Human Digestion: Enzymes, Absorption, and Assimilation

Posted on Feb 1, 2025 in Biotechnology

Why is Digestion of Large Food Molecules Essential?

Firstly, the food we eat is made up of many compounds produced by other organisms. Not all of these are suitable for human tissues. Therefore, these compounds have to be broken down and reassembled so that our bodies can use them. Secondly, food molecules must be small enough to be absorbed by the villi in the intestine through diffusion, facilitated diffusion, or active transport. Consequently, large food molecules need to be broken down into smaller ones for absorption to occur.

The Need for Enzymes in Digestion

Enzymes are needed in the process of digestion as they are the biological catalysts that break down large food molecules into smaller ones so that these can eventually be absorbed. Digestion can occur naturally at body temperature; however, this process takes a very long time as it happens at such a slow rate. For digestion to increase in these circumstances, body temperature would have to increase as well. However, this is not possible as it would interfere with other body functions. This is why enzymes are vital, as they speed up this process by lowering the activation energy required for the reaction to occur, and they do so at body temperature.

Source, Substrate, Products, and Optimum pH for Digestive Enzymes

Functions of the Stomach, Small Intestine, and Large Intestine

The stomach is an important part of the digestive system.

• It secretes hydrochloric acid (HCl), which kills bacteria and other harmful organisms, preventing food poisoning.
• It provides the optimum conditions for the enzyme pepsin to work (pH 1.5 – 2).
• It secretes pepsin, which starts the digestion of proteins into polypeptides and amino acids. These can then be absorbed by the villi in the small intestine.

The small intestine is where the final stages of digestion occur. The intestinal wall secretes enzymes, and it also receives enzymes from the pancreas. However, the main function of the small intestine is the absorption of the small food particles resulting from digestion. It contains many villi, which increase the surface area for absorption.

The large intestine moves the material that has not been digested from the small intestine and absorbs water. This produces solid feces, which are then egested through the anus.

Absorption vs. Assimilation

Absorption occurs when food enters the body as food molecules pass through a layer of cells and into the body’s tissues. This occurs in the small intestine, which has many villi that are specialized for absorption. Assimilation occurs when the food molecules become part of the body’s tissue. Therefore, absorption is followed by assimilation.

Villus Structure and its Role in Absorption and Transport

The structure of the villus is very specific:

• There are a great number of them, increasing the surface area for absorption in the small intestine.
• Villi have their own projections called microvilli.
• The many microvilli further increase the surface area for absorption.
• Microvilli have protein channels and pumps in their membranes to allow rapid absorption of food by facilitated diffusion and active transport.
• The villi contain an epithelial layer that is only one cell layer thick so that food can pass through easily and be absorbed quickly.
• Blood capillaries in the villus are very closely associated with the epithelium so that the distance for the diffusion of food molecules is small.
• This thin layer of cells contains mitochondria to provide the ATP needed for the active transport of certain food molecules.
• A lacteal branch at the center of the villus carries away fats after absorption.