[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe translocation of solutes in the phloem this includes the ideas of active loading and mass flow we've already seen that plants produce the monosaccharide glucose during photosynthesis in the leaves all parts of the plant require glucose for respiration the glucose produced in the leaves is first converted to the disaccharide sucrose sucrose is less reactive than glucose and is less likely to react with other molecules now molecules such as sucrose which are made as a result of photosynthesis are called assimilate assimilate can also include amino acids assimilates such as sucrose are transported around the plant in the phloem and the transport of assimilates in the phloem is referred to as translocation now a key idea you need to understand is that assimilates such as sucrose are transported from sources to sinks sources are where the assimilates are produced for example photosynthesizing leaves other sources include storage organs such as tubers which can release their carbohydrate stores when they're needed sinks are regions where assimilates are required sinks include Roots which carry out active transport and therefore have a high rate of respiration storage organs can also act as sinks when they're refilling their carbohydrate stores other sinks include growing regions such as shoots which contain dividing memory stem tissue now how assimilate such a sucrose are transported in the phloem is not well understood we're going to look at an active model for this process in order to understand this you need to understand the structure of the phloem we looked at this in a previous video and if you haven't watched that video then you need to watch it now I'm showing you here a source which in this case is a photosynthesizing leaf the source is connected by the phloem to a sink which in this case is the root now the key idea you need to understand is that at the source sucrose is loaded into the phloem by an active process I'm showing you here a close-up of the cell membrane of the phloem companion cell a protein on the cell membrane of the companion cell uses ATP to pump hydrogen ions out of the cytoplasm and into the spaces of the cell wall this process is active transport and this creates a concentration gradient for hydrogen ions with more hydrogen ions on the outside of the cell membrane the hydrogen ions can now flow through a co-transporter protein down the concentration gradient back into the cell this inward flow of hydrogen ions is coupled to an inward flow of sucrose into the companion cell companion cells have a large number of mitochondria which provide the ATP needed for the active transport of hydrogen ions foldings on the cell membrane also increase the surface area for the proteins involved so because of this transport process the concentration of sucrose in the companion cells is high the sucrose can now diffuse through the plasmidas MARTA from the companion cells into the sieve tube element cells and this means that we now have a high concentration of sucrose in the sieve Cube element cells the effect of this is to lower the water potential inside the sift Cube element water now moves into the sift tube element by osmosis from nearby tissues including the xylem vessels this now increases the hydrostatic pressure inside the sieve tube element as a result the phloem sap now moves up or down the sift tube element towards the sink this bulk movement of phloem sap is called mass flow at the sink the sucrose moves out of the save Cube element and is converted to glucose for use in respiration or in the case of storage organs the sucrose is converted to starch as the sucrose leaves this increases the water potential in the sift tube element causing water to move out of the safe tube element by osmosis some of this water will go back into the xylem and join the transpiration stream now there are several lines of evidence to support this active model of movement in the phloem for example the rate of flow of sucrose in the phloem is much faster than could take place by diffusion alone and if we inhibit the companion cell mitochondria then translocation stops okay so hopefully now you should be able to describe translocation of solutes in the flower [Music] thank you [Music]