Translocation

• Glucose made in the leaves by photosynthesis is converted to sucrose which is less reactive than glucose and so is less likely to react with other molecules.
• The products made from the glucose made by photosynthesis such as sucrose and amino acids are called assimilates. Assimilates can also be other organic molecules such as hormones.
• The transport of assimilates in the phloem is called translocation.
• Assimilates are transported in the translocation system from source to sink.
• Sources are the locations where assimilates are produced such as photosynthesising leaves and stems, storage organs and food stores in leaves.
• Sinks are the regions where assimilates are required such as roots, storage organs, meristem tissues, active cells and developing food stores.
• Active loading is the process of loading sucrose into phloem vessels.
• A protein pump on the surface of companion cells uses ATP in active transport to move Hydrogen ions from inside the companion cell, out into the surrounding tissue, forming a concentration gradient of H+ ions.
• The hydrogen ions outside of the companion cell now move by facilitated diffusion with co-transporter proteins down the concentration gradient back into the cell whilst simultaneously coupled with the movement of sucrose into the companion cells.
• The transport of sucrose into the companion cell increases the concentration of it within the cell, forming a concentration gradient that causes sucrose to move from companion cells into sieve tube elements via plasmodesmata.
• The high concentration of sucrose within the sieve tube elements causes the water potential of the phloem sap to decrease, causing water to move into the sieve tube elements by osmosis from surrounding tissues and the xylem.
• The water flowing into the sieve tube elements by osmosis causes a increase in the hydrostatic pressure inside the sieve tube elements and as a result phloem sap is moved up or down the plant towards the sink.
• Companion cells have many mitochondria to provide the ATP needed for the active transport of hydrogen ions.
• Foldings on the cell membrane of the companion cell provide a large surface area for the cotransporter proteins involved in the loading of sucrose.
• The bulk movement of phloem sap is called mass flow.
• At the sink, sucrose contained within cell sap moves out of the sieve tube element and is turned back into glucose, which can be used by cells for respiration.
• In some storage organs, the sucrose delivered from the sieve tube elements is converted into starch.
• As the sucrose leaves the sieve tube elements, the water potential in them rises, causing water to move out of the phloem and back into the xylem and surrounding tissues by osmosis.
• One piece of evidence supporting the active model of translocation is that the rate of flow of sucrose is significantly faster than the flow that would ordinarily be possible by diffusion alone.
• If we inhibit the mitochondria in companion cells from respiring and producing ATP, then translocation is stopped, indicating that some part of the process must be active.
• Within the phloem, assimilates can be moved by symplast or apoplast pathways.
• Aphids are insects which insert their stylet into the sieve tube elements of the phloem to feed from exuding phloem sap.
• Plant leaves can be sealed in a container with carbon dioxide composed of radioactively labelled carbon-14, causing photosynthesis to occur producing glucose composed of this carbon.
• This radioactive glucose then is converted into many assimilates also composed of this isotope. Aphids use their stylet to tap into the phloem, and its body is cut leaving phloem sap to still flow through the stylet.
• Phloem sap is collected from flowing through the stylet, where its contents can be analysed for the presence of radioactive carbon-14. Stylets placed at various points along the stem can show the rate of movement of phloem sap.