9.2: Transport in the Phloem of Plants

Question	Answer
What is translocation?	Translocation is the transport of organic solutes in a plant via the phloem, utilising osmosis, passive diffusion, facilitated diffusion and active transport.
From where and where to are organic solutes transported?	From sources to sinks.
How does the process of translocation occur?	Phloem is composed of sieve tubes, which are composed of sieve tube cells. These cells are separated by perforated walls called sieve plates. Companion cells are usually close by, and organic solutes are transported into the phloem through companion cells using symplastic or apoplastic route.
What are sources?	Areas where sugars and amino acids are loaded into the phloem.
What are sinks?	Are areas where the sugars and amino acids are unloaded and used.
How does phloem transport biochemicals?	Phloem transports biochemicals bidirectionally. Flow of phloem sap are active processes due to pressure gradients.
Difference between sources and sinks	Sources: - Photosynthetic tissues Mature green leaves Green stems - Storage organs that are unloading their stores E.g. in germinating seeds Sinks: -Roots that are growing/absorbing mineral ions using energy from cell respiration - Parts of the plants that are developing and growing food stores. e.g. developing fruits Developing seeds Growing leaves
How are organic compounds loaded into the phloem sieve tubes at the source?	Active transport is used to load organic compounds into phloem sieve tubes at the source.
What is phloem loading?	When sucrose/ sugars are brought into and out of phloem (from source to phloem and from phloem to sink).
What are the 2 different ways phloem loading can occur?	- Apoplast route - Symplast route
What is the apoplast route?	Sugars travel through cell walls from mesophyll cells to companion cells, and sometimes sieve cells.
What is the symplastic route?	Sucrose travels between cells through connections between cells called plasmodesmata.
How does the incompressibility of water allow transport by hydrostatic pressure gradients?	- The build up of sucrose and other carbohydrates draws water into the companion cell through osmosis (due to low water potential). - Water flows from areas of high pressure to low pressure at sinks. - Sucrose is withdrawn from phloem to be utilised as energy. - The loss of solute causes a reduction in osmotic pressure thus causing water that carries solute to sink to be drawn back into transpiration stream.
What are the three main functions of phloem?	- Loading of carbohydrates at sources - Transport of carbohydrates (long distances) - Unloading of carbohydrates at sinks
What are the 3 features of a sieve tube?	- Sieve tubes are composed of specialised cells known as sieve tube cells. - Sieve tube elements (unlike the xylem) are living, even though they do not have a nucleus or cytoplasm. - Sieve tube cells are closely associated with companion cells.
Why do sieve tube cells have to be living?	Sieve tube cells must be living as they depend on its membrane to maintain concentration levels of sucrose and organic molecules, that have been carried out by active transport.
How does companion cells work?	- Companion cells perform many genetic and metabolic functions of sieve tube cells. - Sucrose is transferred from sieve tube cells to companion cells via apoplastic route - Plasmodesmata connect cytoplasm of companion cell to sieve tube cell to allow this to happen. - Active transport is needed in the companion cell to produce oligosaccharides.
Why do phloem sieve tubes have rigid cell walls?	To allow them to withstand the pressure of flow of phloem sap within the cell.
What is a sieve tube cell?	They are living specialised cells used for the purpose of transporting sucrose and organic compounds to form a continuous flow of nutrients.
What are sieve plates?	Sieve plates are perforated cell walls that separate individual sieve tube cells. This allows a lower resistance to the flow of phloem sap.
How is sucrose moved into phloem at source?	Apoplastically: Through cell walls, from mesophyll to companion cell, or Symplastically: Sucrose travelling between cells.
Why and how does osmosis occur within the phloem?	High concentrations of sucrose causes water to flow from high pressure regions to low pressure regions. Water moves into the phloem from the xylem via osmosis due to phloem having a low water potential.
What is a phloem?	A vascular tissue in a plant that carries organic and inorganic ions bidirectionally from sources to sinks.
What are sieve tubes?	Sieve tubes are located in the phloem and it is a tube comprised of sieve cells thought to be responsible for translocation of material from sources to sinks.

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9.2: Transport in the Phloem of Plants

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	Created by Jasmine Wells almost 9 years ago