Class 7 Science Chapter 9: Life Processes in Animals Notes

What is this chapter about?

Living beings need life processes such as nutrition, respiration, circulation, excretion and reproduction to stay alive. This chapter looks closely at two of them. Nutrition: how complex food is broken down and absorbed, in humans and in animals such as cows and birds. Respiration: how we breathe, what happens to the oxygen we take in, and how fish, frogs and earthworms get their oxygen.

The journey of food

Animals eat many kinds of food: bees and sunbirds suck nectar, infants drink milk, a python swallows its prey whole, and some water animals filter tiny particles. Food contains complex components such as carbohydrates, proteins and fats, which the body can use only after they are broken into simpler forms. This happens in the alimentary canal, a long tube from the mouth to the anus.

In the mouth, teeth crush and chew the food (mechanical digestion) and saliva softens it and starts breaking starch into sugar, which is why a chapati tastes sweet after long chewing. Activity 9.1 shows it with iodine, which turns starch blue-black: boiled rice turns blue-black, but chewed rice shows little or no blue-black.

Stomach and small intestine

The tongue pushes the soft food into the food pipe (oesophagus), whose walls contract and relax in waves to move it to the stomach. The stomach churns the food and adds a juice that breaks down proteins, an acid that helps with this and kills many harmful bacteria, and mucus that protects the stomach lining. Much of this was first seen in 1822, when a doctor, William Beaumont, studied digestion through a wound that never closed in the stomach of a patient, Alexis St. Martin.

The small intestine is about 6 m long, the longest part of the canal. It receives bile from the liver, which neutralises the stomach acid and breaks fats into tiny droplets, and juice from the pancreas, which works on carbohydrates, proteins and fats. Its own juice finishes the job. Thousands of finger-like projections on its inner lining absorb the nutrients into the blood, which carries them around the body. Read more in nutrition and digestion in animals.

Large intestine and gut health

The undigested food moves into the large intestine, about 1.5 m long but wider. It absorbs water and some salts, leaving semi-solid stool, which is stored in the rectum and leaves through the anus: egestion. Fibre in fruits, vegetables and whole grains keeps the large intestine working well, and its bacteria break down fibre and make useful nutrients; fermented foods such as curd, buttermilk, kanji, gundruk and poita bhat support them. The Charaka Samhita advised easily digestible food and spices such as ginger and cumin.

Digestion in other animals

Cows and buffaloes chew grass a little and swallow it; after partial digestion in the stomach, it comes back to the mouth to be chewed slowly again. This is rumination, and such animals are ruminants; a cow chews for about 8 hours a day. Birds have no teeth: their gizzard grinds food, often with grit. Each animal’s canal suits the food it eats.

Breathing

Breathing is taking air in (inhaling) and pushing it out (exhaling). Air enters through the nostrils; hair and mucus in the nasal passages trap dust, so it is better to breathe through the nose. It passes down the windpipe, which branches into the two lungs, and the branches end in tiny sacs called alveoli. The rib cage protects the lungs. When we breathe in, the ribs move up and out and the diaphragm moves down, giving the lungs more space; when we breathe out, they move back and push air out. A bottle model with balloons (the lungs) and a rubber sheet (the diaphragm) shows this.

What we breathe out

When breath is blown through lime water it turns milky, but room air pushed in with a pichkari does not, so exhaled air has more carbon dioxide. Inhaled air has nearly 21% oxygen and 0.04% carbon dioxide; exhaled air has nearly 16–17% oxygen and 4–5% carbon dioxide.

Gas exchange and respiration

In the thin-walled alveoli, oxygen passes into the blood and carbon dioxide passes out of it to be breathed away. The blood, pumped by the heart through blood vessels (the circulatory system), takes oxygen and nutrients all over the body. There, respiration releases energy: glucose + oxygen → carbon dioxide + water + energy. Breathing is a physical process; respiration is a chemical one.

How other animals breathe

Birds, elephants, cows, lizards and snakes breathe with lungs, though their lungs differ. Fish use gills to take oxygen dissolved in water. Tadpoles use gills; adult frogs use lungs on land and their skin in water. Earthworms breathe through their moist skin. Read respiration in animals. Answers to all the questions are in the NCERT solutions for Chapter 9.