Translocation

Question 1

What are the main sources in a plant? (Where sugars come from)

Answer 1

• Green leaves + green stem
• Storage organs (tubers + tap roots, e.g. potatoes)
• Food stores in seeds as they germinate

Question 2

What are the main sinks? (where sugar goes)

Answer 2

• Roots that are growing and/or absorbing mineral ions
• Meristems that are dividing
• Any part of plant laying down food stores

Question 3

What are the 2 main stages of translocation?

Answer 3

Phloem loading + phloem unloading

Question 4

What is the process of phloem loading? 1st part

Answer 4

• Sucrose enters phloem (active loading)
• Companion cells use ATP to actively transport H ions into surrounding tissue, creating diffusion gradient

Question 5

What is the process of phloem loading? 2nd part

Answer 5

• H ions return to cell via cottansporter protein that transports sucrose into dompanion cells
• Conc of sucrose in companion cells increases

Question 6

What is the process of phloem loading? 3rd part

Answer 6

• Sucrose diffuses out companion cells into sieve tube elements via plasmodesmata
• Water potential inside tube reduced
• Cuases water to enter via osmosis, increasing hydrostatic pressure of sieve tube
• Water moves down sieve tube from high to low pressure

Question 7

How are companion cells adapted for their role in active loading?

Answer 7

• Many infoldings of cell membrane to increase num of transport proteins
• High mitochondrial density to provide ATP needed for active transport

Question 8

What is phloem unloading? 1st part

Answer 8

• Sucrose removed from sieve tube by active transport/facilitated diffusion, into sinks
• Increases WP in sieve tube, + water leaves sieve tube by osmosis, reducing pressure in phloem at sink

Question 9

What is a result of the mass flow of water from the source to sink down hydrostatic pressure gradient?

Answer 9

Supplies assimilates like sucrose to where they need to be