Seeds that pass through the digestive tracts of birds and mammals must have hard seed coats to resist stomach acids and digestive enzymes.
Stomach acids and digestive enzymes help out in the digestion of food and other hard substances such as seed coats.
Pepsin is an elementary digestive enzyme that digests proteins in food and is secreted by the gastric cells.
Moreover, the lining of the stomach contains parietal cells which release hydrochloric acid in order to reduce the pH.
In sum, digestive enzymes and fluids are essential for the breakdown of complex molecules of proteins and food particles.
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