[Music] so what is digestion well digestion is the chemical and mechanical breakdown of food well it does it convert large insoluble molecules into small soluble molecules but then can be absorbed into the blood once they're absorbed into the blood then they can be transported around the body and assimilated into it into cells and be used but we've got to break them down into small enough molecules before that can happen and that happens in the process of digestion now you need to know all the organs that are involved in the digestive system on this diagram you can see ones that are labeled with black arrows are the ones which the food will travel through on its journey through the digestive system so it goes in the mouth through the esophagus stomach into the duodenum of the small intestine then the ileum round into the colon down into the rectum and out through the anus that's the journey the food takes but there's also organs involved that we call accessory organs that we need to help us with this process of breaking down food and those are the ones labeled with green arrows here the saliva glands the liver the gallbladder the pancreas potentially the appendix as well so these are accessory all instead ejection so I'm going to tell you a story I'm going to tell you a story of the digestion of a cheese sandwich it's quite a so typical food that you might eat it contains most of the nutrients that we talked about in the nutrition video such as carbohydrates fats and protein vitamins minerals fiber and water so a look at how the cheese sandwich travels through the digestive system and what happens to it along the way so stage one is that it pulses into the mouth and the mouth two forms of digestion happen in the mouth a lot of people don't think to ejection happens in the mouth it's just where we put our food and then we swallow it but both types of digestion happen mechanical digestion where you physically chew up the food into smaller chunks so that enzymes have a bigger surface area to break the food down and then chemical digestion which is where the enzymes come along and chemically break down the morgue from big molecules into smaller small soluble molecules so in the mouth we got chemical digestion going on because in your saliva you have the enzyme amylase and amylase breaks down starch a big carbohydrate into smaller molecules called maltose so in our cheese sandwich this is the bread that is now being digested but in your mouth before you've even swallowed in a thing the teeth are working away to give a large surface area for that digestion to work the food will be masticated which means chewed and it will form a nice smooth a ball which we call a bolus of food which will hopefully be easily swallowed down the esophagus which is stage two other digestion system now no digestion happens in esophagus it is just a muscular tube which contracts to force the food down into your stomach which is stage 3 so the cheese sandwiches doesn't even look like a cheese sandwich anymore after the first couple of stages and it's now in the stomach where it may be held for several hours more mechanical digestion happens here because the stomach is a muscular bag so it kind of churns up the food and mixes it around and it mixes it around with two things firstly hydrochloric acid which is secreted by the stomach lining now a lot of people think hydrochloric acid is there to digest the food because it's acid so it must break things down but that's not true the acid really is there to kill bacteria and protectors from food poisoning bacteria and other microorganisms that are on the food which are inevitable there always gonna be some on our food will hopefully be killed at this stage before it goes any further into the jet system where absorption happens so it won't causes any any problem if there are too many then you might get food poisoning you might need to be sick or it might cause diarrhea in the stomach there is also an enzyme and this is called type of enzyme called a protease enzyme this one's called pepsin and it breaks down protein into amino acids so in our cheese sandwich is going to be the cheese cheese is high in protein ok and so it's breaking down the proteins in the cheese and releasing in amino acids that pepsin works really really well in the pH one caused by the acid it's especially special enzyme that in acidic conditions when normal enzymes won't be working they're a sphincter holds the food in the stomach until it's ready to be released into the first part of the small intestine called the duodenum so stage 4 is the duodenum several enzymes are now added into the duodenum by the pancreas the pancreas is one of our accessory organs and it secretes a juice called pancreatic juice which travels through a duct into the duodenum and it gets released into your Idina now the pancreatic juice contains three enzymes amylase trypsin and lipase also added at this stage is bile which has come down the bile duct from the gallbladder where it's been stored the file is alkali so it neutralizes that acid that has come out of the stomach which is very important bile is not an enzyme its function is to emulsify large lipids what that means gives them a bigger surface area for the enzyme to break them down so they're quite hard to break down lipids so we look the three enzymes were amylase which is going to break down dar ch further into maltose who are trypsin which will break down more proteins into amino acids we've got lipase which with the help of the bile will break down lipids into fatty acids and glycerol then the food moves through the gut via a process of peristalsis peristalsis are waves of muscle contraction so the gut little intestine is made up of circular and longitudinal muscles which work to squeeze the food squeezes behind the food to move the food along and the muscular wave the ileum is the next part of the small intestine now by this point the food has finally broken down into small enough soluble molecules because of the mechanical digestion in the mouth and the stomach and all that chemical digestion done by all those enzymes along the way so that molecules are now nice and small and finally we can actually absorb the food into our blood and start to use that food in our body the ileum is highly adapted for absorption of these molecules it the inside of the ileum is folded into these lovely finger-like projections called villi so this gives the small intestine a very high surface area for absorption only that these villi actually have on their surface tiny little micro villi so the villi have villi on top of them each villus contains a network of blood vessels and a lacteal blood vessels join up to form the hepatic portal vein which leads to the liver so all the food went to gets absorbed goes to the liver and the liver can help get rid of any toxins that are in the food in the blood before it goes round the rest of the body and the lacteals absorb the products of the fat digestion which go into the lymphatic system so it goes into slightly different system to be transported around so all those food molecules get absorbed in the ileum we're moving now on towards the end of this story and we're in the the large intestine now and all of the amino acids have got broken down from the cheese and the sugars have got broken down from the from the bread and the fats in the butter from our cheese sandwich they've all been broken down and absorbed all we've got left now really is water and that dietary fiber has come from the the lettuce probably in our sandwich okay at this point we need to absorb the water so that's what happens in the large intestine in the colon it all the water gets absorbed here all that's left is the dietary fiber which is what we call feces now that gets stored in the rectum and then expelled through the anus when necessary that process is called ejection shouldn't be confused with excretion excretion is the removal of metabolic waste from the body this isn't a metabolic waste a waste from chemical reaction this is just leftover food so this is what we call a gesture now it's very important that you know all the enzymes the class of the enzyme the name of the enzyme where you find it what it breaks down and what it produces and this table summarizes that so make sure you learn that properly the last thing to really look at is a practical it's important one and it's about how you investigate the energy content in food now food is a total energy content dependent on the amount of carbohydrate fat and protein all of those things can contribute to how much energy is in that food and we measure that energy in kilojoules now scientists can burn food in a calorimeter to work out how much Chinese energy is in it so when you look at the back of a back of a packet of of some food you should be able to find some information about how much energy is in the food which it's being worked out in special apparatus called calorimeter x' but we can do a kind of a basic version of that in the lab and we can do it using water if we set fire to some food and use it to heat water and measure how much the water temperature increases we know how much energy was in the food because we know how much energy it takes to heat up water by one degree so we can do a bit of maths and work backwards by looking at the change in temperature to see how much energy was in the food in the first place so the way to do this is you're gonna have to take the food sample and weight it we need to have the mass of the food initially we then want to place 20 centimeters cubed of water into a boiling tube and measure that temperature of that water at that point and record that in the table as well then we set fire to the food and hold it carefully on a mountain needle underneath that test tube and we do this until the food completely no longer to burp and no longer burns its completely burnt to a crisp and then we stir the water and we take the final temperature using that information we can then plug it into a formula now we know that it takes 4.2 joules of energy to raise one gram of water which is one centimetre cooler water by one degree C so if we put it into this equation if we do look at our change in temperature so the final temperature minus the start temperature times it by 20 because that's how many grams of water we had we're 20 centimeters cubed same with 20 grams and we know that each one it takes 4.2 joules of energy times it by that and divide it by the mass of food we had in grams that will give us our answer as to how much energy there was in the food sample