The Digestive System
A series of connected organs whose purpose is to break down, or digest, the food we eat. Food is made up of large, complex molecules, which the digestive system breaks down into smaller, simple molecules that can be absorbed into the bloodstream. Enzymes play are important for the complete digestion of food.
The scope of this article is to understand the mechanism involved to break down food. It is important to take special note for the role of each enzyme involved. If you suffer symptoms associated with food intolerance, especially bloating and cramps, you would be able to supplement your diet with the missing or lacking enzyme. One can lack enzymes either through decreased production of an enzyme, e.g. lactose intolerance, or due to surgical removal of part of the small intestine or e.g. the pancreas, which produces enzymes.
The Digestion's Mechanism
Digestion is the process by which the body breaks down food into absorbable nutrients. The body absorbs and assimilates everything that we ingest. Digestion includes physical actions such as chewing and peristalsis (involuntary contraction and dilation of muscles to force forward movement), as well as the chemical actions of enzymes, bile and acids.
As soon as we place food in our mouth the digestive system starts to work by biting and chewing. Only one digestive enzyme, amylase (for starch), is present in the mouth. However, food is normally not in the mouth long enough to permit complete digestion and little nutrient absorption takes place in the mouth.
The primary function of the stomach is to break large proteins into
smaller peptides and peptones. Pepsin is an enzyme produced in the acidic
environment of the stomach (1.5 to 7.0 pH) which digests proteins
into smaller peptides of varying lengths. Other enzymes such as gelatinase
(for gelatin) digest specific proteins.
Amylase (the enzyme that breaks down starch) is inactivated, or destroyed, when stomach pH falls below 6.5. Before this happens however, up to 50% of starches may be partially broken down. Some fat is emulsified (broken up into smaller pieces) in the stomach by bile acids, and the enzyme lipase, to aid digestion in the small intestine. At the average stomach pH of 2.0, however, most fat is formed into large globules that pass unchanged into the small intestine. Small amounts of sucrose (table sugar) may be broken apart into glucose and fructose by acid hydrolysis from the bile acids in the stomach.
|The digestive system.|
Virtually all absorption of nutrients (macronutrients such as carbohydrates
and fats and micronutrients such as vitamins and minerals) occurs in the
intestines. Absorption of nutrients is, in fact, the primary function
of the small intestine. Most carbohydrates, for instance, are absorbed
in the small intestine. The pancreatic enzymes secreted into the small
intestine also contain amylase which breaks down starches into a disaccharide
(two sugars joined together) called maltose. In the intestines, enzymes
such as maltase and lactase break disaccharides into single sugars (monosaccharides),
such as glucose. When adults and older children do not have enough lactase
to digest lactose (the sugar in milk) lactose intolerance results and
milk cannot be completely digested.
When food leaves the stomach, digestion is completed in the small intestine with the help of the enzymes secreted by the pancreas (lipase, amylase, protease, maltase, trypsin and chymotrypsin). The pH of food mass increases from about 2.0 to 6.5 (still slightly acidic), as it passes from the stomach through the small intestine to the colon. Very few nutrients, except water, is absorbed by the large intestine (colon).
An enzyme is a protein that acts as a catalyst in a biological reaction. It binds itself to a substance and converts it into another substance. Enzymes are very specific in their functions, which is why there are different enzymes for different biological reactions. In the case of digestion, distinct forms of food require specific enzymes. Unless proteins, fats and carbohydrates (sugars) are reduced to smaller absorbable components they will remain in the digestive track. The following list gives a glance for what each enzyme breaks and where it is produced:
- Amylase - starches into maltose (a disaccharide): saliva and pancreas - released into the small intestine
- Lipase - fats (lipids): in stomach and pancreas - released into the small intestine
- Pepsin - proteins into absorbable peptides and peptones: stomach
- Gelatinase - gelatin: stomach
- Maltase - maltose into monosaccharides: produced by the pancreas - released into the small intestine
- Lactase - lactose into monosaccharides: small intestine
- Trypson - proteins into peptides and amino acids: pancreas - released into small intestine
- Chymotrypson - proteins into peptides and amino acids: pancreases - released into small intestine
The following chart explains where action and what action takes place in a fully functional digestive system:
|mouth||Amylase in saliva||mechanical digestion of all food (teeth chewing & grinding) chemical digestion of carbohydrates begins with amylase|
|oesophagus||none||food passes from mouth through the oesophagus to the stomach|
|mechanical digestion of food (churning of stomach walls) and chemical digestion of proteins begins with pepsin while lipase breaks some of the fats and gelatinase breaks down gelatins|
|small intestine||pancreatic juice
bile, lipase, maltase, lactase
|chemical digestion of carbohydrates, proteins, &
lipids continues & is completed
the small, soluble nutrients (sugars, amino acids, fatty acids) are absorbed
|large intestine||none||water is absorbed from unusable, indigestible wastes (faeces)|
Persons who suffer from food intolerance may suffer from weight loss and malnutrition if kept ignored.
There are several causes why one does not produce enough enzymes or nothing at all. Most lactose intolerant persons do not produce enough lactase, hence by supplementing their diet with commercially available lactase enzyme they avoid suffering the symptoms (see Lactose Intolerance). On the other hand persons who had their pancreas removed may suffer symptoms because they may not be able to produce enough enzymes such as amylase, lipase and maltase amongst others (see Pancreatitis). Once again, today these enzymes are also available (see Products page).
Read also how food is absorbed by means of the Transport Mechanism.