so in the previous video we were talking about how some substances can move across the phospholipid biolia easily such as nonpolar particles but slightly polar particles such as water and extremely polar particles such as ions glucose and amino acids will have greater difficulty passing through the body layer so in this particular video we are going to be looking at the types of diffusion and if you remember diffusion is just a net movement of particles from a region of higher concentration to lower concentration or down the concentration gradient due to random motion of particles so we're going to see in this particular video how particles diffuse across the cell membrane the first type of diffusion we are going to be looking at is something known as simple diffusion some books will just call it diffusion and simple diffusion is just when the particles diffuse across the phospholipid bilayer directly and only nonpolar particles can pass through the phospholipid bilayer very easily such as oxygen and in the previous video we also mentioned carbon dioxide fatty acids and steroids so they can move through the bilayer without any problems whatsoever but you also remember that based on this diagram glucose amino acids ions and other polar molecules cannot pass through the phospholipid bilayer directly because these particles are polar so if that's the case then how do this particular substances move across the cell membrane well in that case they will have to rely on another type of diffusion referred to or known as facilitated diffusion you see facilitated diffusion is what happens when the particles diffuse across the cell membrane but instead of passing through the phospholipid bilayer what exactly happens is they will actually pass through something known as a transport protein so anytime the particle diffuses through the membrane through transport proteins we will call that facilitator because the word facilitated just means to make a process easier and that's what transport proteins do the transport proteins actually make it easier for Polar substances to cross the cell membrane because they couldn't cross through the phospholipid bilayer directly both simple diffusion and facilitated diffusion they do have similarities in in the case where they both do not require ATP and they the particles move down the concentration gradient from a higher concentration to lower concentration so this is just an example of passive transport that is happening right here now with that being said we want to look at facilitated diffusion further in detail and what you have to know for a levels is facilitated diffusion is diffusion of particles using the transport proteins and you must know that there are two possible type of transport proteins in facilitated diffusion the first type of transport protein is referred to as something called as a channel protein now what exactly is a channel well you can imagine a channel to be a kind of like a like you know when you when you Google the word Channel what you'll get is like a river or a body of water traveling through to land masses as you can see in this diagram here and to use these diagrams we can actually kind of um we can make an analogy of these diagrams to describe the channel protein you see if I were to draw out a closed section of the cell membrane you'll see that the purple color structure over there is the general protein now as you can see the ions there's a higher concentration of ions on one region and a lower concentration of ions on the region below but what actually happens here is as you can see in the second diagram the channel protein opens and when the channel protein opens it will actually give a passage known as a water filled paw a pore is just like a hole okay so when the channel protein opens the water filled pore also referred to as a hydromophilic channel it allows the ions to pass through down the concentration gradient because the ions can dissolve in water so it can just go through the water filled paw without any problems and it allows the ions to cross the membrane down the concentration gradient and this is referred to as facilitated diffusion something very important to know about channel protein is that they are extremely specific in my diagram here at the top in the Box you can see that there is a sodium ion channel protein and a potassium ion channel protein the reason why we are quite specific in naming them is because the sodium ion channel protein only allows sodium ions to diffuse through and the potassium ion channel protein only allows potassium ions to diffuse when we are studying chapter 15 coordination we will be looking at sodium ion channels potassium ion channels are calcium ion channels all these channel proteins are specific to What ions they allow to diffuse through them so that's what I just want you to understand about channel proteins an extra bit of information that you do need to memorize for the exam is it's sometimes just good to know that the channel protein is not actually like a sandwich or two pieces of bread that gets separated I know in my diagram here it looks like you know the the purple color just looks like two pieces of hot dog buns and then you just separate them but that's not exactly what happens the channel protein itself actually looks a bit like this and what happens is uh in my three-dimensional drawing as you can see this channel protein is closed but when it opens you see a tiny hole in the middle over there and that tiny hole will form a tube or a hollow passage where that is referred to as the water filled paw and when there is the water filled paw or the hydrophilic passage that's the thing that allows ions to pass through down the concentration gradient the second type of transport protein in facilitated diffusion is referred to as a carrier protein now the thing about carrier protein is it looks a bit weird it looks quite different from the channel protein where in the highlighted region there is a site complementary to a specific substance some students will ask well is this an enzyme then can I call it an active site no this is not an enzyme so you can just only in enzymes we refer to it as active sites to catalyze a chemical reaction but this site in the carrier protein is not to catalyze a chemical reaction so what does this particular highlighted region do then well really simple as you can see here there is this there are these red color particles that they are trying to pass through and they cannot pass through the phospholipid bilayer because they are polar so what happens is one of the particle binds to the complementary sign and when it binds to the complete treatment to reside it causes the protein look at what happens to the carrier protein the shape of the carrier protein automatically changes and when it changes the carrier protein will then flip and when it flips or changes its shape it will cause the particle to then be able to move across the membrane down the concentration gradient and once it releases the particle to the other side it will then return back to its original shape that's what we have to understand for this and just like channel proteins carrier proteins are also very specific for example the one the red color carrier protein is an amino acid carrier protein and the one in green is a glucose carrier protein as you can see the shape of the amino acid is complementary to the amino acid carrier proteins and the shape of glucose is complementary to the glucose carry protein so this is how polar substances such as ions glucose and amino acids can pass through the cell membrane because remember they were blocked by the phospholipid bilayer due to their polarity but in this case what actually happens is because the cell membrane is not just made out of phospholipids they are also made out of proteins such as carrier proteins and channel proteins these allow specific polar molecules or polar particles to be transported across the cell membranes easily as well that's how it goes