In this article, we will discuss digestion, ingestion, mechanical and chemical digestion, and egestion in detail. So, let us get started.

Digestion

Digestion refers to the breakdown of biological molecules such as carbohydrates, proteins, and fats into smaller soluble substances so that they can be absorbed into the bloodstream.

The enzymes that are critical for digestion include amylase, protease, and lipase. These enzymes will be discussed in detail later in this article.

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Ingestion

Food enters our digestive system through our mouth and this process is referred to as ingestion. Once we put food in our mouth, we start chewing it using our teeth. Our chewed food becomes a ball of food known as bolus. This bolus is passed down the oesophagus and into the stomach.

Peristalsis

Peristalsis refers to the process by which food is passed through our digestive system. In this process, two sets of muscles present in our gut play their role. These muscles are:

  • Circular muscles: These muscles minimize the diameter of the gut when they contract.
  • Longitudinal muscles: These muscles reduce the length of the gut as they contract

Both circular and longitudinal muscles work together to generate wave-like contractions. They push the bolus through the gut using their squeezing action.

Mechanical and Chemical Digestion

We have already discussed that the process of digestion breaks down large, insoluble food molecules into smaller, water-soluble molecules by employing mechanical and chemical processes. Mechanical digestion involves chewing in the mouth, churning of food in the stomach, and emulsification. On the other hand, different digestive enzymes are involved in chemical digestion. These enzymes are proteins that act as biological catalysts. First, let us discuss mechanical digestion in detail.

Mechanical Digestion

Mechanical digestion refers to the breakdown of food molecules into smaller pieces without causing a chemical change in the food molecules.

This process is primarily carried out through:

  • the chewing action of teeth
  • churning action of stomach
  • emulsification of fats through bile in the duodenum

The bone of the jaw holds the teeth firmly. Teeth chew the food to increase its surface area so that when it is exposed to saliva in the mouth and other digestive juices in the body, it could be broken down faster.

Teeth are of various shapes and sizes the allow them to perform various functions:

  • Incisors: Chisel-shaped teeth that perform the functions of biting and cutting.
  • Canines: Pointed teeth that tear, hold and bite food
  • Premolars and molars: They are large teeth having flat surfaces with ridges at the edges. They are used for chewing and grinding food.

In the next section of the article, we will discuss chemical digestion.

Chemical Digestion

Half of the food is digested mechanically through chewing, churning, and emulsification so that large food particles are converted into smaller pieces and surface area is increased for enzymes to act on. Usually, chemical digestion takes place where bonds that hold large molecules together are broken down to convert molecules into smaller ones.

Enzymes produced in different parts of our digestive system control chemical digestion. The three main types of digestive enzymes are:

  • Carbohydrase
  • Proteases
  • Lipases

Carbohydrases

  • Carbohydrases are responsible for breaking down carbohydrates into simple sugars.
  • Amylase is a type of carbohydrases that breaks down starch.
  • The enzymes amylases are produced in the mouth and pancreas. They are secreted into the duodenum
  • The purpose of amylases is to digest starch into smaller sugars.

Proteases

  • They are the type of enzymes that are responsible for breaking down proteins into amino acids in the stomach and small intestine.
  • The digestion of proteins takes place in the stomach and duodenum where two main types of enzymes, pepsin, and trypsin are produced.
  • Pepsin is produced in the stomach, whereas trypsin is produced in the pancreas and secreted into the duodenum.

Lipases

  • Produced in the pancreases and secreted into the duodenum, the lipases enzymes are responsible for digesting lipids into fatty acids and glycerol.

Hydrochloric acid

Our stomach produces many fluids which are collectively known as gastric juice. One of the fluids produced by our stomach is known as hydrochloric acid. The purpose of this fluid is to kill bacteria in the food and provide an acid pH for enzymes to work in the stomach.

The role of bile

Cells in the liver produce a fluid known as bile which is stored in the gall bladder. Bile performs the following functions:

  • It is alkaline and it basically neutralizes hydrochloric acid coming from the stomach.
  • The enzymes present in our stomach are more alkaline and have a higher optimum pH than those present in the stomach.
  • Bile breaks down large drops of fats into smaller ones through a process known as emulsification. The larger surface area enables lipase to break down lipid chemically into fatty acids and glycerol.

Remember that emulsification is a part of mechanical digestion rather than a chemical one. This is because emulsification breaks down fats into smaller ones, but it does not break down the bonds which is the main essence of chemical digestion.

In the next section of the article, we will discuss assimilation and egestion in detail.

 

Assimilation and egestion

There are varying outcomes for digested and undigested food once they are passed through the alimentary canal (gut).

Assimilation

Assimilation refers to the movement of digested food molecules into the cells of our body where are employed. For instance:

  • Glucose provides energy for the process of respiration’
  • Amino acids are employed to create new proteins
  • The role of the liver is crucial in assimilation because it converts glucose into glycogen and amino acids into proteins. The liver is employed in the process of deamination. It involves the removal of the nitrogen-containing element of amino acids to create urea. It then releases energy from the remaining amino acid.

Egestion

The majority of the water and contents of the gut are absorbed into the small intestine. When the contents reach the end of the small intestine, the majority of the digested food is already absorbed.

The rest of the material contains:

  • Bacteria (both living and dead)
  • Water
  • Cells from the gut lining
  • Indigestible substances such as cellulose from cell walls of plants

The first part of the large intestine is the colon which absorbs most of the remaining water. This results in semi-solid waste materials known as feces that are stored in the rectum which is the last part of the large intestine. Anus moves these feces out of the body through a process known as egestion.

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Rafia Shabbir