hi guys in this video we'll be looking at fibrous proteins collagen keratin elastin and then we'll finish with a summary so fibrous proteins are another category of proteins alongside globular proteins and they have particular features in common so it's the 3d tertiary structure and the quaternary structure of a protein which results in a formation of a fibrous protein and this is in the same way as a globular protein so fibrous proteins tend to have very similar features in common with each other and it's all designed based on its primary sequence and tertiary structure of amino acids so the fibrous proteins that we can see in the body have some similar properties to each other a lot of the time they contain very long polypeptide chains with many many amino acids but these chains have repeating sequences of amino acids so with the globular protein usually we would see various different sequences in various patterns so there wouldn't be really any repeating units in the case of a fibrous protein you can see here that we get repeating parts of the amino acid sequences so it could be abc followed by abc again and this repeating unit would keep going all the way along the chain so the primary sequence of fibrous proteins tend to have some sort of pattern or recognition to them as well as this the amino acids themselves tend to have a non-polar r group or non-polar variable group so the proteins tend to be insoluble in water so remember for an amino acid this general structure stays the same with the amino carboxyl and hydrogen groups but it's the r group which can vary and this is just an example of one of those r groups which are nonpolar as we find in fibrous proteins because it's nonpolar it cannot interact with water and so it doesn't dissolve in fluids like plasma or the cell cytoplasm so we tend to find fibrous proteins outside of water or away from it finally another common feature is that the polypeptide chains are able to form fibers which make the proteins very very strong so the individual polypeptides are like the strings of a rope if we unwind the rope right down to its smallest level the polypeptides individually are very weak so the individual fibrous protein chains are quite weak but as we wind them up with each other and they're grouped on various layers what we end up with is a strong fibrous protein and hence we tend to call this unit where we've got multiple polypeptides wrapped around each other a fiber and these are very very tough so the fibrous proteins because of their nature tend to have more structural roles in the body as opposed to globular proteins which have more metabolic and functional roles and some of these examples include collagen which is important for forming tendons and giving hard structure to a lot of structures like bones and walls of arteries we have a fibrous protein called keratin which is found in the fingernails and then we have a fibrous protein called elastin which is important in making tissues stretchy so a fibrous protein by definition is a very long strong and insoluble protein which often has a structural role in organisms so one of the examples of a fibrous protein is known as collagen and it's one of the most abundant proteins in the body the fibers that form from collagen are very very strong fibers so it's normally used to provide strength to many parts of the body and various different tissues for example the collagen material is found in artery walls and the reason why it's found here is because its strength allows the vessels to prevent from bursting under their high pressure so looking through an artery blood vessel here we know that as the heart pumps blood into the artery it does so under high blood pressure which would normally damage the vessel but within the artery wall we have fibers of collagen and this is able to withstand that pressure and therefore stop the artery from bursting open which is important because otherwise it would lead to some sort of bleeding collagen is also found in making tendons which connect muscles to the bones allowing the skeleton to move so here we have a diagram of the upper arm here's the muscle for example the tricep muscle and in order to connect to the bone it needs a fibrous element which is very very strong and that element is called a tendon and it's through the tendon that when the muscle contracts it moves the bone at a particular joint and allows us to perform many different movements the tendon has to be tough because the muscle will be pulling on it very very strongly and it has to be tough enough to stick to both the muscle and the bone so collagen is found in tendons as well and finally the other place that we see collagen a lot of the time is in bone bone is obviously a very very strong structure it's found in the skeleton to keep our cells upright and give our organs some protection like in the rib cage so it has to be very very tough and very rigid and collagen is used to provide this material another important example of a fibrous protein in the body is keratin so keratin is another example of a fibrous protein which is very hard and it's very very strong so hardness means obviously that it's difficult to damage for example this metal girder represents hardness and obviously being stronger means that it's harder to break as well so we see keratin therefore in parts of the body which are hard such as the fingernails or horns and hooves on animals so the fingernails have keratin and on animals like a goat for example we would see it on the hooves and on the horns and these tend to be places which are exposed to forces and mechanical damage and therefore they need to be hard and strong the primary structure so the order of amino acids in keratin contains a high amount of cysteine and cysteine is an amino acid of the 20 which contains sulfur in its r group so here's an amino acid known as cysteine and you can see that the r group contains this cysteine atom right on the end and they tend to be paired with a hydrogen too this is important because amino acids with sulfur in them can form disulfide links between two polypeptide chains this makes the whole molecule very hard and very strong so here's a molecule of keratin and it's made up of two polypeptide chains and what connects these two are these disulfide bridges which lie between cysteines in the chain and there's a lot of cysteines in each chain disulfide links are covalent and so they're very very strong so overall this keratin molecule is very hard and very strong and as these all line up together the fingernails and the horns and hooves of a goat and structures like this are very tough and very strong because the forces find it very hard to break these disulfide links another example of a fibrous protein is elastin so elastin is a final example and it has elastic properties which means it can stretch and it can recoil so just in the same way that an elastic band can be stretched and then as the force is removed it can sort of recoil up together elastin does the same thing and the reason elastin is a stretchy molecule is because it coils up when it's not being tensed under pressure the elastic molecules coil up and they also cross link to each other which keeps them together so here is a tissue with some elastin in it and one of these green coils is an elastin molecule and you can see that it takes up this kind of coiled structure and because they're cross-linked to each other they're not all going to just separate and fly off whenever the tissue is exposed to a pressure so now if you imagine this tissue is pulled from either end and stretched out what you end up with is this kind of structure here where all of the elastin has been stretched but it hasn't been broken apart because of these important cross links keeping the tissue together so this means the tissue is intact both when it's recoiled and when it's stretched and we're obviously very elastic creatures because we have a lot of soft tissues which must maintain their shape elastin is found in the lungs to help them inflate and deflate during ventilation so as we breathe in oxygen comes in and the lungs have to be able to expand and fill up with air to replenish that air and so the tissue surrounding the alveoli must be elastic and obviously as we deflate it's allowed to recoil and shrink again to push that air back out and expel it elastin is also found in the bladder to help it expand when it's holding urine so usually the bladder is quite small but as it fills with urine it needs to be able to expand and so elastin is found in the walls to be able to stretch and recoil when it empties and finally elastin is also found in the blood vessel walls just like collagen is found here too where it helps to maintain the blood pressure by stretching and recoiling so in the arteries in particular when the blood enters the artery it's at very high pressure and this high pressure is going to push against the wall of the artery if the artery was very rigid then the pressure would be quite damaging but because there's elastin molecules found in the tissue it can expand and as the pressure drops it can recoil and these happen one after each other and the recoiling helps to maintain that blood pressure so that the blood can get squeezed on to the next area hey guys i hope you enjoyed the video if you are looking for an amazing a level biology resource join me today in my series of engaging bite size video tutorials just click the snap revise smiley face and together let's make a level biology a walk in the park