the last type of transport across the cell membrane is known as bulk transport before we talk about transport in detail I want to do a little bit of igcsc or O-level revision now if you studied all levels this is just an example by the way I do need you to memorize this one when you studied all levels you will know that there is a type of white blood cell called lymphocytes and the function of lymphocyte is to produce and release the antibodies and antibodies are this large protein molecules are supposed to bind to the antigens on the pathogen pathogens are just harmful microorganisms so Point here is antibodies are just things that our lymphocyte produces to fight off harmful pathogens that are invading our body but here's the thing the antibody has to meet the pathogen as you can see in this Arrow the problem is antibodies are extremely large and they cannot just easily move out of the cell now some students will say well maybe they can just diffuse through the bilayer no they can't because they are quite polar can they use a carrier protein no because the antibodies are significantly larger than the carrier protein so the question here is how is the antibody going to be released out of the cell then some of my students will say well maybe the cell membrane just breaks and lets the antibodies out but if the cell membrane break then your lymphocytes might die so so we have a problem here right so how are the antibodies going to go out of the cell with that in mind let's talk about the first part of bulk transport which is known as exocytosis now in exocytosis what actually happens is remember when the cell produces the proteins okay for example antibodies it can be any other proteins for that matter but I'm going to use antibodies as my example yeah the proteins once they are produced by the cell it will go through the Golgi body and one of the function of the Golgi body is to package the protein what does it mean by packaging the protein they will put the proteins into vesicles basically now the thing here is once the protein is put inside the vesicle the vesicle will then move towards the cells of its membrane as as I'm putting over here and what happens here the vesicle membrane touches the cell surface membrane and when it touches the cells of its membrane something pretty spectacular happens what is going on here the vesicle membrane and the cell surface membrane will fuse with each other and when they fuse with each other the look at the second diagram the vesicle becomes a part of the cell surface membrane and what happens to the antibodies or the large proteins inside the vesicle they will then go out of the cell so this is how the cell is able to release or secrete out extremely large substances that normally cannot cross through the cell membrane by itself so what happens over here is the vesicles will fuse with the cell surface membrane and push the substances out of the cell common mistakes that students will say is the vesicle is released out of the cell no that is wrong the vesicle is not the thing that is released out of the cell the content of the vesicle was released out of the cell in this case the antibodies so this process is known as exocytosis and of course it also requires ATP this is a very important thing that we have to understand about exocytosis now endocytosis however again just let's do another Revision in all levels you might have studied another white blood cell known as a phagocyte I always tell my students that phagocyte is like Pac-Man all it does is just eat substances right it engulfs and eats them but look at this phagocyte and look at the bacteria the pathogen the pathogen is too large so how is the phagocyte able to eat it this is another type of bulb Fun Spot called endocytosis and in endocytosis what happens then is it is just the bulk movement of liquids of solids into the cell for example as you can see there's a cell here with its cell surface membrane and it has a solid substance an example of a solid substance is a bacterium and let's say my cell this very large cell wants to ingest the bacteria what it does is the cell surface membrane will fold inwards and it will engulf it and forms a vesicle containing the substance so this is one way endocytosis can happen another way endocytosis can happen is sometimes my cell may require an extra amount of water and dissolved substances so again same thing the cell surface membrane Falls inwards and it will suck in the water and the dissolved substances represented in the red dots and then it forms a vesicle inside the cell that's basically it you have to understand that your cell surface membrane is a very fluid and flexible structure so it's able to fold and change its shape to form a kind of mouth to take in the external substances whether it's solid like the bacterium or liquid like water and dissolved substances endocytosis is further divided into two which is phagocytosis and pinocytosis and phagocytosis is just a way of saying it is cell eating where it is taking in a solid substance or if it's pinocytosis then it's the bulk movement of liquids into the cell but whether it's pinocytosis or phagocytosis both of them involve the folding in of the cell surface membrane to form vesicles and it's also very important to know that it requires ATP so this is all we have to know about bulk transport