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Lesson Note
Subject: Biology
Topic: Tissue and Supporting System.
Lesson Objectives: By the end of the lesson, the learners should be able to:
1. Explain Digestion in Birds;
2. Explain digestion in man.

Birds do not have teeth but horny 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

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. It is then through the proventriculus to the gizzard where gastric juice churns the food and breaks it up into smaller units. This is preceded by the grinding action of the muscles of 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.

Heterotrophic organisms get their food from their autotrophic counterparts or depend on other heterotrophic organisms.
The food is taken in (ingestion), broken down into simple, soluble and diffusible substances through some chemical and mechanical processes. This is referred to as digestion.

               Digestive System in Man

The digested food is eventually absorbed (absorption) into the body fluids and assimilated while the undigested food is removed (egested).

All holozoic animals have structures for obtaining their food. The structures also help them to capture their preys. Those organisms feeding on large food particles have their bodies modified into structures like claws, teeth, beak etc. However, those feeding on small pieces of food have either fluid feeding or filter feeding structures while saprophytes change their food to digested absorbable form before taking them in. Parasites on the other hand have structures for boring into the bodies of their host.

Most holozoic animals have a digestive pathway called alimentary canal or gut unlike unicellular animals. Their gut consist of two openingsAnterior opening called mouthPosterior Opening called 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 alimentary canal and the associated glands and organs which produce some of the enzymes-rich secretion that bring about digestion. The action of the teeth is the mechanical breakdown of digestion while the digestive enzymes speed up the chemical digestion.

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. Saliva contains an enzyme called 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 of food, the entrance to the trachea must be closed to prevent choking. The wall of the esophagus is muscular and it contracts and relaxes to push each bolus of food downward, this process is called peristalsis.

The muscular wall of the stomach contracts and relaxes forcefully just 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 acidic medium and the acid also assists to kill the bacteria present in food. Rennin causes the coagulation of milk into thick curd (convert soluble caseinogens to insoluble casein). Food stays in the stomach for about 3-4 hours.

The first part of the small intestine is 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, which does not contain 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 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 product of protein is amino acids, fats and oil is fatty acid and glycerol while that of starch are glucose, fructose and galactose.

Glucose, amino acids, fatty or carboxylic acids and glycerol as well as vitamins and mineral salt 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. Also there are finger-like projections called villi (Villus).

                         Diagram Of A 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 lacteal which is surrounded by a network of blood capillaries. The lymph in the lacteal transport fatty acid (carboxylic acid) and glycerol which recombines to form fats in the lacteals. This is then carried by the blood to where they are needed. Excess fats are stored in fat cells to form adipose tissues which are usually found under the skin and around organs.

Why are humans heterotrophic?
Explain the mechanism of digestion after taking a plate of rice and beans.

FILTER FEEDING: This concerns mainly aquatic animals which feed on very tiny organisms in water. They use their sieve like structure to collect their food or prey. Examples of filter feeders are mosquito larva, ducks, prawns etc.
FLUID FEEDING: This concerns animals which feed on fluid materials and so they are called fluid feeders. They include classes of animals namely:
(a) Sucker e.g. bugs, mosquitoes, butterfly, housefly, tsetse fly etc.
(b) Wallowers:These are organisms which wallow in their food e.g. tapeworm. Tapeworm lives within the digested food of its host and absorbs the food directly into the body. Therefore, it does not have alimentary canal. The absorption of its food is through its entire body surface.

This can be studied in five mechanisms:

  • absorption mechanisms e.g. tapeworm
  • Biting and chewing mechanisms e.g. grasshopper
  • Sucking mechanisms e.g. mosquitoes
  • Grinding mechanisms e.g. Man
  • Trapping and absorbing mechanisms e.g. bladder worm.

Done studying? See all previous lessons in biology.
Take a quick test for this lesson.

1. State the various feeding habits animals.
2. Give two organisms each for the above listed habits
3. What is an enzyme?
4. Draw the longitudinal section of a villus.
5. Summarize digestion in a tabular form under the following columnar topics (a) site of digestion (b) juice secreted (c) site of secretion of juice (d) enzymes produced (e) digestion process carried out by enzymes.

Questions answered correctly? Bravo!!

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