This video is brought to you by SucceedSchool.com. With complete lessons and resources, assessments, learning plans and schemes of work for students parents and teachers For revision, for catch-up or just to get ahead in class, visit SucceedSchool.com now for your free trial lessons. SucceedSchool.com, learn to succeed. Hey class I'm Mr. Thornton and I'm going to help you Succeed in your GCSE and IGCSE this lesson The Digestive System. This topic was requested by Asbah Afridi. If you've got a topic you'd like me to cover then just leave a comment below. The digestive system is actually a little funny in the 2016 specifications. If you're studying an IGCSE course you need to know about it, it's as important as ever and I'll be covering everything you need in this video. If you're studying a GCSE course things are a little more complicated. The GCSE specifications expect you to know about the features and functions of the digestive system but they don't expect you to study them at GCSE. They say you should have learned all about it at Key Stage 3. They kind of want to have their cake and eat it because it looks to me like they'll use ideas about the digestive system to frame concepts like enzyme action, fighting infections, homeostasis and transport in animals. I think it's probably worth you spending a few minutes refreshing your memory if you're rusty on it. I can see this turning into one of those sneaky exam questions which everyone complains they didn't know to prepare for. I'm going to assume in this video that you're familiar with enzymes. If you're not please click here to watch a video all about them. The digestive system is an organ system, that is, a group of organs all working together to achieve a particular goal, in this case digestion of nutrients from food and the excretion of waste. It's essentially a long tube, known as the alimentary canal, running from the mouth to the anus with various organs along the way. You need to be familiar with the basic functions of each organ and if you're studying IGCSE you may also need to be able to put them in order or identify them on a diagram like this one. We begin in the mouth, where food enters the body. This is known as ingestion. Food has to be ingested before we can digest it. As I'm sure you're aware we generally chew solid food before we swallow it, breaking it down into smaller pieces. This is called mechanical digestion and it increases the surface area of the food to increase the rate of reactions in the digestive system. Please see my video on rate of reactions for more details on this. Humans have a mixed set of teeth; some sharp for cutting and some flat to grind food. I'll cover teeth in more detail in a future lesson. Another type of digestion also takes place in the mouth. Our saliva, produced by the salivary glands, contains a type of carbohydrase enzyme called amylase, which begins breaking down starchy molecules into glucose as soon as we place food in our mouth. This is chemical digestion. Broadly speaking, when we physically chop up food without changing the actual molecules it's made of, this is mechanical digestion. Performing a reaction on it to change the molecules themselves, for example with enzymes or stomach acid, is chemical digestion. A process called absorption also takes place to a small degree in the mouth. This is where digested nutrients from food, such as glucose in this case, can be absorbed into the bloodstream. It's only possible for small molecules which can dissolve in water, such as glucose dissolved in the water in our saliva, to be absorbed. Absorption mainly happens in the small intestine though, so I'll cover it more when we get there, but I wanted to mention that it also happens in the mouth because I've seen test and exam questions which have asked about this in the past. Once we've chewed our food we swallow it and it passes down a tube called the oesophagus to our stomach. The common spelling of the word in the UK begins OES while the common spelling in the U.S. begins ES. Both spellings are accepted. Waves of muscular contractions pass along the oesophagus to help push food towards the stomach, so humans are even able to swallow food while lying down or hanging upside down, Though obviously it's easier and there's less risk of choking if we're upright. Believe it or not even though we've not made it to the stomach yet we've now seen most of the key concepts of the processes taking place in the digestive system. Mechanical digestion, production of substances like enzymes for chemical digestion, breaking down large pieces of food into tiny soluble molecules for absorption into the bloodstream, and peristalsis pushing everything along. Most of the rest of the journey along the alimentary canal just repeats these themes so we can move pretty quickly from here onwards. Following its journey down the oesophagus the food reaches the stomach; a stretchy muscle lined sack containing digestive juices. These are a mixture of substances like enzymes and acid. The acid is important for two reasons. Firstly it performs some chemical digestion and secondly because it has a low pH, around pH 2, it denatures enzymes which work around neutral pH. This can kill a lot of the bacteria which might be in food. Of course it also means enzymes present in the stomach such as pepsin, which is a type of protease, have to work at low pH. This mixture of digestive juices also contains water. Ultimately all the nutrients will need to be broken down into small enough molecules to dissolve in the water, which acts as a solvent in order for them to be absorbed. The stomach walls churn this mixture for a while and then it passes out of the stomach via the duodenum. This is the first part of the small intestine and it's where bile from the gallbladder, which is situated within the liver, mixes with those digestive juices to neutralise the acid. It also has a second function in that it allows fats from those digestive juices to dissolve in an emulsion. We can simulate that with this mixture of oil and water. This is the oil layer on the top and this is the water layer on the bottom. I've actually added some dye to the water layer so that it's easier to tell the difference between the two. Now as i shake up this mixture of oil and water of course the oil and water mix but they do start to separate out quite quickly. Hopefully you can already see that the two layers are beginning to separate out, with a lighter coloured layer which is mainly oil on the top, a much darker colour layer which is the water on the bottom, and this sort of intermediate section in the middle which is where they are still mixed but they're quite rapidly separating out. Next what I'm going to do is add a little bit of bleach to my mixture. The bleach has a high pH and so this is going to simulate our bile. It's strongly alkaline and I don't actually need to add very much of it. This is just household bleach by the way if you want to try this experiment yourself at home. I'm just adding a couple of drops of it there, and then I put the lid back on and I'm just going to give it a single shake again. This time however although there is a little bit of separation it's happening much more slowly. The mixture is forming more of an emulsion. This is where there are droplets of oil suspended in amongst the water droplets and they are microscopic droplets. That's why it still got this cloudy appearance. Yes there's still quite a lot of water down here at the bottom but there is much more up here mixed in with the oil. The oil is going nowhere near as clear. It's worth noting at this point as well that this cloudy appearance is as a result of the microscopic droplets of oil in here refracting and reflecting all the light which hits it, in lots of different directions. This is very very common of emulsion. This is a characteristic appearance and it is something which does occasionally show up in the GCSE as well. In addition to bile from the gallbladder, the pancreas also adds other digestive chemicals in the duodenum such as more of the enzyme amylase which was first added in the mouth. Remember this is because most of the amylase from the mouth will have been denatured by the stomach acid. Next this mixture of food and digestive juices passes into the main part of the small intestine. This is technically in two sections; the jejunum and the ileum, but the differences between them are pretty subtle and at GCSE and IGCSE level all you need to know about is the basic function and structure of the ileum. Peristalsis again moves the mix of semidigested food and digestive juices along the ileum, with the digestive juices continuing to break down the food until it's in the form of single small molecules ready to be absorbed by the body. The inner surface of the ilium is covered with long finger like projections called villi. The singular noun is villus. These in turn are covered in microvilli. This gives the ileum a huge surface area in a very small volume, increasing the rate at which absorption of nutrients can happen. Absorption remember is where small molecules which are dissolved in water are able to pass from the intestine into the bloodstream, in this case via the villi. Once these nutrient molecules have been absorbed into the blood plasma, and blood plasma is just another water-based liquid, the nutrients dissolve in, once they've been dissolved into there they are transported around the body and then cells can absorb the nutrients and use them for growth and repair in a process called assimilation. The majority of the water from the mixture is absorbed in the small intestine too. Any substances which the body was unable to digest or absorb now pass into the large intestine, the first part of which is called the colon. The colon reabsorbs most of the remaining water and various dissolved electrolytes from the mixture as it pushes the remains of the food along with peristalsis. At the end of the colon is another section of the large intestine called the rectum, where whatever's left, faeces to give it its technical name, waits until the next toilet break before passing through the anus in a process called egestion. One final point to be aware of is that what I've just described isn't always exactly how things work. A good example of this is diarrhoea which is when water isn't effectively reabsorbed, which causes watery faeces to be egested. This failure to reabsorb water can make diarrhoea lead to rapid and potentially dangerous dehydration if the sufferer doesn't drink plenty of water. There can be many causes of diarrhoea, but a historically important one is cholera. This is a bacterial infection spread through drinking water which has been contaminated with faeces. The bacterium produces a toxin which causes chloride ions to be secreted in the small intestine. This causes water to move from the blood back into the intestine by osmosis. This significantly raises the water content of the faeces. Thankfully modern plumbing and water treatment make cholera a thing of the past for most of us, so if you do have diarrhoea it's probably something else. All the same I feel I should mention that if you do have diarrhoea then drink plenty of water and see a doctor if your symptoms persist. LIGHTNING ROUND! We chew food in our mouth which is mechanical digestion, plus enzymes from the salivary glands like amylase begin the chemical digestion. The food passes down the oesophagus, helped along by peristalsis, into the stomach acid. The low pH there denatures enzymes in bacteria and is also necessary for our own enzymes like pepsin to work. From there bile from the gallbladder neutralizes the acid in the duodenum and emulsifies fats, and more enzymes are added by the pancreas. Peristalsis again passes the mixture through the ileum, the main part of the small intestine, where nutrients are absorbed into the bloodstream across the ileum's huge surface area, due to the villi and microvilli. The nutrients are dissolved and transported in the blood plasma to cells where they're assimilated and the indigestible waste passes into the large intestine. The colon reabsorbs water and electrolytes, the rectum stores the faeces until you can make it to the toilet, and the faeces is then egested through the anus. Cholera is a bacterial infection from dirty water which causes chloride ions to increase in the small intestine, forcing water out of the blood plasma by osmosis and causing dehydration and diarrhoea. I hope that video really helps you. If it did it'd be great if you let me know in the comments. Remember to Like, Subscribe and hit the bell to get a notification the next time I upload the video. 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