9.2 (phloem transport)

Active transport

a transport mechanism where ions or molecules move against a concentration gradient, this movement requires energy.

Hydrostatic pressure gradient

the stress that develops when solutions containing different concentrations of solute in a common solvent are separated by a membrane that is permeable to the solvent, but not the solute.

Incompressibility of water

the incapability of water to lose volume in response to pressure.

Osmosis

the diffusion of fluid through a semi-permeable membrane from a solution with a low solute concentration to a solution with a higher solute concentration until there is an equal concentration of fluid on both sides of the membrane.

Phloem

the food-conducting tissue of vascular plants that conducts synthesized nutrients to different parts of the plants.

Sieve tubes

an element of phloem tissue consisting of a longitudinal row of thin-walled elongated cells with perforations in their connecting walls through which food materials pass.

Reduced organelles in sieve elements

Absence of cell structures (including nucleus, cytoskeleton, golgi, ribosomes and vacuole) frees the lumen to conduct a large volume of sap

Companion cells

Metabolic support cells (containing all the standard organelles) provide biomolecules (e.g. enzymes) necessary to maintain life functions in the sieve elements

Plasmodesmata

Openings between the sieve elements and companion cells allow communication and support from companion cells

Sieve plate

Pores through the horizontal cells that join sieve elements allow sap to flow freely

Cell membrane

Presence of a fully functional cell membrane in sieve elements that contains specialised protein pumps provides the structures needed to control the composition of sap

Sources

photosynthesising tissues and storage organs that are exporting sugars to other parts of the plant.

Sinks

organs that cannot produce (sufficient) sugars and need them for respiration or storage.

phloem loading

The process by which soluble carbohydrates (sugars) enter the phloem. Requires active transport.

Transverse section of a young dicot stem (shoot)

Transverse section of a dicot root

Transverse section of monocot root.