Chemical Digestion
Introduction
Chemical digestion is the process of reducing food to a size where the nutrients can be absorbed into the bloodstream. This procedure begins in the mouth, continues in the stomach, and ends in the small intestines. Digestive enzymes are the primary coordinators of chemical digestion. These enzymes break down macromolecules into smaller particles in order for the body to use the energy. The enzymes can be found all across the digestive system, as well as inside cells, especially in lysosomes (primarily to maintain cellular survival).
Mouth
Chemical digestion first begins in the mouth where the food arrives and encounters saliva. In the oral cavity, salivary glands secrete the saliva with a multitude of enzymes and substances in order to assist in the digestion and disinfection of the food. Some of the important enzymes include:
Lingual Lipase: Begins the digestion of lipids/ fats and has an optimum PH level of 4.5-5.4.
Amylase: Initiates the digestion of carbohydrates. Amylase breaks down complex carbohydrates and even simple sugar into smaller chains. Surprising, amylase is commonly secreted by fungus too.
Lysozyme: Since most food contain other substances, eg. viruses and bacteria, lysozyme offers a limited, yet helpful, antiseptic role in the digestive system.
Haptocorrin: Helps with the absorption of vitamin B12. Haptocorrin protects the vitamin from the acidic conditions in the stomach after the vitamin leaves its original carrier protein.
Lingual Lipase: Begins the digestion of lipids/ fats and has an optimum PH level of 4.5-5.4.
Amylase: Initiates the digestion of carbohydrates. Amylase breaks down complex carbohydrates and even simple sugar into smaller chains. Surprising, amylase is commonly secreted by fungus too.
Lysozyme: Since most food contain other substances, eg. viruses and bacteria, lysozyme offers a limited, yet helpful, antiseptic role in the digestive system.
Haptocorrin: Helps with the absorption of vitamin B12. Haptocorrin protects the vitamin from the acidic conditions in the stomach after the vitamin leaves its original carrier protein.
Stomach
When the food arrives in the stomach, different hormones allow the body to release enzymes to further digest the food. These enzymes are called gastric enzymes. Amylase from the oral cavity is still at work digesting the carbohydrates while the lingual lipase is replaced by the gastric lipase. This stage of digestion usually takes between one to two hours before the food is released into the small intestines. The stomach plays an important role in both mechanical digestion (mixing and crushing food) and chemical digestion (enzymes and acids). This organ produces many enzymes, hormones and compounds, with different functions for each. Here are some examples:
Pepsin (Enzyme): It is the main gastric enzyme in the stomach and is responsible for breaking down the protein into tiny particles, such as peptide fragments and amino acids. Pepsin is produced in the stomach cells, first in its inactive form called pepsinogen, which is later activated by the stomach acid. Unlike lipid and carbohydrates, protein begins digestion in the stomach instead of the oral cavity.
Hydrochloric Acid: It is a highly corrosive and strong acid that mainly functions to dissolve protein, destroy bacteria and viruses in the food, and to activate pepsin. In layman terms, it is referred to as simply "stomach acid". It is produced by cells called parietal cells that line the stomach. In order to protect itself from the highly corrosive acid, the stomach is lined with mucus, which is changed every two weeks.
Mucin: The stomach has a high priority on killing bacteria and viruses using its acidic conditions, but is also responsible for protecting itself from its own acid. The solution for this is by secreting mucin and bicarbonate to create a protective layer on the sides of the stomach. Mucin and bicarbonate are created in mucus cells. Another solution is by having a quicker cell turnover rate in order to regenerate dead cells.
Gastrin: It is an important hormone produced by the "G-cells" in the stomach. These cells produce gastrin when food enters the stomach and after the stomach is exposed to protein. These hormones arouse the parietal cells to produce hydrochloric acid and intrinsic factor (helps with absorption of vitamin B12).
Gastric Lipase: Has the same task as the lingual lipase (digest lipids) and together, comprise the two acidic lipase. Both of these lipase do not require bile acid or colipase in order to function. Gastric lipase has a pH level of 3-6.
Pepsin (Enzyme): It is the main gastric enzyme in the stomach and is responsible for breaking down the protein into tiny particles, such as peptide fragments and amino acids. Pepsin is produced in the stomach cells, first in its inactive form called pepsinogen, which is later activated by the stomach acid. Unlike lipid and carbohydrates, protein begins digestion in the stomach instead of the oral cavity.
Hydrochloric Acid: It is a highly corrosive and strong acid that mainly functions to dissolve protein, destroy bacteria and viruses in the food, and to activate pepsin. In layman terms, it is referred to as simply "stomach acid". It is produced by cells called parietal cells that line the stomach. In order to protect itself from the highly corrosive acid, the stomach is lined with mucus, which is changed every two weeks.
Mucin: The stomach has a high priority on killing bacteria and viruses using its acidic conditions, but is also responsible for protecting itself from its own acid. The solution for this is by secreting mucin and bicarbonate to create a protective layer on the sides of the stomach. Mucin and bicarbonate are created in mucus cells. Another solution is by having a quicker cell turnover rate in order to regenerate dead cells.
Gastrin: It is an important hormone produced by the "G-cells" in the stomach. These cells produce gastrin when food enters the stomach and after the stomach is exposed to protein. These hormones arouse the parietal cells to produce hydrochloric acid and intrinsic factor (helps with absorption of vitamin B12).
Gastric Lipase: Has the same task as the lingual lipase (digest lipids) and together, comprise the two acidic lipase. Both of these lipase do not require bile acid or colipase in order to function. Gastric lipase has a pH level of 3-6.
Small Intestines
After leaving the stomach, the food is now a thick liquid called chyme. This liquid will move through the small intestines, where it will be exposed to several different liquids and enzymes. One of the most important fluids is bile, which changes fat into fatty acids and glycerol. During this time, the small intestine is absorbing the nutrients from the chyme and is introducing them into the blood stream. The last stage of chemical digestion happens in the final area of the small intestines, called the ileum. In this space, two enzymes are released called protease and carbohydrase, which complete the digestion of proteins and carbohydrates. The remaining food will leave the small intestines and enter the colon where it will be processed as waste, thereby ending the chemical digestion process. The following are an in-depth look at some of the enzymes and hormones used during this stage:
Secretin: It is an hormone produced by the duodenal "S cells" in order to control the acidity of the chyme while inside the small intestines and stomach. It helps regulate the pH levels by preventing the parietal cells from producing more gastric acid while stimulating the produce of bicarbonate. Fun Fact: Secretin was the first hormone to be identified.
Protease: Are enzymes that catalyzes the breakdown of protein into amino acids. Since proteins are long chains of amino acids, these enzymes break them down into peptides (smaller chains of amino acids) and eventually into individual amino acids. This allows the small intestines to finish the protein digestion. The word equation for protease is:
protease
proteins -----------> amino acids
Carbohydrase: These enzymes breakdown the carbohydrates into simple sugars. From the point of eating, carbohydrase digest complex starch molecules into simpler maltose molecules. Inside the small intestines, the maltose is disintegrated further into glucose. The word equation for carbohydrase is:
amylase* maltase*
starch -----------> maltose ------------> glucose
Protease: Are enzymes that catalyzes the breakdown of protein into amino acids. Since proteins are long chains of amino acids, these enzymes break them down into peptides (smaller chains of amino acids) and eventually into individual amino acids. This allows the small intestines to finish the protein digestion. The word equation for protease is:
protease
proteins -----------> amino acids
Carbohydrase: These enzymes breakdown the carbohydrates into simple sugars. From the point of eating, carbohydrase digest complex starch molecules into simpler maltose molecules. Inside the small intestines, the maltose is disintegrated further into glucose. The word equation for carbohydrase is:
amylase* maltase*
starch -----------> maltose ------------> glucose
*Amylase and maltase are examples of carbohydrase.