continuous tube with little resistance

walls thickened with lignin → strengthens and supports against tension

pits → allow lateral diversion when blocked

Question 2

define transpiration

Answer 2

loss of water vapour from mesophyll cells through the stomata

Question 3

describe the cohesion tension theory of transpiration

Answer 3

transpiration occurs in leaves

water potential gradient forms across the leaf

negative pressure / tension created, pulling water up

cohesion between water molecules maintains continuous columns and adhesion between water and xylem creates and inward pull, reducing diameter

Question 4

what factors affects transpiration

Answer 4

light intensity – increases stomatal opening, increasing transpiration

temperature – increase kinetic energy increasing evaporation and diffusion rates
humidity – humidity reduces water potential gradient

wind speed – wind removes water vapour near the stomata, increasing water potential gradient and transpiration rate

Question 5

how would you measure the rate of transpiration using a potometer?

Answer 5

1. cut plant underwater (prevent air entering the xylem)
2. ensure the apparatus is airtight (sealing with petroleum jelly)
3. form air bubble into the capillary tube and record its movement over time using a ruler.
4. rate = distance moved / time taken

Question 6

what is the transpiration stream?

Answer 6

movement of water through the xylem and mesophyll cells

Question 7

describe the structure of the phloem

Answer 7

• sieve tube elements with few organelles → living cells joined end to end, for little resistance
• sieve plates → contain lots of holes so solutes can pass through
• associated companion cells → contain nucleus and mitochondria for active loading of sucrose into phloem
• plasmodesmata → connects cells for communication

Question 8

describe the mass flow hypothesis

Answer 8

1. at sink; active transport loads sucrose from mesophyll cells to sieve tube elements in phloem via companion cells
2. water potential in phloem decreases → water moves in via osmosis from xylem
3. volume increases so hydrostatic pressure increase
4. at source; active transport unloads sucrose into the sink via companion cells
5. water potential in phloem decreases → water moves by osmosis back into the xylem
6. volume decreases so hydrostatic pressure decreases

hydrostatic pressure gradient created between source and sink / mass flow of sucrose down a pressure gradient

Question 9

what evidence is there for the mass flow hypothesis?

Answer 9

radioactive tracers (carbon-14)
plants radioactively labelled CO₂ → shows that organic substances move through the phloem

ringing experiments
removing a ring of bark (containing phloem) causes sugar to accumulate → supports downward flow of solutes.

Question 10

what evidence is there against the mass flow hypothesis?

Answer 10

sieve plates
their function is undetermined → surely they would impede movement

different solutes move at different speeds
why?