Life Processes in Animals | CBSE Class 7 Science Notes

This note covers animal feeding, human digestion, nutrient absorption, removal of undigested food, digestion in cows and birds, human breathing, gas exchange, respiration, circulation, and breathing structures in other animals.

What are life processes, and why must animals digest food?

Life processes are the processes that sustain living beings. Two of these are nutrition, the obtaining and use of food, and respiration, the conversion of nutrients into usable energy. Nutrients are food components that support energy needs, growth, repair, and other body functions.

How do animals obtain food?

Animals obtain food in different ways. Bees and sunbirds suck nectar, the liquid food in flowers. Human infants and the young of many other animals feed on their mother's milk. Pythons swallow the animals they catch as prey.

Some aquatic animals, meaning animals living in water, filter tiny food particles from the surrounding water. These examples show that animals differ both in the food they eat and in how they take it into their bodies.

Carbohydrates, proteins, and fats are complex food components. Before the body can use them, they must be broken down into simpler forms. Obtaining food is therefore the beginning of a longer process that makes its nutrients available to the body.

Definition: Digestion is the breakdown of complex food components into simpler forms inside the body.

What route does food follow in humans?

The alimentary canal is the long tube through which food passes, beginning at the mouth and ending at the anus, the opening through which undigested waste leaves. Different parts move food, mix it with digestive juices, absorb nutrients, or help remove waste.

Digestive juices are secretions that help break down food components. A secretion is a substance released by a body part. As food moves along the canal, these juices act on it at different places rather than completing the whole process in the mouth.

The sequence is mouth, oesophagus or food pipe, stomach, small intestine, large intestine, and anus. The oesophagus carries food to the stomach, a chamber that churns food. The intestines are the parts of the canal that follow the stomach; they continue digestion and absorb useful substances.

The liver and pancreas are associated structures that supply digestive secretions. The rectum is the lower part of the large intestine where stool, the semi-solid undigested waste, is stored before removal.

The drawing shows the digestive organs within a human outline. Labels identify the mouth, oesophagus or food pipe, liver, stomach, pancreas, small intestine, large intestine, rectum, and anus. The small intestine is drawn coiled inside the surrounding large intestine.

See Fig. 9.1 in your NCERT textbook

How do teeth, the tongue, and saliva begin digestion?

Mechanical digestion is the initial breakdown of food into fine pieces by crushing and chewing. Teeth perform this work in the mouth. Food is partially digested there, so chewing begins digestion without completing the whole process.

Saliva is the fluid released into the mouth that moistens food and contains a digestive juice. Moistening makes chewed food softer and easier to swallow. The tongue mixes food with saliva and helps push the softened food towards the food pipe.

Why can chewed chapati or rice taste sweet?

Starch is a type of carbohydrate present in chapati and rice. The digestive juice in saliva helps turn starch into sugar, a simpler food component. This explains why these foods begin to taste sweet after prolonged chewing.

A small piece of chapati or a bite-sized portion of boiled rice can be chewed for 30 to 60 seconds to observe the change. The change in taste is connected with saliva acting on starch, alongside the breaking of food into smaller pieces.

How does the iodine activity test saliva's action?

Iodine solution is used to test for starch because it produces a blue-black colour in the presence of starch. The two test tubes below are labelled A for unchewed boiled rice and B for chewed boiled rice.

1. Place one teaspoonful of boiled rice in test tube A. Place one teaspoonful of boiled rice chewed for 30 to 60 seconds in test tube B.
2. Add 3 to 4 millilitres of water to each tube. A millilitre, written mL, is a unit used to measure liquid volume.
3. Observe and record the starting colour of each rice-water mixture before adding the testing solution.
4. Add 3 to 4 drops of iodine solution to each tube with a dropper. Mix the contents of each tube separately and observe their colours.

The unchewed rice in A turns blue-black. The chewed rice in B either does not change colour or turns only a very light blue-black colour. No change indicates that starch is no longer present; a slight change indicates a very small amount remains.

Note: Do not replace the two possible outcomes for chewed rice with a claim that it always shows no colour change. If colour remains, repeating the activity with increased chewing time allows further investigation.

Oral hygiene means keeping the mouth clean. Brushing teeth and cleaning the tongue twice a day, along with rinsing the mouth with water after each meal, helps prevent tooth decay and an unpleasant smell.

How does food move through the food pipe and stomach?

The oesophagus, or food pipe, is a long, flexible tube linking the mouth to the stomach. After the tongue pushes softened food into it, the walls gently contract and relax. Contracting means tightening; relaxing means loosening again.

What moves food forward?

Contraction and relaxation produce a wave-like movement that pushes food towards the stomach. This movement takes place throughout the alimentary canal and moves food forwards. Thus the canal actively moves food as well as providing a passage for it.

Saliva's moistening action and the tongue's mixing action prepare food for swallowing. The food pipe then moves that softened food onwards. These functions form a sequence, with each part contributing to the journey started by chewing in the mouth.

What happens inside the stomach?

The stomach is the part of the alimentary canal between the food pipe and small intestine. Its walls contract and relax to churn food. Churning mixes food with substances released from the stomach's inner lining.

This secretion contains digestive juice, acid, and mucus. Here, acid is the stomach secretion component that helps protein breakdown and kills many harmful bacteria, which are small living organisms. Mucus is the protective material that shields the stomach lining from the acid.

Bacteria are small living organisms; some are harmful, while others help digestion. The stomach's acid kills many harmful bacteria. That description does not mean every bacterium is harmful or that the stomach acid kills them all.

Food becomes a semi-liquid mass, meaning a partly liquid mixture, in the stomach. It remains only partially digested. The next stage takes place after this mixture enters the small intestine, where further digestive secretions act on it.

How does the small intestine digest food and absorb nutrients?

The small intestine is the longest part of the alimentary canal. It is almost 6 metres long. Its name does not mean that it is shorter than the large intestine. Partially digested food enters it from the stomach.

Which secretions act here?

Three sources supply digestive secretions: the small intestine's own inner lining, the liver, and the pancreas. Together, their actions continue the processing of food that has already been chewed, mixed with saliva, and churned in the stomach.

The liver produces bile, a mildly basic secretion. Basic here describes its ability to neutralise acid, meaning counteract its acidic nature. Bile neutralises acids in food arriving from the stomach and divides fats into tiny droplets, making fat digestion easier.

The pancreas produces pancreatic juice, a basic secretion that also helps neutralise acids in the food. It breaks down carbohydrates, proteins, and fats. The small intestine's own digestive juice further breaks down fats, proteins, and partially digested carbohydrates into simpler forms.

How do digested nutrients reach the body?

Definition: Absorption is the passage of digested nutrients from the small intestine into the blood in its wall blood vessels, the tubes through which blood travels.

Blood vessels are tubes through which blood travels. The small intestine has a thin inner lining with thousands of finger-like projections, or small outgrowths. These increase the surface area available for efficient nutrient absorption.

1. Partially digested food enters the small intestine from the stomach, bringing food components that require further breakdown.
2. Secretions from the liver, pancreas, and intestinal lining act on the food and continue the digestive process.
3. Digested nutrients pass through the thin lining and its finger-like projections into the blood in nearby vessels.
4. The blood carries the absorbed nutrients to different parts of the body, where they support energy needs, growth, repair, and proper functioning.

Digestion and absorption describe different events. Digestion makes complex food components simpler. Absorption moves the digested nutrients into the blood. Most nutrients are digested and absorbed in the small intestine before the remaining material moves into the large intestine.

What happens to undigested food in the large intestine?

The large intestine follows the small intestine. It is about 1.5 metres long, so it is shorter than the small intestine. The word “large” refers to its greater width. Undigested food reaches it after most nutrients have been digested and absorbed.

How are water absorption and egestion connected?

The large intestine absorbs most of the remaining water and some salts from the undigested food. Here, salts are substances absorbed along with water from this material. As water is removed, the waste becomes semi-solid and is called stool.

The rectum, the lower part of the large intestine, stores stool until the body is ready to remove it. Stool then leaves through the anus. Egestion is this expulsion of undigested waste from the body.

The route through the final parts of the canal therefore has an order: water and some salts are absorbed, stool is stored, and stool is expelled. Absorbing useful material and removing undigested waste are different contributions to the digestive process.

How do the two intestines compare?

Fibre is a food component present in fibre-rich foods such as fruits, vegetables, and whole grains. These foods help the large intestine function properly by making stool easier to pass. Various small living organisms, including bacteria, also occur in the large intestine.

These organisms help break down undigested food, especially fibre, and produce essential nutrients. Their role shows why bacteria should not all be described as harmful. The digestive system includes useful activities beyond the breakdown that occurs in the stomach.

How do cows and birds digest food differently from humans?

Animals have differences in the structure and function of their alimentary canals that suit different foods and ways of digesting them. Human chewing begins with teeth, but the processing of food in cows and birds illustrates other arrangements.

What is rumination?

Rumination is the return of partially digested food from the stomach to the mouth for further chewing. Animals that do this are ruminants. Grass-eating animals such as cows and buffaloes partly chew grass before swallowing it.

1. A cow partially chews grass and swallows it into the stomach, where the next stage of food processing begins.
2. The swallowed food undergoes partial digestion in the stomach rather than completing its entire digestive journey there.
3. The partially digested food returns to the mouth, where it is chewed gradually and thoroughly.
4. The thoroughly chewed food passes down the alimentary canal again for further digestion.

This explains why a cow can be seen chewing when it is not actively grazing or taking in new food. The material in its mouth has returned from its stomach. A cow spends about 8 hours a day chewing food.

How can birds break down food without teeth?

Birds have no teeth, but they have a chamber called the gizzard. Contraction and relaxation of its walls break down food. This often happens with help from grit, the small stones that birds swallow.

The word “often” matters: the role of grit must not be turned into a claim that every bird uses stones on every occasion. The central action described is the movement of the gizzard walls, with grit often assisting that action.

A bird is drawn with its digestive tract visible inside the body. Labels identify the oesophagus, stomach, gizzard, and intestine. The gizzard is shown as a distinct labelled chamber in this tract.

See Fig. 9.7 in your NCERT textbook

These examples connect structure with function. Human teeth crush and chew food in the mouth; a cow brings partially digested food back for more chewing; and a bird uses movements of its gizzard to break food down.

What pathway does air follow through the human respiratory system?

Breathing is taking air into the body and sending air out. Taking air in is inhalation; sending it out is exhalation. The respiratory system is the group of body parts involved in breathing and gas exchange, the transfer of oxygen and carbon dioxide between air and blood.

Oxygen is the gas used in the breakdown of glucose to release energy. Glucose is a simple sugar. Carbon dioxide is a gas produced in this process and carried away from the body. Breathing helps supply oxygen and remove carbon dioxide.

How does air reach the lungs?

1. Air enters through the nostrils, the pair of nasal openings through which we inhale and exhale.
2. It passes into the nasal passages, the small passages inside the nose. Tiny hairs and mucus help trap dust and dirt from incoming air.
3. Air travels through the windpipe, the tube leading towards the lungs. The windpipe divides into two branches, one entering each lung.
4. Inside the lungs, the branches divide into smaller and finer branches ending in alveoli, the small balloon-like sacs where gas exchange occurs.

The lungs are the paired breathing organs reached by these air passages. The rib cage, the bony structure surrounding them, protects them. Together, the passages and lungs provide a route through which air reaches the gas-exchange surfaces.

Why does breathing through the nose help?

Hairs and mucus in the nose trap dust and dirt, which is why breathing through the nose is advised. Their filtering action acts before the inhaled air reaches the finer branches and air sacs within the lungs.

A lot of dust is filtered from inhaled air, but this does not mean every unwanted particle is removed. Small infectious particles, meaning particles that can cause infection, can often still get through to the lungs. Nasal filtering should not be described as complete protection.

The route of air and the movement of the chest are connected but different parts of breathing. The passages provide the route. Movements of the ribs and the muscle below the lungs change the space in the chest and move air.

How do the ribs and diaphragm produce inhalation and exhalation?

The diaphragm is a dome-shaped muscle below the lungs. Its movement works with movement of the ribs to change the space inside the chest. These changes allow air to enter during inhalation and leave during exhalation.

What changes during each phase?

During inhalation, the ribs move up and outwards while the diaphragm moves downwards. The space inside the chest increases and air enters the lungs. During exhalation, the ribs move down and inwards while the diaphragm moves upwards.

These movements reduce the space inside the chest during exhalation and push air out. The direction of diaphragm movement must therefore be linked with the change in chest space: downwards increases space, while upwards reduces it.

What does the bottle model demonstrate?

The model uses a transparent bottle with its bottom removed, a hollow Y-shaped tube, two balloons, and a rubber sheet. The two balloons are attached to the forked ends of the tube, and the straight end passes tightly through the bottle lid.

Rubber bands secure the balloons, clay seals the opening in the lid, and a rubber sheet is fastened tightly over the open base. The airtight arrangement allows the effect of moving the sheet to be observed through the transparent bottle.

The balloons represent the lungs, and the rubber sheet represents the diaphragm. Pulling the sheet down inflates the balloons; releasing it upwards deflates them. The two observations connect movement of the sheet with the entry and exit of air.

Two chest drawings compare inhalation and exhalation. Arrows show air drawn in or forced out, ribs moving outwards or back, and the diaphragm moving downwards or upwards. The labels connect these movements with the two phases.

See Fig. 9.10 in your NCERT textbook

How do gas exchange and respiration supply usable energy?

Gas exchange is the movement of oxygen into the blood and carbon dioxide out of it at the lungs. Fresh air reaches the alveoli during breathing. Their thin walls are surrounded by fine blood vessels, bringing blood close to the air inside.

What happens at an alveolus?

An alveolus is one air sac; alveoli is the plural. Blood brings carbon dioxide from the body to these sacs, where it passes into the air. At the same time, oxygen passes from the air sacs into the blood for transport around the body.

The drawing shows a group of air sacs and an enlarged single alveolus beside it. A surrounding blood vessel and arrows indicate oxygen moving towards the blood and carbon dioxide moving into the alveolus.

See Fig. 9.12 in your NCERT textbook

How is breathing different from respiration?

Respiration is the chemical process that releases usable energy from nutrients. In human respiration, oxygen helps break down glucose. The products are carbon dioxide and water, and energy is released for activities such as walking, running, playing, and thinking.

Glucose + Oxygen → Carbon dioxide + Water + Energy

This word equation summarises the process. The plus sign joins substances or outcomes listed together; the arrow means “produces”. Glucose and oxygen are used in the process. Carbon dioxide and water are formed, and energy is released.

Breathing is a physical process: it moves air into and out of the lungs. Respiration is a chemical process occurring inside the body: it changes substances and releases energy. Breathing supplies oxygen for respiration and helps remove the carbon dioxide produced.

How does exhaled air differ from inhaled air?

Not all inhaled oxygen is used up. The outgoing air still contains oxygen, but contains more carbon dioxide than the incoming air. A percentage, written %, expresses an amount out of one hundred. The gas percentages are approximate, as indicated by “nearly”.

Lime water is a testing liquid that turns milky or cloudy when it reacts with carbon dioxide. In a teacher demonstration, equal amounts of freshly prepared lime water are placed in two test tubes. Surrounding air enters one through a syringe or pichkari; exhaled air enters the other through a straw.

In this demonstration, the lime water receiving exhaled air turns milky, while the other does not. The conclusion is that exhaled air contains more carbon dioxide. It does not establish that surrounding air has no carbon dioxide.

How are substances transported, and how do other animals breathe?

The circulatory system transports nutrients, oxygen, and other substances around the body. It consists of the heart, blood, and blood vessels. The heart is the organ that pumps blood through these vessels, supplying body parts and carrying waste products away.

How do the body systems work together?

The digestive system makes nutrients available through digestion and absorption. The respiratory system brings oxygen into the body and removes carbon dioxide. Blood connects these activities by carrying absorbed nutrients and oxygen to different body parts and transporting waste products away.

These systems therefore work in coordination. Digestion makes simpler nutrients available, gas exchange transfers oxygen into the blood, and circulation transports these materials. Respiration then uses oxygen to break down glucose and release energy inside the body.

Which structures do other animals use?

A habitat is the environment in which an animal lives. Breathing structures differ among animals and suit their habitats. Birds, elephants, lions, cows, goats, lizards, and snakes breathe through lungs, although their lung structures are quite different.

Most aquatic animals, such as fish, use gills, specialised structures richly supplied with blood vessels. Across the gills, oxygen and carbon dioxide are exchanged between the blood and gases dissolved in water. “Most” must not be changed to “all”.

Amphibians, such as frogs, live both on land and in water. Their breathing structures can differ with their life stage. A tadpole, the young stage of a frog, uses gills, while the adult uses lungs on land and skin in water.

Earthworms exchange gases through their moist skin. The comparison shows that lungs are one solution among several. Gas exchange can take place across lungs, gills, or skin, depending on the animal and, in frogs, its stage and surroundings.

Glossary

• Life processes — Processes that sustain living beings, including the obtaining of nutrients and the release of usable energy.
• Digestion — The breakdown of complex food components into simpler forms that the body can use.
• Alimentary canal — The long tube through which food passes, extending from the mouth to the anus.
• Mechanical digestion — The initial crushing and chewing of food into fine pieces by the teeth.
• Saliva — Fluid in the mouth that moistens food and contains a digestive juice that acts on starch.
• Oesophagus — The flexible food pipe whose contracting and relaxing walls move swallowed food towards the stomach.
• Bile — The mildly basic liver secretion that neutralises acid and divides fats into tiny droplets.
• Absorption — The passage of digested nutrients from the small intestine into blood vessels in its walls.
• Egestion — The removal of undigested waste from the body through the anus after storage in the rectum.
• Rumination — The return of partially digested food from the stomach to the mouth for further chewing.
• Gizzard — A chamber in birds whose contracting and relaxing walls break down food, often helped by swallowed grit.
• Breathing — The physical process of inhaling air into the lungs and exhaling air from them.
• Diaphragm — A dome-shaped muscle below the lungs whose movements change the space inside the chest.
• Alveoli — Small balloon-like sacs in the lungs where oxygen and carbon dioxide are exchanged with the blood.
• Respiration — The chemical process that converts nutrients into usable energy; in humans, oxygen helps break down glucose.

Common errors and misconceptions

• Misconception: Digestion begins in the stomach. Correct: It begins in the mouth, where teeth break food into fine pieces and saliva starts breaking down starch into sugar.
• Misconception: Chewed rice must show no colour change with iodine. Correct: It may show no change or only a very light blue-black colour if a very small amount of starch remains.
• Misconception: The small intestine is shorter than the large intestine. Correct: The small intestine is almost 6 metres long; the large intestine is about 1.5 metres long and wider.
• Misconception: Stomach acid kills all bacteria. Correct: It kills many harmful bacteria. Various bacteria in the large intestine help digestion and produce essential nutrients.
• Misconception: Every bird's food breakdown requires swallowed stones. Correct: Gizzard walls contract and relax to break down food, often with help from grit.
• Misconception: Breathing and respiration mean the same thing. Correct: Breathing physically moves air; respiration chemically breaks down nutrients and releases usable energy.
• Misconception: Exhaled air contains no oxygen. Correct: Not all oxygen is used up. Exhaled air contains nearly 16 to 17% oxygen and more carbon dioxide than inhaled air.
• Misconception: Frogs use the same breathing structure throughout life. Correct: Tadpoles use gills; adult frogs use lungs on land and skin for gas exchange in water.

Exam-style questions with model answers

Q1. Define digestion and absorption in humans, giving one separate definition for each process. [2 marks]

1. Digestion is the breakdown of complex food components into simpler forms inside the body.
2. Absorption is the passage of digested nutrients from the small intestine into the blood vessels in its walls.

Q2. Test tube A contains unchewed boiled rice; test tube B contains boiled rice chewed for 30 to 60 seconds. After iodine is added, A turns blue-black while B shows only a very light blue-black colour. Iodine turns blue-black with starch. Explain A's result, B's result, and saliva's role. [3 marks]

1. The blue-black colour in test tube A indicates that starch is present in the unchewed boiled rice and reacts with the iodine.
2. The very light blue-black colour in test tube B indicates that starch is still present, but in a very small amount.
3. Saliva contains a digestive juice that helps break starch into simple sugars during chewing, explaining why the chewed sample contains much less starch.

Q3. Describe four contributions to food processing or protection in the human stomach: the stomach walls, digestive juice, acid, and mucus. [4 marks]

1. The stomach walls contract and relax, churning food so that it mixes with the secretion released by the inner lining.
2. The stomach's digestive juice breaks down proteins present in food into simpler components as digestion continues.
3. The acid helps in protein breakdown and kills many harmful bacteria; this does not mean that it kills every bacterium.
4. Mucus protects the stomach lining from the acid, preventing damage while food is being processed in the stomach.

Q4. Trace food through the human digestive system in six stages: mouth, food pipe, stomach, small intestine, large intestine, and removal through the anus. Include the role of the rectum in the final stage. [6 marks]

1. In the mouth, teeth crush and chew food into fine pieces, while saliva moistens it and begins breaking starch into sugar.
2. The tongue pushes softened food into the food pipe, whose walls contract and relax in waves to carry it towards the stomach.
3. The stomach churns food and mixes it with digestive juice, acid, and mucus, producing a partially digested, semi-liquid mass.
4. In the small intestine, digestive secretions continue food breakdown. Digested nutrients pass through the lining into blood, which transports them around the body.
5. The large intestine receives undigested material and absorbs most of the remaining water and some salts, making the waste semi-solid stool.
6. The rectum stores stool until the body is ready to remove it. Its expulsion through the anus is called egestion.

Q5. A breathing model has two balloons representing lungs and a rubber sheet representing the diaphragm. Pulling the sheet down inflates the balloons; releasing it upwards deflates them. Explain inhalation and exhalation through five points covering ribs, diaphragm, chest space, airflow, and the model's observations. [5 marks]

1. During inhalation, the ribs move up and outwards. During exhalation, they move down and inwards, contributing to changes in the chest.
2. The diaphragm moves downwards during inhalation and upwards during exhalation. It is the dome-shaped muscle located below the lungs.
3. Space inside the chest increases during inhalation because of the rib and diaphragm movements, and decreases during exhalation.
4. Air enters the lungs when chest space increases. When the space decreases during exhalation, air is pushed out of the lungs.
5. The balloons inflate when the sheet is pulled down, corresponding to inhalation, and deflate when it is released upwards, corresponding to exhalation.

Q6. Inhaled air contains nearly 21% oxygen and nearly 0.04% carbon dioxide. Exhaled air contains nearly 16 to 17% oxygen and nearly 4 to 5% carbon dioxide. Compare the oxygen amounts, compare the carbon dioxide amounts, and assess the claim that all inhaled oxygen is used up. [3 marks]

1. Exhaled air contains a smaller percentage of oxygen: nearly 16 to 17%, compared with nearly 21% in inhaled air.
2. Exhaled air contains a larger percentage of carbon dioxide: nearly 4 to 5%, compared with nearly 0.04% in inhaled air.
3. The claim is incorrect because oxygen remains in exhaled air. The figures show that not all the oxygen taken in during breathing is used up.

Q7. Explain gas exchange at the human alveoli in four points: their wall and blood-vessel arrangement, movement of oxygen, movement of carbon dioxide, and the difference between breathing and respiration. [4 marks]

1. Alveoli are small air sacs with thin walls surrounded by fine blood vessels, bringing the blood close to the air inside them.
2. Oxygen passes from the alveoli into the blood, which carries it to different parts of the body.
3. Blood brings carbon dioxide from the body to the alveoli, where the gas passes into the air for removal.
4. Breathing physically brings air in and sends it out; respiration chemically uses oxygen to break down glucose and release energy.

Q8. Identify the gas-exchange structure in each case and describe its role: fish in water, tadpoles, adult frogs on land and in water, and earthworms. Give one point for each case. [4 marks]

1. Fish use gills, which have a rich supply of blood vessels, to exchange oxygen and carbon dioxide with gases dissolved in water.
2. Tadpoles breathe through gills during this young stage of a frog's life, before the adult breathing arrangement develops.
3. Adult frogs use lungs to breathe on land and their skin for gas exchange when they stay in water.
4. Earthworms exchange oxygen and carbon dioxide through their moist skin, illustrating another breathing arrangement suited to an animal's way of life.

Key takeaways

• Digestion begins in the mouth: teeth break food into fine pieces, while saliva starts converting starch into sugar.
• The stomach churns food and adds digestive juice, acid, and mucus; its contents remain partially digested.
• The small intestine receives digestive secretions and primarily absorbs digested nutrients through its thin lining into the blood.
• The large intestine absorbs most remaining water and some salts; stool is stored in the rectum before egestion.
• Ruminants bring partially digested food back for chewing, while birds break down food through movements of the gizzard.
• Inhalation increases chest space as ribs move up and outwards and the diaphragm moves downwards.
• Alveoli exchange gases with blood; respiration uses oxygen to break down glucose, forming carbon dioxide and water and releasing energy.
• Breathing structures vary: fish and tadpoles use gills, adult frogs use lungs or skin, and earthworms use moist skin.

Test yourself

Why can chapati begin to taste sweet after prolonged chewing?

Saliva contains a digestive juice that helps break the starch in chapati into sugar, producing the sweet taste.

What pushes food through the food pipe towards the stomach?

Its walls contract and relax in a wave-like movement that pushes the softened food forwards.

How does mucus help protect the stomach?

Mucus protects the stomach lining from the acid in the stomach secretion, preventing damage.

Why are finger-like projections useful in the small intestine?

They increase the surface area for efficient absorption and allow digested nutrients to pass into the blood.

Why might a cow chew when it is not actively grazing?

It may be chewing partially digested food brought back from the stomach to the mouth during rumination.

Which movements increase space inside the chest during inhalation?

The ribs move up and outwards, while the diaphragm moves downwards, increasing chest space so air enters the lungs.

Does a milky lime-water result with exhaled air prove that inhaled air contains no carbon dioxide?

No. The comparison indicates more carbon dioxide in exhaled air. Inhaled air also contains carbon dioxide, nearly 0.04%.

What connects absorbed nutrients and oxygen with the body parts that need them?

The circulatory system transports them in blood. The heart pumps blood through blood vessels to different parts of the body.