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Topic 9 Plant Biology 9.2 Transport in the Phloem 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. Translocation occurs from source to sink Plants transport organic compounds from sources to sinks. o Phloem = vascular tissue that transports sugar. o Composed of sieve tubes which are made of columns of cells called sieve tube cells. These cells are closely associated with companion cells. o Transports organic compounds around the plant, which is called phloem translocation o Phloem moves material from source to sink o o o SOURCE = place that has a surplus of organic compounds (ie. Leaves) SINK = place that needs organic compounds (ie. Roots) Sometimes sinks turn into sources, or vice versa. For this reason the tubes in phloem must be able to transport biochemicals in either direction Sources Photosynthetic Tissues • Mature green leaves • Green stems Storage organs that are unloading their stores: • Storage tissues in germinating seeds • Tap roots or tubers at the start of the growth season Sinks Roots that are growing or absorbing mineral ions using energy from cell respiration. Parts of the plant that are growing or developing food stores: • Developing fruits • Developing seeds • Growing leaves • Developing tap roots or tubers Phloem Loading • Active transport is used to load organic compounds into phloem sieve tubes at the source. o o o Sucrose is the most prevalent solute in phloem sap. 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. Phloem loading – the process by which plants bring sugar into the phloem. Two mechanisms… Phloem Loading o Apoplast route - 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. A concentration gradient of sucrose is established by active transport. H+ 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. Phloem Loading o Symplast route – much of the sucrose travels between cells through connections between cells called plasmodesmata. 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 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. Incompressibility of water allows transport by hydrostatic pressure gradients o o The buildup of sucrose and other carbohydrates draws water into the companion cell through osmosis (from xylem). 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 or converted into starch. The loss of solute causes a reduction in osmotic pressure and the water that carried the soute to the sink is then drawn back in to the transpiration stream in the xylem. • https://www.youtube.com/watch?v =MxwI63rQubU Application • Structure-function relationships of phloem and sieve tubes. o o o Composed of sieve tubes, which are composed of columns of specialized cells called sieve tube cells. These cells are living although they have reduced cytoplasms and no nucleus. Sieve tube cells are closely associated with companion cells. Companion cells perform many of the genetic and metabolic functions of the sieve tube cell and maintain viability. Lots of mitochondria in companion cell to support active transport of sucrose. Infolding of the plasma membrane increases phloem loading capacity. Plasmodesmata connect the cytoplasm of companion cells with the sieve tube cells Skill • Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively labelled carbon dioxide. ****Complete DBQ on pg 418 Skill • Identification of xylem and phloem in microscope images of stem and root.