Hence, the contraction wave starts to have a stronger and stronger grinding effect. On its lower side, your stomach is narrower, with thicker muscular walls. While the chyme is being mixed at the top of the stomach, while our small protein loses its 3D shapes and even starts to break apart, losing some of its peptide bonds, the wave of contraction reaches the bottom of the stomach. Overall in the digestion process, pepsin accounts for 10-20% of protein digestion 2, 3. Pepsin starts to break down peptide bonds between amino acids 2, 6, creating smaller and smaller polypeptides. Interestingly enough, once pepsin is activated, it also acts as an autocatalyzer, an accelerator, on pepsinogen conversion 2, 5! Leading to an even faster activation rate of pepsin! Like other proteins, it will end up being digested. In presence of hydrogen ions, as soon as pH is below 6, pepsinogen loses a short series of amino acids and becomes the active protease pepsin. This is because pepsinogen is also a protein, hence its 3D structure is also sensitive to pH. While flowing into the stomach, while being mixed with the acidic chyme, pepsinogen undergoes a transformation. Precisely, in the gastric pits, another kind of cell, the chief cells, have also added to the gastric juice a precursor of a protease, pepsinogen. This is because acidity disrupts the weak hydrogen bonds that maintain the protein folded 1.Īt this stage, however, amino acid chains are still intact, as pH does not break the strong peptide bonds between amino acids (most of the time) 1.īut, this unfolding already makes protein much more vulnerable to protease attacks ! It mixes the bolus with gastric juice, creating a mixture called chyme, where hydrogen ions and enzymes diffuse evenly around proteins 2.īeing now soaked into the chyme, under such a strong acidity, our small protein starts to denature, meaning that it begins to lose its 3D structure. These waves of contraction, called peristalsis, move at a speed of about 1 cm per second 2.Īt the top of the stomach, the contraction wave has a churning effect. When the bolus that you propelled into your esophagus lands in the stomach, it discovers a strange cavity whose walls are slowly contracting in waves traveling from top to down. Yes, you really have hydrochloric acid in your stomach! The release of this very acidic juice by the gastric pits strengthens the acidity in the stomach to about pH 2 3, 5. Hidden in these pits are cells, the parietal cells 2– 4, that secrete a digestive juice, the gastric juice.Īmong other components, this juice contains hydrogen ions and chloride ions, that is, hydrochloric acid. The walls of the stomach contain small indentations, called gastric pits. Instead, they play a role in regulating the blood flow in the salivary glands 2.Įventually, this mechanical action of chewing with your teeth and mixing with your tongue creates a mixture called the bolus, which you propel to the esophagus when swallowing.īut even before you swallow, your stomach is already diligently preparing for the next step. Yet, saliva does contain proteases, that is, enzymes that break down proteins.īut these proteases, called kallikreins are not directly involved in the digestion process of protein. In contrast, big chunks of entangled proteins, whose peptide bonds are difficult to reach by enzymes, are more difficult to digest. So, if protein structure is not affected by mastication and mixing, why are we saying that protein digestion starts in the mouth?īecause when food fragments are small and coated with saliva, they are easily moved down by your esophagus, your stomach processes them more efficiently, and their surface area in contact with enzymes is much greater 2.Ĭhewing proteins is thus an important step that prepares for the chemical digestion process later in the stomach and small intestine. Nor does it break the weaker bonds, such as hydrogen bonds 1, that maintain the protein chain folded in secondary and even tertiary 3D structures. It does not break the peptide bonds 1, the strong linkages between amino acids, that create the chain of amino acids called the primary structure of the protein. But this mechanical breakdown does not affect the molecular structure of the proteins.
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