6. Sieve tube elements and Sieve cells
Sieve Tube Elements Sieve Cells
Type of Plants Angiosperms Gymnopserms
Junction Open channels, sieve Blocked with membranes
plates
P-protein present absent
Associated cells Companion cells Albuminous cells
7. P-protein – in all dicots, many monocots, absent
in gymnosperms;
- seal off damaged sieve elements
Callose – longer solution to damaged sieve tube
Wound callose – efficient seal of damaged sieve
plates
Companion cells – transport of photosynthate
from producing cells to sieve elements;
take over some metabolic function during
differentiation of sieve elements; supply
energy
8. Specialized Companion cells
1. Ordinary companion cells – have chloroplasts
2. Transfer cells – have wall ingrowths which
increase the surface area of the plasma
membrane and enhance solute transfer
3. Intermediary cells- with numerous
plasmodesmata which connect them to
sorrounding cells
9. Patterns of Translocation
Sources –areas of supply;
include photosynthetic parts or
exporting storage organ
Sink-areas of metabolism or storage ;
nonphotosynthetic organs; ex. Roots,
tubers, developing fruits, immature leaves
10. Factors which affect Translocation:
1. Proximity
2. Development
3. Vascular connections
orthostichy- vertical row of leaves
Anastomoses – vascular interconnections,
alternative pathway in cases of interferences in
the translocation path
11. Translocated Materials
1. Water – solvent of carbohydrates
2. Nitrogen- in the form of amino acids and
amides
3. Plant hormones – auxin, gibberellins,
cytokinins, abscisic acid
4. Proteins- P-proteins ( wound sealing); protein
kinases( phosphorylation);
thioredoxin(disulfide reduction); ubiquitin(
protein turnover); chaperones(protein
folding); protease inhibitors ( against insects)
12. 5. Inorganic solutes – K, P, Mg, PO4, Cl
Reducing Sugars – not generally translocated in
the phloem : aldehyde and ketone
Nonreducing sugars - sucrose
13. Rates of Movement
1. Velocity – linera distance travelled per unit
time
2. Mass transfer rate- quantity of material
passing through a given cross sectionof
phloem per unit time
* Transport velocities in the phloem exceed rate
of diffusion over long distances
14. Mechanism of Translocation
phloem loading – movement of photosynthate
from producing cells into sieve elements
phloem unloading – from sieve elements to
sink cells
Pressure-Flow Model: by Ernst Munch
A flow of solution in the sieve elements is
driven by an osmotically generated pressure
gradient between source and sink.
