23.3: Transport of sugars in phloem

Translocation

The process by which sugar generated from photosynthesis is transported throughout the plant.

Phloem

The tissue that transports sugars from source to sink in plants. Composed of sieve-tube elements and companion cells

Sieve-tube elements

Specialized cells arranged end to end to create long tubes for sugar transport. Connected via many plasmodesmata. Lacks nuclei, ribosomes, golgi apparatus and cytoskeleton due to sieve plates

Companion Cells

Cells located alongside sieve-tube elements that provide support and substances necessary for the sieve-tube elements to function. Connected via plasmodesmata

Phloem Sap

composed of sugar, amino acids, hormones, and minerals.

Pressure Flow Hypothesis

Explains how sap moves via bulk flow (like in xylem)

1. sucrose and other solutes actively transport into companion cells from source and flow into sieve tube elements via plasmodesmata (loading) which requires ATP and moves into phloem through either apoplast or symplast

2. sucrose accumulates into sieve making more negative solute potential (higher concentration than surrounding cells) causing water to enter sieve from adjacent xylem by osmosis

3. water increases turgor pressure = more positive pressure potential at source end of sieve = fluid content pushed to sink end where pressure is lower

4. sucrose enters sinks both passively and actively via companion cells = lowers sucrose concentration in sieves = less negative solute potential = greater water potential

5. water leaves phloem by osmosis into xylem = reinforces pressure gradient (difference in solute concentration between sources and sinks creates different pressure potential along sieves = bulk flow)

Apoplastic pathway

Transport through the extracellular space and cell walls.

Symplastic pathway

Transport through the cytosol.