Chemical digestion occurs when the body uses enzymes to break down certain molecules, such as carbohydrates. This process allows the bloodstream to absorb smaller molecules to transport around the body.

As food travels from a person’s mouth and into their digestive system, the body uses enzymes to break it down into smaller molecules. The name for this process is chemical digestion. It primarily takes place in the mouth, stomach, and small intestine.

Once chemical digestion has taken place and the bloodstream has absorbed these smaller molecules, cells in the body can use them in metabolism. This refers to chemical processes within cells, such as converting these small molecules into energy.

This article discusses chemical versus mechanical digestion, why chemical digestion is necessary, and the different parts of the digestive system where chemical digestion occurs.

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Digestion begins with the ingestion of food and drink and ends with defecation.

The process occurs throughout the gastrointestinal tract and has two main forms: mechanical and chemical.

Mechanical digestion

When a person ingests food, their body must break it down into smaller particles through mechanical digestion. This allows the next step of digestion, which scientists call chemical digestion, to occur.

Mechanical digestion begins in the mouth when a person chews their food. It then continues in the stomach with churning and mixing actions that break the food into smaller pieces.

Chemical digestion

The digestive system uses chemical digestion to break down certain molecules. This occurs so a person can absorb these molecules into the bloodstream.

The digestive system does this through a process healthcare professionals call hydrolysis. During hydrolysis, the digestive system uses water and digestive enzymes to break down these molecules.

The body uses chemical digestion to break down the complex molecules of:

The process turns them into much smaller molecules, including:

After chemical digestion, the bloodstream absorbs these tiny molecules so cells in the body can utilize them.

Nutrients are chemical substances the body needs to carry out basic functions.

The six major classes of nutrients that are essential for human health are:

  • water
  • carbohydrates
  • proteins
  • fats, which healthcare professionals may call lipids
  • vitamins
  • minerals

The following nutrients are too large to absorb into the bloodstream:

  • carbohydrates
  • lipids
  • proteins

This means that the body must break these nutrients into smaller molecules during chemical digestion so that they can absorb into the bloodstream and travel to cells around the body.

If a person eats a healthy balanced diet, chemical digestion helps them use all these nutrients and prevents malnutrition.

Malabsorption and food intolerance

Malabsorption occurs when the body does not properly absorb nutrients from food. It can occur for several different reasons. One common cause of malabsorption is a lack of digestive enzymes.

If a person lacks certain enzymes, it can cause them to develop intolerances to certain foods.

Lactose intolerance is an example of a food intolerance that occurs due to low levels of enzymes. Lactose is a sugar that is naturally present in most dairy products.

If a person has lactose intolerance, their small intestine makes low levels of lactase, the enzyme that breaks down lactose. If a person has low lactase levels, they cannot digest all of the lactose they eat or drink.

A person with lactose intolerance who consumes it may develop the following symptoms within a few hours:

Chemical digestion occurs in several different parts of the digestive system.

The mouth

Chemical digestion in the mouth is minimal compared with other parts of the digestive system. However, it does take place here.

The salivary glands produce an enzyme called salivary amylase, which helps digest carbohydrates into maltose and maltotriose.

The lingual glands produce an enzyme called lingual lipase, which helps break down certain fats, called triglycerides, to form substances called diacylglycerols and monoacylglycerols.

Healthcare professionals call this partially digested food a “bolus.” After sufficient digestion, the person swallows the bolus and passes it into the esophagus.

The stomach

Once the bolus passes through the esophagus, it enters the stomach, where a more significant amount of chemical digestion occurs.

Oxyntic glands in the main body of the stomach secrete hydrochloric acid. One main purpose of hydrochloric acid is to create a hostile environment that destroys pathogens that may enter the stomach through the mouth. Its other key purpose is to denature proteins to help enzymes break them down.

Pepsin is the enzyme in the stomach that breaks down proteins into peptides and amino acids.

The oxyntic glands also secrete the enzyme gastric lipase, which helps break down fats into fatty acids and monoacylglycerols so they can absorb into the bloodstream.

The small intestine

Most chemical digestion occurs in the small intestine.

The pancreas produces several digestive enzymes that function more effectively away from the highly acidic conditions of the stomach. The pancreas secretes these enzymes into the small intestine, where they carry out chemical digestion.

Pancreatic amylase is an enzyme that helps digest starch into maltose and maltotriose. The pancreas also creates pancreatic lipase and another important enzyme called colipase. These help break down triglycerides to form diacylglycerols and monoacylglycerols.

The small intestine also produces the enzyme lactase. Lactase breaks lactose down into glucose and galactose.

The large intestine

The large intestine produces no digestive enzymes. This means no chemical digestion takes place in the large intestine.

Instead, the large intestine absorbs water and electrolytes and eliminates feces.

Digestion begins with the ingestion of food and drink and ends with defecation. Mechanical and chemical digestion are the two main types. Mechanical digestion occurs so the body can then carry out chemical digestion.

Chemical digestion uses certain enzymes to break down nutrients, such as carbohydrates, proteins, and fats, into smaller molecules. This occurs so a person can absorb these molecules into the bloodstream. The process occurs in the mouth, stomach, and small intestine.