mass transport in plants

Question 1

what does the xylem transport?

Answer 1

water and mineral ions

Question 2

what does the phloem transport?

Answer 2

dissolved sugars

Question 3

what is transpiration?

Answer 3

movement of water through the xylem
starts with the evaporation of water down the water potential gradient

Question 4

what is translocation?

Answer 4

movement of solutes like dissolved sugars through the phloem

Question 5

what is the structure of the xylem?

Answer 5

long tube like structures, made from dead elongated cells joined end to end
there are no end walls

Question 6

in what direction does the movement of water occur?

Answer 6

only upwards

Question 7

in what direction does the movement of dissolved sugars occur?

Answer 7

both up and down

Question 8

how does cohesion and tension occur?

Answer 8

water is lost via evaporaiton at the leaves, this creates TENSION and more water is pulled into the plant via the roots
water molecules are COHESIVE and stick together so a whole collumn of water moves upwards from the roots to the leaves

Question 9

what factors affect the rate of transpiration?

Answer 9

light intensity
temperature
humidity
wind

Question 10

how does light intensity increase the rate of transpration?

Answer 10

the stomata are open when it gets light to let in CO2 for photosynthesis so more water is able to leave via the open stomata

Question 11

how does temperature increase the rate of transpiration?

Answer 11

the higher the temperature, the more kinetic energy the water molecules have so the more water molecules evaporate off the leaves of the plant and increases the water potential gradient

Question 12

how does humidity increase the rate of transpiration?

Answer 12

a LOWER humidity increases the rate of tranpiration as the air is more dry so the water potential gradient between the leaf and the air is increased

Question 13

how does the wind affect the rate of transpiration?

Answer 13

the windier it is the increased rate of transpiration, as the air movement blows water molecules off of around the stomata so the water potential gradient is increased and rate of transpiration increases

Question 14

what is the structure of the phloem like?

Answer 14

cells arranged in a tube structure, with sieve tubes and a companion cell for each sieve tube which carries out the living functions for the sieve tube cells

Question 15

what is a ‘source’ and a ‘sink’ in translocation?

Answer 15

a source is where the solutes are produced (high concentration)
a sink is where the solutes are used up (lower concentration)

Question 16

what is the mass flow hypothesis?

Answer 16

1. the companion cells load sucrose into the phloem sieve tubes using active transport
2. this decreases the water potential in the sieve tubes where sugars are made
3. water moves into the sieve tubes by osmosis from the companion cells and adjacent xylem
4. this increases the hydrostatic pressure in the phloem
5. at the sink, the sucrose moves out of the phloem by facilitated diffusion and water follows by osmosis meaning there is a lower hydrostatic pressure at the sink
6. this results in the mass flow of the sieve tube contents from the source to the sink

Question 17

why are water molecules cohesive?

Answer 17

because they are polar so hydrogen bonds form between the water molecules

Question 18

what does adhesion mean?

Answer 18

the water molecules are attracted to lignin, so pull in the xylem walls

Question 19

what is the cohesion tension theory?

Answer 19

1. water evaporates from the the leaves at the top of the xylem
2. this creates tension which pulls more water into the leaf
3. water molecules are cohesive so when some are pulled into the leaves, others follow
4. adhesion means the water molecules can pull the xylem inwards as the water molecules move up
5. this means the whole collumn of water moves up
6. more water enters the the stem through the roots