What is the difference between physical and chemical digestion? 

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Physical digestion is also known as mechanical digestion.  During mechanical digestion, food is physically broken down into smaller pieces.  The mouth is a good example for where mechanical digestion occurs.  The teeth grind, cut, and shred food into smaller pieces.  This increases the overall surface area of the food being eaten which is important for chemical digestion.  

Chemical digestion is done with various chemicals and will chemically break down the food into simpler and simpler molecules.  Two broad categories of chemicals used in chemical digestion are acids and enzymes.  Things like hydrochloric acid and amylase are specific examples.  Amylase will be used in the mouth.  That means the mouth is capable of doing both chemical and mechanical digestion.  Saliva contains the amylase enzyme which specifically targets sugars and starches.  Teeth tear the food into smaller pieces, which gives the food a greater surface area for the amylase to attack.  

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