Life Processes in Animals
Class 7 Science — Chapter 9 Notes
1. Introduction
Animals eat different types of food depending on their habitat and habits. For example:
Bees and sunbirds suck nectar from flowers.
Human infants and the infants of many other animals feed on their mother’s milk.
Snakes, such as pythons, swallow the animals they catch.
Some aquatic animals filter tiny food particles from water.
All animals, including humans, obtain energy from food. This energy is essential for carrying out life processes. Food contains complex components such as carbohydrates, proteins, and fats. These components must be broken down into simpler forms before the body can use them.
This breakdown of food takes place in a long tube called the alimentary canal, which starts at the mouth and ends at the anus. As food passes through this canal, digestive juices secreted by different organs help convert it into simpler substances. These nutrients are then absorbed and transported to various parts of the body for energy production, growth, repair, and other life processes.
2. Nutrition in Animals
Animals obtain energy from food, but the way food is broken down and used varies among different species. To understand this process, let us study digestion in humans.
3. Digestion in Human Beings
Digestion is the process of breaking down complex food into simpler substances. It occurs in the alimentary canal, a long tube that extends from the mouth to the anus.
The following stages describe how digestion takes place in human beings from beginning to end.
1. Beginning with the Mouth Cavity
Digestion begins in the mouth, where food enters and is broken down into smaller pieces by the teeth through crushing and chewing. This is called mechanical digestion.
When you think of your favourite food, your mouth produces more saliva. Saliva moistens the food and makes it easier to swallow.
Saliva contains the enzyme salivary amylase, which begins breaking down starch, a carbohydrate, into simpler sugars. This is why starchy foods such as chapati or rice may taste sweet when chewed for some time.
After chewing, saliva moistens the food, making it soft and easy to swallow.
The tongue mixes the chewed food with saliva and pushes it into the food pipe or oesophagus.
The oesophagus is a long, flexible tube that connects the mouth to the stomach.
Food moves down the oesophagus by a wave-like movement called peristalsis. During peristalsis, the walls of the food pipe contract and relax gently.
This movement continues throughout the alimentary canal and pushes food forward for digestion.
The walls of the food pipe contract and relax in a wave-like motion to push food down into the stomach.
The walls of the stomach contract and relax to churn the food and mix it thoroughly.
The stomach lining secretes digestive juice, acid, and mucus.
Digestive juice helps break down proteins into simpler substances.
The acid provides the acidic medium needed for protein digestion and helps kill harmful bacteria.
Mucus protects the stomach lining from being damaged by the acid.
Food is partially digested in the stomach and changed into a semi-liquid mass that is ready for the next stage of digestion.
4. Small Intestine
After leaving the stomach, partially digested food enters the small intestine. It is about 6 metres long, making it the longest part of the alimentary canal.
The small intestine receives digestive secretions from:
Its own inner lining.
The liver, which produces bile.
The pancreas, which produces pancreatic juice.
Bile is mildly basic. It helps neutralise stomach acid and breaks fats into tiny droplets, making them easier to digest.
Pancreatic juice is also basic. It helps neutralise acid and contains substances that help break down carbohydrates, proteins, and fats.
Digestive juices from the small intestine further break down fats, proteins, and carbohydrates into simpler substances.
Nutrients are absorbed into the blood through the inner lining of the small intestine.
The lining has thousands of finger-like projections called villi. These projections increase the surface area for efficient absorption.
Absorbed nutrients provide energy, help in growth and repair, and support the proper functioning of the body.
After nutrient absorption in the small intestine, undigested food moves into the large intestine.
The large intestine is about 1.5 metres long. It is shorter but wider than the small intestine.
Its main function is to absorb water and some salts from undigested food.
This process changes the waste into a semi-solid form called stool.
Stool is stored in the rectum until the body is ready to eliminate it.
The waste is expelled through the anus in a process called egestion.
Eating fibre-rich foods, such as fruits, vegetables, and whole grains, helps keep the large intestine healthy and makes stool easier to pass.
4. Digestion in Other Animals
Not all animals digest food in the same way as humans. Their digestive systems are adapted to their diets and habitats.
1. Ruminants
Ruminants are grass-eating animals. They partially chew grass and swallow it into their stomachs, where it undergoes partial digestion.
The food is then brought back to the mouth for thorough chewing. This process is called rumination.
Ruminants may spend about 8 hours a day chewing their food.
After thorough chewing, the food moves down the alimentary canal for further digestion.
Birds lack teeth but have a special muscular stomach chamber called the gizzard.
The gizzard breaks down food by contracting and relaxing. Small stones, called grit, swallowed by birds may help grind the food.
Animals show variations in their digestive systems according to the types of food they eat.
Nutrients from digested food help build and repair the body or are broken down to release energy.
The process of using nutrients to release energy is called respiration.
Respiration is the process by which animals use oxygen to break down nutrients such as glucose and release energy. Carbon dioxide and water are formed as products of this process.
Breathing is a physical process that brings oxygen into the body and removes carbon dioxide. Respiration is a chemical process that takes place in cells.
Breathing is the process of inhaling, or breathing in, oxygen-rich air and exhaling, or breathing out, air containing more carbon dioxide.
Breathing is essential for survival. Without breathing, humans cannot live for more than a few minutes.
All living organisms exchange gases with their surroundings. Plants and animals also carry out respiration.
The respiratory system is the body system responsible for breathing and gas exchange.
Key Components of the Human Respiratory System:
Nostrils: Openings in the nose through which air is inhaled and exhaled.
Nasal passages: Passages after the nostrils lined with tiny hairs and mucus that trap dust and dirt. Breathing through the nose is better than through the mouth due to this filtration.
Windpipe (Trachea): A tube that carries air from the nasal passages to the lungs and divides into two branches.
Lungs and Alveoli: The windpipe branches inside the lungs into finer tubes ending in tiny balloon-like sacs called alveoli, where gas exchange occurs.
Protection: The lungs are protected by the rib cage, a bony structure surrounding them.
(i) Inhalation
When you breathe in, or inhale, your chest expands as your ribs move up and outwards.
The diaphragm is a dome-shaped muscle located below the lungs. It moves downwards during inhalation.
This movement increases the space inside the chest and allows air to enter the lungs.
When you breathe out, or exhale, your ribs move down and inwards, while the diaphragm moves upwards.
This reduces the space inside the chest and helps to push air out of the lungs.
In the activity described above, the balloons represent the lungs and the rubber sheet represents the diaphragm.
7. What Do We Breathe Out?
When we exhale, the air we breathe out contains more carbon dioxide than the air we inhale. This can be demonstrated through an experiment with lime water:
Lime water is a clear solution that turns milky when it reacts with carbon dioxide.
Two test tubes containing equal amounts of fresh lime water are taken (Test Tube A and Test Tube B).
Air similar to inhaled air is passed through test tube A using a syringe.
Exhaled air is blown through a straw into test tube B.
Test tube B turns milky because of its reaction with carbon dioxide, while Test tube A remains clear.
This indicates that exhaled air contains a higher concentration of carbon dioxide than inhaled air.
Fresh air enters the lungs during breathing and fills tiny sacs called alveoli.
Alveoli have thin walls and are surrounded by blood vessels.
Carbon dioxide from the blood passes into the alveoli and is removed from the body during exhalation.
Oxygen from the alveoli passes into the blood and is carried to the entire body.
Oxygen is used to break down glucose from food and release energy in a process called respiration.
Glucose + Oxygen ➔ Carbon dioxide + Water + Energy
Inhaled air contains about 21% oxygen and 0.04% carbon dioxide.
Exhaled air contains about 16–17% oxygen and 4–5% carbon dioxide.
Breathing is a physical process involving the intake and release of air.
Respiration is a chemical process inside cells that produces energy.
Both breathing and respiration are vital for survival.
Breathing and respiration are related but occur at different levels in the body.
Breathing is the simple act of moving air in and out of the lungs, whereas respiration is the chemical process inside cells that releases energy from food.
The circulatory system transports nutrients, oxygen, and waste products throughout the body.
It consists of the heart, blood, and blood vessels.
The heart pumps blood through blood vessels to deliver oxygen and nutrients and remove waste products.
Respiration in Other Animals
Different animals live in different habitats and have different breathing mechanisms suited to their surroundings:
Lungs: Animals such as birds, elephants, lions, cows, lizards, and snakes breathe through lungs, though lung structures vary.
Gills: Aquatic animals such as fish use gills to extract dissolved oxygen from water and release carbon dioxide.
Skin and Gills: Amphibians such as frogs use gills as tadpoles, lungs as adults on land, and moist skin for gas exchange in water.
Moist Skin: Earthworms breathe through their moist skin, which allows direct exchange of oxygen and carbon dioxide.
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