ICSE Class 7 Biology Chapter 05 Ingestion Digestion Absorption and Assimilation in Humans

Ingestion, Digestion, Absorption and Assimilation in Humans

Syllabus

1. Types of teeth and their functions - in human beings. Care of the teeth. The pattern of teeth in herbivores, carnivores and omnivores.

2. The digestive system/process in human beings - drawing a simple, labelled diagram of the alimentary canal.

3. Absorption and assimilation in human beings.

Observing their own teeth (E).

Learning to brush teeth correctly (E).

Observing the digestive system on a model, if available (D).

Introduction to Nutrition

You have already learnt in the previous chapter that the food we eat includes seven categories of substances: carbohydrates, proteins, fats, mineral salts, vitamins, water and roughage. Out of these, the first five, namely the carbohydrates, proteins, fats, mineral salts and vitamins are called nutrients. Water and roughage though not included in the category of nutrients, play a very important role in our body.

Nutrient is any substance required by an organism as food for growth and body activities.

Water is essential for carrying out various life processes. It is also needed to dissolve substances so that they are transported in the body.

Roughage is the indigestible portion of food. It largely consists of cellulose of the plants. The fruits, vegetables and the outer coats of the seeds and grains are particularly rich in roughage. The roughage enables the food to move easily through the intestines.

The whole process of nutrition is conducted in five steps.

(i) ingestion (ii) digestion (iii) absorption (iv) assimilation and (v) egestion

The process of ingestion starts when food is taken into the mouth cavity. The main organs in this cavity are the teeth, tongue and salivary glands. The food is cut into small pieces by the teeth which is helped by a movable muscular tongue. While the food is being chewed, it is mixed with saliva produced by the salivary glands.

The Mouth

The mouth is bordered by upper and lower lips. The lips help in closing the mouth during swallowing.

Types of Teeth

There are two types of teeth in bony animals.

(a) Homodont Teeth (homo: similar, dont: teeth): In animals like fishes, frogs, reptiles, etc., all the teeth are similar in size and shape. They are homodont teeth. These teeth are not meant for biting or grinding the food, but they simply help in capturing the prey.

(b) Heterodont Teeth (hetero: different): Mammals including humans, have heterodont teeth. These teeth are different in their size, shape and functions. Some teeth are used for biting, some for tearing and some for crushing and chewing. These teeth vary according to the nature of food of the animal, e.g. herbivorous animals like cows feed on plant matter for which they have flat and broad cutting teeth. Carnivorous animals like dogs and cats feed on the flesh of other animals for which they have large, pointed teeth to tear the flesh.

The Human Teeth

Based on their different shapes and functions, human teeth are of four kinds (Fig. 5.2):

1. Incisors are the four front teeth at the middle of each jaw. They are chisel-shaped for biting and cutting.

2. Canines are one on either side of the incisors in each jaw. They are pointed for tearing the food.

3. Premolars are two on each side of canines in each jaw. They help in crushing and grinding the food.

4. Molars are the last three teeth on each side in each jaw. They have broad uneven surface for finer crushing and grinding.

The last molar on each side in each jaw is called the Wisdom Tooth. The wisdom tooth appears at the age of 17-20 years. In some people, one or more wisdom teeth may not come out of the gums at all.

The human teeth develop in two sets during the life-time. The first set called the milk teeth or the temporary teeth, consisting of 20 teeth (8 incisors, 4 canines and 8 premolars only), appear in young children. The temporary teeth start falling out around the age of 5-6 years to be replaced by the permanent set by about 12-13 years of age. The molars appear only once in the permanent set.

An adult human normally has 32 teeth in all, 16 in each jaw. You as a child may have a fewer (usually 28). The teeth cut and break the food into smaller pieces and also grind the food into a pulp so that it can be swallowed easily. The teeth also help in speaking.

Activity 1

Stand against a mirror fitted on a wall. Wide open your mouth, and observe your teeth.

Observe your teeth in the upper jaw.

The front four teeth are chisel-shaped. These are the incisors meant for biting and cutting.

Look at one tooth on either side of the incisors in each jaw. Observe their shape and note how they differ from the incisors? Are they pointed. Yes/No.

These teeth are known as canines. These are meant for tearing the food.

Two teeth on either side of canines are the premolars meant for crushing and grinding the food.

Observe the teeth behind the premolars. These are again broad in shape with an uneven flat surface. These are also meant for grinding and crushing the food. These are molars.

Count the number of teeth in one half of each jaw:

Upper jaw Incisors =

Canines =

Premolars =

Molars =

Lower jaw Incisors =

Canines =

Premolars =

Molars =

Structure of a Tooth (Fig. 5.3)

The top portion of the tooth which can be seen is called the crown. The white, hard layer which surrounds the crown is called the enamel. Enamel is the hardest substance in the body. Dentine is the bone-like hard substance just below the enamel. It supports the tooth. Dentine has a hollow space filled with a soft material called the pulp. It has nerves and blood vessels. The root is the lower part of the tooth which is fixed in the jaw and is surrounded by cement.

The Aching Tooth

Oral hygiene is essential for good health. If you do not clean your teeth properly, bacteria present in the mouth act on the food particles trapped between the teeth. The acid produced by the bacteria slowly corrodes the enamel of the teeth and forms cavities known as caries (Fig. 5.5). The bacteria enter the pulp through these holes and start spreading. This process gives out a foul smell and causes toothache.

When the food is swallowed, the bacteria enter the stomach along with the food and cause indigestion.

Small bits of food get stuck between the teeth. A yellow, sticky substance called plaque forms on the teeth. It has sugar and bacteria. Hardened plaque is called tartar.

A tooth ache can be very painful. Sometimes, if the bacteria invade the gums, the tooth has to be pulled out.

If you take good care of your teeth, they can last a life-time. Sparkling white teeth add charm to your face and your smile.

Care of the Teeth

One should brush one's teeth and gums twice a day - once in the morning and then before going to bed (Fig. 5.6).

We eat, drink and speak during the day. This constant movement slows down the growth of bacteria. When you sleep at night, the undisturbed bacteria can damage your teeth. So, it is important to brush your teeth before going to bed everyday.

You must rinse your mouth after each meal. Very hot and very cold things should be avoided.

Chocolates and sweets promote the growth of bacteria in the teeth. Extra care should be taken to rinse the mouth after eating too much of sweets or too many chocolates.

The Pattern of Teeth in (i) Herbivores, (ii) Carnivores and (iii) Omnivores

The pattern of teeth differs in different mammals in relation to their type of food. (a) The herbivores like the cows, horses, sheep (Fig. 5.7), etc. eat grasses and other herbs which need lot of chewing. For this they have large molars with broad and flat grinding surface.

(b) The carnivores or meat eaters like the dog (Fig. 5.8) and tiger, have long and powerful canines to tear flesh. Their molars carry raised conical projections to cut their food (like scissors) into smaller pieces.

(c) The omnivores or "all type" eaters like the humans, have all types of well developed teeth - incisors for biting and cutting, canines for tearing and premolars and molars, both for crushing and grinding (see Fig. 5.2)

Tongue

The tongue is a fleshy muscular organ attached at the back to the floor of the buccal cavity.

The tongue helps in several ways:

1. Manipulates the food while chewing.

2. Helps in tasting the food.

3. Helps in mixing the watery secretion (saliva) with the food.

4. Helps in cleaning the teeth.

5. Helps in speaking.

Activity 2

Locating the taste buds on the tongue

1. Take the following solutions separately in four different test-tubes. (a) Sugar solution, (b) salt solution, (c) lemon juice and (d) crushed neem leaf.

2. Blindfold one of your classmates and ask her/him to take out the tongue and keep it in straight and flat position.

3. Take clean toothpicks to put the above samples one by one on different areas of the tongue as shown in the figure given here. Use a new toothpick for each sample.

4. Ask the classmate which areas of the tongue could detect the sweet, salty, sour and bitter substances (Fig. 5.9).

The Salivary Glands

What you call the watering of the mouth is actually a secretion of salivary glands in your mouth.

The saliva is secreted by three pairs of salivary glands (one below the tongue, second at the base of the ear, and third on the inner side of the angles of the lower on jaws (Fig. 5.10).

Saliva is a fluid containing water, salts and a slimy mucus. It also contains a substance (an enzyme) called amylase which convert starch into maltose.

Functions of Saliva

1. It moistens and lubricates the mouth cavity and the tongue to make speaking and swallowing easy.

2. It acts as a solvent, dissolving some part of food to stimulate taste.

3. It cleans the mouth and destroys germs.

4. It initiates digestion by converting starch into a simpler form - the maltose.

A piece of bread or boiled rice when chewed thoroughly starts tasting sweet. The tasteless starch changes into sweet maltose.

Teacher's Note

Just like your body needs to brush teeth twice daily for hygiene, your digestive system works continuously to break down food and convert it into energy. Understanding this process helps you make healthier food choices every day.

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