The Digestive System

1
The Digestive System

• Unit 13

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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. 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) --gt
  Region for physical breakdown--gt Region for
  chemical digestion --gt Region for additional
  digestion and nutrient absorption --gt Region for
  waste storage --gt 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 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 arent
  completely broken down here because the enzyme
  cant 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 gallbladders 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

• 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. 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.
  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 ----gt maltose
Esophagus Moistening, peristalsis nothing
Stomach Moistening, churning, peristalsis, some water absorption (pepsin) Protein --gt polypeptides
Small intestine Peristalsis, fat emulsification, most nutrient absorption, some water absorption (amylase) Starch --gt disaccharide (protease) Protein --gt polypeptide (peptidase) Polypeptide --gt amino acids (maltase, lactase, sucrase) Disaccharide --gt monosaccharide
Large intestine Most water absorption, peristalsis, waste elimination Some vitamins synthesized