Describe Current Theories About Processes Responsible For Th

The function of xylem and phloem in transport

Is mainly to carry materials for photosynthesis to the cells and move the products away from the cells to other parts of the plant. In small plants this may be achieved through diffusion and active transport however in larger plants specialized vascular tissue has developed to serve this function. The vascular system consists of xylem and phloem and the movement of materials from one part of the plant to another is known as translocation.

Xylem – transpiration stream theory

The transpiration stream in xylem occurs due to physical forces that result in water and ions being moved by passive transport. A column of water is sucked up by the stem, by the evaporation pull of transpiration – this is known as the transpiration stream. Once water has been absorbed into the roots (osmosis) along with mineral ions (diffusion and active transport), these substances move into the xylem. A small amount of root pressure results from the continual influx of ions and water – forcing the solution upwards, this is insufficient by itself. Most of the upward movement is a result of the transpiration stream – which is water is drawn up the xylem to replace water which is lost due to transpiration at the leaves.

Evidence for this theory :
- Xylem are hollow and narrow – very little resistance to the flow of water
- The physical properties of water contribute to a continuous stream. Adhesive forces (between water and xylem walls) lead to capillarity (water rises up the narrow bore of the xylem) and cohesive forces (between water molecules) form a continuous stream
- A concentration gradient exists. The surface of the leaf has high osmotic pressure (low water concentration) due to transpiration. The centre of the lead has a low osmotic pressure

Explanation

Water loss at the surface of the leaf results in the osmotic movement of water across from adjacent internal cells into those that have just lost some water. This osmotic flow continues across the leaf – until it reaches the xylem tissue. When water molecules leave the xylem and move along the concentration gradient, this creates a tension in the column of water rising up the xylem. Due to the properties of adhesion and cohesion the water column does not break and so the entire water column is not pulled upwards. The combination of adhesive and cohesive forces, together with the suction pull of the transpiration – creates the transpiration stream. Mineral ions dissolved in the water are carried along by the transpiration stream and can move out by active transport – to reach the tissues where they are needed.

Phloem – pressure flow theory

Translocation in phloem tissue moves products of photosynthesis by active transport. The flow of materials in phloem is an active process that requires energy. The mechanism of flow is driven by an osmotic pressure gradient, generated by difference in sugar and water concentrations. It involves the active loading of sugar into the phloem at one end (source) and then the active unloading from the phloem into surrounding tissues at the other end (sink). The loading of sugar into the phloem attracts water to flow in (due to the differences in osmotic pressure) and the offloading at the sink causes the water to flow out of the phloem.

Loading at the source

There are two theories for the loading of amino acids, sucrose and other minerals at the source.
- Symplastic loading – sugars and other nutrients move in the cytoplasm from the mesophyll cells to the sieve elements through plasmondesmata (strands of cytoplasm that pass through pits into the cell walls)
- Apoplastic loading – sugars and other nutrients move along a pathway through the cell walls until they reach the sieve element. They then cross the cell membrane to enter the phloem tube. These sugars pass into the sieve cell by active transport

Offloading at the sink

Materials flow to the sink. At the sink (roots or flowers) sugars and material are removed from the phloem by active transport. As sugars move out of the phloem, they draw water out with them (osmosis). This results in a lower osmotic pressure (due to the higher water concentration) in the phloem at the sink

Pressure flow

This difference in osmotic pressure between the source and the sink in the phloem drives the phloem sap to flow. The direction of the flow depends on where the sink areas are in relation to the source. Water can move into the phloem by osmosis at any point along the gradient. The flow is continuous as sucrose is continually added at one end and removed at the other.