Digestion of Proteins: Study with Video Lessons, Practice Problems & Examples

Protein digestion primarily aims to produce amino acids for synthesizing new proteins. It begins in the stomach, where mechanical digestion and denaturation occur, aided by stomach acid and pepsin, which breaks proteins into polypeptides. In the small intestine, proteases like trypsin and chymotrypsin further hydrolyze these polypeptides into free amino acids. These amino acids are then absorbed through intestinal cells into the bloodstream, facilitating their delivery to various cells for metabolic processes, including anabolism and energy production.

Digestion of Proteins Concept 1

Hey, everyone. In this video, we're going to take a look at the digestion of proteins. Now here we're going to say that the main purpose of protein digestion is to produce amino acids for the synthesis of new proteins. And, we're going to say that digestion begins in the stomach, but final hydrolysis takes place inside the small intestine. Now, here we're going to segment our protein digestion in points a, b, c, and d. If we take a look here, in point a, we're going to say after mechanical digestion in the mouth, food enters the stomach. And once there, the stomach acid denatures our proteins. Here, we're going to say pepsin breaks large proteins into polypeptides. If we take a look here at our image, we have our protein source as this piece of meat, the steak. Here we're going to say point a is where we have denaturation and partial hydrolysis of that protein. We go to point b where we're dealing with proteases in the small intestine. Here, we're talking about trypsin, chymotrypsin, and other types of proteins found within the small intestine. They hydrolyze our proteins to amino acids. So here we have the denatured proteins and polypeptides. We go into point b where we have the enzymes in our small intestines. Here, we have now free amino acids. We're going to say the amino acids are directly absorbed into the bloodstream through your intestinal cells. So here are free amino acids, they're being absorbed through the intestinal cells, so this is part c, and they're going to be delivered to cells through the bloodstream. So this would be our part d here. So when we're looking at protein digestion, keep in mind these different parts, parts a, b, c, and d.

Digestion of Proteins Example 1

Which of the following statements is incorrect about the digestion of proteins?

• A. Denaturation in the stomach makes the peptide bonds accessible to proteases. That's true; remember that we're going to have the denaturation of our protein within the stomach. This has to occur so that later on the proteases can start to hydrolyze those proteins into amino acids. So, this is true.
• B. Pepsin in the stomach breaks large proteins into smaller polypeptides. Yes, that is true. Pepsin is going to be the enzyme that we find within our stomach, and it's going to break down our larger proteins into polypeptides.
• C. Proteases in the small intestines hydrolyze proteins and polypeptides into free amino acids. That is also true.
• D. Here, protein digestion begins in the mouth by enzymes in the saliva. Remember, your saliva doesn't have enzymes that are going to help with the breaking down of our macromolecules. So this part is false. So, out of all the options, the answer has to be option D. Yes, mechanical digestion can begin in the mouth, but saliva doesn't have the necessary enzymes to start breaking down and denaturing our proteins. That's going to happen later on within the stomach.

Which of the following statements is correct about food digestion?

• A. Lipids are completely hydrolyzed inside the stomach through acidic hydrolysis.
• B. Unlike lipids, monosaccharides and amino acids are directly absorbed into the bloodstream through intestinal cells.
• C. Similar to proteins, large polysaccharide chains are also denatured in the stomach. 
• D. Carbohydrates and amino acids are digested to produce energy while lipids enter anabolic pathways.

Here’s what students ask on this topic:

What is the role of stomach acid in protein digestion?

Stomach acid, primarily hydrochloric acid (HCl), plays a crucial role in protein digestion. It denatures the proteins, which means it unfolds their complex structures, making them more accessible to digestive enzymes. This denaturation process is essential because it exposes the peptide bonds within the protein molecules. Additionally, the acidic environment activates pepsinogen into pepsin, an enzyme that breaks down proteins into smaller polypeptides. Without stomach acid, the initial steps of protein digestion would be significantly less efficient, hindering the overall process of converting proteins into absorbable amino acids.

How do proteases in the small intestine contribute to protein digestion?

Proteases in the small intestine, such as trypsin and chymotrypsin, are essential for the further breakdown of polypeptides into free amino acids. These enzymes are secreted by the pancreas in an inactive form and are activated in the small intestine. Trypsin and chymotrypsin specifically target peptide bonds within the polypeptides, breaking them down into smaller peptides and eventually into individual amino acids. This process is crucial because only free amino acids can be absorbed through the intestinal cells into the bloodstream, where they are transported to various cells for protein synthesis and other metabolic functions.

What happens to amino acids after they are absorbed in the small intestine?

After amino acids are absorbed in the small intestine, they enter the bloodstream through the intestinal cells. These free amino acids are then transported to various tissues and cells throughout the body. Once inside the cells, amino acids can be used for several purposes, including the synthesis of new proteins, which are essential for growth, repair, and maintenance of body tissues. Additionally, amino acids can be used in metabolic pathways to produce energy or serve as precursors for other important biomolecules. This efficient absorption and distribution system ensures that the body has a constant supply of amino acids for its various needs.

Why is pepsin important in the digestion of proteins?

Pepsin is a crucial enzyme in the digestion of proteins. It is produced in the stomach in an inactive form called pepsinogen, which is activated by the acidic environment created by stomach acid (HCl). Pepsin specifically breaks down large protein molecules into smaller polypeptides by cleaving peptide bonds. This initial breakdown is essential because it prepares the proteins for further digestion by other proteases in the small intestine. Without pepsin, the efficiency of protein digestion would be significantly reduced, making it harder for the body to obtain the necessary amino acids for various metabolic processes.

How does mechanical digestion in the mouth aid in protein digestion?

Mechanical digestion in the mouth, primarily through chewing, plays a supportive role in protein digestion. Chewing breaks down food into smaller pieces, increasing the surface area available for enzymatic action. This process makes it easier for stomach acid and digestive enzymes to access and break down the proteins more efficiently. While mechanical digestion does not directly break down proteins into amino acids, it is a crucial first step that facilitates the subsequent chemical digestion processes in the stomach and small intestine.