Translocation in plants

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Translocation in plants

  1. 1. Translocation
  2. 2. What is it?    Translocation is the movement of organic substances made by the plant which are called assimilates. These are things such as sugars and other chemicals made by plant cells and are transported in the phloem tissue. Assimilates are transported in sieve elements, which work closely together with companion cells to achieve translocation. The part of the plant that releases the assimilate into the phloem is called a source, and the part of the plant that removes the assimilate from the phloem is called a sink.
  3. 3. Sieve Elements    Figure 1 shows a sieve tube. A sieve tube is made up of elongated sieve elements that are joined at the ends to form one continuous column. Sieve elements are cells that are found in phloem tissue, they have non- thickened cellulose walls and they are also very narrow, often having a diameter between 10 and 15µm. A sieve element has a similar composition to any other plant cell. However, they do not have a nucleus, ribosomes and a tonoplast. A sieve elements walls are made of cellulose so that substances can move laterally as well as vertically. When the end walls of 2 sieve elements meet, a sieve plate is formed. This is made up of the walls of both sieve elements and has many large pores, allowing water and solutes to pass through to the next sieve element. Sieve plate Companion cell Sieve tube Sieve tube element Cytoplasm Figure 1
  4. 4. Companion cells   Each sieve element has at least 1 companion cell alongside it. Companion cells have a similar structure of a normal plant cell as they have a cellulose cell wall, a plasma membrane, cytoplasm, a vacuole and a nucleus, but they have a larger number of mitochondria and ribosome as these cells are very metabolically active. Also, the vacuole of a companion cell is relatively small instead of large. Many plasmodesmata pass through the walls between sieve elements and companion cells, providing a direct pathway between the cytoplasm of the companion cell and the cytoplasm of the sieve element.
  5. 5. How translocation occurs    Inside a sieve tube is phloem sap. This moves by mass flow, so a pressure difference needs to be created in order to achieve mass flow in the phloem, meaning that the plant has to use its own energy. This pressure difference is the outcome of active loading of sucrose into sieve elements at the place from which the sucrose is to be transported (normally a photosynthesising leaf). This movement of sucrose decreases the water potential in the sap inside the sieve tube, which causes water to move into the sieve element down a water potential gradient, by the process of osmosis. In the leaf (source), water moves into the sieve tube and moves out of it in the root (sink) and a pressure difference is created. This pressure difference causes the liquid inside the tube to flow from the higher pressure area to the lower one.
  6. 6. Loading sucrose into the phloem   Some sugars produced by photosynthesis are converted into sucrose. This sucrose is transported to other parts of the plant by the phloem. Sucrose is a soluble substance, so it dissolves in water in the mesophyll cells, and moves out of the cell and across the leaf by either the apoplast or symplast pathway. The sucrose is then transferred into the companion cells by active transport and then into the sieve tube, through the plasmodesmata.
  7. 7. Apoplast and Symplast pathway: what's the difference?  Apoplast: Water soaks into wall and then seeps from cell wall to cell wall, and through spaces between the cells without ever entering a cell.  Symplast: The water moves from cell to cell by osmosis or through plasmodesmata and actually enters the cells.
  8. 8. Unloading sucrose from the phloem  This occurs in any tissue that requires the sucrose. The sucrose usually moves out of the phloem and into the tissue by facilitated diffusion and it is then converted into another substance to maintain a concentration gradient.

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