SS2 FIRST TERM: WEEK 1: DIGESTIVE SYETEM

DIGESTIVE SYSTEM

The digestive system is a series of connected organs that break down food into small, simple molecules that can be absorbed into the bloodstream. These nutrients are then transported to body cells for energy, growth, and repair.

All animals possess a digestive system because they must consume organic food. In contrast, plants do not need a digestive system since they produce their own food through photosynthesis using sunlight, water, and carbon dioxide.

DIGESTION

Digestion is the process of breaking down complex food substances into simple, soluble, and              absorbable forms through mechanical and chemical actions.

It occurs in two phases:

1. Mechanical digestion, where food is physically broken into smaller pieces by teeth or similar         structures.

2. Chemical digestion, where enzymes produced by glands break food molecules into forms that can be absorbed and transported around the body.

ALIMENTARY CANAL

This is a tubular passage between the mouth and the anus, including the organs through which food passes for digestion and elimination as waste. The alimentary canal is also called the digestive tract or gut.

In most holozoic animals, digestion and absorption of food take place in the alimentary canal or gut. A simple unicellular animal does not have an alimentary canal.

TYPES OF ALIMENTARY TRACT

 There are different types of alimentary tracts in animals. However, there is no definite alimentary canal in plant. This includes:

1. Intracellular alimentary tract: this is a digestion that takes place inside the cell, within a food vacuole. E.g. unicellular organism like amoeba

2. Extracellular alimentary tract: this is a type of digestion that occur in multi-cellular organisms. The mouth opens into a sac-like gut cavity where food is digested. E.g. Hydra.

3. Simple alimentary tract for simple digestion of food. E.g. Birds.

4. Compartmentalised alimentary tract by ruminants for digestion of cellulose. E.g. ruminants like cow.

5. Complex Human alimentary tract; a long tube stretching from the mouth to the anus. Most of it is coiled up in the abdominal cavity. It is divided into several regions: mouth, oesophagus, stomach, and intestines. Each region plays a role in the digestion and /or absorption of food. 

PARTS OF THE MAMMALIAN ALIMENTARY CANAL

A typical mammalian alimentary canal consists of the following parts, namely: mouth, pharynx, oesophagus, stomach, small intestine, rectum and anus. All these parts can be found in most vertebrates. However, there are modifications of the parts of the alimentary canal in various, animals. In some animals, a structure may be reduced or enlarged, while in others, some parts may be absent. The modifications reflect the various modes of feeding and types of diet.

 

DESCRIPTION AND FUNCTIONS OF THE PARTS OF THE MAMMALIAN ALIMENTARY CANAL

1. Mouth and Teeth: Digestion begins in the mouth. Different teeth (incisors, canines, premolars, and molars) chew food into small particles, increasing surface area for enzyme action.

2. Tongue: The tongue tastes food, moves and mixes it with saliva, and forms it into a bolus for swallowing.

3. Saliva: Saliva lubricates food, aids swallowing, dissolves food substances, and contains the enzyme ptyalin, which begins the digestion of cooked starch into maltose. It is slightly alkaline, providing a suitable medium for enzyme action.

4. Pharynx: The pharynx is a short passage that connects the mouth to the oesophagus and directs swallowed food into it.

5. Oesophagus (Gullet): This is a long muscular tube that carries food to the stomach by peristalsis. No digestion occurs here.

6. Stomach: The stomach mainly stores food and continues digestion. Its muscular walls churn food and mix it with gastric juice, which contains hydrochloric acid, pepsin, rennin, and mucus. Hydrochloric acid kills microbes and provides an acidic medium for pepsin to digest proteins into peptones, while rennin curdles milk. Food stays in the stomach for about 3–4 hours and becomes a semi-liquid called chyme, which passes through the pyloric sphincter into the small intestine. In some animals, the stomach is modified (e.g. crop or gizzard in birds and insects), and in ruminants, it has four compartments: rumen, reticulum, omasum, and abomasum.

SMALL INTESTINE:

Most digestion and absorption occur in the small intestine, a long, narrow tube about 6 m long. Chyme is moved by peristalsis through its three parts: duodenum, jejunum, and ileum.

In the small intestine, bile from the liver emulsifies fats, while pancreatic juice and intestinal enzymes digest carbohydrates, proteins, and fats into simple absorbable molecules. Mucus from intestinal glands protects the intestinal walls.

Undigested residues pass into the large intestine, where water is absorbed and waste is formed. The large intestine is about 1.5–1.8 m long, and food remains there for 12–24 hours before elimination.

THE LARGE INTESTINE:

The large intestine serves several important functions.

I. It absorbs water—about 6 liters (1.6 gallons) daily—as well as dissolve salts from the residue passed on by the small intestine.

II. In addition, bacteria in the large intestine promote the breakdown of undigested materials and make several vitamins, notably vitamin K, which the body needs for blood clotting.

III. The large intestine moves its remaining contents toward the rectum, which makes up the final 15 to 20 cm (6 to 8 in) of the alimentary canal. The rectum stores the faeces— waste material that consists largely of undigested food, digestive juices, bacteria, and mucus—until elimination. Then, muscle contractions in the walls of the rectum push the faeces toward the anus. When sphincters between the rectum and anus relax, the faeces pass out of the body.

DIGESTION IN BIRD

Birds do not have teeth but beak which they use for feeding. In many birds, the feet also show adaptation for feeding.

The alimentary canal of the bird consists of the following:

1. Oesophagus 2. Crop 3. Proventriculus

4. Gizzard 5. Small intestine 6. Caeca 7. Cloaca 

 

MECHANISM OF DIGESTION IN BIRDS

The bird swallows their food whole and store in the crop. In the crop, it is softened by secretion from the wall of the crop. The food passed through the proventriculus to the gizzard where gastric juice churns the food and breaks it up into smaller units. It is passed to the gizzard [a strong muscular bag]. Small stone in the gizzard also assists in the grinding of the food.The digestion is completed in the small intestine by the action of intestinal and pancreatic juice. The absorption also occurs here and the solid waste passed through the anus into the cloaca.

 DIGESTION IN INSECTS

The alimentary canals of insects consist of three major parts:

v The fore gut [mouth, pharynx, oesophagus crop, gizzard]

v Mid gut [stomach]

v Hind gut [intestine, ileum, colon, rectum, anus]

 

Insects such as grasshoppers feed on leaves. They use their mouth parts to cut and crush the leaves. Saliva is introduced or poured into the leaves from the salivary gland. The saliva helps to soften the leaves and the chewed food is in the crop, and broken up further into small pieces.

The foregut and the midgut secrete enzymes rich juice into the midgut where digestion and absorption occurs.

The hindgut is for water absorption. Only solid faeces pellets are egested from the anus after the food waste has been collected through the malpighian tubules joining the mid and hindgut.