so um in the previous video what we were looking at was the overview of how sucrose and amino acids were transported along the flm from a source to a sink and in that particular video I did mention a process known as active loading and I did say that active loading is a slightly complex process that deserves its own video by the way for this video we are just going to be focusing on the ACT loading of sucrose because that's what you require to know for the exam I know that the source produces amino acids and also sucrose molecules but usually in the purpose of the exam uh they would just focus on how sucrose is transported in the flow up uh but if a question asks you how amino acids were transported it's the same way anyway so active loading basically means the sucrose is transported from the source to the flm SE tube element in this case here what we are going to talk about is the process of active loading what exactly happens during this time because it's not just like a simple diffusion that happens from a high to low concentration the process is a little bit more complicated than that so remember in the process of active loading it involves three places it involves the mesophile cells which is the source uh the source meaning to say it produces the sucros and amino acids or it gives the sucros and amino acids and it also involves the companion cell and the flum SE tube element remember the companion cell in the flum sea tube element make up the flum tissue so what I'm going to do here is I'm going to draw out the mesop cell I'm just going to make it bigger and I'm going to represent the mesophi cell in green the green color cell and I'm also drawing out the companion cell right here adjacent to the companion cell is the flum sea tube element and how do I know that's the flum SE tube element you can see the sea plates and the lack of cytoplasm it has very little cytoplasm not as much as the companion cells now I know that the mesophile cell and the companion cell are supposed to have organel like mitochondria uh nucleus rough ER and such but for the purpose of this video I'm going to remove all the organel because it will become too messy if I were to draw out all the organel but bear in mind that the mesophile cell and companion cells do have important organel all right so in this case what else we're going to do is we're going to focus on the companion cell you see on the cell surface membrane of the companion cell it will have two transport proteins the first transport protein it has is a hydrogen ion sucrose Co transporter which is a type of carrier protein now if you remember going back to chapter 4 I did mention a little bit about Co Transporters I will put a link up at the top right corner of the screen if you would like to go and visit that video again Co transporter molecules are just basically transport proteins that transport two substances at the same time in this case it is transporting hydrogen ion and sucrose at the same time not just one by the way and this Co transporter protein follows the hydrogen ion concentration gradient not the sucrose concentration gradient what I mean by that I'm going to explain that in a while uh just just hold on the second transport protein that the companion cell also has is the proton pump or also a hydrogen ion pump in this video I'll use the word proton or hydrogen ion interchangeably because they're the same things protons are just hydrogen ions and in the exam you can use the word hydrogen ion you can use the word proton no problems and when you see the word pump a pump is a type of transport protein that will only carry out active transport so how are these transport proteins involved in the process of active loading you see the first thing we're going to do is I'm going to draw out some hydrogen ions inside the companion cells and I'm also going to draw out some hydrogen ions in the cell wall of the companion cell and the mesophile cell you just have to understand that they exist between these two places now so far so good and then the next thing we just have to talk about is the mesophile cells and the mesophile cells because they are a source they will produce the sucrose which I've represented in the pink triangles and what the mesophile cell will do is the mesophile cell will just transport the sucrose into the cell wall now this is not the process of active loading yet this is just setting up the stage for active loading to happen so how does the sucr move all the way to the FL SE tube element then now to start the process of active loading what we have to do is notice what I'm highlighting I'm actually highlighting the hydrogen ion outside the companion cell and also the hydrogen ion inside the companion cell and you notice that the concentration of hydrogen ion outside the cell and the concentration of hydrogen ion inside the cell are the same so what the big deal for this well the big deal is what exactly will happen here is the companion cells mitochondrian will produce some ATP and the ATP will provide Power or energy to the proton pump and remember I told you earlier that the pump carries out active transport so when the pump which I've highlighted when the pump receives ATP it will pump hydrogen ion from the cytoplasm into the cell wall as I'm showing you here look at the hydrogen ion they are moving from an area of low concentration to high concentration or against the concentration gradient that's the first thing that is supposed to happen so in the exam you can just say that for process number one in active loading active transport of hydrogen ion from companion cells to the cell wall occur and it uses the proton pump why is this important is because it creates a proton gradient now what do I mean by a proton gradient look the concentration of hydrogen ion outside the companion cell is much higher than the concentration of hydrogen ion inside the cell so this is now referred to as a proton gradient so in this case here what exactly will happen is the hydrogen ion will diffuse passively down the concentration gradient through the co transporter molecule which I've represented in the yellow yellow arrow but remember the coold transporter doesn't just transport hydrogen ion the beautiful thing about this is as hydrogen ion moves through the Cod transporter one sucrose molecule will also follow at the same time another hydrogen ion Moves In from high to low concentration and the sucrose also follows and then the third one another hydrogen ion goes in and another sucrose molecule follows you might be thinking isn't is just like diffusion of sucrose why why does the companion cell go through all this trouble here's where it becomes a bit more interesting now look at the concentration of sucrose outside the cell and inside the cell there are three sucrose outside the cell and there are three sucrose molecules inside the cell so in this case diffusion of sucrose is no longer supposed to happen but the beauty is the hydrogen ion still has a concentration gradient because the outside still has a higher hydrogen ion concentration and the inside has a lower hydrogen ion concentration so hydrogen ion continues to diffuse through the co transporter and as it continues diffusing through the co transporter the sucrose molecule follows without bothering about the sucrose concentration gradient that's what I meant earlier when I said that the hydrogen ion sucros cor transporter only follows the Hy hen ions concentration gradient it doesn't care about the sucrose molecules concentration gradient at all as long as one hydrogen ion moves through the cor transporter by diffusion down the concentration gradient a sucrose molecule will automatically follow and it doesn't have to expand any ATP in this process so therefore process number two is hydrogen ion passively diffuses into the companion cell through the co transporter and takes in sucrose at the the same time and the co transporter only follows the hydrogen ion concentration gradient and in this case of course the sucrose molecules are now inside the companion cells they can easily diffuse into the SE tube element through the plasmo Desma that's how active loading is supposed to take place process number one number two and number three so there is active transport of hydrogen ion uh in the proton pump but then Co transporter Co transporter molecules will take over to help the sucrose molecules enter the companion cells so if you're not so sure let's try this again so process number one ATP Powers up the proton pump and hydrogen ions are actively transported from the companion cell cytoplasm into the cell wall this creates a proton gradient where there's a high concentration of hydrogen ion in the cell wall and a low concentration of hydrogen ion in the companion cells and as the hydrogen ion diffuses into the companion cell through the co transporter it will take in the sucrose molecules as well and the sucrose molecules will then diffuse into the flo sea tube element through the plasmo Desma that is the entire process of active loading that we have to know