Digestive System – SS2 Biology Lesson Note (Second Term)

Digestion is the breakdown of ingested food into simple, soluble and diffusible substances by mechanical and chemical processes. The digestive system is made up of the alimentary canal and the associated glands and organs that produce enzyme-rich secretions. The digested food is absorbed and assimilated, and the undigested part is egested.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Describe feeding and digestion in Hydra, flatworms, the earthworm, insects and birds.
2. List the parts of the alimentary canal of the earthworm, insects and birds.
3. Describe digestion in the mouth, stomach and small intestine of man, naming the enzymes and their products.
4. Explain how the small intestine is adapted for absorption.
5. Name the feeding mechanisms in animals with one example of each.

How does Hydra feed?

Hydra feeds on small aquatic organisms, e.g. crustaceans. It captures its food with structures called nematocysts found on its tentacles. The food is then passed into the mouth, while the digestive enzymes are secreted into the enteron by the gland cells.

The digestion is partly extracellular, and the digested food is absorbed into the body. The undigested food is sent back into the enteron and released through the mouth. Digestion in Hydra is therefore both intracellular and extracellular.

How does digestion occur in flatworms (tapeworm and planaria)?

Tapeworm is a parasitic worm which has no alimentary canal. It feeds on the digested food of the host. The planarian, however, is a free-living flatworm which feeds on small animals.

Planaria has a simple alimentary canal with one opening, the mouth, present in the ventral part of its body cavity. The pharynx of the planarian can be extended out of the mouth during feeding. The pumping action of the pharynx sucks in pieces of food, which enter the pharynx and move into the small intestine.

Digestion is partly extracellular and partly intracellular. The branched intestine enables the digested food to diffuse to all parts of the body, while undigested food is egested through the mouth.

How does digestion occur in the earthworm?

Parts of the alimentary canal

The alimentary canal of the earthworm is a tube with two openings:

1. The mouth, through which the food enters
2. The anus, through which the undigested food leaves the body

The alimentary canal of the earthworm has the following parts:

1. Mouth and thin-walled buccal cavity for food ingestion
2. A muscular walled pharynx which secretes mucus to lubricate food particles
3. A narrow tubular thin-walled oesophagus
4. A thin-walled crop for storing food
5. A thin-walled muscular gizzard where food is ground against small stones to break it up
6. A long straight intestine in which extracellular digestion occurs

Mechanism of digestion in earthworm

The prostomium is the mouth of the earthworm. Since it has no jaws or teeth, the earthworm uses its muscular pharynx to suck in soil containing food. The food particles and soil go through a long oesophagus into a round organ called a crop. The crop stores the food temporarily.

The food is then forced into a very muscular organ called the gizzard. The gizzard contracts and expands, causing grains of sand and food to rub together. In this way, the food is ground up.

Food is digested in the intestine, which stretches from segment nineteen to the end of the worm. Here enzymes chemically break down the food. The digested food is then absorbed by the blood circulating through the intestinal walls. The earthworm’s digestive system can be thought of as a tube within a tube.

The complicated organs of the digestive system take up most of the anterior half of the earthworm’s body. The earthworm ingests large amounts of soil, which has organic matter in it. The useless inorganic matter goes through the worm with no change. This is often left on the surface of the ground in the form of castings.

As the earthworms feed, they loosen the soil, making it easier for air and water to enter. Their wastes also add to soil fertility. If there were no earthworms, the soil could not support crops, grasses and other plants.

How does digestion occur in insects?

The alimentary canals of insects consist of three major parts:

1. The foregut [mouth, pharynx, oesophagus, crop, gizzard]
2. Mid gut [stomach]
3. 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 poured onto the leaves from the salivary gland and helps to soften them. The chewed food goes into the crop, where it is broken up further into small pieces.

The foregut and the midgut secrete enzyme-rich juice into the midgut, where digestion and absorption occur.

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.

How does digestion occur in birds?

Birds do not have teeth but horny beaks 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 its food whole and stores it in the crop. In the crop, the food is softened by secretion from the wall of the crop. It then passes through the proventriculus to the gizzard.

In the gizzard, gastric juice churns the food and breaks it up into smaller units, helped by the grinding action of the muscles of the gizzard [a strong muscular bag]. A 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. Absorption also occurs here, and the solid waste passes into the cloaca and out of the body.

What are digestion and the digestive system in man?

Heterotrophic organisms get their food from their autotrophic counterparts or depend on other heterotrophic organisms.

The food is taken in (ingestion) and broken down into simple, soluble and diffusible substances through chemical and mechanical processes. This is referred to as digestion. The digested food is eventually absorbed (absorption) into the body fluids and assimilated, while the undigested food is removed (egested).

How do holozoic animals feed and digest food?

All holozoic animals have structures for obtaining their food. The structures also help them to capture their prey. Those organisms feeding on large food particles have their bodies modified into structures like claws, teeth and beaks. Those feeding on small pieces of food have either fluid-feeding or filter-feeding structures. Saprophytes change their food to a digested, absorbable form before taking it in. Parasites have structures for boring into the bodies of their host.

Most holozoic animals, unlike unicellular animals, have a digestive pathway called the alimentary canal or gut. Their gut consists of two openings:

1. Anterior opening called the mouth
2. Posterior opening called the anus

A typical alimentary canal is adapted for breaking food into smaller units, producing digestive secretions and absorbing digested food and water.

A digestive system is made up of an alimentary canal and the associated glands and organs which produce some of the enzyme-rich secretions that bring about digestion. The action of the teeth is the mechanical breakdown of food, while the digestive enzymes speed up the chemical digestion.

How does food pass through the human body?

Food passes through the following process in man:

Ingestion → Digestion → Absorption → Assimilation → Egestion

Food is ingested in the mouth, and the teeth grind the food into smaller units. Chemical digestion also begins in the mouth. Saliva contains an enzyme, ptyalin, that acts on cooked starch to convert it to complex sugar (maltose). Saliva is a watery, slightly alkaline substance secreted by the salivary gland.

The tongue mixes the food with saliva and rolls it into a ball (bolus), which is then swallowed. The food passes down into the stomach through the gullet (oesophagus). During swallowing, the entrance to the trachea must be closed to prevent choking.

The wall of the oesophagus is muscular. It contracts and relaxes to push each bolus of food downward. This process is called peristalsis.

How is food digested in the stomach?

The muscular wall of the stomach contracts and relaxes forcefully, churning the food. The gastric juice mixes with the food. Gastric juice contains two important enzymes, pepsin and rennin, as well as hydrochloric acid for activating pepsinogen into pepsin.

Pepsin digests protein into peptones and polypeptides, which are intermediate products in protein digestion. Pepsin works best in an acidic medium, and the acid also assists in killing the bacteria present in food. Rennin causes the coagulation of milk into thick curd (converting soluble caseinogens to insoluble casein). Food stays in the stomach for about 3 to 4 hours.

How is food digested in the small intestine?

The duodenum

The first part of the small intestine is the duodenum. The pancreas secretes pancreatic juice, which contains digestive enzymes. The liver produces bile, which is stored in the gall bladder. Bile is a greenish liquid that emulsifies fat. It contains no digestive enzymes.

The pancreatic juice contains three important enzymes:

• Amylopsin converts starch to maltose
• Trypsin converts protein to polypeptides
• Lipase converts lipids to fatty acids and glycerol

The ileum

The latter part of the small intestine is the ileum. Here the wall of the intestine secretes five important enzymes:

• Maltase converts maltose to glucose + glucose
• Sucrase converts sucrose to glucose + fructose
• Lactase converts lactose to glucose + galactose
• Erepsin converts polypeptides to amino acids
• Lipase converts fats and oil to fatty acids and glycerol

In man, the digestion of food ends in the small intestine. Hence the end products of protein are amino acids, those of fats and oil are fatty acids and glycerol, and those of carbohydrates are glucose, fructose and galactose.

How is food absorbed and assimilated?

Glucose, amino acids, fatty (carboxylic) acids and glycerol, as well as vitamins and mineral salts, are absorbed in the small intestine. For efficient absorption, a large surface area is needed. To ensure this, the wall of the small intestine has folds and furrows. There are also finger-like projections called villi (singular: villus). The inner surface layer (epithelium) of each villus is thin.

This allows the absorption of the end products of digestion, which takes place by either diffusion or active transport. The absorbed food substances are carried away through the blood vessels and lymphatic vessels (containing blood and lymph respectively).

In each villus, there is a blind lymphatic tube called a lacteal, which is surrounded by a network of blood capillaries. The lymph in the lacteal transports fatty acid (carboxylic acid) and glycerol, which recombine to form fats in the lacteals. This is then carried by the blood to where it is needed. Excess fats are stored in fat cells to form adipose tissues, which are usually found under the skin and around organs.

What are the feeding mechanisms in animals?

Filter feeding

This concerns mainly aquatic animals which feed on very tiny organisms in water. They use their sieve-like structures to collect their food or prey. Examples of filter feeders are mosquito larvae, ducks and prawns.

Fluid feeding

This concerns animals which feed on fluid materials, and so they are called fluid feeders. They include two classes of animals:

1. Suckers, e.g. bugs, mosquitoes, butterflies, houseflies and tsetse fly.
2. Wallowers: These are organisms which wallow in their food, e.g. tapeworms. Tapeworm lives within the digested food of its host and absorbs the food directly into the body. Therefore, it does not have an alimentary canal. The absorption of its food is through its entire body surface.

What are the modifications and mechanisms of feeding in animals?

Feeding in animals can be studied in five mechanisms:

1. Absorption mechanisms, e.g. tapeworm
2. Biting and chewing mechanisms, e.g. grasshopper
3. Sucking mechanisms, e.g. mosquitoes
4. Grinding mechanisms, e.g. man
5. Trapping and absorbing mechanisms, e.g. bladder worm

Evaluation

1. Name the three enzymes in pancreatic juice and state what each one converts.
2. State the function of the gizzard in the earthworm and in the bird.
3. What is peristalsis?
4. Describe two ways in which the small intestine is adapted for absorption.
5. List the five mechanisms of feeding in animals, with one example of each.

Assignment

1. State the region in the alimentary canal where the chemical digestion of the following food substances begins: (a) carbohydrate, (b) protein, (c) lipids.
2. What role does pancreatic juice play in digestion?