Nature of science:
Developments in scientific research follow improvements in apparatus—experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available.
Understandings:
The movement of organic molecules in plants
Organic molecules move in plants via the phloem. Unlike the xylem, phloem is made up of living cells
Phloem is mostly made up of sieve tube members and their companion cells. Sieve tube members are connected to one another by sieve plates to form sieve tubes. Sieve tubes are referred to by many as sieve elements. The sieve plates have pores that allow the movement of water and dissolved organic molecules throughout the plant. Companion cells are actually connected to their sieve tube members by plasmodesmata.
Whereas xylem cells conduct water and minerals only upwards from the roots, phloem cells transport their contents in various directions. However, the direction of movement is based on a single principle: the movement is from a source to a sink. A source is a plant organ that is a net producer of sugar, either by photosynthesis or by the hydrolysis of starch. Leaves are the primary sugar sources. A sink is a plant organ that uses or stores sugar. Roots, buds, stems, seeds, and fruits are all sugar sinks. It is possible for some structures to be both a source and a sink.
The movement of organic molecules in plants is called translocation. The organic molecules are dissolved in water and the solution is referred to as phloem sap. The organic molecules of the phloem sap include:
The pressure-flow hypothesis
The phloem sap can move as fast as 1 m per hour. The best explanation at present
for the movement of phloem sap is the pressure-flow hypothesis. It includes the following processes.
The loading of sugar into the sieve tube at the source, and the removal of sugar at the sink, is accomplished by active transport. This active transport is a chemiosmotic process involving proton pumps and specialized membrane proteins called cotransport proteins that can allow both passive and active transport. The companion cells of the phloem are involved with the active transport process. Only the loading and removal of sugar from the sieve tube members requires energy: the actual transport in the tube is a passive process. It is passive because it involves transport along hydrostatic pressure gradients. Hydrostatic pressure is produced by compression of a liquid in a confined space or by the addition of solute particles to a liquid in a confined area. As water is relatively incompressible, adding solutes to a limited space filled with water increases pressure. Two areas with different hydrostatic pressure produce a hydrostatic pressure gradient. Water with its dissolved solutes will move from the higher pressure area to the lower pressure area.
Developments in scientific research follow improvements in apparatus—experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available.
Understandings:
- Plants transport organic compounds from sources to sinks.
- Incompressibility of water allows transport along hydrostatic pressure gradients.
- Active transport is used to load organic compounds into phloem sieve tubes at the source.
- High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.
- Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.
- Application: Structure–function relationships of phloem sieve tubes.
- Skill: Identification of xylem and phloem in microscope images of stem and root.
- Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide.
The movement of organic molecules in plants
Organic molecules move in plants via the phloem. Unlike the xylem, phloem is made up of living cells
Phloem is mostly made up of sieve tube members and their companion cells. Sieve tube members are connected to one another by sieve plates to form sieve tubes. Sieve tubes are referred to by many as sieve elements. The sieve plates have pores that allow the movement of water and dissolved organic molecules throughout the plant. Companion cells are actually connected to their sieve tube members by plasmodesmata.
Whereas xylem cells conduct water and minerals only upwards from the roots, phloem cells transport their contents in various directions. However, the direction of movement is based on a single principle: the movement is from a source to a sink. A source is a plant organ that is a net producer of sugar, either by photosynthesis or by the hydrolysis of starch. Leaves are the primary sugar sources. A sink is a plant organ that uses or stores sugar. Roots, buds, stems, seeds, and fruits are all sugar sinks. It is possible for some structures to be both a source and a sink.
The movement of organic molecules in plants is called translocation. The organic molecules are dissolved in water and the solution is referred to as phloem sap. The organic molecules of the phloem sap include:
- sugars (sucrose is the most common, and sugars account for most of the phloem)
- amino acids
- plant hormones
- small RNA molecules (this is a recent finding and may explain how cells that are far apart in a plant can communicate)
The pressure-flow hypothesis
The phloem sap can move as fast as 1 m per hour. The best explanation at present
for the movement of phloem sap is the pressure-flow hypothesis. It includes the following processes.
- Loading of sugar into the sieve tube at the source. This reduces the relative water concentration in the sieve tube members, causing osmosis from the surrounding cells.
- The uptake of water at the source causes a positive pressure, referred to as hydrostatic pressure, in the sieve tube, which results in a flow (bulk flow) of the phloem sap.
- This hydrostatic pressure is diminished by the removal of sugar from the sieve tube at the sink. The sugars are changed at the sink to starch. Starch is insoluble and exerts no osmotic effect.
- Xylem recycles the relatively pure water by carrying it from the sink back to the source.
The loading of sugar into the sieve tube at the source, and the removal of sugar at the sink, is accomplished by active transport. This active transport is a chemiosmotic process involving proton pumps and specialized membrane proteins called cotransport proteins that can allow both passive and active transport. The companion cells of the phloem are involved with the active transport process. Only the loading and removal of sugar from the sieve tube members requires energy: the actual transport in the tube is a passive process. It is passive because it involves transport along hydrostatic pressure gradients. Hydrostatic pressure is produced by compression of a liquid in a confined space or by the addition of solute particles to a liquid in a confined area. As water is relatively incompressible, adding solutes to a limited space filled with water increases pressure. Two areas with different hydrostatic pressure produce a hydrostatic pressure gradient. Water with its dissolved solutes will move from the higher pressure area to the lower pressure area.