9.2 Transport in the Phloem Plant Science 9.2 Transport in the Phloem

Review Active Transport Passive Transport Photosynthesis Osmosis

Transport in the phloem Where does photosynthesis happen in the plant? The phloem moves sugar

Transport in the Phloem Structure-Function relationship Sieve tubes Made of sieve tube cells Sieve plate Reduced cytoplasm No nucleus Ridged walls Companion cells Genetic and metabolic functions Mitochondria Folding of membrane Transport proteins Plasmodesmata

Transport in the Phloem Identification of Phloem in cross sections of stem and root

Transport in the phloem Source to sink Translocation  When organic compounds are produced in leafs in excess, the leaf is the source. Sink can be flowers, fruits, roots When organic compounds are released from storage, the storage structure is the source Sink can be fruits, flowers

Transport in the Phloem Phloem loading Apoplast Through cell walls Active Transport Symplast Between cells plasmodesmata

Transport in the phloem Apoplast A. . proton pumps move H+s into extracellular environment (hydrolysis of ATP) B. Creation of proton gradient outside the cell C. H+ moves passively into the cell via a co- transport protein that takes sucrose with it D. Sucrose builds up in sieve tube

Transport in the Phloem Increase in solute (sucrose) Water moves from xylem to phloem Water uptake in phloem Phloem sap movement Pressure gradient is maintained by unloading of sucrose at sink

Transport in Phloem The solutes are unloaded Sap solution becomes hypotonic  Water is drawn out of the phloem and back into the xylem This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source

Transport in the Phloem Incompressibility of water allows transport along hydrostatic pressure gradients. at normal temperature, the compressibility of water is so low that it can be considered incompressible therefore, any change in pressure at one end of the transport vessel will result in an equal change in pressure at the other end of the transport vessel this allows phloem sap to move up or down, with or against gravity, as it moves from source to sink

Translocation Rate Aphids are a group of insects that feed on sap extracted from phloem

Translocation Rate Aphids can be used to collect sap at various sites along a plant's length and thus provide a measure of phloem transport rates

Factors Affecting Translocation Rate The rate of photosynthesis The rate of cellular respiration The rate of transpiration The diameter of the sieve tubes