Translocation by

Translocation by https://padlet.com/johanna_turnbull/translocation 6. Describe the pressure flow model of translocation. en-us 2021-08-31 05:32:38 UTC 2021-09-07 07:51:17 UTC hello@padlet.com https://padlet.net/icons/png/2618.png Pressure-Flow Model of Translocation https://padlet.com/johanna_turnbull/translocation/wish/1714014914 The pressure flow model describes the movement of assimilates in three steps:
Step 1: Phloem Loading:
To begin, sucrose produced in mesophyll cells (or another source) is moved into sieve flow elements by companion and source cells to make it more concentrated. This can occur via the symplastic or apopoplastic pathways. Symplastic transport occurs through multiple plasmodesmata down a concentration gradient and is passive. In apopplastic transport, photosynthates travel between the cell wall and cell membrane before being drawn through the cell membrane via active transport using a hydrogen pump.

Step 2: Mass flow in Phloem:
- As sugar is concentrated in phloem, water follows suite, moving out of the adjacent xylem cells into the phloem where it generates osmotic water potential. This high turgor pressure and drives water (and dissolved assimilates) through the phloem (sieve elements), down the pressure gradient towards a sink. This process is termed mass flow.

Step 3: Phloem Unloading:
- Once assimilate reaches a sink, imported solutes are unloaded from the sieve elements. This can also occur via the aforementioned symplastic or apopoplastic pathways. Once unloaded, the sugars are transported to a cell in the sink where they are stored or metabolised.

In order for translocation to occur via the pressure-flow model, several rules have to be met. Firstly, sieve plate pores must be unobstructed and a positive pressure gradient must be present. Secondly, simultaneous bidirectional transport in a single sieve element cannot occur. Finally, great expenditures of energy are not required in order to drive translocation in the tissues along the path.

]]> 2021-09-03 00:51:48 UTC https://padlet.com/johanna_turnbull/translocation/wish/1714014914 Pressure-Flow Model of Translocation #2 https://padlet.com/johanna_turnbull/translocation/wish/1716514377 The Pressure flow model of translocation provides a description of how carbohydrates are moved from sites of origins (sources) to sites of consumption (sinks). Carbohydrates are made in the mesophyll cells of plants and are transported to the outer vascular bundle via symplastic pathways, traveling through the connecting plasmodesmata to the phloem cells. Once reached, the carbohydrates can either continue syplastically to the sieve tube elements or apoplastically, either way solutes are pumped into the sieve-tube element actively by the companion cell which increases the cells osmotic water potential also forcing water from the xylem tubes to be passively pumped into the sieve tube as well. This process is called phloem loading. Once the solutes are packed and ready, they flow down the concentration gradient created in loading via mass flow. Upon reaching areas of low pressure (i.e. sinks), the solutes are unloaded and actively pumped across the sieve tube membrane into the sink. As for the water, it is passively pumped back into the xylem to continue its journey around the plant.

]]> 2021-09-04 10:28:01 UTC https://padlet.com/johanna_turnbull/translocation/wish/1716514377 Translocation answer https://padlet.com/johanna_turnbull/translocation/wish/1716543857 The pressure flow model consists of 3 steps- phloem loading, bulk flow and phloem unloading. In phloem loading, the sucrose is produced in the mesophyll cell of the plants leaf during photosynthesis and is then transported through the cells using the cell cytoplasm and plasmodesmata in a process known as the symplastic pathway. This pathway is relatively slow so some plants will also use the apoplastic pathway, travelling outside the cell between the cell wall and cell membrane, as this is faster. Eventually the sucrose will reach the companion cells and the phloem where a H pump will transport the sucrose into the cell and this will cause a very concentrated area in the phloem.

The concentrated solutes in the phloem will cause water to passively move from the xylem as the osmotic water potential in the phloem is now lower than in the xylem. This will cause the start of Step 2- Bulk flow. The water that flows into the phloem to even out the concentration gradient will now start to proceed downwards and out towards photosynthate sink areas. Bulk flow is caused by the change in the turgor pressure as the solutes flow from areas of high concentration to lower concentrations. When the solute reaches an area that is signaling a need for sucrose, step 3 will begin- phloem unloading. Here the sucrose will start to move into the companion cells and out into the sink using the symplastic and apoplastic pathways. This causes the solute concentration in the phloem to drop and cause the osmotic water potential to increase and the water will passively move out of the phloem and back into the xylem where the water potential is lower.

]]> 2021-09-04 11:42:36 UTC https://padlet.com/johanna_turnbull/translocation/wish/1716543857 Translocation https://padlet.com/johanna_turnbull/translocation/wish/1717830010 The Pressure-Flow model is the process of translocation in plants. It moves materials around the plant from the source to the sink – usually over long distances. The flow is driven by the osmotically driven pressure gradient between the source and the sink. It occurs via 3 steps:

1. Phloem loading: the photosynthate movement from mesophyll cells to the sieve elements. This can occur via the faster but more energy expensive pathway, the apoplastic pathway, or through the slower passive pathway, the symplastic pathway.

2. Mass flow in phloem: the movement of photosynthates into the sieve tube element creates a concentration gradient which passively draws water into the phloem from the xylem, known as osmosis. The increase in water creates a pressure gradient that results in a mass flow .

3. Phloem unloading: When the mass flow reaches the sink, the photosynthate elements are unloaded. The drop in concentration also causes the water to move back into the xylem.

]]> 2021-09-05 22:58:28 UTC https://padlet.com/johanna_turnbull/translocation/wish/1717830010 Translocation https://padlet.com/johanna_turnbull/translocation/wish/1717830481 The pressure flow model describes the movement of mass flow throughout a plant, from the movement of carbohydrates (sucrose) from a source, to sinks. The process can be described through 3 steps:

1) Phloem loading: Surcrose that has been produced in source cells is moved into sieve elements. This occurs via either symplastic transport through multiple plasmodesmata down a concentration gradient (passive) or via apoplectic loading through cell walls (active). The loading of molecules drives up the concentration gradient and has the generation of a pressure gradient form

2) The mass flow in Phloem: the generation of osmotic water potential from high concentration of solutes forms high tutor pressure, forces water through the phloem into different sieve elements

3) Phloem unloading: Once solutes reach a sink destination, they are unloaded from the phloem into companion cells and can occur via apoplectic and symplastic pathways. When they are unloaded, the sugars can be stored or metabolised for repair or growth]]> 2021-09-05 22:59:21 UTC https://padlet.com/johanna_turnbull/translocation/wish/1717830481 Translocation https://padlet.com/johanna_turnbull/translocation/wish/1718777192 During phloem loading, a source actively transports sugars into the companion cells through apoplastic or symplastic pathways and into the sieve tube cells. Water moves by osmosis across the osmotic water potential from the xylem into the sieve tube cells which creates a region of high pressure. The sap moves up or down according to mass flow and once at the sink actively transports the sugar during phloem unloading, which makes the water passively leave the phloem due to the concentration gradient of sugar in the water.

]]> 2021-09-06 07:16:23 UTC https://padlet.com/johanna_turnbull/translocation/wish/1718777192 aj992 https://padlet.com/johanna_turnbull/translocation/wish/1721235654 Phloem sap flows from source to sink, moving through the sieve tubes of angiosperms by bulk flow driven by positive pressure, known as pressure flow. The building of pressure at the source and reduction of that pressure at the sink cause sap to flow from source to sink.

]]> 2021-09-07 07:51:17 UTC https://padlet.com/johanna_turnbull/translocation/wish/1721235654