What Is Chemical Digestion?

Medically Reviewed by Mahammad Juber, MD on June 01, 2022
5 min read

Chemical digestion is one of the six main functions of your digestive system. This system consists of your digestive or gastrointestinal tract and accessory organs. 

During chemical digestion, your food is broken down into very small pieces that your cells can absorb. Your cells will use the nutrients available from chemical digestion to make energy and new cells. 

This process is called metabolism.  

The organs that make up the digestive tract include your: 

  • Mouth
  • Stomach
  • Small intestine
  • Large intestine
  • Pharynx — This is a tube that extends from the back of your nose to the top of your esophagus.
  • Esophagus — This is a tube that extends from the base of your pharynx to your stomach.
  • Stomach

Your tongue and teeth are accessory components to the system that are found in your mouth. You also have a number of organs and glands that contribute fluids to your digestive tract. These fluids are necessary for chemical digestion. 

The organs and glands that make or interact with these fluids include your: 

  • Salivary glands
  • Liver
  • Gallbladder
  • Pancreas

Once food enters your mouth, it begins the process that we call digestion. Nutrients are absorbed from the digested food. Anything that can’t be absorbed or broken down is eliminated as waste. 

Your digestive tract performs six functions every time you eat. These functions are: 

  • Ingestion. This is the process of putting food into your mouth. 
  • Mechanical digestion. This is the process of physically breaking up large pieces of food so they’re more manageable. Chewing and the churning motions in your stomach are the main forces involved in this process. 
  • Chemical digestion. This is the process where complex molecules like proteins, fats, and carbohydrates are broken down into smaller pieces that your body can use. It requires special proteins called enzymes. 
  • Movements. There are many movements that keep food going through your digestive system. Some of these help with both mechanical and chemical digestion. Once your food has been swallowed, the movements are — for the most part — unconsciously controlled by your nervous system. 
  • AbsorptionThis is the point where nutrients are able to pass through the cell membranes in the lining of the small intestine and into the capillaries of your blood and lymph systems.  
  • Elimination. This is the process of removing indigestible waste products from your body in the form of feces or urine. Feces are formed in your large intestine and eliminated through your anus. 

Where Does Chemical Digestion Occur?

Chemical digestion takes place throughout your digestive tract.

Chemical digestion begins in your mouth with your saliva. The process continues in your stomach and is completed in your small intestine. The majority of chemical digestion takes place in your small intestine. 

The chemical reaction that’s central to chemical digestion is called hydrolysis. It’s the process of incorporating water molecules into larger molecular structures. This breaks them down into smaller parts and makes it much easier for your cells to absorb the nutrients in your food. 

Hydrolysis is a very slow process on its own. Your body makes enzymes, though, that speed up the process by facilitating the integration of water molecules into your proteins, fats, and carbohydrates. 

Some foods begin their chemical digestion when they’re still in your mouth. Saliva is capable of chemically breaking down some large molecules, like carbohydrates, but it isn’t good at digesting proteins.

The chemical digestion of proteins begins in your stomach. Carbohydrate and lipid digestion continues in the stomach (lipids are the chemical components of fat). 

Your stomach releases gastric juices that begin to break down all of the food you’ve eaten. You can also absorb some substances in your stomach, including aspirin and some alcohol. 

Once your food is broken down, your stomach stores it in an acidic liquid called chyme. Chyme is gradually released into the small intestine. 

Your small intestine is a long, highly-segmented structure that’s covered in microscopic strands called microvilli. There are approximately 200 million microvilli per square millimeter of your intestine. 

The microvilli helps increase the surface area in your intestine. They help make chemical digestion and absorption more efficient. Microvilli are covered in brush border enzymes that finish digesting your proteins and carbohydrates.

To finish chemically digesting fats, your small intestine relies on bile that’s made in your liver. Your gallbladder stores and concentrates your bile. It releases it when the small intestine needs it. Bile is necessary to turn fats into tiny lipid droplets. 

You also need a juice that’s made by the pancreas to complete chemical digestion. This helps buffer the acidic chyme and allows the digestive enzymes to work at optimal levels. 

Mechanical digestion takes place alongside chemical digestion. The main difference is that mechanical digestion is a physical process where grinding, tossing, and churning break your food into smaller pieces. It’s the process that breaks down the really large pieces of food so enzymes can do their work. Chemical digestion, meanwhile, actually changes the chemical composition of the food that you’ve eaten. 

Both mechanical and chemical digestion begin in your mouth and continue in your stomach. 

Chewing is the first step to mechanical digestion. The process continues with rolling motions in your stomach. These motions also help put the food in contact with the gastric juices and other fluids that are needed for chemical digestion. 

The various segments of your small intestine also help create rolling motions and increase the rate of digestion.  

Your body is absorbing nutrients at the same time and in many of the same areas where chemical digestion is occurring — namely, the small intestine. Digested nutrients move through the membranes of the cells that line your small intestine. From there, they make their way to the rest of your body, where they can be used to support cells, create energy, and make new cells. 

After your body has finished digesting your food and absorbing its nutrients, the food moves into the large intestine. This organ pulls water out from the digestive fluids until a solid fecal matter is formed. 

At this point, the only things left are the parts of your food that your body couldn’t digest or absorb. Fiber is an example of a food product that remains throughout the digestive process.

Feces can remain in your large intestine for one to two days before naturally occurring contractions move them along to your anus. Here, waste is removed from your body in the process called elimination