Big Ideas

Digestion is the breakdown of food into simpler molecules that can be absorbed by the body. The digestive system is actually a long hollow tube called the gastrointestinal tract, or GI tract. The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Several major glands include the salivary glands, the pancreas, and the liver.

The first task of the digestive system is to break down food into a fine pulp (mechanical digestion). When the food is physical broken down, digestive chemicals break the food down into small molecules (chemical digestion).

The mouth is the organ in which the process of digestion begins. It starts the process of mechanical digestion by grinding the food with teeth. Also in the mouth, an enzyme called salivary amylase begins to break down long starch molecules into maltose. Once the food is ready to be swallowed it is gathered into a ball called a bolus and sends to the part of the bolus called the pharynx.

Once in the pharynx, the bolus next passes down the esophagus. The bolus is moved down the esophagus by rhythmic muscles contracts called peristalsis. At the end of the esophagus is a thick ring of a muscle called a sphincter which allows food to enter the stomach but not from the stomach back into the esophagus.

Once the food enters the stomach it is mixed with the digestive juices. The stomach produces stomach acid; hydrochloric acid, and pepsin. Stomach acid activates the pepsin that digests protein into smaller proteins. Proteins are long chains of amino acids that are essential for cell growth and repair.

Next, from the stomach the food moves into the small intestine where it continues to be digested by enzymes produced in the pancreas. In the small intestine, the pancreas empties digestive juices that break down carbohydrates, fats, and protein. The liver produces bile that is stored in the gallbladder between meals and emptied into the small intestine. The small intestine is where most of the nutrients are absorbed and thus is the most important organ in the digestive system.

The undigested materials continue to the large intestine where the remaining water is absorbed and eventually the waste material is expelled through the rectum. Also, fats (lipids) are molecules that are stored for future energy uses mainly because they are difficult to digest.


Drawings of the human digestive system

Our models of the digestive system

Chemistry review and food

Food indicator tests

  • Benedicts solution
  • Iodine
  • Burrets solution
  • Sudan IV

Tests of own food

Fast food diets

Molecular models of food

Protein model

Carbohydrate model



Fat model

Enzyme activity

The main components of the digestive system are the esophagus, stomach, small intestine, large intestine, liver, gallbladder, pancreas, and bile duct

The three types of foods are proteins, carbohydrates, lipids

Food is broken down into simpler forms which then can be used by the body

A protein is a chain of amino acids which help with cell growth and repair

Carbohydrates give the cells energy. They are composed of long chains of sugars

Enzymes help to speed up reactions and are important in the digestive system

Digestion takes place in the stomach, small intestine, large intestine, liver, gallbladder, pancreas, ducts

Certain foods take longer to digest – fats are the longest – often stored as energy.

Lipase, pepsin, maltase, carboxypeptidase, and amylase are enzymes that break down food

Digestive System

Digestion is the process in which food and liquid are broken down into their smallest parts so that the body can use them


L2.p2A Describe how organisms sustain life by obtaining, transporting, transforming, releasing, and eliminating
matter and energy. (prerequisite)

Identify the general functions of the major systems of the human body (digestion, respiration,
reproduction, circulation, excretion, protection from disease, and movement, control, and coordination)
and describe ways that these systems interact with each other

B2.2f Explain the role of enzymes and other proteins in biochemical functions (e.g., the protein hemoglobin
carries oxygen in some organisms, digestive enzymes, and hormones).

B2.2g Propose how moving an organism to a new environment may infl uence it ability to survive and predict
the possible impact of this type of transfer.