Photosynthetic products must obviously be transported to non-photosynthetic tissues of the plant. Otherwise organs like the roots would never receive a supply of organic molecules for respiration. The main product of photosynthesis that is transported by plants is the sugar sucrose. Sucrose is a disaccharide and is ideal for transporting in solution because it is highly soluble.
The main theory that explains how sucrose is transported in the phloem is known as the Mass Flow Hypothesis. At AS and A level it is the only theory looked at in detail. However it is useful to know that there are limitations to the theory, and some experimental data does not fit in with the theory. You could be presented with exam questions that asks you to interpret data, and decide whether the data supports or undermines the mass flow hypothesis.
The major steps in the mass flow theory are as follows:
- Active transport / active movement of sucrose occurs into phloem cells
- This causes the water potential of phloem contents to become more negative than the surrounding cells.
- Therefore water follows the sucrose by osmosis from adjacent cells.
- This means the hydrostatic pressure in phloem cells increases.
- The increased pressure forces the contents of the phloem cells to be pushed along the phloem tubes. This is mass flow.
- At another part of the plant (a sink e.g. the roots) the sugars are removed from the phloem by active transport so the gradients are maintained.
There are two main experimental techniques that have been used extensively to provide information about the transport that occurs in the phloem.
The first is radioactive labelling. In this technique plants are supplied with 14CO2 and a light source. The 14C is a radioactive isotope of carbon that can be detected. When the plant photosynthesises some of the CO2 is used to synthesise sugars (sucrose). By measuring the levels of radioactivity in the phloem around the plant the path of the phloem can be determined.
The second method is more cunning. It is difficult to get a sample of phloem sap out of a phloem sieve tube cell. This is because normally when a sieve tube is punctured with a measuring probe, the holes in its end walls quickly plug up. However, aphids (an insect also called greenfly) can insert their mouth parts without triggering this response. Figure 8 shows when it punctures a sieve tube, sap enters the insect's mouth parts under pressure. The right hand diagram shows phloem sap will continue to exude from the mouthparts even after the aphid has been cut away from them!
|Figure 8 : Using aphids to investigate phloem sap