Question

3. Explain how phloem loading occurs (both mechanisms), what results, and how this causes the movement of water and sucrose through the phloem 4. Describe long-distance transport in phloem through the Munch pressure-flow model 5. Describe the process of phloem unloading. 6. Predict "source" vs. "sink" relationships in plant parts, given a description of environmental influences and metabolic status.

Answer 1

Answer 3

Pressure flow / mass flow hypothesis of food / sucrose translocation given by E. Munch.

This is the most accepted theory of food conduction in plants. According to it foot translocation occurs in between source and sink in order of turgor pressure gradient that is from high turgor pressure to low Turgor pressure.

The Accepted mechanism used for the translocation of Sugars from source to sink is called the pressure flow hypothesis. As glucose is prepared at the source (by photosynthesis) it is converted to sucrose (a disaccharide). The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport.

This process of loading at the source produces a hypotonic condition in the phloem. Water in the adjacent Xylem moves into the phloem by Osmosis. As turgor pressure builds up the phloem sap will move to areas of lower pressure. At the sink osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the phloem sap and into the cells will used the sugar - converting it into energy, starch, or cellulose. As Sugars are removed, the osmotic pressure decreases and water moves out of the phloem into the xylem.

To summarise, the movement of Sugars in the phloem begins at the source, where Sugars are loaded (actively transported) into sieve tube. Loading of the phloem set up a water potential gradient that facilitates the mass Movement in the phloem. Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates. Cytoplasmic strands pass through the holes in the sieve plates, so forming continuous filaments. As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins, and the sap moves through the phloem. meanwhile, at the sink, incoming Sugars are actively transported out of the phloem and removed as Complex carbohydrates. the loss of solute produces a high water potential in the phloem and water passes out returning eventually to xylem.

High T.P./High YW -H20- Carriers Glucose O.P., T.P. 1 Yw1 Source - Phloem Loading Sucrose ATP Low T.P. Low YW O.P.V, T.P.V Yw ADP Carriers Sucrose -HO- ATP Sink - Phloem Unloading Starch, Consumed Xylem Phloem Fig.: Sucrose Translocation in Plants

Answer 2

e cotransport of a proton with sucrose allows movement of sucrose against its concentration gradient into the companion cells. occurs. From the companion cells, the sugar diffuses into the phloem sieve-tube elements through the plasmodesmata that link the companion cell to the sieve tube elements