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 bulk flow in phloem

1. sugars actively transport into companion cells from source (requires ATP) and flow into sieve tube elements via plasmodesmata (loading)

2. sugar concentration increases in sieve = more negative solute potential = lower water potential = water enters phloem from xylem via osmosis

3. increased turgor pressure = more positive pressure potential at source end = content pushed to sink (lower pressure)

4. sugars enter sinks passively and actively via companion cells = lowers sugar concentration in sieves = less negative solute potential = greater water potential

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

Apoplastic pathway

between cell wall and plasma membrane

Symplastic pathway

Transport through the cytoplasm.