Chapter 10: Phloem Transport

What are the two transport systems of plants?

Phloem (positive pressure) and xylem (negative pressure)

Phloem transport

“source” to “sink”

Source

Net excess of nutrients (carbohydrates)

Sink

Net demand for nutrients (carbohydrates)

What type of tissue is source tissue?

Phototrophic

What type of tissue is sink tissue?

Heterotrophic

When does sucrose transport occur?

Day and night

What does the Calvin cycle lead to the production of during the day?

Glyceraldehyde-3-phosphate (G3P)

What are the two products G3P can be converted into?

• G3P → starch in the chloroplast (transient starch)

• G3P → Goes into the cytoplasm and converts in sucrose

What happens to the transient starch during the night time?

The starch is degraded and turned to sucrose in the cytoplasm

What is the phloem composed of?

Companion cells and sieve elements

Sieve elements

• Membrane lined

• Living cells

• At maturity, they lose most of their organelles including the nucleus (enucleated)

• Connected end to end through sieve plate pores to form sieve tubes

• Hollow living lumen that allow sap flow

Companion cells

• Cytoplasmically dense with ribosomes, endomembranes, chloroplasts and mitochondria

• Maintains the sieve elements

• Plasmodesmata-pore units connect companion cells to sieve elements

Phloem loading

The energized accumulation of solute to lower solute potential and increase pressure potential by osmosis (source to sink transport)

Apoplastic phloem loading

Sucrose from the mesophyll is released to the apoplast (cell wall space) and then accumulated in phloem by energized secondary transport powered by the proton motif force

What allows sucrose to enter the companion cell?

Sucrose proton symporter

What produces ATP in the companion cell?

Mitochondria

What pumps protons into the apoplast?

Proton pump (ATPases)

What does ATP get converted into?

ADP+P

What connects the companion cell and the sieve element?

Plasmodesmata

Symplastic phloem loading

Instead of symplastic isolation (no plasmodesmata), there are lots of specialized, branched plasmodesmata

How do the branched plasmodesmata help facilitate the transport of sucrose?

They allow the diffusion of sucrose into the sieve element but larger oligosaccharides are trapped

Which is most true of phloem companion cells? *IClicker*

A) They are the meristem cells in stems that give rise to xylem and phloem

B) They are dead at maturity

C) They are alive and membrane lined, but otherwise mostly a hollow lumen to

allow sap flow

D) They are cytoplasmically dense & carry out metabolic functions for the sieve

elements

E) They have Kranz anatomy and conduct the Calvin Cycle in C4 plants

D) They are cytoplasmically dense & carry out metabolic functions for the sieve

elements

A is incorrect because companion cells are parenchyma cells and not meristem cells

B is incorrect because they are living at maturity

C is incorrect because it is describing the sieve elements. Companion cells are cytoplasmically dense

E is incorrect because companion cells do not have Kranz anatomy

If I had a compound that specifically inhibited the activity of plasma membrane ATPases, and if I observed that this compound resulted in an immediate cessation of phloem transport when applied to a plant, what type of phloem loading is most likely being employed? *IClicker*

A. Apoplasmic phloem loading

B. Symplasmic phloem loading

C. Phloem loading by transpiration

D. Phloem loading by osmosis

E. Passive phloem loading

A. Apoplasmic phloem loading

Phloem unloading

Solutes are released to sink cells through plasmodesmata or are released to the apoplast (cell wall space) for subsequent uptake into adjacent cells

What are the sink organs in phloem unloading?

Immature leaves

What are the source organs in phloem unloading?

Mature leaves

Allocation

The balance among metabolic processes within source cells determines how much photosynthate is available for export to other parts of the plant

Partitioning

A plant’s sinks compete for a share of the available photosynthate. Allocation within sink cells can influence sink strength