Classwork Series and Exercises {Basic Science – JSS2}; Digestive System

Week 4

Topic: Digestive System

Introduction

Digestion is the process by which food materials are broken down into smaller pieces which can be absorbed into the body cells. Digestion of food starts from the mouth and ends in the large intestine. The unwanted food materials, however, go out of the body from the large intestine through the anus. All the organs that food passes through, including those that secrete enzymes that act on it, such as the gall bladder, pancreas and salivary glands, make up the digestive system. The food taken in passes through five parts of the alimentary canal which consist of the mouth, oesophagus (gullet), stomach, the small and large intestines.

Parts of the Digestive System

The digestive system consists of the mouth, gullet (or oesophagus), stomach, duodenum, small intestine and large intestine.

1. Mouth: This contains the teeth, lips, tongue and glands. It is the beginning of the digestion process.
2. Oesophagus (gullet): This is a long tube connecting the mouth and the stomach. As a result of the peristaltic movement (peristaltic is a wave-like motion) of the oesophagus, the softened food is forced down into the stomach.
3.  Stomach: A sac-like organ with sphincters at the entrance and exit.
4.  Duodenum: The first and short part of the small intestine nearest to the stomach.
5. Small intestine: This is made up of involuntary muscles that can relax and contract automatically like that of the oesophagus.
6. Large intestine: This is a tube of about 1/5 of the length of the small intestine.

Digestive Process at various Stages

Mouth

Food is broken down by the teeth into smaller pieces and is mixed with saliva from the salivary glands. Saliva contains the enzyme, ptyalin, which starts starch digestion in the mouth, converting starch to sugar (maltose). The softened food is then swallowed and goes into the oesophagus.

Stomach

The movement of the muscles in the stomach wall helps in mixing or churning the food very well into a semi-fluid substance called chime. The chyme is passed into the duodenum by peristalsis. In the stomach, the food is churned up as a result of rhythmic contraction and relaxation of its muscular walls. The gastric glands of the stomach wall produce gastric juice containing hydrochloric acid, which neutralizes alkaline in food from the mouth, thus stopping the action of ptyalin. Pepsin in gastric juice digests proteins. Rennin is also produced in the stomach of young animals (mammals) to coagulate milk protein. Digestion of protein starts in the stomach. Food from the stomach does not normally go back into the oesophagus because of a valve that lies between the beginning and the end of the stomach.

Duodenum

The gall bladder stores bile and empties its contents into the duodenum. Bile neutralizes the acidity of the food in the stomach and stops the action of gastric juice.bile contains bile salts which emulsify fats. The following enzymes are produced, act on food substances to produce end products

1.  Peptidase acts on protein to give amino acid
2. Amylase acts on starchy food to give glucose
3.  Lipase acts on fats and oil to give fatty acid and glycerol

Small Intestine

The intestinal walls secrete intestinal juice that contains enzymes that complete starch and protein digestion. Crepsin acts on left over protein in the small intestine to give amino acid. The digested food is absorbed through the walls of the small intestine into the blood. The small intestine is made up of the duodenum, jejunum and ileum.

Large Intestine

The large intestine is a tube of about 1/5 of the length of the small intestine and twice its diameter. It has three main parts;

•  Ascending colon
•  Transverse colon
•  Descending colon

The unabsorbed food materials move out from the small intestine into the large intestine where excess water is re-absorbed into the blood stream, leaving a semi-solid substance known as faeces. The faeces then passes out of the body through the anus. Vegetable and roughages are digested in the large intestine by the action of bacteria.

Absorption of Food

Absorption is the transfer of end product of digestion from the small intestine into the blood stream. The food materials are absorbed by the villi on the walls of the small intestine.

End products of protein digestion (amino acids) and starch digestion (glucose) are absorbed directly into the blood stream through the finger-like projections called villi. The end products of fats and oil digestion (fatty acids and glycerol) are absorbed into the lacteal before going to the blood stream.

Absorption also takes place in the large intestine. This is the final absorption stage in which a large quantity of water is absorbed into the blood stream.

The Diagram of Villi

Summary of digestion processes and section

Organs	Digestive juice	Enzyme	Food digested	Products of Digestion
Mouth	Salivary gland	Ptyalin	Starch	Maltose
Stomach	Gastric juice	Renin Pepsin	Protein	Peptones Coagulation of milk
Pancreas	Pancreatic juice	Lipase Amlyse Peptidase Protease	Fats and Oil Starch Peptones	Fatty acid Maltose Amino acids
Small intestine	Intestinal juice	Erepsin Lipase	Fats and oil Maltose Amylase	Fatty acid and glycerol Glucose, amino acid

Simple Food Tests:

Food	Test	Observation
Starch	Add iodine solution	Blue black colouration is obtained
Reducing sugar (glucose, fructose)	Add Fehling’s solution and warm Add Benedict’s solution	Red precipitate Yellow precipitate
Protein	Add Million’s reagent solution and heat with biuret’s solution and warm	White precipitate is a deep reagent when cold, but on heating a red colour is obtained A violet colour is obtained
Fats and Oil	Rub on blotting or filter paper Add sudan III solution Add osmic acids and oil	Only spot or translucent stain Distinct red colouration  A brownish black precipitate