Duodenum
Pancreas and Gall bladder
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Small Intestine
Small Intestine
Chyme leaves the stomach through the pyloric sphincter and enters the Duodenum. The pyloric sphincter only allows about half a teaspoon (3mL) of Chyme will pass into the duodenum at a time.
The Duodenum is the first part of the small intestine.
The small intestine consists of 3 parts
Duodenum
Jejunum
Ileum
The duodenum is highlighted blue in the diagram.
Notice how short it is compared to the rest of the intestine.
The duodenum is shorter than your ruler. It is only about 20cm.
That is tiny when you consider that the small intestine, when uncurled, is about 7 meters long (22 feet). (The length of our small intestine is your height x 3.5)
Despite its short length, a whole lot is happening
Neutralization
Neutralization
As the chyme enters the duodenum it needs to be neutralized. This serves two functions:
It stops the acid from damaging the cells of the duodenum.
It stops the pepsin from functioning, again protecting the cells of the duodenum from digestion
The duodenum is lined with mucus, this is only here and not in the other parts of the small intestine. The function of the mucus is to protect the duodenum from the acid chyme as it enters the duodenum
Cells near the pyloric sphincter release Hydrogen Carbonate to neutralize the acid - this is only a small amount. Hydrogen Carbonate is very similar to baking soda (which is Sodium Carbonate). Hydrogen Carbonate is part of the mucus that lines the stomach and the small intestine
These cells also stimulate the pancreas to release Hydrogen Carbonate (a large amount). The Carbonate reacts with the hydrogens in the acid, forming carbon dioxide and water. In the pipe that connects the pancreas and the gall bladder the pH is around pH8
The Hydrogen Carbonate that pours into the duodenum through the two ducts neutralizes the chyme and lifts the pH to 6. This deactivates the pepsin.
Helico pylori can survive in the duodenum, due to the acid that keeps entering, but as they need an acidic environment, they generally do not survive any further into the small intestine.
Pancreas and its Juice
Pancreas and its Juice
This is a very good and short (3min) clip outlining the functions of the pancreas
Another major phase of chemical digestion occurs in the duodenum with all of the 3 metabolizable nutrients continuing their digestion.
Fresh enzymes are released from the Pancreas
The Pancreas releases roughly a liter of Pancreatic Juices per day ( although this varies from 800mL to 1.5 Liters)
This is mostly water, then Hydrogen Carbonate, Amylase, Trypsin and Lipase. Due to the Hydrogen Carbonate this juice is on the basic side of neutral; pH 7.1 - pH 8.2
98% of the Pancreas is used for this function (exocrine). Only 2% is used for making Insulin and other hormones (endocrine).
The Pancreas secretes several enzymes. The ones of most interest for this topic are: pancreatic Amylase, Trypsin and Lipase
Starch continues its digestion with fresh alpha Amylase from the Pancreas
Proteins continue their digestion, with Trypsin that is released from the pancreas turning peptides into amino acids.
Lipids are digested with the enzyme Lipase released from the pancreas
Other enzymes contained in the pancreatic juice include Ribonuclease and Deoxyribonuclease. These cut up the DNA that is part of the food you eat from plants and animals, thus giving you the nucleotides that your cells need.
The exact quantities of the enzymes secreted changes depending on your diet. A diet high in fat will increase lipase production, whilst a diet high in starch will stimulate amylase production
Pancreatic Alpha Amylase
Pancreatic Alpha Amylase
Pancreatic alpha amylase does the same job as salivary amylase
Amylase cuts starch into simpler sugars like glucose
Well, it cuts starch into maltose which is just 2 units of glucose
The reason we can't just rely on the Salivary Amylase is because it is denatured in stomach acid and then digested by pepsin
Between 70% and 50% of starch digestion occurs using pancreatic amylase
All animals have pancreatic amylase - herbivore, carnivore and omnivore - so DO NOT talk about this in your exam
Naturally, omnivores and animals that have high starch diets have the highest levels of pancreatic amylase
It is here that dogs have had the massive increase in amylase compared with wolves
If you missed Salivary Amylase and the extra information all about amylase, click on the link below
Trypsin
Trypsin
Protein digestion = Protease = Enzymes that digest proteins
Different proteases have different sites that they cut on the proteins, thus having several different protease increases the efficiency of digestion.
The stomach makes Pepsin, as such its optimum pH for activity is around pH 2. A high pH will cause it to denature.
The pancreas makes Trypsin. It is released into the duodenum as part of the pancreatic juices which contain high levels of hydrogen carbonate as well as bile (which is very basic at around pH8). Thus trypsin's optimum pH is around pH 8
Trypsin continues the job that was started by pepsin
Stomach = Pepsin
(think peptic ulcers as these are in the stomach)
Duodenum = Trypsin
(think inTesTines = Trypsin. Or think tummy - as most of what we call tummy is intestines)
The pancreas itself is protected from the action of trypsin as it is secreted in an inactive form called trypsinogen. Effectively, the "-ogen" part is cut off once it is in the small intestine. Once the "-ogen" part is cut off the trypsinogen, then it becomes the active protease; trypsin. Trypsin can then cut off "-ogen" off other trypsinogens to get more trypsin.
Trypsin isn't the only protein digesting enzyme released form the pancreas. There is also Carboxypeptidase which does the same job, just with a different binding site. Chymotrypsin and elastase also digest proteins, again with different binding sites. Collectively these are called Proteases - protein cutting enzymes.
Lipase
Lipase
Lipase cuts up lipids
Lipase digests Lipids
Triglyceride is the type of fat found when we cook meat
A Triglyceride is a Glycerol Unit with 3 (Tri-) Fatty acid chains attached
A triglyceride is a very large molecule
Lipase cuts 2 of the Fatty Acid chains off
This leaves the glycerol with one fatty acid chain - this is called a Monoglyceride
So Lipase chemically digests a Triglyceride into a mono-glyceride and two Free Fatty Acids
This clip is about a washing product. However, it mechanism of action is very similar to the team work of bile and lipase in digestion. Have a watch
This is an excellent overview of the digestion of fat - and its short; 3 minutes
This is an in-depth look at the digestion and absorption of lipids - you don't have to watch it
For lipase to work on Fat, it needs to get to it and bind to it. However, as fat is not water soluble, it is hard for lipase to get to it. This is where bile comes in. The bile salts 'emulsify' the fat, breaking the big unreachable globs down into tiny globs that have 'dissolved' due to the bile salts attaching to the fat and hiding it from the water. This allows lipase to get to the fat and digest it. Without bile this would not be possible.
Pancreatic Lipase works best at a basic or alkaline pH of around pH 8
Pancreatic Lipase does about 70% of the total digestion of fat.
(these also salivary and gastric lipase, but we won't worry about them)
There is a drug called Orlistat is used for obesity. It works by blocking the action of Lipase, thus the patient has loose oily poops as the fat from their diet is not absorbed and is instead excreted
Have a watch over this experiment. Fatty acids chains are acidic, so as they are cut off the triglyceride, the pH of the solution drops, the solution becomes more acidic. Thus, if you have universal indicator in the solution, it will slowly move towards neutral. The solution starts of with a basic colour due to pancreatic lipase working best in a solution with an alkaline pH of pH 8
A couple of clips about Triglycerides. For one of them you'll have to open on YouTube due to its privacy settings
Bile and the Gall Bladder
Bile and the Gall Bladder
Bile is responsible for the 'emulsification' of the fats and oils that you eat
This allows the fat and oil to mix with the water in the intestines
Because the fat is now dissolved in the water of the intestines, Lipase can get to it and start to cut it into smaller pieces. Some of the Bile stays with these digested pieces to help them to be absorbed in the villi and move into the Lacteals
Bile is made by the liver
It is mostly water with 1% other things. These other things are Bile Salts, Bilirubin and fats
It is the Bile Salts that emulsify fats and oils
It is the Bile Salts that emulsify the Lipids
Once emulsified Lipase from the pancreatic juices can digest the Lipids.
The bile that is wrapped around the fat has a high pH, around pH 8. This assists the Lipase work.
The Bile Salts remain attached so that they can assist in absorption of the lipids.
Because the Bile Salts remain attached to the digested fats and oils as they are absorbed, the bile salts themselves are recycled and reused. As indicated in the image below
Fat and Water do not mix. So fat is not soluble.
So, what we need is the fat to be 'emulsified'
Detergent and soap are emulsifiers. If you eat a roast chicken and fried chips for dinner with your bare hands, your hands will become greasy due to the fat from the chicken and the oil from the chips. So, you go to wash your hands. First attempt, you just wash your hands with water. Your hands will still feel greasy, you can keep washing for several minutes with just water but your hands will continue to feel greasy. This is because the fat and oil will not dissolve into the water, so it stays stuck on your hand.
So what do you do? You'll use some soap, within seconds the fat and oil are covered by the soap, they dissolve in water and rinse away.
You can see this when you clean your roasting dish or the dish from your air fryer.
So what is happening?
Emulsifiers surround the fat molecule, hiding it from water, and thus allowing it to dissolve into the water
The soap action of bile is so effective, that there is a clothes washing soap that is made from slaughtered cow bile
The bile is being made continuously, so it needs somewhere to be stored until it is needed to dissolve Fat. For this, it is stored in the Gall Bladder
The Fuse School Clip above is an excellent overview of the action of Bile
Bilirubin is made form the breakdown of old blood cells. The iron is recycled and the rest of the molecule is broken down and excreted in the bile. Bacteria act on the Bilirubin in the large intestine creating Stercobilin which is brown - this makes feces brown
Bile and Lipase = Working together to digest fat
Bile and Lipase = Working together to digest fat
In the image below you can see the big fat globule that is floating in the intestine (in its lumen which is like the inside of the pipe). This big fat globule is insoluble, so lipase cannot get to it.
The bile salts attach to it and break it into smaller globules that are water soluble (because the bile salts hide the fat from the water)
Then the water soluble Pancreatic Lipase can get nice and close and cut it into even smaller pieces
The Pancreatic Lipase cuts the Triglycerides (Triglyceride = Glycerol with 3 fatty acid chains) into a monoglyceride (Monoglyceride = Glycerol with 1 fatty acid chain, usually the 'middle' chain) and two 'free' fatty acid chains
These can now pass into the cells that line the intestine where they are packaged with a loop of protein (making a lipoprotein, fat and protein ball)
This then moves into the Lacteal (and some in to the blood)
From the Lacteal it moves into the lymphatic system and then moves up through the abdomen and the chest to just behind the collar bone where it empties into veins that connect to the vena cava. From here it goes to the heart, then to the lungs, back to the heart and then off throughout the body
Fat Digestion
Fat Digestion
Triglycerides + Bile Salts + Lipase = Emulsification + Monoglyceride + Free Fatty Acids
A series of images showing the same as above but in different ways
Have a look at them and see if you can figure out what each is showing
Digestive Enzymes Recap
These few clips provide a very good overview of all of the digestive enzymes - good for a recap
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