hello and welcome to learn a little biology for free with mrs. trick today we're going to be going through translocation looking at how organic substances such as sugars are moved on mass around plants so this is quite a challenging topic often for students so you might want to have some paper and pen at the ready just to note down any key points as we go through so the first thing is just thinking about where these organic substances come from and that's taking us to what is the function of the leaves and one of the key functions is for face synthesis so just looking here at the very basic word equation or photosynthesis carbon dioxide plus water creates glucose and oxygen and stay is this sugar component the glucose that were interested in looking at how that is transported so glucose and other sugars which are made are organic substances because they contain carbon and all of the cells of a plants require glucose and organic substances for respiration so the phloem is responsible for transporting the organic substances made in the leaf during photosynthesis up and down the plants so that all of the cells have a source so the flowing tissue then this is what we said is responsible for the transports and phloem tissue is made up of two key cells we have sift cheap elements and companion cells so in this diagram we can see in the middle are the sieve tube elements so these cells all lined up together and on either side we have the companion cells so some of the features of a sieve tube element cell the end walls are perforated so we've got these holes in the end and that's so we can have a continuous flow of the sugar solution they are living cells but they actually don't contain a nucleus and there are very few organelles inside much like the xylem so that you have a hollow tube to ensure that you can have maximum flow of these sugary solution now translocation implants does require active transport so they don't have very many organelles and the safety of elements but the companion cells do and the companion cells are responsible for providing the ATP which is required for the active transport of the sucrose the organic substances into the sieve-tube element and then it can be transported on mass so this is all linked to what we call the mass flow hypothesis and it's linked to the idea of there's always a source which is where the organic substance is created and in this example we're thinking about sucrose and that is created in the leaves during photosynthesis the sink is where the suit Crow's is going to be transported to induced sucrose is going to be used by the respiring cells so that is where it's going to be transported to we're going to go through exactly how we go from the production in the source cell to the sucrose or could be glucose transported into the sieve tube elements all the way to the sink cell but first let's just think about this source to sink mass flow hypothesis in a bit more detail using a basic model before we go on to looking at all the cells in the plant so I've simplified it here just looking at our source cell which is a leaf cell which is going to be photosynthesizing that will be connected to a sink cell which will be any respiring cell and the phloem is always next to the xylem which will be transporting water in this model we're thinking about if we just had it in a tank of water and what would happen but actually instead of it being a tank of water you could imagine that as all of the other surrounding plant cells around the leaf and the respiring cells so because the photosynthesizing leaf cell is going to be creating glucose or sucrase or other organic substances those are all soluble and that will lower the water potential of the source cell and as a result the water from the surrounding cells or in this model the tank of water will move into the source cell by osmosis at the sink so that cell is going to be respiring and therefore using up lots of the soluble sugars so there won't be as many soluble sugars left in the sink so so that cell will have a more positive water potential compared to its surroundings and that will mean water will move out of the sink cell to the surrounding cells or to the xylem by osmosis that will have an impact on the pressure of the two cells so the source cell will have an increased hydrostatic pressure because more liquid is moving into it whereas the sink cell will have a decreased hydrostatic pressure because the water and liquid is moving out so because the source cell comparatively has a higher pressure that will force the liquid the sugary solution inside of the source cell up and out of the phloem towards the sink cell and that will then move this sugary solution on mass to the respiring cells that require that sugar so that's the model of mass flow hypothesis and what we mean by going from the source cell to the sink cell but we're going to add in the actual details of the role of the companion cell and the sieve tube element and also the xylem in translocation so step one we're going to start here on the diagram at the source so we're these sucrase is created in phase synthesis in the leaves and the first thing then is once we've created those organic substances such as sucrose they get transported into the companion cell and this is going to be by diffusion because we are going from a high concentration where the sucrose is created to a lower concentration in the companion cell it's facilitated diffusion though because sucrose and glucose can't simply diffuse through and plasma membranes they have to go via a carrier or a channel protein now at the same time we also have active transport hydrogen ions from the companion cell into the space in the cell wall which is just here in this light green section and that's going to create a concentration gradient of there being lots and lots of hydrogen ions or protons within the cell wall the reason that's important is the way sucrose is transported from the companion cell into the sieve tube elements of the phloem is by co-transport and it has to be Co transport with the hydrogen ions so now we've got lots of lots of hydrogen ions in the cell wall they can attach to the co transporter protein to transport back into the companion cell and in doing that the sucrose can attach to the carrier protein and be transported into the sieve tube elements so now we've moved a sucrose into the sieve tube element we need to think how does that sucrose get transported along the sieve tube elements and this is the bit where it's linked to pressure changes so the first thing is now we've got lots of dissolved sucrase in the sieve tube elements it lowers the water potential in this section of the phloem the xylem lies directly next to the phloem and as a result water moves into the sieve tube elements by osmosis because we've got water now moving in we've got lots and lots of liquids we've got very very high volume and that creates a high hydrostatic pressure and that high hydrostatic pressure then forces the liquid towards an area of lower hydrostatic pressure so we need to look at then why is there a lower hydrostatic pressure by the sink cells and how do or how does the sucrose then get transported into the sink so so in the sink so that is where respiration is occurring so the glucose and sucrose is constantly being used up in respiration or it could be being converted and I'm turning in soluble starch more secrets gets actively transported from the sieve cheap element into the sink so and because we are now transporting lots and lots of sweet crates into the sink so the water potential decreases so the water that was in the sieve cheap element will move by osmosis into the sink so some of it will actually move bars Moses back into the xylem as well because as the water and as their sucrase leaves the phloem that's area when I have a more positive water potential the result of this there is there is now a lower volume at this section of the phloem and because the volume is lower the hydrostatic pressure is lower and that's how we continually maintain this high hydrostatic pressure at this section of the Civet rebelling near the saucer and a lower hydrostatic pressure in the sieve tube element near the sink so now finally you can actually do investigations to identify is it definitely the phloem in which the organic substances are being transported in a plant so one of these methods is called traces traces and the traces involves tracking or tracing the location of radioactively-labeled carbon so what they would do is they'd have a particular plant than investigating that have it isolated and only provide carbon dioxide which has been radioactively-labeled every period of time that plant will be absorbing that calm dioxide through its stomata by diffusion that carbon dioxide will be used by faith synthesis and will create the sugars so that glucose and C craze in the plant now all of the carbons within those sugars will have come from this radio actively labeled carbon dioxide so now all the sugar in the plant will be radioactively-labeled as well and this radioactively-labeled carbon it is placed on x-ray film it turns the x-ray film black so that means if we were to take thin slices through the stem place it on the x-ray film we can then identify which parts of the stem contain the sugar and therefore identify which cells these sugar has been transported in if you could also do that with a whole stem rather than just a slice place an x-ray film and you can see the area where the phloem is that would be highlighted by the radioactive sugars and turn black on the film to show you that the phloem is where the sugars are and therefore it's proof that these sugars are being transporting the phloem another way to investigate this translocation is called wringing experiments and in these experiments they'll take a ring of the bark and phloem off the tree and that's what we can see here that complete ring has been removed what you'll find happening over time then is this part of the tree above the ring will start to swell and that's because all of the phloem have been removed so that sugary solution can't be transported any further down so it starts to swell just above the ring now to prove that is a sugary solution you can take samples of the liquid I haven't tested for sugar and in doing that they've proven that it is the phloem that transports sugars because when they take a ring of those phloem out the sugars don't get transported any further so in summary and the mass transport of organic substances is known as trans vacation most transfer of organic substances such as sucrose or glucose implants and is from a source which is the leaf to the sink which is the respiring cells and that movement is due to changes in hydrostatic pressure the high hydrostatic pressure at the source site is created by the active transport of sucrose into the safety element which lowers the water potentials of water then moves in bars Moses traces and ring and experiments can be used to investigate the transport of these organic substances and in the new spec papers those have been quite common application questions looking at the tracer results or ring and experiments so that's it for translocation and that's one of our other lessons for the year 13 learn a level biology for free and I'm going to link the disaccharides video just so you can have a recap about sucrose as well as the year thirteen learned channel in case this is part of your loop of revision don't forget subscribed as well click it for all the latest videos as they are being uploaded