Your cells need small molecules: simple sugars, amino acids and fatty acids. A sandwich is made of big molecules. The digestive system is a long tube that breaks food down and lets those small molecules into the blood. Its organs are a good example of Cells, Tissues, Organs and Systems.
Key idea
Food travels mouth → esophagus → stomach → small intestine → large intestine. Teeth and the stomach break food into pieces; enzymes break the pieces into small molecules. Most nutrients are absorbed in the small intestine. The large intestine absorbs water.
Why it works
Digestion has two parts. Mechanical digestion (chewing, the stomach churning) makes pieces smaller, so enzymes have more surface to work on. Chemical digestion uses enzymes: saliva starts breaking starch in your mouth, the stomach starts on proteins with acid and enzymes, and the small intestine finishes the job with enzymes from the pancreas and bile from the liver. Its inner wall is covered in tiny fingerlike folds (villi) that give a huge surface for absorbing.
Worked example: Where does it start?
Chew a plain cracker for a minute and it starts to taste a little sweet. Why?
- Crackers are mostly starch, a chain of sugar units.
- An enzyme in saliva breaks starch into smaller sugars.
Answer: Digestion has already started in the mouth.
Worked example: Put it in order
Put these in the order food reaches them: small intestine, esophagus, large intestine, mouth, stomach.
- Food enters at the mouth.
- It is swallowed down the esophagus into the stomach.
- Then the small intestine, then the large intestine.
Answer: Mouth, esophagus, stomach, small intestine, large intestine.
Common mistake
Students think digestion begins in the stomach and that most absorption happens there. Digestion begins in the mouth. The stomach mixes and starts on proteins, but most nutrients pass into the blood through the walls of the small intestine.