so for chapter 4 what we are going to be looking at is a cell membrane and transport before we go into before we go into the chapter I want to look at a bit of Revision in chapter one if you remember in a eukaryotic cell I did say that the eukaryotic cells such as animal cells and plant cells have a cell surface membrane and they also have internal membranes forming membrane-bound organelles which means to say there are structures within the eukaryotic cell that have their own membrane for example the nuclear membrane Golgi apparatus membrane vesicle membrane the tonoplas which is known as the vacuole membrane in plant cells even the mitochondria and chloroplasts and endoplasmic reticulum they have their own set of membranes as well so what exactly are all these membranes what is it made out of and why are these membranes so important one of the importance of the membranes that I might have mentioned in chapter one is it allows for separation of function for example lysosomes contain hydrolytic enzymes and if there was no membrane the hydrolytic enzymes can go and easily destroy the nucleus called geopolitas and other structures and we don't want that to happen that is one reason why membranes are important within the cell than in Chapter 2 we looked at a particular type of lipid known as phospholipids and phospholipids were made up of one polar phosphate head attached to the glycerol which is attached to two nonpolar fatty acid tails and I did say that the polar phosphate head is hydrophilic which means to say they can interact with water and the nonpolar fatty acid tails are hydrophobic which means to say they cannot interact with water so if we were to ask the question are phospholipids water soluble they technically are water soluble because of the Polar phosphate head but not all parts of the phospholipid is water soluble the polar phosphate head is soluble in water but the nonpolar fatty acid tails are not soluble in water so what does this have to do with cell membrane the reason why phospholipids are important is because they are the main build building blocks of the cell membrane what do I mean by that how is a membrane formed the basic structure of the membrane imagine for a second you have many phospholipid molecules I've simplified the phospholipids to its hydrophilic head and hydrophobic Tails as you can see there and imagine then if we just basically put them in water now when you put the phospholipids in water I did tell you that if you look at the diagram on the right you will notice that the polar phosphate head which is the hydrophilic part is able to interact with water because the head is polar and water is also polar therefore they can bond with each other however the tail does not want to interact with water because the tail is non-polar so what happens then as you can see that phospholipid molecule will bend a little bit so that the head is facing the water this is pretty straightforward but the teal however does not want to interact with water and as all nonpolar molecules go they interact with other nonpolar molecules which are the Tails thus they form a configuration which is quite peculiar where the head faces the water and the Tails face each other and when the Tails face each other we call this something known as the hydro phobic interaction which I've also mentioned in lipids and proteins so what's the big deal so why is this important then you see when you put many phospholipid molecules in water the first thing that will happen is it will form a spherical structure now I know that it looks circular here but remember this is a cross section a two-dimensional diagram in reality it will be a sphere because if I had drawn the sphere you would not have been able to see the Tails or what's going on inside so that's why I'm doing a cross section so as you can see here what happens is the hits will face the outer environment which is the watery environment but the Tails will not face it so you might go oh this is good but remember within a cell membrane there is also liquid inside the cell and that liquid inside the cell which I've highlighted in that blue color circle inside this diagram is the cytoplasm so the Tails cannot interact with the cytoplasm too because the cytoplasm is mostly water so what's the solution to that problem the solution to that problem is what happens is the phospholipids will then form another layer where the heads are also facing the internal environment or the internal watery environment so in this peculiar case what actually happens is that's how the cell membrane forms the cell membrane forms with phospholipid hits the heads of the phospholipids interacting with the outer watery environment and the polar phosphate heads also interacting with the inner watery environment causing the tails to interact with each other we call this as something known as the phospholipid bilayer and I'm also going to label outside meaning outside the cell membrane and in which means inside the cell membrane that is how the cell membrane is actually formed so if we were to just take a section of the bilayer what we just have to describe if a question asks you in the exam how do phospholipids form a membrane all you just basically have to say is the hydrophilic heads or the polar heads interact with the water that is outside and inside this cell and the hydrophobic Tails interact with each other and thus this actually causes the formation of the phospholipid by layer the reason why we call it a bilayer is because there are two layers of phospholipid one which I've highlighted in yellow and one which I've highlighted in pink and you must also know that the phospholipid bilayer which is the basis of all cell membranes the width of the phospholipid bilayer is only about seven to eight nanometers this tells us that the cell membrane can only be seen using electron microscope because anything because the light microscope is unable to visualize or resolve anything smaller than 200 nanometers so these are all division for chapter one so in 3D theoretical drawing remember earlier I said it was going to be a hassle to draw so what I'm going to do is I'm just going to show you a 3D view of a cell if we could easily visualize the phospholipid by layers it will all just look at all you'll just basically see at the polar phosphate heads that's all you'll be able to look at in reality we can't because it's too small so this is just a theoretical drawing of a cell surface membrane but if we were to cut the cell into half we would be able to see like an orange that has been cut into half we would be able to see the skin of the cell which is the phospholipid bilayer and the highlighted Parts where you can see this greenish lines those are the Tails and if we were to just zoom into the cross section we would be able to see the phospholipid by layer this is how a membrane is actually formed in summary to form a membrane whether it's the cell surface membrane are the internal membranes of a eukaryotic cell you need something called phospholipid molecules the phospholipid molecules have heads which are hydrophilic and can interact with water and they also have tails which are hydrophobic and avoid the water thus due to the nature of the phospholipid being like this because some parts want to interact with water and some parts do not want to interact with water this causes many phospholipid molecules when put in a watery environment to form something known as the phospholipid bilayer and that is the fundamental structure of the cell membrane