3.3.4.2- Mass Transport in plants

Question 1

translocation- why is hydrostatic pressure lower at sink than source? (3)

Answer 1

1) sucrose is used in respiration at the sink, or stored as insoluble starch
2) so more sucrose AT > sink cell, causing decrease in WP
3) Osmosis of water from STE> sink cell, decreasing w volume & Hydrostatic pressure in STE

Question 2

Translocation process (4)

Answer 2

1- sugars created by Phs diffuse down conc gradient to companion cells via FD
2- Co-transport of sucrose with H+ ions> sieve tube element
3- Increase of s in STE, lowers WP; water enters p from x by osmosis
4-Increase in water volume in STE increases HYDROSTATIC pressure, causing liquid to be forced down> sink (where HP lower)

Question 3

Source > sink explanation

Answer 3

1. source cell= photosynthesising leaf cell (has low WP due to sucrose from PhS & Increased HP)
2. sink cell= respiring cell (Has high WP as sucrose used in r,& decreased HP)
3. Pressure difference causes movement of sugars from source> sink

Question 4

2 key cells in phloem tissue- for translocation

Answer 4

1) sieve tube elements- living, no nucleus, few organelles

2) companion cells- provide ATP for AT of organic substances

Question 5

movement of water up xylem/ transpiration (5)

Answer 5

1) water vapour evaporates out of stomata on leaves; creates lower p
2) More water pulled up xylem to replace it (moves due to negative pressure)
3) Due to H bonds between water molecules they are COHESIVE, creating a column of water within xylem
4) water molecules also adhere to walls of xylem, helping to pull water columns upward
5) As column pulled up x, it creates TENSION, pulling the xylem in to become narrower

Question 6

3 aspects of Cohesion-tension theory=

Answer 6

1) Cohesion
2) Adhesion
3) Root pressure

Question 7

MF- Transpiration- Cohesion- Tension processes (1+3,2,3

Answer 7

1.-water is DIPOLAR (slightly - O, slightly + H2)
-this enables H bonds to form between H & O of different water molecules
-This creates cohesion BETWEEN water molecules
-Thus water travels up xylem as continuous water column
2-Water adheres to xylem walls increasing movement up
3-As water moves into roots by o, it increases V of liquid in root so pressure in root increases; this p increase forces water above roots upwards

Question 8

Factors that affect the rate of transpiration (4)

Answer 8

1) Light intensity (H>more stomata open> larger SA for evaporation> more T)
2) Temperature (H>more ek>F movement of molecules> more evaporation> more T)
3) Wind (H>more humid air & its wv blown away> WP gradient maintained)
4) Humidity (M>higher WP outside leaf>lower WP gradient>less T)