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transport in the phloem of plants
Essential idea: Structure and function are correlated in the phloem of plants.
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:
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:
- 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.
translocation occurs from source to sink
Plants transport organic compounds from sources to sinks.
PHLOEM: is a tissue found throughout plants, including stems, roots and leaves. Phloem is composed of sieve tubes. Sieve tubes are composed of columns of specialized cells called sieve tube cells. Individual sieve tube cells are separated by perforated walls called sieve plates. Sieve tube cells are closely associated with companion cells. The function of phloem include loading of carbohydrates; transport of the carbohydrates sometimes over long distances; and unloading of the carbohydrates at sinks.
Phloem transport organic compounds throughout the plant. The transport of organic solutes in a plant is called translocation. Phloem links parts of the plant that needs a supply of sugars and the other solutes such as amino acids to other parts that have a surplus.
Sources: Areas where sugars and amino acids are loaded into the phloem
Sinks: Where the sugars and amino acids are unloaded and used.
Sometimes sinks can turn into sources, and vice versa. For this reason the tubes in phloem must be able to transport biochemicals in either direction and there are no valves or central pumps in phloem.
The table bellow classifies parts of the plant into sources and sinks
PHLOEM: is a tissue found throughout plants, including stems, roots and leaves. Phloem is composed of sieve tubes. Sieve tubes are composed of columns of specialized cells called sieve tube cells. Individual sieve tube cells are separated by perforated walls called sieve plates. Sieve tube cells are closely associated with companion cells. The function of phloem include loading of carbohydrates; transport of the carbohydrates sometimes over long distances; and unloading of the carbohydrates at sinks.
Phloem transport organic compounds throughout the plant. The transport of organic solutes in a plant is called translocation. Phloem links parts of the plant that needs a supply of sugars and the other solutes such as amino acids to other parts that have a surplus.
Sources: Areas where sugars and amino acids are loaded into the phloem
Sinks: Where the sugars and amino acids are unloaded and used.
Sometimes sinks can turn into sources, and vice versa. For this reason the tubes in phloem must be able to transport biochemicals in either direction and there are no valves or central pumps in phloem.
The table bellow classifies parts of the plant into sources and sinks
phloem loading
Active transport is used to load organic compounds into phloem sieves tubes at the source.
SUCROSE: is the most prevalent solute in phloem sap. Sucrose is not as readily available for the plant tissues to metabolize directly in respiration and therefore makes a good transport form of carbohydrate as it will not be metabolized during transport
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 the cells walls from mesophyll cells to the cell walls of companion cells, and sometimes sieve cells where the sucrose transport protein then actively transport the sugar in. This is referred to as apoplast route.
In this case a concentration gradient of sucrose is established by active transport. Which is achieved by a mechanism whereby II ions are actively transported out of the companion cell from surrounding tissues using ATP as an energy source. The build up of H+ 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
In other species much of the sources travels between cells through connections between cells called plasmodesmata. This is referred to as the symplast route. Once the sucrose reaches the companion cell it is converted to an oligosaccharide to maintain the sucrose concentration gradient.
Pressure and water potential differences play a role in translocation
Incompressibility of water allows transport by hydrostatic pressure gradients.
The build up of sucrose and other carbohydrates draws water into the companion cell through osmosis. The rigid cell walls combined with the incompressibility of water results in a buid-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 form 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.
The build up of sucrose and other carbohydrates draws water into the companion cell through osmosis. The rigid cell walls combined with the incompressibility of water results in a buid-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 form 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.