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Acids, Bases, and Solutions
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Section Summary
Digestion and pH
Key Concepts
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Why must your body digest food?
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How does pH affect digestion?
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Acids, Bases, and
Solutions
Foods must be broken down into simpler substances that your body can
use for raw materials and energy. The process of digestion breaks down the
complex molecules of food into smaller molecules.
Digestion involves two parts—mechanical and chemical digestion.
Mechanical digestion is a physical process in which large pieces of food are
torn and ground into smaller pieces. The size of the food is reduced, but the
foods aren’t changed into other compounds. Chemical digestion breaks
large molecules into smaller molecules. Some molecules provide your body
with energy. Others serve as building blocks for the molecules in muscle,
bone, blood, skin, and other organs. Chemical digestion takes place with the
help of enzymes, catalysts that speed up reactions in living things. Some
digestive enzymes work at a low pH. For others, the pH must be high or
neutral.
Digestion begins in your mouth. Your teeth chew and mash the food. The
food gets wet with a fluid called saliva. Saliva has a pH near 7, the neutral
point. Saliva contains amylase, an enzyme that helps break down the
carbohydrate starch into smaller sugar molecules. Amylase works best when
the pH is near 7.
Next, the food is swallowed and arrives in your stomach. This muscular
organ starts the chemical digestion of foods that contain protein. Cells in the
lining of your stomach release enzymes and hydrochloric acid. The pH in
your stomach is very acidic—about 2. The main enzyme that works in your
stomach is pepsin. It helps break down proteins into small molecules called
amino acids. Pepsin works best in acids.
Your stomach empties its contents into the small intestine. Here, other
digestive fluids containing bicarbonate ions surround the food. These ions
create a slightly basic solution—the pH in the small intestine is about 8.
Enzymes in the small intestine complete the breakdown of carbohydrates,
fats, and proteins. These enzymes work best in a slightly basic solution.