Hi everyone and welcome to Miss Estrich Biology and in this video I'm going through the A-level biology explanation of digestion and the absorption. This is for AQA A-level biology but it applies to most exam boards and one of the key things with this topic is that you need to be really particular with the keywords that you're using to make sure that you get those marks and I emphasize this throughout the video so you can make notes on just that. But if you want that all done for you then don't forget that you can get hold of my A-level notes and my flashcards which cover all the key terms and key marking points using the link below straight to my website but for now I'll leave you to this lesson.
So just to go through the definition of digestion first of all it's where you have large insoluble molecules hydrolyzed into smaller soluble molecules which can then be absorbed across cell membranes and then into the bloodstream. So we're going to go through digestion of carbohydrates, lipids and proteins, which are the three that you need to know for AQA biology in A-level. So the two enzymes that you need to know about are amylases, and these are all different types of amylases enzymes, and membrane-bound disaccharidases. And you'll notice that pretty much all enzymes end in A-S-E.
So if you do ever see ... a name ending in A-S-E in biology, it means it's an enzyme. So amylase is produced in the pancreas, which we can see just a tiny bit of here in yellow, that's the pancreas, but it's also produced in the salivary glands which are up here and for that reason when they're producing the salivary glands it's secreted into the mouth. So that is where digestion of carbohydrates begins in the mouth. And then once those amylases have started to hydrolyse the carbohydrate polysaccharides, say, for example, starch into the disaccharides such as maltose by hydrolyzing glycosidic bonds, that is in swallowed.
It goes into the stomach and then into the small intestines. But it's this very first bit of the small intestine, the duodenum, which is where the next stage of digestion occurs. And with.
in the duodenum and the ileum, this is where we have the membrane-bound disaccharidases, and those then are able to hydrolyse the disaccharides into monosaccharides. And for example, you've got two examples of these membrane-bound disaccharidases, meaning it's an enzyme that can hydrolyse a disaccharide. And you can get sucrase, which can hydrolyse sucrose into fructose and glucose, and lactase, which can hydrolyse lactose into galactose and glucose.
And that will be occurring in the duodenum and then into the ileum as well. Proteins then, there are three key sets of enzymes that can hydrolyse the polymer, which is proteins. You can have endopeptidases. Endo meaning inside.
So endopeptidases are enzymes which can hydrolyse the peptide bonds between amino acids that are within the centre of the polymer chain. You also get exopeptidases, exo meaning on the outside. So these are the enzymes which can hydrolyse the peptide bond between the amino acids right at the end of the polymer chain.
Now, you'll also get to the point where you probably only have two amino acids still joined together. And that's what membrane bound dipeptidases are hydrolyzing. And they can hydrolyze the peptide bond between just two amino acids or a dipeptide.
Just to point out where this is occurring then. So the protein digestion starts in the stomach, which is just here. Then all of that churned up and partially digested protein material moves down into the duodenum or duodenum and into the ileum.
And that's where the remaining digestion of proteins occurs. Lastly, then the digestion of lipids. And this is slightly different because it does involve enzymes for chemical digestion. But it also involves some physical breakdown as well using biosolids.
And we'll go on to that in a bit more detail in the next slide. So the enzyme lipase or lipase is produced in the pancreas and that will then secrete that enzyme into the duodenum and into the ileum. And lipase is able to hydrolyse the ester bond in triglycerides. And that's the bond which holds the glycerol and the three fatty acids together. So when it hydrolyses those three ester bonds that occur in triglycerides, you end up with fatty acids and glycerol.
But actually, you often get monoglycerides and fatty acids. So the next step then is just referring to these bile salts. Now this actually occurs before the lipase action and bile salts are produced in the liver, which is this large organ here.
It gets stored in the gallbladder and then through the bile duct it's delivered into the small intestines or the duodenum first. And those bile salts can emulsify the lipids. meaning it splits large droplets into many, many, many tiny droplets. And eventually those will then go towards making micelles.
So we're going to go through the lipids in more detail because there is quite a bit more to say about them. So I pointed out you have physical and chemical digestion. Physical is looking at the emulsification and micelle formation.
Chemical is the action of the enzyme. So physical, the lipids get coated in the bile salts. which are being released from the gallbladder where it was made in the liver.
Those bile salts coat the lipids and it causes them to split up into tiny droplets. And that's what we call an emulsion. You need to know why that is an advantage to digestion.
And AQA are very specific in their mark scheme. So it's the fact that you get many small droplets and that creates a large surface area. However, the reason that is an advantage is if you've got a larger surface area, you'll get a faster hydrolysis action by lipase.
Because if you're providing a large surface area, more of the enzymes can attach. You get more enzyme substrate complexes and therefore a faster action. And that's the key idea. You do have to point out you get faster action or faster hydrolysis. And that's where it links to the chemical digestion.
The physical digestion provides the large surface area for that lipase to be able to hydrolyse the lipid into monoglycerides and fatty acids. So how this links then to the micelles, because we've gone through how you end up with these fatty acids and monoglycerides, but how do we then get this next stage where those form the micelles? So firstly is what is a micelle? So it is a sphere or a vesicle and it's made up of the fatty acids and monoglycerides that are produced from that hydrolysis or digestion but it's also incorporating in the bile salts as well and these spheres or these micelles they are able to deliver the fatty acids and monoglycerides to the epithelial cells in the ileum. they'll then release the fatty acids and glycerol which can move into the epithelial cell by diffusion so that's it for the digestion next is how do all of those digestive food molecules get absorbed and all of the absorption is happening within the cells lining the ileum which is part of the small intestines and the key thing and this is similar to gcsc you have these villi which is the folding of that ileum surface.
And on the villi, they are covered in several microvilli cells. And zooming in, we can see these microvilli cells. They have even further foldings to increase the surface area for more rapid absorption. But also, it's not shown in this diagram, but in the middle of these villi, there is a capillary network, and that enables the concentration gradient to be constantly maintained, and also you have a very, very short diffusion distance.
So if we go specifically through how each of the molecules are absorbed. Now monosaccharides, which are the products of digestion of the carbohydrates, and amino acids, which are the products of digestion of the proteins, they are both absorbed in the same way, and that is by co-transport. Now I'm going to link...
up here my full video on co-transmorts because I'm not going to go through it in this one. Just in brief here's some key information. So the glucose, an example of a monosaccharide and amino acids are absorbed as we said by co-transport which is a type of active transport now some will be absorbed by facilitated diffusion but you could get to the point where you have more sugar inside of the epithelial cells and inside of the blood compared to the lumen and therefore you won't be able to absorb that sugar or amino acids and that's why co-transport a type of active transport is also essential The actual process as I said check out the video on that.
So the lipid absorption follows on from what we've already said so we've already gone through how you get the macelles forming and then just to go into the extra detail so those macelles are carrying the fatty acids and the monoglycerides and they deliver that to the cells lining the ileum or the epithelial cells and that delivers the fatty acids and monoglycerides which can then simply diffuse through that plasma membrane and that is because they have that non-polar nature meaning they are lipid soluble. Once those have then entered the cell they need to be formed back into triglycerides so they can be used within the body and that is what is happening inside of the Golgi apparatus or the Golgi body. sometimes in the endoplasmic reticulum. And that's what this diagram is showing us here.
We've got the fatty globules are being processed and quite often when they're processed, they are combined with a protein. And if that does happen, we then call it a chylomicron. So when a triglyceride is combined with a protein, that's our chylomicron.
The chylomicron is then released inside of a Golgi vesicle, which we can see here. The Golgi vesicle then moves towards the other end of the epithelial cell and it is released by exocytosis, meaning is then just released from the cell. The vesicle fuses with the membrane and that releases the contents. How that then gets absorbed into the rest of the body is through this lacteal or sometimes it's called a lymph vessel and the lipids which have then been formed or our chylomicron are absorbed inside the lacteol and they'll get transported around in the lymph and that does eventually drain into the capillary system so those are the key things that you need to know about absorption and digestion