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Digestive Enzymes

Have you ever thought about what happens to the food after you have taken it into your mouth?
How those big steak pieces end up into small amino acids… those rice ends up giving you glucose? How the fatty food you eat end up into fatty acid and glycerol?… and where do all the extra things go?

So today I’m going to tell you everything about digestion…


In the mouth, the food is digested physically by your teeth, and also by the saliva produced by the salivary glands in your mouth. Saliva contains an enzyme called salivary amylase (ptyalin). Salivary amylase keeps the pH in the mouth basic. Salivary amylase will digest starch in your food to maltose, which will later get digested to glucose in the Duodenum.

S. Amylase
Starch ————-> Maltose
Note: not all amylum (starch) is digested to maltose in the mouth; some are digested in the stomach (very little) and some in the duodenum

The food is mixed with the saliva and is now called a bolus. The bolus travels down the esophagus by peristalsis movement. A wave of contraction is made on the walls of the esophagus just above the food bolus. This pushes the bolus down. By these waves, the food is passed into the abdomen (stomach) through the cardiac sphincter present at the end of the esophagus and just above the abdomen.

The cardiac sphincter also prevents the regurgitation of food from the stomach back into the esophagus.


When peptides are detected in the stomach, hormone gastrin is released into the bloodstream. This hormone causes the gastric glands in the lining of the stomach wall to produce gastric juice. Gastric juice is an acid juice (pH 1 – 3). Its main components are:
1. Hydrochloric Acid (HCl)
2. Renin
3. Pepsinogen
4. Gelatinase
5. Gastric Amylase
6. Gastric Lipase

HCl keeps the stomach pH acidic. As HCl is a strong acid, it will kill most of the bacteria present in the food. It has another main function. That is to convert pepsinogen into pepsin.

HCl: Pepsinogen ——–> Pepsin

Renin is an enzyme that has the function of digesting only milk proteins to peptides.

Renin: Milk Proteins ———–> Peptides

Pepsinogen is activated by HCl into Pepsin. Pepsin digests other proteins present in the food to smaller peptides fragments.

Pepsin: Other Proteins ———-> Peptides

Gelatinase digests type I and type V gelatin and type IV and V collagen, which are proteoglycans in the meat.

Gastric amylase digests starch that was not digested in the mouth.

Gastric lipase acts on butterfat. It digests it into fatty acids and glycerol.

These are all the digestive processes that take place in the stomach. The remaining food, now called chyme, is passed through the abdomen into the duodenum through the pyloric sphincter present at the lower end of the abdomen.


Brunner’s Glands are duodenal glands located throughout the duodenum. The main function of this gland is to produce an alkaline solution (contain bicarbonate ion) in order to :
1. To protect the duodenum from the acidic content of chyme
2. To provide an alkaline condition for the intestinal enzymes to be active, thus enabling absorption to take place
3. To lubricate intestinal walls

There are also 3 juices secreted in the Duodenum:
1. Bile Juice
2. Pancreatic Juice
3. Intestinal Juice

Bile juice is secreted by the gall bladder. After being secreted by the gall bladder it travels through the bile duct that opens into the duodenum. Bile juice contains bile salt that helps to emulsify fats.  Fats are broken down into small globules that are easily digested by the enzyme lipase present in the pancreatic juice.

The pancreatic juice is secreted by the pancreas. It travels through the pancreatic duct that opens into the duodenum. The enzymes present in the pancreatic juice are as follows:

  • Trypsinogen, an inactive form of trypsin, is activated by the enzyme enterokinase present in the intestinal juice. It digests proteins.
  • Chymotrypsin
  • Steapsin (Pancreatic Lipase) acts on fats converting them into fatty acids & glycerol.
  • Carboxypolypeptidase, the enzyme that converts peptides into amino acid
  • Pancreatic amylase digests starch

Intestinal juice is secreted by the intestinal walls, they contain the enzymes:

  • Entirokinase – activates trypsinogen to tryspsin
  • Eripsin – converts polypeptides to amino acids
  • Maltase – digests Maltose to glucose
  • Sucrase – digests sucrose into glucose and fructose
  • Lactase – digests lactose into glucose and galactose


The walls of the ilium are made up of fingerlike projections known as villi. Villi are made of up microvilli. Therefore, ilium has a very large surface area. The main function here is to absorb Vitamin B12, bile salts, and the products of digestion. Cells lining the ilium also secrete protease and carbohydrase enzymes responsible for the final stages of protein and carbohydrate digestions. The villi contain large numbers of capillaries which take the amino acids and glucose produced by digestion to the liver through the hepatic portal vein.

Lacteals are small lymph vessels that are present in villi. They absorb fatty acids and glycerol, the products of fat digestion. The lacteals transport the fatty acids and glycerol to the lymphatic system for filtering. The fatty acids and glycerol are combined with the blood as lymph joins blood at the right and left subclavian veins.


The large intestine is mainly responsible for storing waste, reclaiming water, maintaining the water balance, and absorbing some vitamins, such as vitamin K. The large intestine is divided into the cecum, colon, rectum, and anal canal.

  • Cecum
  • Colon – its primary purpose is to extract water from the feces
  • Rectum – The rectum ampulla acts as a temporary storage facility for feces. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is absorbed. If defecation is delayed for a prolonged period, constipation and hardened feces results.
  • Anal Canal – the feces are passed along from the rectum into the anal canal and they are then eliminated out of the body via the anus.


Contributed by: Amrik, October 15, 2006

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