Transcript Sugar Transport - mvhs

Sugar Transport
AP Biology
Unit 5
Sugars in a plant
• Plants do cellular respiration AND
photosynthesis
• The sugar produced in photosynthesis is
then transported to other cells
– For storage
– For cellular respiration
Phloem
• The function of the phloem
is to transport organic
nutrients (sugars) through
the plant.
• Cells that make up the
phloem:
– Sieve Tube
Members/Elements
– Companion Cells
Formation of the Phloem
1. Plasmodesmata (junctions between
cells) enlarge to form pores
2. Membrane of central vacuole
(tonoplast) disappears in sieve tube
members
3. Nucleus degrades. Companion cell
supports cellular function.
* Because the sieve tube members are
hollow, they need the companion
cells to sustain them.
Translocation
• Transport in the phloem = translocation
• The phloem transports substances from
“sources” to “sinks”
– Source = where sugar is produced/broken down
– Sink = where sugar is used/stored
• Translocation occurs due to pressure flow.
Pressure Flow
•
The solution in the phloem
is a combination of sugar
and H2O
1. Sugar is actively
transported into the sieve
tube elements from the
source cell
2. Water from xylem flows
into the phloem by osmosis
–
Additional water + sugar =
higher pressure region
Pressure Flow (continued)
3. At the phloem near the sink cell,
sugars are actively transported
into the sink cell from the
phloem  lower pressure region
4. Due to the lower solute
concentration in the phloem near
the sink, H2O flows back into
the xylem.
5. Solution in the phloem flows
from HIGH pressure (near
source) to LOW pressure.
Translocation
• Transport in the phloem will stop if the
phloem cells are killed.
• Translocation can occur both up and down
the stems and petioles