ICSE Class 9 Biology Chapter 11 Digestive System

Chapter 11: Digestive System

Syllabus

The structure of a tooth, different types of teeth. Structure of a tooth to be discussed with the help of a diagram. Functions of different types of teeth. Dental formula for an adult.

Digestive System of Organs, digestive glands and their functions (including enzymes and their functions in digestion; absorption, and assimilation of digested food).

Organs and glands of the digestive system and their functions with reference to digestion, absorption and assimilation. Brief idea of peristalsis.

11.1 Why The Digestive System

The food that we eat cannot be utilized as such in the body. It must be changed into a soluble form to get absorbed by the blood for distribution in the body. Certain foods, like cane-sugar are already soluble in water, but they require a breaking down of their molecules into smaller units so that they could pass through the cell membranes of the wall of the gut.

Digestion is the break-down of naturally occurring foodstuffs into diffusable form.

OR

Digestion is any change which makes the foods soluble and of such chemical nature that they can be absorbed through living membranes.

Enzymes

Enzymes play a key role in the digestion of food taken in. There are hundreds of enzymes in addition to those involved in digestion, but the general characteristics of all enzymes are same.

Characteristics Of An Enzyme

1. It is a protein and is, therefore, destroyed by heating.

2. It acts only on one kind of substance called the substrate i.e. it is specific.

3. It always forms the same end-product(s) from the substrate.

4. It only affects the rate of a chemical reaction and always speeds up the reaction.

5. Like a catalyst it can be used again and again.

6. It acts best only at a particular pH, i.e. at a particular degree of acidity or alkalinity.

7. It acts best within a narrow temperature range, usually between 35° and 40°C which is also called optimum temperature.

11.2 The Digestive System

The digestive system consists of:

(i) alimentary canal and (ii) digestive glands

The alimentary canal (Fig. 14.1) is a muscular tube which starts with the mouth and ends at the anus. It is about 9 metres long and is highly coiled in certain regions especially in the small intestine. Its various regions, shown in Fig. 13.1, are different both in structure and function. In addition to the digestive glands located in the lining of the various regions of the digestive tube, two large digestive glands, are the liver and pancreas, are also associated with it, and three different salivary glands are associated with the mouth cavity.

The various organs of the digestive system are described as follows:

11.2.1 The Mouth

The mouth or the mouth cavity is the space where the food is chewed and mixed with saliva. Its front limits are formed by the upper and lower lips. The lips help in (i) closing the mouth, (ii) sucking and sipping liquids, (iii) speaking and also in (iv) perceiving certain sensations, especially those of touch and heat.

A muscular tongue helps in (i) manipulating the food while chewing and mixing it with saliva, (ii) tasting, (iii) cleaning the food particles from the teeth after eating and also in (iv) speaking.

11.2.2 The Teeth - Dentition

The teeth have a very special role - they cut and break the food into smaller bits. The small sized bits have a relatively larger surface for the enzymes to act on for better digestion. The teeth help in speaking (How? Find out for yourself - try to speak 'thick', 'talwar', 'tandoor', 'thin', 'through', 'the', etc.). Teeth also add to facial beauty.

An adult human normally has 32 teeth. These teeth (Fig. 11.2) are different in shape and perform different functions as follows:

(a) Incisors are the four front teeth in the centre of each jaw. Their cutting edges are broad and sharp like a chisel. They are used for biting and cutting.

(b) Canines are one on either side of the incisors in each jaw. These are conical and sharply pointed for holding and tearing the food.

(c) Premolars are two on each side in each jaw next to the canines. Each premolar has two hill-like projections or cusps on its surface, and hence known as bicuspid. Premolars help in grinding and crushing the food.

(d) Molars are the last three teeth on each side in each jaw. They have a larger surface than the premolars. They are principal grinders and crushers of food. The last molar of each side in each jaw is called wisdom tooth. The wisdom teeth are so called because they appear last at an age of about 17-20 years when the human body is reaching maturity. In this manner the human or mammalian teeth are different in shape and are called heterodont (hetero: different, dont: teeth) as opposed to homodont (homo: similar) teeth of other animals like those of the lizard and frog.

"Confusion" over the name "molar"

The dentists usually call the premolars of a child as "molars" (literally meaning "grinders"). But, since the grinders appear in two sets-first the ones that are shed and regrown (premolars), and second those that appear only once as permanent grinders (molars), it is scientifically more appropriate to give two separate names to the two kinds as premolars and molars right from the beginning (to avoid any confusion).

Mammalian teeth appear in two sets during life. In humans, the first set, or milk teeth, consists of 20 teeth (all, but not the molars) which start growing through the gums (sometimes painfully) when the child is about 7-8 months old and are completed when he is about 2 years old. These temporary (or the deciduous teeth) fall out as a result of their roots being dissolved away in the jaw and are completely replaced by the permanent teeth by about 12 years of age. (Fig. 11.3).

The number of permanent teeth of mammals is usually indicated in a formula in which the number of incisors (i), canines (c), premolars (pm), and molars (m) is given strictly in the same order for one-half of each jaw. The human dental formula is as follows:

Human child up to about 2 years:

\[\frac{2, 1, 2, 0}{2, 1, 2, 0} = 20\] (milk teeth)

Human adolescent up to about 17-20 years:

\[\frac{2, 1, 2, 2}{2, 1, 2, 2} = 28\] (permanent teeth)

Human adult:

\[\frac{2,1, 2, 3}{2, 1, 2, 3} = 32\] (permanent teeth) with wisdom teeth added

11.2.3 Structure Of A Tooth (Fig. 11.4)

The general structure of all types of teeth is the same. Each tooth consists of a crown or the part exposed above the gum and the root or the part embedded in a cup-like socket of the jaw bone. The root consists of a single process or fang as in incisors and canines, or of two processes or fangs as in premolars and lower molars and three in upper molars. The neck is a slight constriction between the root and the crown. In a vertical section, a tooth shows the following parts.

Enamel or the "ivory" is the material which covers the crown. It is the hardest substance in the body.

Dentine forms the bulk of the tooth. It is harder than bone but not as much as the enamel. It has minute canals through which run the strands of cytoplasm of the cells in pulp cavity.

Cement is another bone-like structure covering and fixing the root in position.

Pulp is the soft connective tissue contained in the central space (pulp cavity) of the tooth. It consists of blood capillaries, lymph vessels and nerve fibres which are continuous below those of the body through the opening of the pulp cavity at the base of the root.

Progress Check

1. Given below is a jumbled sequence of the different parts of the human alimentary canal. Rewrite them in correct sequence. Stomach, Oesophagus, Large intestine, Small intestine, Pharynx.

2. Name the following:

(i) The three subdivisions of the small intestine.

(ii) The three subdivisions of the large intestine.

(iii) The three salivary glands.

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