Question

Where are proteins digested? A. Mouth and stomach B. Stomach and large intestine C. Stomach and small intestine D. Small intestine and large intestine

Short answer

C. Stomach and small intestine

Step-by-step solution

Understanding Protein Digestion

Protein digestion begins in the stomach, where the enzyme pepsin breaks down proteins into smaller peptides.

Digestion in the Stomach

In the stomach, hydrochloric acid activates pepsinogen to pepsin, which starts the digestion process by breaking down protein molecules.

Digestion in the Small Intestine

The digestion of proteins continues in the small intestine with the help of pancreatic enzymes, such as trypsin and chymotrypsin, which further break down peptides into amino acids which can be absorbed.

Review Options

By reviewing all the options, it is clear that the correct answer must include both the stomach and small intestine, as these are the primary sites of protein digestion.

Choose an Answer

The correct answer is C. Stomach and small intestine.

Key concepts

enzyme pepsin

Pepsin is a critical enzyme for protein digestion. It is secreted by the stomach lining in an inactive form called pepsinogen.
Pepsinogen is activated to pepsin when it comes into contact with hydrochloric acid in the stomach.
This activation process ensures that pepsin only starts breaking down proteins when it reaches the stomach, preventing damage to the tissues.
Once active, pepsin begins breaking down protein molecules into smaller peptides.
This is the first step in converting proteins into forms that the body can absorb and utilize.
Pepsin is most effective in the highly acidic environment of the stomach, with a pH range of 1.5 to 2.
Without pepsin, the proteins we consume wouldn't break down efficiently, hindering our body's ability to make use of them.

hydrochloric acid

Hydrochloric acid (HCl) plays a vital role in protein digestion. It is secreted by the parietal cells in the stomach lining.
One of its main functions is to create an acidic environment in the stomach, which is crucial for the activation of pepsinogen into pepsin.
The acidic pH also helps to denature proteins, which means unfolding them from their complex structures.
This unfolding makes the proteins more accessible for enzymatic activity, especially for pepsin.
HCl also serves as a barrier against pathogens, killing many harmful bacteria and microorganisms present in the food.
Despite its corrosive nature, the stomach lining is protected by a thick layer of mucus, preventing the acid from causing harm.

pancreatic enzymes

In the small intestine, pancreatic enzymes take over the digestion of proteins. The pancreas secretes these enzymes into the duodenum, the first part of the small intestine.
Trypsin and chymotrypsin are two main pancreatic enzymes that further break down the smaller peptides formed by pepsin into even smaller peptide fragments and amino acids.
These enzymes are initially secreted as inactive precursors, trypsinogen and chymotrypsinogen, to prevent the pancreas from digesting itself.
Once they enter the small intestine, they are activated by other digestive enzymes, continuing the process of protein breakdown.
This process ensures that proteins are broken down into amino acids and small peptides that can be absorbed by the intestinal lining and transported into the bloodstream.

amino acids

Amino acids are the end products of protein digestion. Once proteins are broken down into amino acids and small peptides by pepsin and pancreatic enzymes, they are ready for absorption.
The lining of the small intestine contains specialized cells that absorb these amino acids through active transport and diffusion.
Amino acids enter the bloodstream and are transported to various cells in the body, where they are used for multiple functions.
They serve as building blocks for new proteins, which are essential for growth, repair, and maintenance of body tissues.
Amino acids also play vital roles in producing enzymes, hormones, and other necessary molecules.
Without efficient protein digestion and absorption of amino acids, our bodies wouldn't function properly.