3- Mass transport in plants

Question 1

Xylem structure

Answer 1

• Dead tissue / no organelles
• Hollow - minimal resistance to flow of water and ions
• Long tubes - water and ions can be transported over long distances
• Pits - allows lateral movement of water and ions to adjacent vessels
• Lignin - strengthens cell wall to make it more rigid providing support to the xylem and makes it impermeable to water

Question 2

Function of phloem

Answer 2

-Transports amino acid for protein production
-Transports fatty acids and glycerol for phospholipid production
-Transports to growing areas
-Transports to storage areas
-Transports sugar to cells that don’t photosynthesise

Question 3

Structure of phloem

Answer 3

• Made of sieve tubes made of sieve elements
• Few sieve plates to prevent restriction to flow of sugar
• Few organelles at the periphery of sieve tubes to prevent obstruction to flow of sugars
  -Each sieve tube has companion cell beside it to carry out living functions for the sieve element cells (many mitochondria which respire to release energy for the AT of sucrose into phloem)

Question 4

Cohesion theory

Answer 4

TTUCA
T- Transpiration- water evaporates from leaves from the stomata decreasing water potential in leaves
T- Tension -This draws water from xylem into leaf down water pot grad creating tension in xylem
U- Up - This tension pulls water up -
C -Cohesion water is pulled up as a column of water because water molecules are joined together by cohesive hydrogen bonds
A -Adhesion - Water molecules are maintained by adhesion - water molecules are attracted to xylem wall

Question 5

Factors affecting rate of transpiration

Answer 5

Environmental:
- Light
- Humidity
- Air movement
- Temperature
Others:
- No. of stomata
- No. of leaves
- SA of leaves

Question 6

Mass flow hypothesis

Answer 6

1 - Glucose is produce by photosynthesis in leaf cells then joined with fructose in condensation reaction, forming a glycosidic bond to produce sucrose
2 - Sucrose moves from leaves to phloem down conc grad by AT using energy from hydrolysis of ATP.
3 - High conc of sucrose at top end of phloem lowers water pot so water moves in by osmosis through pits of xylem causing a high hydrostatic pressure
4 - Pressure gradient is formed from high to low end which pushes sugars along sieve tubes towards the sink- This is called mass flow/ translocation
5 - At lower end, sugar moves by FD into sink increasing water potential in phloem so water moves back into xylem by osmosis lowering hydrostatic pressure in the xylem.

Question 7

What is a source and a sink

Answer 7

• Source - A part of plant where sugars are made
• Sink - A part of plant that needs sugars

Question 8

Evidence for mass flow in the phloem

Answer 8

• Ringing experiments
• Radioactive tracers
• Aphid mouthparts
• Respiratory inhibitors