Terminology - Re. Digestive System

Some words and expressions necessary for an introductory-level understanding of the human digestive system and digestive processes concern specific processes such as peristalsis. Others refer to specific substances, or types of matter, found in the digestive system, e.g. enzymes and proteins.

This list is in alphabetical order. It is intended to assist students of first-level courses in biology, human biology, anatomy & physiology and health sciences.

This page of short definitions is intended for convenient revision.
Further information is available via the links.


Amino Acids

Amino Acids are organic compounds that contain both:

  • an amino group (-NH2)
  • a carboxyl group (-COOH)

Amino acids are fundamental constituents of all proteins.

Breakdown of the proteins in the body yields amino acids, specifically:
alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, valine.

Some amino acids can be synthesised by the human body but essential amino acids must be obtained from protein in the diet.



Bile is a thick alkaline fluid that is secreted by the liver and stored in the gall bladder.

It is ejected into the duodenum via the common bile duct. It does not flow continuously but is released at intervals.

Bile may vary in colour (e.g. yellow, green, brown) according to the proportions of bile pigments. Leithin, cholesterol and bile salts are also present in bile.

What it does:

Bile salts help to emulsify fats in the duodenum so that they can be more readily broken-down by pancreatic lipase into fatty acids and glycerol. Bile salts also form compounds with fatty acids. Bile also helps to stimulate peristalsis in the duodenum.



There are many different carbohydrates. The term carbohydrate refers to any one of a huge group of compounds that contain the elements carbon (C), oxygen (O) and hydrogen (H) and have the general formula Cx(H2O)y. Examples of carbohydrates include sugars and starch.

Why are they important?

Carbohydrates are an important source of energy. They are produced by plants, in which carbohydrates form important structural and storage materials, e.g. cellulose and starch, respectively. Carbohydrates are ingested into the human body as food (incl. sugary drinks). Carbohydrates are one of the three main parts of the human diet, the others being fat and protein.

How does the body process, use, and store carbohydrates?

All carbohydrates ingested as part of the diet are eventually broken-down by the body into the simple sugar glucose - which can participate in energy-generating metabolic processes. Excess carbohydrates, ingested but not needed by the body immediately are stored in the liver and muscles in the form of glycogen.



Disaccharides are carbohydrates that consist of two linked monosaccharide units.

The most common disaccharides include maltose, lactose and sucrose.
(The -suffix "ose" indicates that these are sugars.)



Enzymes are types of proteins that, in small quantities, increase the rate of biological reactions without being used-up in the reactions themselves. That is, enzymes can act as catalysts (see below).

Enzymes form within living cells and may act either within the cell or ouside it.

Many enzymes are unstable and are easily de-activated, e.g. by heat or by other chemicals.

Certain enzymes are essential for normal functioning and development of the human body. Failure in the production or activity of even a single enzyme can lead to serious problems / disorders.

How do enzymes act as catalysts ?

Enzymes act by binding with the substance involved in the reaction (called the "substrate") and converting it to another substance (called the "product" of the reaction).

Particular enzymes catalyse specific reactions. There are many different enzymes that assist in many of the huge range of different biochemical reactions. Even when considering the specific enzyme that could help with a particular reaction, understand that each enzyme needs certain conditions in order to act as a catalyst with the maximum efficiency possible for it. Such conditions can include a particular range of:

  • temperature(s)
  • pH
  • presence of specific other enzymes called 'co-enzymes'
  • lack of particular inhibitors i.e. substances that reduce or prevent the particular reaction

Names of specific enzymes:

The names of many enzymes end with the suffix "-ase". They are generally named according to either the substrate on which they act (e.g. lactase), or according to the type of reaction they catalyse (e.g. hydrolase).



Fats are substances that contain one or more fatty acids in the form of triglycerides.

They are also the main form in which energy is stored in the body as adipose tissue.

Fats are one of the 3 main parts of the human diet - the others being carbohydrates and proteins.

Uses and effects of fats:

A healthy diet should include sufficient essential fatty acids for the efficient absorption of fat-soluble vitamins via the intestines. Fats in the body itself provide several important functions including insulation beneath the skin (in the subcutaneous tissue) and around organs such as the kidneys. However, excessive deposition of fat in the body can lead to obesity and its many consequences and risks.


Fatty Acids

Fatty Acids are the basic constituents (parts) of many important lipids (fats), including e.g. triglycerides. In terms of their chemistry, fatty acids are organic acids that have long straight hydrocarbon chains and an even number of carbon atoms.

In the same way as amino acids, some fatty acids can be synthesized (formed) in the body but others, called "essential fatty acids" must be obtained from the diet, i.e. ingested within foodstuffs - including beverages.

Examples of fatty acids include:

  • palmitic acid,
  • oleic acid,
  • stearic acid.


(also known as 'glycerin')

Glycerol is a clear, viscous (meaning very thick e.g. flows more like treacle than like water - but note that the term "viscous" conveys no information about colour or transparency) liquid that can be produced by hydrolysis of fats and mixed oils, e.g. as a by-product in the manufacture of soap.

What does it do:

Glycerol has various uses, such as an emollient in some skin products, as a sweetening agent in some pharmaceutical products, and as a laxative (e.g. in the form of suppositories). The use of glycerol for its laxative effects is relevant to the digestive system.



Insulin is a protein hormone produced by the beta cells in the islets of Langerhans within the pancreas.

Secretion of insulin is stimulated by a high concentration of sugar in the blood.

What it does:

Insulin is important for regulating the amount of sugar (glucose) in the blood. Lack of sufficient insulin causes diabetes mellitus - in which excessive amounts of sugar are present in both the blood and urine. Diabetes mellitus can be treated with injections of insulin.



Monosaccharides are simple sugars that have the general chemical formula (CH2O)n.

Monosaccharides can have between 3 and 9 carbon atoms but the most common ones have 5 or 6.

How monosaccharides are described:

Monosaccharides are classified according to the number of carbon atoms they have:

  • trioses have 3 carbon atoms,
  • tetroses have 4 carbon atoms,
  • pentoses have 5 carbon atoms,
  • hexoses have 6 carbon atoms.



Peptones are large protein fragments that result from the action of enzymes on proteins in the initial stages of breaking-up proteins.



Peristalsis is a wave-like movement (motion) that progresses along some of the hollow tubes of the body that have circular and longitudinal muscles, such as the intestines. Peristalsis happens involuntarily and is induced by distension of the walls of the tube.

What it does:

Immediately behind the distension the circular muscle contracts. In front of the distension the circular muscle relaxes and the longitudinal muscle contracts - which pushes forward the contents of the tube.



Polypeptides are substances whose molecules consist of three or more amino acids linked together by peptide bonds. (Note that the prefix "poly-" indicates "many", implying a structure consisting of multiple units attached together.)

An example of a polypeptide is a protein because all protein molecules are polypeptides.



Proteins are a category of compounds formed from the elements carbon (C), hydrogen (H), Oxygen (O) and Nitrogen, and in some cases also Sulphur (S) and Phosphorus (P). There are many different protein molecules. They have complex structures formed by one or more chains of linked amino acids.

Why are proteins important?

Protein is one of the three main parts of the human diet - the others being fat and carbohydrates. Proteins are essential as chemical "building-blocks" within the body because they form the material structures of many tissues, muscles and organs. Proteins are also important because of their roles regulating bodily functions, enzymes and hormones.

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