1. The Digestive System
Unit 13

2. Digestion in Animals
Some animals, such as cnidarians and flatworms, have only one opening to their body. That means everything going into the opening also comes out the same opening.

3. More complex animals have two openings so that food can enter one opening and exit another.
The opening it enters is called the mouth and the opening it exits is called the anus.
The presence of an anus provides for a longer digestive tract in animals, which ensures more thorough, efficient digestion.

4. Digestion can be divided into many stages
Digestion can be divided into many stages. At each stage, food particles are broken down until they are the right size for the next stage, continuing until the particles are small molecules.
Mouth (food is evaluated and moistened) --> Region for physical breakdown--> Region for chemical digestion --> Region for additional digestion and nutrient absorption --> Region for waste storage --> Anus (waste is expelled)

5. Digestion in Humans
In the mouth are three pairs of salivary glands, which secrete enzyme-rich saliva.
The glands are stimulated by the taste, smell, sight, or thought of food.
Saliva contains mucus to moisten the food and an enzyme that starts breaking down starch into maltose.
Fats and proteins are not chemically digested in the mouth.

6. Chewed, moistened, slightly digested food leaves the mouth and enters the pharynx, where it is swallowed.
The pharynx opens into the esophagus. Food is moved further down the digestive tract through peristalsis (muscle contractions). No food is broken down here.

7. The stomach is a hollow organ at the end of the esophagus
The stomach is a hollow organ at the end of the esophagus. The stomach has 3 main functions:
1) The walls churn food to physically break it down. Here, the food is in the form of an acidic liquid called chyme.
2) It secretes acid and certain enzymes that begin breaking up protein. Proteins aren’t completely broken down here because the enzyme can’t break the bonds between all amino acids.
3) It stores the food, so that it can be released into the small intestine.

8. The two openings into the stomach (esophagus-stomach and stomach-small intestine) are controlled by tight rings of muscle, called sphincters. The first is the esophageal sphincter and the second is the pyloric sphincter.
These muscles allow food and chyme to move in and out in a slow-release, controlled manner.

9. The small intestine is the part of the digestive tract where chemical digestion is completed. Also, most of the nutrient molecules are absorbed here.
The small intestine in humans is very long (~7m or ~23ft) to ensure that the food stays in the organ long enough to be completely digested and absorbed.

10. Two organs that are not directly part of the digestive tract but that play an important role are the gallbladder and the pancreas.

11. The gallbladder’s function is to store bile produced by the liver
The gallbladder’s function is to store bile produced by the liver. As soon as proteins or fats are detected in the intestine, a hormone is released that stimulates the gallbladder to empty the bile into the small intestine through the common bile duct.
Bile contains bile salts, which
break up fats into small globules
in a process called emulsification.
The bile salts also aid in the absorption
of digested fats.

12. There are also four enzymes secreted by the pancreas.
The function of the pancreas is the release of sodium hydrogen carbonate, similar to baking soda or Tums, to neutralize the acidic chyme in the small intestine.
There are also four enzymes secreted by the pancreas.
One of them is similar to the salivary enzyme that converts starch to maltose.
The second helps convert fats to fatty acids and glycerol.
The third acts on the polypeptides from the stomach.
The fourth converts nucleic acids to nucleotides.

13. Now, the intestine contains simple and complex nutrients
Now, the intestine contains simple and complex nutrients. Only the proteins have been fully digested. Fats, carbohydrates, and nucleic acids have been partially digested.
Fat and nucleic acid digestion is completed by enzymes from the intestinal walls.
Carb digestion is completed by enzymes located in the cell membranes lining the intestine.
By the time digestion is complete, all food molecules have been converted to smaller molecules that can pass across cell membranes.

14. Finally, food moves into the large intestine
Finally, food moves into the large intestine. Here, most of the water absorption occurs. Through peristalsis again, waste is moved down the rectum and out the anus in a process called defecation.

15. Summary of Human Digestion
Organ
Major Phys. Actions
Major Chem. Actions
Mouth
Chewing, grinding, moistening
(amylase)
Starch ----> maltose
Esophagus
Moistening, peristalsis
nothing
Stomach
Moistening, churning, peristalsis, some water absorption
(pepsin)
Protein --> polypeptides
Small intestine
Peristalsis, fat emulsification, most nutrient absorption, some water absorption
Starch --> disaccharide
(protease)
Protein --> polypeptide
(peptidase)
Polypeptide --> amino acids
(maltase, lactase, sucrase)
Disaccharide --> monosaccharide
Large intestine
Most water absorption, peristalsis, waste elimination
Some vitamins synthesized