Digestive system Flashcards
The gastrointestinal tract
4.5 metres long. Continuous tube from the mouth right through to the anus. Conditions in the digestive tract essential for the digestive process can be tolerated in the lumen that would never be tolerated inside the body proper. For example, the pH of the stomach can get as low as 2. In the lower GI tract there are quadrillions of microorganisms that are beneficial and harmless but would be lethal if inside the bloodstream.
Comprised of the:
Mouth - which is concerned with mastication which is basically just chewing and breaking the food into smaller pieces. It also releases some enzymes to start carbohydrate digestion.
The pharynx, or throat, and eosophagus are concerned with swallowing and transporting the food from the mouth to the stomach.
The stomach begins the digestion process proper releasing acid and other factors as well as mixing and churning the food. No foodstuffs are absorbed in the stomach but some lipid soluble substances such as alcohol and aspirin are.
The small intestine is made up of the duodenum, jejunum and ileum, and it’s where the majority of digestion and absorption occur with the help from secretions from the bile duct and pancreas as well as with enzymes located on the wall of the intestine.
The large intestine is made up of the ascending, transverse and descending colon. It absorbs the last of the water and salt and converts the luminal contents into faeces, or poo, which end up in the rectum and anus.
The accessory organs
These are comprised of the salivary glands, the pancreas, the liver, and the gallbladder.
The gastrointestinal tract structure
The innermost layer is called the mucosa, which is made up of the mucous membrane which is a protective surface barrier comprised of epithelial tissue which can contain specialized cells for absorption or secretion.
The lamina propria which is a thin middle layer of connective tissue containing tiny blood vessels and lymphoid tissue.
The muscularis mucosa which is a very thin, sparse layer of smooth muscle which upon contraction can expose different areas of surface folding.
The submucosa is a thick layer of connective tissue which gives elasticity and contains larger blood and lymph vessels and a nerve network.
The muscularis externa which is the smooth muscle on the outermost layer of the tube. In moist regions this is made up of two layers; the inner circular layer which decreases the diameter for contraction, and the outer longitudinal layer which decreases the length on contraction. There’s another nerve network here called the myenteric plexus.
The serosa which is a thin layer of connective tissue that secretes serous fluid for lubrication. This prevents friction between the digestive tract and the surrounding viscera.
Four basic digestive functions
Motility:
The muscular contractions that mix and move food forward along the digestive tract. This is predominantly due to smooth muscle contraction. These movements can be propulsive movements, such as peristalsis, or mixing movements, such as segmentation.
Secretion:
Includes both exocrine and endocrine functions in both the gastrointestinal tract and accessory organs. There is so much secretion going on in the digestive tract that even though we only ingest about two and a half litres a day through food and drink almost 10 litres of fluid enters the small intestine due to secretions. The mouth and pharynx and eosophagus secrete mucous. The stomach secretes gastric juice which is comprised of hydrochloric acid, pepsin, mucous and intrinsic factor. The small intestine secretes mucous, salt, small intestinal enzymes, and the large intestine secretes mucous. The accessory organs include salivary glands, and these secrete lysozyme, which is antibacterial, and amylase, which is a digestive enzyme. The pancreas secretes trypsin, amylase, lipase and other pancreatic juice, and the liver and gall bladder secrete bile salts, alkaline secretion and bilirubin.
Digestion:
The breakdown of the three macromolecules; protein, carbohydrate and fat. Although this is a complex process that is different for the three macromolecules, overall digestion of any of these is accomplished by enzymatic hydrolysis which literally means water breaking. It’s a chemical process where a molecule is cleaved into two parts by the addition of molecules of water via the catalytic actions of enzymes.
Absorption:
Absorption of the small units of digestion end-products which are absorbed with water and electrolytes. These are then transferred into the blood and lymph. The primary site of absorption is the small intestine and it involves a different process for each of the macromolecules.
Digestion of carbohydrates
Digestion, or breaking down food, is both mechanical and biochemical.
The very first step is mechanical with mastication (chewing). It uses teeth and muscles in the mouth and tongue to grind and break up food into small pieces. This increases the surface area for the food to expose it to enzymes and help begin biochemical digestion. We also start to stimulate our taste buds here and this is important because it not only makes our food taste good but by stimulating them it kicks in a reflex to increase salivary, gastric, pancreatic and bile secretion, which are all important for digestion and absorption.
Saliva is comprised of 99.5% water and 0.5% electrolytes and proteins. The water and mucus help flush away food residue, foreign particles and old epithelial cells that have shed from the mucosa, as well as moistening the food particles which helps hold them together. It also acts as a solvent. Saliva contains lysozyme, which is an enzyme that is antibacterial in nature and it does this by breaking down the bacterial cell walls. Saliva also has bicarbonate buffers which neutralize acids in food as well as acids produced by bacteria in the mouth and so this stops us having to get fillings at the dentist.
It also starts by a chemical digestion of fats with lingual lipase and it gives us the first stage of biochemical digestion of carbohydrates with amylase. This enzyme breaks down big, long carbohydrate molecules into shorter chains of carbohydrates.
no matter what macromolecule we’re digesting it’s all basically accomplished by enzymatic hydrolysis, which is just a chemical process where a molecule is cleaved into two parts by the addition of a molecule of water via the catalytic actions of enzymes. By adding water at the bond site enzymes in the digestive secretions break down the bonds that hold the molecular subunits within the nutrient molecule together. Hydrolysis replaces the water molecule and that frees the small absorbable units. Digestive enzymes are specific in the bonds that they can hydrolyze.
Maltose is a disaccharide carbohydrate. The two sugar molecules that make up maltose are held together by a bond and when enzymatic hydrolysis adds water it breaks that bond that holds them together and makes them two singular glucose molecules.
In the stomach, hydrochloric acid is secreted here, but it actually doesn’t directly break down carbohydrates. In fact, the hydrochloric acid inhibits the saliva amylase that we’ve swallowed so biochemical digestion actually stops here. A lot of mixing and churning happening here though which the stomach is especially well designed for. Most of the digestive tract has just two muscle layers, but here at the stomach we have three layers. Along with the standard inner circular and outer longitudinal layers we also have a middle oblique layer which allows the stomach to churn the contents of the food and make sure that it’s all liquid. And this liquid is now called chyme which will now enter the small intestine.
And so here in the small intestine amylase comes back in again but this time instead of salivary amylase we have pancreatic amylase, secreted obviously by the pancreas into the duodenum of the small intestine. Most of the carbohydrate that we ingest is in the form of polysaccharides. These chains of interconnected glucose molecules are commonly ingested as starch, which is derived from plant products, or glycogen, derived from meat sources. Amylase from both saliva and the pancreas breaks down these big polymers to disaccharides. It also includes sucrose and lactose which we would ingest in things like table sugar or milk.
We ingest other carbohydrates we can’t digest ourselves. Polysaccharides like cellulose and raffinose are found in plants that we eat but cannot be digested by our own bodies. We just don’t have the right enzymes. So they can either form the indigestible fibre, or bulk of our diets, or we can get some help from the gut microbiome to break it down. Our gut microbiome is found in our large intestine and it is comprised of a hugely wide diverse commensal, which means good, bacteria population and the gut microbiome is essential for our health. So if we eat food that is high in carbohydrates that our bodies don’t have the correct enzymes to break down our microbiome will do it for us. This produces gases like carbon dioxide, hydrogen, methane and sulfur. An example of this type of food includes baked beans.
in the small intestine we have a specialized mucosa which is made up of projections called villi which are covered in small intestinal epithelial absorptive cells which are called enterocytes. And on these enterocytes are all of these tiny little projections called microvilli which together make up the brush border. In these brush borders are lots and lots of enzymes called disaccaridases. Some of these disaccaridases include maltase, which breaks maltose into two glucose molecules. We also have sucrase, which breaks down sucrose into one glucose and one fructose molecule, and we also have lactase, which breaks down lactose into one glucose and one galactose molecule. So now that we have broken down that piece of bread into our monosaccharides we can finally absorb it.
We absorb carbohydrates indiscriminately, as in we absorb what’s there not what we need, and the small intestine is so well equipped for this because one, it has incredibly large surface area and two, the epithelial cells, remember they’re called enterocytes, have specialized transport mechanisms. So the large surface area of the small intestine is because of folds on folds on folds on folds. It starts with circular folds and coming off these folds are projections called villi, and these villi have a lamina propria core which have got blood vessels in them and lymphatic vessels. On these villi we have a covering of enterocytes and if we take just one of those enterocytes and have a look at the top border we can see those microvilli we were talking about, which are the tiny, tiny hair projections on every single cell of the villus and these make up the brush border. When we put all of this together it makes a huge surface area which makes it perfect for efficient absorption. So on these enterocytes they also have specialized transport mechanisms that allow them to absorb the end products of that piece of bread. Glucose and galactose monosaccharides are absorbed into the epithelial cell with sodium and energy-dependent secondary active transport through a symporter called the SGLT, meaning sodium-glucose cotransporter symport. This is driven by a sodium gradient so we therefore need to also have a pump in their cells that swaps sodium out of the cell and potassium into the cell to help keep the sodium concentrations inside the cell low. This means that we can use the low sodium concentration inside the cell to bring the monosaccharides in with them. The monosaccharide fructose enters the cell through a protein called GLUT-5 in a passive manner, which just means it doesn’t require ATP to function.
So now we have the end products of that piece of bread inside our enterocytes and finally the monosaccharides can exit the cell at the basal membrane by passive facilitated diffusion via a protein called GLUT-2. Blood capillaries are right next to these cells and so the monosaccharides then enter the capillary by simple passive diffusion.
Digestion of protein
We ingest proteins as peptides which are made up of amino acids with peptide bonds between them. At one end we have the amino terminal end and at the other end we have the carboxyterminal end. The two key players involved in the digestion of protein are the pancreas and the stomach. The stomach secretes gastric juice. This gastric juice contains pepsinogen and hydrochloric acid. The hydrochloric acid kills bacteria, the bad guys, not the good commensal ones, a bit like the salivary lysosome. Hydrochloric acid however also denatures proteins. Hydrochloric acid is secreted from parietal cells in the stomach and we generally don’t like exposing ourselves to something with such a low pH like hydrochloric acid. So our clever bodies make sure that we actually don’t come into contact with the hydrochloric acid at all. The epithelial cells of the stomach are impenetrable to hydrochloric acid. Partly because they have very, very close joins within each other called type junctions that stop the hydrochloric acid getting through the gaps between the cells. We also have cells in the stomach that secrete mucus and this mucus acts as a buffer that stops the acid from physically penetrating the cells. Bicarbonate is also secreted and this neutralizes the acid and inactivates pepsin near the surface of the cells so that there is a neutral environment here that will not damage the lining cells of the stomach.
The acid itself doesn’t actually break down protein but it’s pretty important in protein digestion. Number one, it denatures proteins. That just means that the protein structure uncoils from its highly folded form to expose more of the peptide bonds for enzyme attack. Hydrochloric acid also activates pepsinogen, a type of proenzyme secreted by chief cells in the stomach. When pepsinogen is released into the stomach hydrochloric acid activates it to pepsin. This acts as a self activating loop called an autocatalytic process where the new pepsin molecules act on other pepsinogen molecules to activate more pepsin. Pepsin initiates protein digestion by splitting certain amino acid linkages in proteins to release peptide fragments, which are small amino acid chains. Pepsin works best in an acidic environment which is kindly provided by hydrochloric acid. So the reason for this is because pepsin digests proteins and obviously we are made of protein, therefore we have to store and secrete pepsin in the inactive form of pepsinogen until it reaches the acidic environment of the stomach so that it doesn’t start breaking us down.
Now the other key player is the pancreas. The pancreas secretes proteolytic enzymes into this small intestine. Again, given that these enzymes break down protein, which is what we are made of, they are secreted in their inactive form which then must be activated in the small intestine where we have mucus to protect our lining cells of the small intestine. The proteolytic enzymes that the pancreas secretes are; procarboxy-peptidase, chymotrypsinogen and trypsinogen. These proteins are secreted into the duodenum of the small intestine and here embedded in the luminal membrane of the cells is an enzyme called enteropeptidase. When trypsinogen comes into contact with enteropeptidase it converts it into trypsin, the active form, which then further breaks down proteins.
As we saw with pepsin this is another example of an autocatalytic reaction where the trypsin helps convert more trypsinogen into trypsin. This trypsin then also converts the other two pancreatic enzymes, chymotrypsinogen and procarboxypeptidase, into their active forms, chymotrypsin and carboxypeptidase. Now let’s get back to the structure of the peptide protein and look at it in a bit more detail. The stomach and pancreatic enzymes that we were just talking about are called endopeptidases, endo meaning inside, pept meaning peptide, and ase meaning enzyme. Or if we put it all together, an enzyme that breaks down, or hydrolyzes, internal peptide bonds. This results in two smaller peptide chains. We then need to hydrolyze, remember that means break down, the external bonds of the two smaller peptide bonds. So we use exopeptidases, exo meaning outside. Or an enzyme that hydrolyzes external peptide bonds found at the carboxyterminal ends. This is helped by another enzyme called aminopeptidase which is found in the brush border of the small intestine. This results in amino acids and small peptides which are now ready to be absorbed.
So we know that the small intestine is the ultimate for absorbing, so it’s no surprise that we absorb proteins here too. We actually absorb everything here pretty much. Similar to what we saw with the monosaccharides in the previous video, we absorb amino acids across enterocytes with symporters and these symporters are selective for different amino acids. These amino acids are absorbed into the epithelial cells by means of sodium and energy- dependent secondary active transport. Small peptides are absorbed with another sodium dependent carrier in a process known as tertiary active transport. Tertiary meaning third, in reference to a third link step ultimately being driven by energy used in the first step. In this case the symporter is simultaneously transporting both hydrogen and the peptide from the lumen into the cell. This is driven by hydrogen moving down its concentration gradient, from high concentration to low, and the peptide moving against its concentration gradient, from low concentrations to high. The hydrogen gradient is established by an antiporter in the luminal membrane that is driven by sodium moving into the cell, down its concentration gradient, and hydrogen moving out of the cell, against its concentration gradient. The sodium concentration gradient that drives the antiporter in turn is established by the energy dependent sodium potassium pump at the basolateral membrane. So you can see that glucose, galactose, amino acids and small peptides all get that free ride in on the energy expended from the sodium transport. For the small peptides that enter the cell that weren’t hydrolyzed by the amino peptidases in the brush border they can then be broken down into their amino acids by intra, meaning inside, cellular peptidases, or intracellular peptidases. Just like in the previous video the amino acids then entered the blood circulation by simple diffusion.
Digestion of fat
Fat digestion is more complicated due to fat being insoluble.
Fat comes in the form of large fat globules. These large fat droplets are composed lots of triglycerides. These triglycerides go down to the stomach and get turned into liquid chyme. The liquid chyme then enters the first part of the small intestine, the duodenum. Once the chyme enters the duodenum, or the small intestine, it stimulates enteroendocrine cells, which are hormone cells in the lining of the small intestine and these will then release a pair of hormones. These hormones then tell the gallbladder, which is that little green structure at the bottom of the liver, to contract and it will then squirt bile through the bile ducts back into the duodenum and it is this bile that helps us break down fat. But it’s not actually the gallbladder that makes the bile. All the gallbladder does is store it and concentrate it. It is in fact the largest accessory organ, the liver, that makes the bile and this bile is the missing ingredient your body needs to attack fatty food. In part that’s because that isn’t water-soluble and since your insides are mostly water, fat will clump together becoming hard to digest. To keep fat from clumping together we need an emulsifier, so bile comes in to keep big hydrophobic fat molecules from sticking together. Having multiple smaller droplets rather than one large droplet allows a larger surface area for lipid hungry enzymes to move in and break the fats down into fatty acids and monoglycerides that you can then digest and absorb.
Another crucial accessory organ is the pancreas, a gland that kind of looks like a fistful of cottage cheese stuffed into a plastic bag. The pancreas also does lots of important things for your body, especially related to the endocrine system, but for the purposes of today just know that it brews up powerful enzyme cocktails that are also triggered by the same two hormones that trigger the gallbladder to release bile. Pancreatic juice has trypsin and in peptidases in there, amylase, which helped us break down the bread into smaller glucose and fructose units, nucleases in there, which burst the nucleic acids that are in DNA and RNA into nucleotides, and you also have lipases, and these are the enzymes that hydrolyze triglycerides into free fatty acids and monoglycerides. The lipases come into the duodenum and start to break down the triglycerides that make up the smaller fat droplets that the bile has emulsified for us. They do this with hydrolysis, where triglycerides are broken down into monoglycerides and free fatty acids.
In the monomers the end product is still not water-soluble. Micelles are water-soluble particles formed by bile salts and other bile constituents that can carry the end products of fat digestion with their lipid soluble interiors. Once these micelles reach the luminal membranes of the epithelial cell the monoglycerides and free fatty acids passively diffuse through the micelles through the lipid component of the epithelial cell membranes to enter the interior of these cells. As these fat products leave the micelles and are absorbed across the epithelial cell membranes the micelles can then go on pick up more monoglycerides and free fatty acids for absorption. Bile salts will keep emulsifying fats along the length of the small intestine until all the fat is broken down and absorbed and the bile is then reabsorbed in the ileum, which is the terminal segment of the small intestine.
So we’ve finally got the monomers inside the cell but the story is not over yet. Once inside the cells the monoglycerides and free fatty acids are resynthesized back into triglycerides which then aggregate and get coated with a layer of lipoprotein from the endoplasmic reticulum to form water-soluble chylomicrons. The chylomicrons are then extruded through the basal membrane of the cells by exocytosis. However, unlike the other macronutrients, protein and carbohydrate that we looked at, chylomicrons cannot across the basement membrane of capillaries so instead they enter the lymphatic vessels, the central lacteals in the villi.
Mouth
The mouth performs both mechanical and chemical digestion functions. Saliva contains an enzyme (amylase) that begins the breakdown of carbohydrates.
Liver
The liver produces bile to digest lipids, and converts carbohydrates to glycogen for storage. It also converts amino acids into proteins such as albumin.
Pancreas
The pancreas produces three substances:
Amylase to digest carbohydrates
Lipase to digest lipids
Protease to digest proteins
Stomach
The role of the stomach is to churn food and to produce acids and pepsin to digest proteins. The stomach also produces intrinsic factor to help with the absorption of B12.
Gall bladder
Within the gall bladder, bile (from the liver) is stored and concentrated to emulsify lipids, by breaking them into tiny droplets.
Small intestine
Within the small intestine, the villi help with nutrient absorption. Carbohydrates and proteing are absorbed into the blood, and lipids are absorbed into the lymph system via the lacteals.
Chyme is created in the?
Stomach
Pepsinogen, a digestive enzyme, is secreted by the:
chief cells of the stomach.
The ingestion of a meal high in fat content would cause which of the following to occur?
The ingestion of a meal high in fat content would cause which of the following to occur?
You have just eaten a meal high in complex carbohydrates. Which of the following enzymes will help to digest the meal?
Amylase
Molecule cleaved into two parts with the addition of a water molecule
Hydrolysis
Breaks down (digests) fat
Gastric and pancreatic lipase
If a macronutrient is not digested what would happen?
Absorption would not occur or would be impaired
One feature that increases surface area for absorption
Microvilli
Enzyme that breaks disaccharides into monosaccharides
Disaccharidases
Emulsifies fats
Bile
Proteins and fats are needed for building components of cells, therefore the preferred energy source is
Carbohydrates
