Class 7 Science Chapter 2 ‘Nutrition in Animals’ Notes by Sciक्षक

Before starting with Class 7 Science Chapter 2 i.e. ‘Nutrition in Animals’, I am hoping that you have completed Chapter 1 of the 7th class. If not, then you can go through its Notes and NCERT Exercise Solutions whose links have been provided below. ⤵️

Table of Content

Introduction

We know that animals are heterotrophic in nature meaning they don’t prepare their own food but they directly or indirectly depend upon plants for their nutrition.

Nutrition includes the food we need, the mode of food intake, and how it is utilised in the body.

The main components of food are:

• Carbohydrates
• Protein
• Fats
• Minerals
• Vitamins
• Roughage or Fibers
• Water

Our body can’t directly use these complex components of food like carbohydrates, protein, fats, etc. It breaks them down into simpler forms before using them and this breakdown of complex components of food into simpler ones is known as digestion.

Different Ways of Taking Food

The ways of taking in food vary from animal to animal. e.g. snakes like pythons swallow their prey, bees siphon the nectar, lamb sucks on their mother’s tits for milk, etc.

Some of these common modes of feeding are:

1. Scraping

In this mode of feeding the animals rub or scrape their teeth or mouth against the food item. This mode of feeding is quite common in animals like snails and slugs.

2. Chewing

In this mode of feeding the animals have well-developed teeth and they break down large pieces of food into the smaller by chewing them. This mode of feeding is quite common in animals like human beings, ants, tigers, etc.

3. Siphoning

In this mode of feeding the animals suck their food directly from the food sources using their tubular mouth. This mode of feeding is quite common in butterflies, hummingbirds, honeybees, etc.

Siphoning is like drinking the fruit juice that you have poured into a glass with the help of a straw.

4. Sucking

In this mode of feeding the animals pierce the food item to suck its internal content. This mode of feeding is quite common in leeches, mosquitoes, etc.

Sucking is similar to you piercing a mango Frooti Tetra pack with the straw and then drinking it with the straw.

5. Sponging

This mode of feeding is quite common in those animals who can’t feed on solid food but rather consume liquid or semi-solid food like houseflies. They liquify the solid food with the help of their saliva.

Table 2.1 (Class 7th Science NCERT Textbook) Various Modes of Feeding

6. Special Mode of Food Intake in Starfish

They mainly feed on animals covered in hard calcium carbonate shells.

• With the help of their arms, they open up the shell and then they pop out their stomach through their mouth.
• The animal inside the shell is consumed directly by the stomach and after eating the stomach goes back into the body and food is slowly digested in the body.

Digestion in Humans

We take food through our mouth, then we digest it, utilise it and defecate the unutilized or undigested food through our anus.

Food passes through a continuous canal or tube that runs from our mouth to the anus. This continuous canal is known as the alimentary canal or alimentary tract.

The various compartments of an alimentary tract are:

• Buccal cavity or mouth
• Oesophagus or food pipe
• Stomach
• Small intestine
• Large intestine
• Anus

As the food travels through these various compartments of the canal the complex food material breaks down into more simpler ones.

The human digestive system is also composed of the associated glands with the alimentary canal that facilitate the digestion of food.

The main digestive system glands are:

• Salivary gland in the mouth
• Liver
• Pancreas

So, the alimentary canal and the associated glands constitute the human digestive system.

Let’s learn about the human digestive system in detail now.

1. The Mouth and Buccal Cavity

The process of taking food into the body via mouth is called ingestion.

The buccal cavity mainly consists of these parts:

• Teeth
• Salivary gland
• Tongue

1.1 Teeth

We chew the food with the help of our teeth and break it down mechanically into smaller pieces.

Human beings have two sets of teeth. One is milk teeth or temporary teeth that grow during infancy and fall in childhood and the second set of teeth are the permanent teeth that replace the milk teeth. These permanent teeth may last throughout life or fall off in old age or maybe due to some illness or accident.

Each tooth is rooted in a different socket in the gums.

Our teeth vary in appearance and function. We have 4 different kinds of teeth.

1.2 Salivary Glands

These are the special organs that are present in our mouth that secret saliva (लार).

Saliva breaks down the starch into simpler sugars or carbohydrates. It moistens the food and makes the chewed food easy to swallow.

1.3 Tongue

It is a fleshy muscular organ that is attached at the back to the floor or the buccal cavity. It is free at the front and can be moved in different directions.

The main functions of the tongue are:

• It helps us to talk.
• It mixes the saliva with the food with chewing.
• It helps in swallowing the food.
• It also helps us experience the various tastes. The tongue has small buds (taste buds) present on it that are sensitive to various tastes and because of it, we can experience the sweet, bitter, sour, and salty nature of a food item.

The food that we chew and after mixing up with saliva it is ready to swallow is called bolus.

Tooth Decay

If we don’t clean or brush our teeth daily then the bacteria present in our mouth start to overgrow due to the presence of remaining food items in our mouth and they release acid which harms our teeth. This process of gradual damage of our teeth by acid is known as tooth decay.

Bacteria grow more rapidly in the presence of sugars, which is why it is advised not to overconsume toffee, sweets, chocolates and cold drinks.

2. The Foodpipe or Oesophagus

The food that we swallow travels from our mouth to our stomach via food pipe or oesophagus.

The oesophagus runs along the neck and the chest. The food is pushed down by the movement of the wall of the oesophagus called peristalsis.

This peristalsis takes place throughout the alimentary canal and pushes the food downwards (mouth to anus).

At times when this food is not accepted by our stomach or we overeat or we are sick, we vomit out the food.

The windpipe and foodpipe run adjacent to each other, but inside the throat the food and air share a common passage. During the swallowing, a flap-like structure called epiglottis closes the passage of the windpipe and guides the food into the oesophagus. But sometimes, when we eat in a hurry or talk while eating and due to some illnesses the food gets into our windpipe and causes hiccups, coughs and choking sensations.

3. The Stomach

It is a J-shaped thick-walled bag-like organ situated just below the left side of the ribs.

It is the widest part of our alimentary canal. It receives food via a food pipe. Food digests here and then passes onto the small intestine.

The inner wall of the stomach secretes mucous (a thick liquid substance), hydrochloric acid and digestive juices.

The mucous lining protects the stomach from acid burns. The acid kills the bacteria that are present in food and also helps in further digestion.

The digestive juices break down the proteins into simpler substances.

Once the food is digested in the stomach it is passed into the small intestine for further digestion and absorption.

The Story Behind, How We Got To Know About The Working of Stomach

The working of the stomach was discovered by a strange accident. In 1822, a man named Alexis St. Martin was badly hit by a shot gun. The bullet had seriously damaged the chest wall and made a hole in his stomach. He was brought to an American army doctor William Beaumont. The doctor saved the patient but he could not close the hole properly and left it bandaged. Beaumont took it as a great opportunity to see the inside of the stomach through the hole. He made some wonderful observations. Beaumont found that the stomach was churning food. Its wall secreted a fluid which could digest the food. He also observed that the end of the stomach opens into the intestine only after the digestion of the food inside the stomach is completed.

© Class 7th Science NCERT Textbook Page No. 16

4. Small Intestine

It is a highly coiled tube that is about 7.5m in length. It receives the secretion from the liver (stored in the gall bladder) and pancreas, and it also secretes digestive juices.

4.1 The Liver

It is a reddish-brown gland situated right beside the stomach. It is the largest gland present in the human body. It secretes bile juices that are stored in a sac-like structure called the gallbladder. Bile juices play a very critical role in the digestion of fats.

4.2 Pancreas

It is a large cream-coloured gland that is located just below the stomach. The pancreatic juice acts on carbohydrates, fats and proteins and changes them into simpler forms.

Carbohydrates ——-> Glucose
Fats —————-> Fatty acids and glycerol
Protein ————-> Amino acid

Absorption of Food in Small Intestine

The nutrients or digested food pass into the bloodstream through the wall of the small intestine. This process is known as the absorption.

The inner walls of the intestine have millions of fine finger-like projections known as villi that increase the area for absorption of digested food.

Each villi has a network of thin and small blood vessels close to the surface.

Now, once the nutrients have been absorbed into the bloodstream they are transported to various organs where they are required.

This process of transportation of absorbed nutrients to their place of requirement is known as assimilation.

Now the food that remains undigested or unabsorbed passes onto the large intestine.

5. Large Intestine

It is a wider and more shorter tube than the small intestine. It is about 1.5m long.

Its main function is the absorption of the water and remaining salts in the undigested food material.

The remaining waste passes into the rectum and it is stored there in the form of semi-solid faeces.

Then this faecal material is removed through the anus from time to time, and this process is called egestion.

Diarrhoea is a medical condition when a person passes a watery stool (faeces) due to problems in the digestive system, illness, bad food, etc. This can be lethal as it can cause dehydration as water and salts are not absorbed by the body. We need to consult a doctor and in the meantime, we have to give ORS (Oral Rehydration Solution) which is a mixture of water, salt and sugar to the person in distress.

That’s all about the human digestive system.

To get a more thorough understanding of the human digestive system you can refer to this video linked below:

Digestion in Grass-Eating Animals

Have you ever noticed that grass-eating animals like buffalos, cows, etc. chew continuously even when they are not eating?

Well, this constant chewing is called rumination.

• These grass-eating animals quickly swallow the grass and store it in their stomach (rumen). Where the grass or cellulose (a type of carbohydrate) is partially digested by the gut bacteria and digestive juices.
• This partially digested food in the rumen is called cud.
• Later this cud returns to the mouth in small lumps and the animal chews it.
• This process of chewing the cud is called rumination and these animals are called ruminants.

Animals like horses and rabbits have a large sac-like structure called Caecum as a part of their alimentary canal. The cellulose food or grass is digested here by the action of certain bacteria that are not present in humans.

Humans can’t digest the cellulose.

Not all animals have a well-developed digestive system like us. But, they all have their unique way of acquiring and digesting food.

Here is an example of such a simple organism and its approach towards digestion.

Feeding and Digestion in Amoeba

Amoeba is a single-celled or unicellular microscopic (very small in size) organism mainly found in pond water. It doesn’t have a well-developed digestive system.

Its cell or body consists of a plasma membrane, a nucleus, vacuoles, cytoplasm, etc.

Amoeba doesn’t have a fixed shape and structure. Its shape and position always keep on changing.

• It extends one or more finger-like projections called pseudopodia (fake feet) for the movement and capturing the food.
• It feeds on the microscopic organisms present in/on pond water.
• It senses the food and it pushes out its pseudopodia around the food particles and engulfs them.
• Then the food is passed onto the food vacuole where the food is digested by the digestive juices or enzymes secreted by the vacuole.
• Then, gradually the digested food is absorbed and used by the body for growth, multiplication and repair.
• The undigested food is thrown out of the body.

Conclusion

The basic purpose and process of digestion are the same in almost all organisms. This digestive system has evolved over thousands of years to meet the basic needs of the body.

References & Credits

• Class 7th Science NCERT Textbook
• Image by brgfx on Freepik
• Image by macrovector on Freepik
• Image by freepik
• Image by Pixel_perfect from Pixabay
• Image by Clker-Free-Vector-Images from Pixabay
• Image by avitalchn from Pixabay

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