so without wasting any time let's let's talk about the function of the flow you see the flum itself it is a vascular tissue so it transports substances as well the function of the flum is to transport sucrose and amino acids just a little bit of reminder for you chapter 2 sucrose is a disaccharide it is a nonreducing sugar and it's made up of alpha glucose and fructose linked together by glycosidic bonds just a little bit of revision all right and the sucrose and amino acids are transported from something called a source to a sink so you might be thinking looking at the plant here I'm showing you Leaf stem and root and some students will go wait what is a source is it a particular part of the plant is it a special part of the plant well a source is just a part of the plant that provides the sucrose and amino acids and a sink is just the part of the plant that receives the sucrose and amino acids so most of the time not always but most of the time the sauce is usually the part of the plant that can produce the sucrose and amino acids through photosynthesis and obviously that part of the plant has to be in the leaf and particularly it will be the mesophile cells uh spongy mesophile cells or pisit mes cells these cells are referred to as the source because they are able to synthesize the sucrose and amino acids therefore they are able to provide it to other parts of the plant that's what the source is now the sink is a part of the plant that usually cannot synthesize or make the suos and amino acids so they have to receive it from somewhere else and for example a particular sink is the root cortex cell because the root cortex cells do not have chloroplasts right because why would they need chloroplast they are underneath the soil they will not be able to receive sunlight so in that case um the root cortex is the sink so the transport of sucrose and amino acids as you can see those yellow color dots for example let's just imagine them to be sucrose and amino acids uh they will be transported from the leaf cell from from the pisit mesophile cell to the root cortex cell via the flum tissue so that's what the flum does the flow makes sure that the sucros and amino acids from the leaf go all the way to the roots where they are required so just taking out the mesophile cell and the root cortex cell sauce and sink respectively remember they are connected to each other through the floor and I told you before in the pr previous video the flm is actually made out of two types of cells which are the companion cells and also the flam C tube so the question here is how exactly do the sucrose and amino acids move from the mesop cells all the way to the uh root cortex cell for the purpose of this video I'm only going to focus on sucrose I know that the FL transports amino acids but how now uh if the question most of the time the question will just focus on sucrose all right but if you need to explain the transport of amino acids just replace the word sucrose with amino acids and it's very much the same but for the purpose of this video I'm just going to focus on suc close only all right so the first thing that actually needs to happen is the mesop cell synthesizes the sucrose which I've represented in the pink triangle and what happens first is a process known as active loading of sucrose active loading just basically means the sucrose is transported from The Source into the flam SE tube element as you can see over there represented by the red arrow now you might be thinking as a student wait isn't that just normal diffusion doesn't it just move from a higher to lower concentration not exactly active loading is a rather complex process and it deserves its own video but for now all you just have to know is active loading of sucrose happens where the sucrose is transported into the flo C tube element from The Source simple stuff first now what happens is remember I told you that the flo SE tube has some cytoplasm right uh so because the sucrose is now dissolved inside the FL SE tube cytoplasm it lowers the water potential in the sea tube element which I've highlighted over there you might be thinking why does the water potential become lower well remember when we studied osmosis in chapter 3 the higher the solute concentration in a particular area the lower its water potential that's just the relationship it has and another very important thing to also understand don't need to memorize this is the flow SE tube element is not just next to a companion cell there will be other parenchima cells that I'm just drawing over that it's just surrounding the flo tissue as well so why is this important is because when the flo sea tube element has a lower water potential the surrounding cells will have a higher water potential by comparison so what's the big deal if it has a higher water potential in the surrounding cells well obviously water from the surrounding cells will then rush into the flo sea tube through a process known as osmosis so why is this a big deal when water rushes in through osmosis in that case look at what happens to the the flam SE tube when the water rushes in the volume of the cell increases obviously the volume of the cytoplasm increases and look at the sea tube element the sea tube element starts to swell up due to the excess water rushing in and because it swells up obviously the pressure inside the SAA tube element will increase and in that case by comparison the FL sa tube element at the bottom has a lower pressure because earlier they both have the same pressure but because water rushed into the SE tube element at the top the sea tube element at the top will now have a higher pressure and by comparison the one at the bottom will have a lower pressure and this creates a pressure gradient which you kind of have seen in the xyl vessels as well pressure gradient just means that one area has a higher pressure one area has a lower pressure it's just a difference in pressure between two areas and therefore when there's a pressure difference a process known as mass flow occurs look at the cytoplasm look at the water the water starts draining downwards uh from a higher pressure to lower pressure and because the sucrose is dissolved together with the water wherever the water goes the sucrose follows this is known as mass flow where the water and solutes move down the pressure gradian and once it reaches the bottom look now the sucrose is at the bottom of the flum and it is near the sink and at this point the sucrose is unloaded into the sink this is how substances are transported in the flm first process that happens is active loading of sucrose into the flam sea tube and the second thing is it lowers the water potential of the flam sea tube thus water rushes in by osmosis at the the floam SE tube will have a higher pressure and this creates a pressure gradient where there's a difference in pressure and the process known as mass flow occurs where the water and the dissolved substances will move uh down the pressure gradient and the sucrose will reach the bottom the destination and it's unloaded into the sink uh for example the sink here is the root cortex but it can be also any other parts of the plant a very important thing that I want you to understand is a lot of times students assume that the sauce is always at the top and the sink is always at the bottom as I'm showing you over here but not necessarily though for example if I were to draw out a fruit here and the fruit requires sugar the sucrose will have to be transported from the leaf towards the fruit which is above right so in this case transportation in the floor is not just always moving from top to bottom in some situations it might also move from bottom to top as I'm showing you in this diagram the sauce is located at the bottom and the sink which is the fruit cell is located ated at the top in this situation so students might go oh my God is this movement going to be different not exactly the same principles actually will apply what I mean by that is number one the mesophile cells will still synthesize the sucrose as usual and the sucrose is actively loaded into the flam SE tube same process happens by the way and once active loading of uh sucrose happens in the flum SE tube there will be more solutes which are the sucrose inside the flum SE tube element so the water potential of that particular Flo sea tube element which I've highlighted it will decrease the water potential will decrease and because the water potential decreases um water from the surrounding cells of the SE tube element will rush into the cell by osmosis and when water rushes into the cell by osmosis that particular SE tube element swells up and when it swells up it's so full of water that it has a very high pressure and it creates a pressure gradient because now the high pressure is at the SE tube element at the bottom and the low pressure by comparison is at the sea tube element at the top so there is a pressure gradient which is the difference in pressure and thus mass flow occurs where the water and the dissolved substances just move along the pressure uh from high to low pressure and in this case it moves upwards because that's what the pressure gradient was like so mass flow of sucrose along the FL SE tube occur and they reach the top which are the fruit cells and in this case active unloading of sucrose takes place for this particular diagram the sauce is at the bottom and the sink is at the top but uh for this particular diagram the sauce is at the top the sink is at the bottom that's all we just have to understand so as students you just have to know that uh movement of sucrose and amino acids in the Flor uh can occur in both directions either top to bottom or bottom to top not at the same time obviously uh and um you just also have to be familiar with the uh particular process active loading lowering the water potential of the FL SE tube water rushing in by osmosis uh creating a pressure gradient and the mass flow these are the key words that you'll have to talk about in the exam do not worry so much about active loading because we will be doing that in the final video for this chapter