Fig. 7.1 the Human Digestive System
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Lesson Seven: The Human Digestive System
Teaching suggestions: Lecture and group work.
Students will work in groups to construct a concept map of the human digestive system.
Let the teacher post unlabelled diagram or model of the organs of the human digestive system. The teacher is required to make students engaged in labeling or indicating the organs of the human digestive system and the associated glands (liver and pancreas).
Fig. 7.1 the Human Digestive System
From their previous knowledge, the students may indicate parts of the digestive system. They can describe the digestive tract as a muscular tube extending through the body which composed of several parts such as the mouth, esophagus, stomach, small intestine, and large intestine. The digestive tract is sometimes called the alimentary tract. It is more commonly referred to as the gastrointestinal (Gl) tract because of the major importance of the stomach and intestine in the process of digestion.
The students should be informed that liver and pancreas are the two glands located out of the digestive tract. But they take part in the processes of digestion of food. Thus, they are not considered as organs of the digestive tract. Such kind of misconception should be avoided.
Let the teacher post a well organized and labeled model or diagram of the digestive system.
Here the students are expected to explain or describe the specific roles of the organs of the digestive system and the associated structures in the process of digestion.
Let the students are grouped into small groups and discuss the roles of the organs of the digestive system. .
Group 1: Digestion of food in the Oral cavity, the enzyme involved, and product/s formed.
Group 2: Digestion of food in the stomach, the enzymes involved, and product/s formed.
Group 3: Digestion of food in the Small intestine, the enzymes involved, and products formed.
Group 4: The roles of liver and pancreas in food digestion.
During presentation of group work, the teacher is expected to give guided instruction. The core concepts of the process of digestion might be explained as follows.
Digestion of food in humans begins in oral cavity. Basically, there are two types of digestion; mechanical and chemical digestion. Mechanical digestion is a physical phenomenon. In the mouth, teeth, jaws and the tongue begin the mechanical breakdown of food. The tongue rolls food during chewing. Chewing breaks down large pieces of food into smaller pieces. This makes food easier to swallow and increases the surface area available for enzyme action. Chemical digestion is a biochemical process whereby enzymes are involved. The presence of food in the oral cavity stimulates the salivary glands to secrete saliva into the oral cavity. Saliva contains
The tongue tastes and manipulates food during chewing and forms it into a bolus, which it pushes to the back of the oral cavity and into the pharynx. The esophagus is a muscular tube whose muscular contractions propel food to the stomach. Mucus moistens food. Swallowing moves food from the mouth through the pharynx into the esophagus and then to the stomach. Pharynx is a common passage for food and air.
What are the advantages of mechanical digestion of food? Let the students discuss the advantages of mechanical digestion.
The esophagus is a muscular tube. It takes food from the throat and pushes it down through the neck, and into the stomach. It moves food by the waves of muscle contraction called peristalsis.
What is peristalsis?
Peristalsis: - is a wave like movement of food along the alimentary canal due to the contractions of the muscles of the digestive organs. Epiglottis closes off the larynx during swallowing, preventing food from entering the respiratory tract.
The two main components of gastric juice are :
Chyme is the liquid substance found in the stomach before passing through the pyloric valve and entering the duodenum. It results from the mechanical and chemical breakdown of a bolus and consists of partially digested food, water, hydrochloric acid, and various digestive enzymes.
Churning action of the stomach:- Food is mixed in the lower part of the stomach by peristaltic waves that also propel the acid-chyme mixture against the pyloric sphincter. Increased contractions of the stomach push the food through the sphincter and into the small intestine as the stomach empties over 1 to 2 hours of period.
Small Intestine: - The small intestine is a coiled tube and long structure provided with large surface area. It is divided into 3 unequal sections: the duodenum, jejunum, and the ileum.
It is the major site for chemical digestion and absorption of nutrients.
The duodenum is the upper part of the small intestine that receives:
The pancreas is a large gland that secretes digestive enzymes and bicarbonate.
What are the enzymes of the pancreatic juice?
Digestive enzymes of the pancreatic juice include:-
It is recommended to ask students the following questions.
A watery bicarbonate rich solution which has a slightly alkaline pH helps to neutralize the acidity of the chyme. The action of bile on lipids is not a chemical phenomenon. It is a kind mechanical or physical process. Bile is used to emulsify lipids. Emulsification is important as it increases the surface areas of lipids for enzymatic actions. As the surface area increases, the efficiency of digestion by enzymatic action increases. Bile is secreted by liver and stored in gall bladder.
Intestinal glands also secrete enzymes that complete the digestion of carbohydrates, proteins and lipids. It is possible to see the enzymes of the intestinal glands from the summary table.
Enzymes that convert disaccharides into monosaccharide are
| Glands | Secretions | Enzymes | Substrate | Product |
|---|---|---|---|---|
| Salivary Glands | Saliva | Salivary amylase | Starch | Maltose |
| Gastric Glands | Gastric Juice | Pepsin | Proteins | Peptones |
| Rennin | Milk proteins | Curdled milk | ||
| Pancreas | Pancreatic Juice | Pancreatic Amylase | Starch | Maltose |
| Pancreatic lipase | Lipids | Fatty acids and glycerol | ||
| Trypsin | Peptides | Amino acids | ||
| Intestinal Glands | Intestinal juice | Sucrase | Sucrose | Fructose and glucose |
| Lactase | Lactose | Galactose and glucose | ||
| Maltase | Maltose | Glucose + Glucose | ||
| Lipase | Lipids | Fatty acids and glycerol | ||
| Erepsin/Peptidase | peptides | Amino acids |
Table 7.2. Summary of food digestion with Enzymes.
Fig.7.2. Organs involved in the process of digestion.
Ask students to tell the end products of digestion of carbohydrates, proteins and lipids
Most digestion takes place in the small intestine. The inner lining of the small intestine is has many folds and is covered with tiny projections called villi. The folding of villi enormously increases the surface area through which food can be absorbed through the intestinal wall and into blood capillaries. Sugars and amino acids enter into the bloodstream via capillaries in each villus. Absorption of glycerol and fatty acid takes places through lacteals or lymph capillaries.