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Terms in this set (16)
* Define translocation, phloem sap, source and sink.
Translocation: transport of organic solutes in a plant
Phloem sap: the substance in which organic compounds are transported from sources to sinks
Source: a site where organic compounds are loaded into the phloem
Sink: a site where compounds are unloaded from the phloem
* List example source and sink tissues.
* State that phloem transport is bidirectional.
* Outline why pressure in the phloem increases due to the movement of water into the phloem.
The build up of sucrose and other carbohydrates draws water into the companion cells through osmosis. The rigid cell walls combined with the incompressibility of water result in a build-up of pressure. Water will flow from this area of high pressure to an area of low pressure. At the sink end, sucrose is withdrawn from the phloem and either utilized as an energy source for such processes as growth or converted to starch. In either case, the loss of solute causes a reduction in osmotic pressure and the water that carried the solute to the sink is then drawn back in to the transpiration stream in the xylem.
* State that sucrose is the most prevalent solute in phloem sap.
Sucrose is the most prevalent solute in phloem sap
* Outline why sucrose is used for phloem transport, as opposed to glucose.
Sucrose is not as readily available for plant tissues to metabolize directly in respiration and therefore makes a good transport form of carbohydrate as it will not be metabolized during transport.
* Describe the active transport of sucrose into the phloem via a co-transport protein.
(Plants differ in the mechanism by which they bring sugars into the phloem, a process called phloem loading.) In some species, a significant amount travels through cell walls from mesophyll cells to the cell walls of companion cells, and sometimes sieve cells, where a sucrose transport protein then actively transports the sugar in. this is referred to as the apoplast route. In this case, a concentration gradient of sucrose is established by active transport. This is achieved by a mechanism whereby hydrogen ions are actively transported out of the companion cell from surrounding tissues using ATP as an energy source. The build-up of hydrogen ions then flows down its concentration gradient through a co-transport protein. The energy released is used to carry sucrose into the companion cell-sieve tube complex.
* State that the phloem becomes hypertonic to xylem due to the active transport of sucrose into the phloem.
The phloem becomes hypertonic to xylem due to the active transport of sucrose into the phloem
* State that water moves into the phloem by osmosis.
Water moves into the phloem by osmosis
State that Jessie Cronshaw is going to blitz this exam
Jessie is going to blow it out of the water and become the best midwife ever
* State that water moves from area of higher pressure to area of lower pressure and that the movement of water also moves the solutes dissolved in it.
* State that the function of phloem includes loading of carbohydrates at a source, transport of carbohydrates through the plant, and unloading of carbohydrates at a sink.
the function of phloem includes loading of carbohydrates at a source, transport of carbohydrates through the plant, and unloading of carbohydrates at a sink
* Outline the structure and function of sieve tube cells, with specific mention of the rigid cell wall and sieve plates.
Sieve tubes are composed of columns of specialized cells called sieve tube cells. (Unlike the vascular elements of xylem,)sieve tube elements are living, thought they do have reduced quantities of cytoplasm and no nucleus. One reason that sieve cells need to be living is that they depend on the membrane to help maintain the sucrose and organic molecule concentration that has been established by active transport.
The rigid cell walls of the sieve tube cell allow for the establishment of the pressure necessary to achieve the flow of phloem in the sieve tube cell.
Individual sieve tube cells are separated by perforated walls called sieve plates. These are the remnants of cell walls that separated the cells. The perforated walls in combination with the reduced cytoplasm means that the resistance to the flow of phloem sap will be lower.
* Outline the structure and function of companion cells, with specific mention of mitochondria and cell membrane infolding.
Sieve tube cells are closely associated with companion cells. This is due in part to the fact that the sieve tube cell and its companion cell share the same parent cell. The companion cells perform many of the genetic and metabolic functions of the sieve tube cell and maintain the viability of the sieve tube cell. The abundant mitochondria in the companion cell support active transport of sucrose. The infolding plasma membrane increases the phloem loading capacity using the apoplastic route.
The accumulation of sucrose in the sieve tube element-companion cell pair requires the presence of active transport proteins or enzyme activity in the companion cells to produce oligosaccharides.
* State two ways xylem cells can be identified in cross sections of stem and root.
Xylem cells are generally larger than phloem cells. (within one vascular bundle) phloem cells tend to be closer to the outside of the plant in stems and roots.
* State that aphids consume phloem sap as the main component of their diet.
Aphids consume phloem sap as the main component of their diet
Aphids penetrate plant tissues to reach the phloem using mouth parts called stylets. If the aphid is anaesthetised and the stylet severed, phloem will continue to flow out of the stylet and both the rate of flow and the composition of the sap can be analysed. The closer the stylet is to the sink, the slower the rate at which the phloem sap will come out.
* Outline how radioactive carbon isotopes are used to study translocation.
Radioactively-labelled carbon within carbon dioxide can be fixed by plants during photosynthesis. It will release radiation that can be detected either using film or radiation detectors. As the carbon is metabolised, it will be found in different molecules within the plant.
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