Digestive System Pt 2 (Bio 11 Unit 3)

The Digestive System of a Bird

• The esophagus carries food to the crop.
• The crop stores food.
• The gizzard has thick muscular walls; birds swallow small pebbles to help grind the food.
• The stomach begins chemical digestion.
• The intestine absorbs digested food.
• Waste is removed through the anus.
• Amylase: an enzyme that breaks down complex carbohydrates.

Importance of Digestion

• Digestive systems enable the absorption of nutrients needed for growth, maintenance, and repair.
• The digestive system and other organ systems are interdependent.
• The four components of digestion are ingestion, digestion, absorption, and egestion.
• In protozoa, digestion occurs in intracellular vacuoles.
• Some simple organisms have a single gastrovascular cavity with one opening, which both stores food and digests it.
• Digestive tracts allow the separation of processes into different regions.

Ingestion

• The digestive tract of adult humans, normally 6.5 m to 9 m long, stores and breaks down organic molecules into simpler components.
• Physical digestion begins in the mouth, where food is chewed and formed into a bolus (the Greek word for ball) by the tongue.

Saliva

• Saliva, the watery fluids produced by the salivary glands, contains amylase enzymes, which break down starches (complex carbohydrates) to simpler carbohydrates.
• Saliva lubricates the food so it can be swallowed, dissolves food particles, and makes it possible to taste what is being eaten.
• The way that we discern flavor is that food particles dissolved in solution penetrate the cells of the taste buds located on the tongue and cheeks.
• Different types of receptors respond to specific flavors; for example, the taste buds are equipped with receptors that have a specific geometry that permit the identification of sweet tastes from carbohydrates.
• Nerve cells for taste are stimulated when receptor sites are filled by chemical compounds with a complementary shape.
• You can find out the significance of dissolving foods by drying your tongue and then placing a few grains of sugar or salt on it; you will not receive any flavor until the crystals dissolve.

Teeth

• The teeth are important structures for physical digestion.
• Eight chisel-shaped teeth at the front of your mouth, called incisors, are specialized for cutting.
• The incisors are bordered by sharp, dagger-shaped canine teeth that are specialized for tearing.
• Next to the canine teeth are the premolars; these broad, flattened teeth are specialized for grinding.
• The molars are found next to the premolars; these teeth tend to be even broader and have cusps that are even more flattened; they are designed for crushing food.
• The last set of molars are the wisdom teeth, so called because they usually do not emerge until about 16 to 20 years of age; often these molars are troublesome and must be removed because there is not enough space for them to grow in.
• Each tooth has two divisions: the root and the crown.
• An enamel crown covers the tooth with calcium compounds and forms the hardest substance in the body.
• Immediately inside the enamel is dentin, a bonelike substance, which is part of the root structure.
• The dentin encases the pulp cavity, which contains nerves and blood vessels.
• Tooth decay is caused by bacteria living off nutrients left on the teeth. These harmful microbes produce corrosive acids that erode a tooth’s structure.
• Infections can also spread to the periodontal membrane, which anchors the teeth to the jawbone; as an infection progresses, the periodontal tissue is slowly destroyed and the teeth loosen.
• The tiny projections on your tongue that give it a velvety appearance are called papillae. It is within the papillae that the majority of taste buds on the tongue are found.
• The most numerous papillae (which are down the middle of your tongue) do not have taste buds; these papillae are involved in sensation.

Esophagus

• Once swallowed, food travels from the pharynx to the stomach by way of the esophagus.
• The bolus of food stretches the walls of the esophagus, activating smooth muscles that set up waves of rhythmic contractions called peristalsis.
• Peristaltic contractions, which are involuntary, move food along the gastrointestinal tract.
• The only points at which food is moved voluntarily along the tract is during swallowing and during the last phase, egestion.
• Peristaltic action will move food or fluids from the esophagus down to the stomach even if you stand on your head.

Ingestion Summary

• Saliva is important in digestion because it:
  • Contains amylase enzymes that initiate carbohydrate breakdown
  • Lubricates the food passage
  • Dissolves food particles
  • Activates the taste buds
• Teeth are necessary for biting, tearing, grinding, and crushing food into smaller particles.
• After food is swallowed, movement through the esophagus is regulated by peristalsis, contractions of smooth muscle.

Activity 6.3.1: Enzyme Amylase on Starch

• Starch is a polysaccharide made of a large number of glucose molecules that are bonded together.
• Glucose molecules are bonded together by dehydration synthesis to form starch.
• The enzyme amylase is used to break the bonds between the glucose molecules in starch and release simpler sugars.

The Stomach and Digestion

• The stomach is the site of food storage and initial protein digestion.
• The movement of food to and from the stomach is regulated by circular muscles called sphincters.
• Sphincters act like the drawstrings on a bag. Contraction of the cardiac sphincter closes the opening to the stomach located nearer the heart, while its relaxation allows food to enter.
• A second sphincter, the pyloric sphincter, regulates the movement of foods and stomach acids to the small intestine.
• The J-shaped stomach has numerous ridges that allow it to expand so that it can store about 1.5 L of food.
• Millions of cells line the inner wall of the stomach; these cells secrete the various stomach fluids, called gastric fluids or gastric juice, that aid digestion.
• Approximately 500 mL of these fluids are produced following a large meal.
• Gastric fluid includes mucus, hydrochloric acid (HCl), pepsinogens, and other substances.
• Mucus provides a protective coating.
• Hydrochloric acid kills harmful substances that are ingested with food; it also converts pepsinogen into its active form, pepsin, which is a protein-digesting enzyme.
• Pepsin breaks the long amino acid chains in proteins into shorter chains, called polypeptides.
• The pH inside the stomach normally ranges between 2.0 and 3.0, but may approach pH 1.0. Acids with a pH of 2.0 can dissolve fibers in a rug!
• It is the high acidity (low pH) of hydrochloric acid that makes it effective at killing pathogens and allows pepsin to do its work.
• A layer of alkaline mucus protects the stomach lining from being digested.
• Pepsinogen moves through the cell membrane and mucous lining, is activated by (HCl), and becomes pepsin.
• The pepsin breaks down the proteins in the food, but not the proteins of the stomach’s cells because these proteins are protected by the mucous layer.

Ulcers

• When the protective mucous lining of the stomach breaks down, the cell membrane is exposed to the (HCl) and pepsin.
• The destruction of the cell membrane leads to a peptic ulcer.
• Beneath the thin layer of cells is a rich capillary network.
• As the acids irritate the cells of the stomach lining, there is an increase in blood flow and acid secretions.
• With this increased blood flow and acid secretion, more tissue is burned, the allergic reaction becomes even stronger, and the cycle continues; eventually the blood vessels begin to break down.
• Most ulcers have been linked to a bacterium called Heliobacter pylori (H. pylori).
• Although diet, stress, and other factors may still play a part in the development of ulcers, this harmful microbe has changed how stomach ulcers are commonly treated.
• Dr. Barry Marshall, an Australian physician, is credited with changing the minds of many in the medical community.
• Prior to Dr. Marshall’s research, scientists believed that microbes were unable to withstand the highly acidic conditions of the stomach.
• However, H. pylori can survive in this harsh environment and often only powerful antibiotics can kill it.
• Dr. Marshall’s research, now conducted in the United States, is attempting to prove a possible link between the microbe and some forms of stomach cancer.
• A simple breath test for the presence of H. pylori is now widely available.

Frontiers of Technology: Ulcers and Lasers

• By 1960, American physicist Theodore Maiman had built the first laser.
• Laser beams have many medical applications; they can be used to remove damaged tissues such as those created by stomach ulcers.
• The laser beam is thinner than most scalpels and provides the added advantage of sealing small blood vessels.
• In addition, the laser may reduce the need for surgery.
• A device called an endoscope can be fitted with a light-emitting glass fiber and then positioned inside a patient’s body.
• The endoscope can be used to view such things as stomach ulcers; tiny forceps, fitted in the endoscope, can even extract small pieces of tissue for a biopsy.

Digestion in the Stomach Summary

1. Sphincter muscles regulate the movement of food into and out of the stomach.
2. Digestive fluids in the stomach include hydrochloric acid (HCl), pepsinogens, and mucus.
   • (HCl) kills pathogens and helps convert pepsinogen into pepsin.
   • Pepsin digests proteins.
   • Mucus protects the stomach from the above two fluids.
3. Ulcers are caused by the breakdown of the mucous lining in the stomach, exposing the stomach to stomach acids; ulcers are linked to Heliobacter pylori.