Day 4 Sugar Transport in Phloem

What is translocation

the movement of sugars by bulk flow in sap

Where are sugars made

photosynthesizing organs, primarily leaves

• (These are sources)

Where are sugars transported

to organs that do not photosynthesize — young leaves, flowers, roots

(These are sinks)

What does sap carry

some minerals, amino acids, hormones, etc.

Whats the pressure flow hypothesis and how is it generated

sugars move from sources to sinks along a water potential gradient

• potential is generated osmotically by manipulating solutes

What are the tasks at the source end of translocation

1. move sugars into phloem

2. generate high water potential in phloem

What is phloem loading

moving sugars into the phloem

What are the tasks at the sink end of translocation

1. move sugars out of the phloem

2. generate low water potential in phloem

Whats phloem unloading

moving sugars out of phloem

Mechanism of phloem loading

1. move sucrose into sieve tube elements

2. increased concentration draws water through osmosis

3. influx of water increases hydrostatic pressure

Is the initial moment of sucrose passive or active

can be passive or active transport

What allows sucrose to move between cells

plasmodesmata

Whats bulk flow of sap

• sugars are dissolved in water (=sap)

• hydrostatic pressure gradient causes bulk flow from sources to sinks

• sugars removed from sap at sinks

• water is pulled back into the xylem by a low potential gradient by transpiration

When is sugar removed from sap

at the sinks

Phloem unloading occurs for…

1. powering metabolism

2. storage

Is phloem unloading passive or active

can be passive or active transport

When does water return in phloem unloading

water returns to the xylem as the osmotic potential in the phloem drops

Seasonal changes effects on sugar transport

sources and sinks can change with the seasons

Sugar transport in growing season

• making, using, and storing sucrose

• sucrose produced by photosynthesis moves to areas where needed

Where is the sugar stored and what is it stored as

storage areas: roots, stems, leaves, and fruits

• stored as starch

Sugar transport in winter and early spring

• using the stored sucrose

• stored sucrose moves to developing shoots or roots

• use of xylem for sugar transport at this time

• can be tapped (get maple syrup)