[Applause] [Music] hello everyone welcome to as biology with Dr Demi I am Dr Demi and in today's video I will be starting with chapter 4 of the as biology syllabus which is cell membranes and transport if you have just stumbled on my channel I encourage you to please check out the other videos that I have done you will see that I am posting the as biology content in chronological order so that it is easy for you to follow so if you have any questions about the content or you would like some clarification make sure to leave me a comment and don't forget to share those videos with your friends so that they are able to revise for the exams in today's video just like I said we will be starting with cell membranes and transport which is chapter 4 of the as biology syllabus the objectives of this video are to help you recall the structure of phospholipids so we discussed this in the first chapter where we were discussing the different things that make up a salt the different components of a salt and we said phospholipids make up the cell membrane we would also be looking at how to identify the two main types of proteins that are in the cell membrane as well as understanding the role of cell membrane components and cell signaling so I hope that you're excited um we'll try to keep this as short as possible but please make sure you pay attention now if you recall what we covered in our first chapter um if you haven't checked out those videos please make sure you do so but we did say that the phosphor lipid is a molecule that has a phosphate head which loves water and two hydrophobic Tails which do not love water phospholipids make up the cell membrane and you can see in the blue diagram here I'm trying not to use the pointer on PowerPoint because I noticed that it is usually out of sync in a video so please just look at the blue diagram and you can see the arrangement of phospholipids in a cell membrane we call that the phospholipid by layer what this basically means is that there are two sets of phospholipids and what you can see there is that the phosphate heads are on the outside while the hydrophobic Tails which are the lipid tails are on the inside and this is how the cell membrane is able to regulate the things that are able to travel through it now when we discuss the cell membrane we use a model called the fluid mosaic model this fluid mosaic model was discovered by two men singer and Nicholson who pictures are on the slide and what they discovered is that when you look at the cell membrane you would see that there are proteins scattered around it the cell membrane itself is not a solid structure it has a bit of a liquid consistency more like an oily consistency if you may and it has proteins that are scattered around it that makes it look like a mosaic especially if you look at it from the top and this is what that Mosaic looks like if you look at the image there and you can see in the cell membrane that it is made up of phospholipids as well as additional components such as carbohydrates you have things called glycoproteins and even cholesterol and here's a fun fact that I usually share with my students in the classroom that if you don't eat enough cholesterol your body will make it cholesterol is an important part of your cell membrane and you will see why as we go along this is a neater image of the uh fluid mosaic model and you can see that this shows you very clearly the different types of proteins that are within the cell membrane phospholipid tails are saturated Tails some of them are saturated some of them are unsaturated and the longer the length of the tail the less fluid the membrane but that is not usually a question that pops up in your cie exams what you do need to know however are the important parts of the cell membrane so you can see there that there is something called a channel protein and you're going to see that we have channel proteins and carrier proteins in the cell membrane we also have what we call glycoproteins a glycoprotein is simply A protein that has a carbohydrate attached to it to it you also have glycol lipids which are lipids attached to a carbohydrate in other words the part that says glyco is referring to a carbohydrate there is also cholesterol just like I said that is an integral part of the cell membrane now there are two types of proteins that you would find in the cell membrane you have what we call the integral or intrinsic proteins and these proteins are found in the inner layer the outer layer or are commonly around the whole of the membrane when they are found around the membrane we call them transmembrane proteins you also have the peripheral or extrinsic proteins which are found on the inner and outer surface of the membrane and are sometimes bound to the integral proteins now this is just to give you information it doesn't necessarily play role in preparing for your exams when you get into the exams chances of them asking you about integral or extrinsic proteins are very very slim what you're likely to encounter are questions about the channel proteins and the carrier proteins so like I have been saying cell membranes are made up of phospholipids cholesterol glycolipids glycoproteins and other membrane proteins such as the channel and the carrier proteins and again phospholipids can be modified chemically to act as signaling molecules and we will see as we go along how cell signaling occurs in the cell membrane but you would find that sometimes you have a glycolipid which means there's a carbohydrate attached to the phosphate head of the phospholipid and what this carbohydrate does is that it acts as a receptor in order to receive signals from other cells but we will get into that in just a little bit now what role does cholesterol play in the cell membrane Chester olol molecules actually have hydrophilic heads and hydrophobic Tails cholesterol is less present in plant cells which is why if you want to eat cholesterol free food you're likely to get them from plant cells and it is also absent from procaryotes and animal cells are regarded as eukaryotic cells while procaryotic cells are more of bacteria procaryotes do not have any cholesterol in them when temperature is low what cholesterol does is that it increases the fluidity of the membrane by preventing close packing of the phospholipid tails and what it does is that that enables the cell to be able to survive a colder temperature and when temperature is high cholesterol would then also regulate the fluidity of the cell membrane the point is that the cell membrane has to maintain some level of fluid consistency if it doesn't do this even molecules that are able to pass through the phospho lipid by layer would not be able to cross through because the membrane would be in a solid state so think of it as trying to penetrate a drop of oil versus a block of butter penetrating a block of butter is definitely harder than penetrating a little bit of oil so the cholesterol in the cell membrane simply tries to maintain the fluidity of the membrane so that the cell doesn't become too fluid and it also doesn't become too solid without cholesterol cells would burst open and so in a way it maintains mechanical stability I bet you didn't think that cholesterol played such an important role in your cell membrane now like I said earlier we have glycolipids and glycoproteins glycolipids being lipids and a carbohydrates while glycoproteins are proteins and a carbohydrate and the glyco part refers to the carbohydrate what the carbohydrate sections do is that they help to stabilize the membrane structure they also help the glycoproteins and the glycol lipids to act as receptors this is very important as I go into cell signaling later on in this video because you will see that cells talk to each other by sending out signals in the form of small molecules those small molecules have to have a receptor to bind to on the surface of the next cell that they're trying to communicate with in order to get a response to the signal you also have glycolipids that act as cell markers or antigens this is important in immunity I'm sure you have heard of autoimmune diseases whereby the immune system attacks the cells of the person or of the host what this means then is that some of the markers on those cells might be faulty causing the immune system to think that they are infected cells or they are strange cells foreign cells and as a result of that the immune system would attack them but I don't want to go into detail of that I want you to wait until we get to chapter 11 to discuss that also like I said we have the transport proteins they are channel proteins as well as carrier proteins just a little something about the cell membrane receptors so you can have what we call the signaling receptors and the signaling receptors just like I explained earlier are part of a signaling system whereby they recognize messenger molecules or what I call the signal molecules and as a result of recognizing the signal molecules and binding to them they can elicit a response either in the form of a chemical reaction within the body or in other ways that ensure that there's a response to whatever change the body is experiencing we also have endocytosis receptors and these are receptors that bind to molecules on the surface so that they can be engulfed by the cell all of these become very very important when we discuss immunity in chapter in chapter 11 but also in chapter 15 when we discuss synapses you will see that the way nerve cells communicate with each other in the nervous system also has to do with The Binding of liens to receptors or of signaling molecules to receptors so how do cells talk to each other well this is basically the summary of it you have a sending cell which sends out a message that message is sent out in the form of a small molecule called a ligand the liond travels to the Target cell which has a receptor in the form of a glycolipid on its cell membrane the Ligon binds to that receptor and as a result of that the receptor is able to transmit the message from the outside into its cytoplasm which would then cause a response so cells communicate by using signaling molecules which are called liance just like I said signal molecules can be hydrophobic so you can have a steroid hormone like estrogen and hydrophobic molecules are able to cross the cell membrane because remember the cell membrane is made up of phosphate heads and hydrophobic Tails those hydrophobic Tails form the Gateway in a way and so whenever it is a hydrophobic molecule that's coming through it is able to pass through however more commonly you would find the signal molecules are hydrophilic which means that they cannot pass through the cell membrane and as a result of that they have to use a different signal in Pathway to communicate the message that they are carrying this is a summary or should I say a stepwise process of what happens in cell signaling so we have the signal arriving at a protein receptor in the cell membrane this protein receptor again can be a glycoprotein or a glycol liid and what happens is that the receptor has a shape that is specific to the signaling molecule so the signaling molecule is able to bind to this receptor once the signalin molecule binds to the receptor it results in a change in the receptor that's a change of the shape of the receptor and that allows the receptor to bind with a new component which is called a g protein a g protein is like a switch within the cell and what it does is that it activates a second messenger so we consider the ligant that arrives at the receptor as the first messenger the second messenger is a group of small soluble molecules that are able to amplify the signal that has been sent and spread the message across the cell once the second messenger simplifies the message and activates um a bigger response or should I say a bigger signal within the cell what will then happen is there will be an activation of enzymes to respond to whatever change there is so if I were to explain this in simple terms I would use um say sugar concentration as an example so when you take a high uh carbohydrate breakfast for example you will likely have an increase in your blood sugar what would then happen is that there is a A liant or a message that is sent to the cells to say listen there is a lot of sugar in the blood and so we need to take up this sugar that liend is usually insulin insulin is secreted and then it binds to receptors in the cell membrane and Insulin will then activate the cells in order to make them take up glucose from the blood and convert it to glycogen that is something that you will learn in deeper detail when we get to chapter 14 besides activating second diens Messengers within a cell in order to elicit a response a receptor can also alter the activity of the cell by opening an ion Channel this is very important when we discuss nervous coordination because you will see that the transmission of impulses through the nervous system is dependent on the opening and closing of ion channels it can also act directly as a membrane bound enzyme whereby if the signal binds to the receptor the receptor itself is able to act as an enzyme and reacts to to the change or to the message that is being communicated it can also act as an intracellular receptor when we say intracellular we mean within the cell so remember I said that some liens or some signal molecules are hydrophobic and are able to cross through the cell membrane directly into the cell so what this means then is that when they cross into the cell membrane they have a receptor on the inside of the cytoplasm that they bind to so receptor cells which are inside the cytoplasm are intracellular receptors and they are able to respond to the signal from inside the cytoplasm and not necessarily from the celf surface membrane this is all that I have for you today on this video it is again a summary of notes that I am sure you have learned in class if there is anything that seems confusing please make sure you ask me a question in the comment section and make sure you watch this video again if you seem confused um share it with your friends and don't forget to hit the Subscribe button thank you for joining me today until next time goodbye [Music]