BSCI 442  
PLANT PHYSIOLOGY  OUTLINE
Dr. H Sze
FALL 2008
Lec. # 11: Phloem transport

 Phloem Slides
Taiz (ch. 10)

Metabolites are transported from source to sink in the phloem.

SOURCE: Tissue/organ that makes or stores food reserves.  A net exporter. 
    E.G. Seed endosperm, Green Photosynthetic leaf.

SINK: Organ/tissue or cell that requires metabolites for energy and for biosynthesis.  A net importer. 
    E.G. Shoot meristem, roots, developing seeds.

 PHLOEM CONTAINS 3 TYPES OF CELLS:
a.  Sieve Tube elements
b.  Companion cells
c.  Phloem parenchyma

PHLOEM LOADING in source tissue is by energy-dependent transport.
Sucrose is actively loaded into companion cell or phloem parenchyma by H+-coupled symport.

PHLOEM UNLOADING is via the apoplast and symplast.  Maybe passive.

LONG DISTANCE MOVEMENT IN PHLOEM IS DRIVEN BY A PRESSURE GRADIENT.

1. A sucrose gradient exists from source to sink.
Loading results in high p (osmotic pressure) & low Yp (osmotic potential). Water moves in.  P increases.

2. At sink, unloading causes decrease in p, increase in Y p. H2O leaves. Turgor pressure decreases.

3.  Solution moves by mass flow under pressure gradient from source to sink.