What is mechanical digestion? Why is chewing food considered a part of mechanical digestion?
Answer: Mechanical digestion is the initial breakdown of food into smaller pieces by physical action. Chewing breaks food into fine pieces without changing it into a new substance, so it is a form of mechanical digestion.
Q2. 2 Marks
A piece of chapati tastes sweeter after it is chewed for a long time. Give the reason.
Answer: Chapati contains starch. Saliva contains a digestive juice that breaks down starch into simpler sugars. Therefore, after prolonged chewing, the chapati begins to taste sweet.
Q3. 2 Marks
Why does food move down the food pipe even when we are not standing upright?
Answer: The walls of the food pipe contract and relax in a wave-like motion. This movement pushes food forward towards the stomach through the alimentary canal.
Q4. 2 Marks
State two functions of the acid and mucus present in the stomach.
Answer: Stomach acid helps break down proteins and kills many harmful bacteria. Mucus protects the stomach lining from the acid and prevents damage.
Q5. 2 Marks
What are absorption and egestion?
Answer:Absorption is the process by which digested nutrients pass from the small intestine into the blood. Egestion is the process by which undigested waste is expelled from the body through the anus.
Q6. 2 Marks
Why is the small intestine well suited for efficient absorption of nutrients?
Answer: The inner lining of the small intestine is thin and has thousands of finger-like projections. These increase the surface area and allow digested nutrients to pass efficiently into the blood.
Q7. 2 Marks
What is rumination? Name one animal that shows rumination.
Answer: Rumination is the process in which partially digested food is brought back to the mouth for gradual chewing. A cow is an example of a ruminant.
Q8. 2 Marks
Differentiate between breathing and respiration.
Answer: Breathing is the physical process of inhaling and exhaling air. Respiration is a chemical process in which oxygen helps break down glucose to release usable energy.
Q9. 2 Marks
Why does lime water turn milky when exhaled air is passed through it?
Answer: Exhaled air contains more carbon dioxide than inhaled air. Carbon dioxide reacts with lime water and makes it turn milky or cloudy.
Q10. 2 Marks
Name the breathing structures used by (a) fish and (b) earthworms.
Answer: (a) Fish use gills. (b) Earthworms use their moist skin for exchange of oxygen and carbon dioxide.
Section C • 3-Mark Questions
Q1. 3 Marks
Explain the role of the liver, pancreas and small intestine in digestion.
Answer: The liver secretes bile, which is mildly basic, neutralises acids from the stomach and breaks fats into tiny droplets. The pancreas secretes pancreatic juice that helps neutralise acids and breaks down carbohydrates, proteins and fats. The small intestine receives these digestive secretions and further breaks down food into simpler forms; its wall then absorbs the digested nutrients.
Q2. 3 Marks
Trace the journey of food through the human alimentary canal from the mouth to the anus. Mention the main function of the small intestine in this journey.
Answer: Food passes through mouth → oesophagus (food pipe) → stomach → small intestine → large intestine → rectum → anus. In the small intestine, digestion is completed and the digested nutrients are absorbed into the blood through its walls.
Q3. 3 Marks
In an iodine test, boiled rice turns blue-black but chewed boiled rice shows no or only a very light blue-black colour. Explain the observation.
Answer: Iodine gives a blue-black colour in the presence of starch. Boiled rice contains starch, so it turns blue-black. During chewing, saliva acts on starch and breaks it into simple sugars. Therefore, chewed rice contains much less starch and shows no or only a slight blue-black colour.
Q4. 3 Marks
Explain inhalation and exhalation with reference to the movement of ribs and diaphragm.
Answer: During inhalation, the ribs move up and outwards and the diaphragm moves downwards. This increases the space inside the chest and air enters the lungs. During exhalation, the ribs move down and inwards and the diaphragm moves upwards, reducing the space and forcing air out of the lungs.
Q5. 3 Marks
How are alveoli adapted for exchange of gases? Why is efficient gas exchange important?
Answer: Alveoli have thin walls and are surrounded by fine blood vessels. Oxygen passes from the air in the alveoli into the blood, while carbon dioxide passes from the blood into the alveoli. This supplies oxygen to body tissues for respiration and removes carbon dioxide.
Q6. 3 Marks
A frog can live both on land and in water. Explain how its breathing mechanism is suited to these two habitats.
Answer: Tadpoles breathe through gills. Adult frogs use lungs for breathing on land and their skin for gas exchange when they stay in water. Thus, different breathing structures help the frog survive in both habitats.
Q7. 3 Marks
Why should we breathe through the nose rather than the mouth? Explain using the structures involved.
Answer: Air entering through the nostrils passes through the nasal passages. Tiny hairs and mucus trap dust and dirt from the inhaled air. Thus, the nose helps filter the air before it reaches the lungs.
Q8. 3 Marks
An athlete has completed a race and is breathing faster than before the race. Give two scientific reasons that can explain the increased breathing rate.
Answer: During running, the muscles need more energy. Respiration requires oxygen to break down glucose and release usable energy. Therefore, breathing becomes faster to supply more oxygen and remove the increased carbon dioxide produced by the body.
Q9. 3 Marks
Compare the breathing mechanisms of fish, earthworms and frogs. How are these mechanisms suited to their habitats?
Answer: Fish use gills, earthworms use their moist skin, and tadpoles use gills. Adult frogs use lungs on land and skin for gas exchange in water. These differences help animals breathe according to their habitats.
Section D • 4-Mark Case/Source-Based Questions
Case Study 1 • Digestion and Saliva
Riya chewed a piece of chapati for about one minute. She noticed that its taste changed from its usual taste to a sweeter taste. Her teacher asked her to compare this with unchewed boiled rice using an iodine test.
(a) Which nutrient component in chapati is responsible for the change being studied?
(b) Which substance in saliva acts on it?
(c) What does iodine indicate in this experiment?
(d) Why would chewed rice show less blue-black colour than unchewed rice?
Answers: (a) Starch. (b) Digestive juice in saliva. (c) Presence of starch by a blue-black colour. (d) Saliva breaks down starch into simpler sugars, reducing the amount of starch present.
Case Study 2 • Journey of Food
A student traced the path of food after it enters the mouth. The food is chewed and mixed with saliva, pushed through the food pipe, churned in the stomach and then sent to the small intestine.
(a) What movement pushes food through the food pipe?
(b) Name the three sources of digestive secretions received by the small intestine.
(c) What is the role of bile?
(d) Where are most digested nutrients absorbed?
Answers: (a) Wave-like contraction and relaxation of the walls. (b) Inner lining of the small intestine, liver and pancreas. (c) It neutralises acids and breaks fats into tiny droplets, making digestion easier. (d) Small intestine.
Case Study 3 • Breathing Model
A model is made using a bottle, two balloons attached to a Y-shaped tube and a rubber sheet at the bottom. When the rubber sheet is pulled down, the balloons inflate. When it is released upwards, the balloons deflate.
(a) What do the balloons represent?
(b) What does the rubber sheet represent?
(c) What happens to the diaphragm during inhalation?
(d) Why do the lungs inflate when the diaphragm moves downwards?
Answers: (a) Lungs. (b) Diaphragm. (c) It moves downwards. (d) The downward movement increases the space inside the chest, allowing air to enter the lungs.
Case Study 4 • Inhaled and Exhaled Air
A teacher passes inhaled air into lime water in one test tube and exhaled air into lime water in another. Only the lime water receiving exhaled air turns milky.
(a) Which gas causes the lime water to turn milky?
(b) Which sample contains more of this gas?
(c) What does this experiment show about inhaled and exhaled air?
(d) Why does the amount of carbon dioxide increase in exhaled air?
Answers: (a) Carbon dioxide. (b) Exhaled air. (c) Exhaled air contains more carbon dioxide than inhaled air. (d) Respiration produces carbon dioxide as glucose is broken down.
Case Study 5 • Different Animals, Different Breathing Structures
A teacher asks students to compare a fish, an earthworm and a frog. The students recall that these animals live in different habitats and use different structures for gas exchange.
(a) What structure does a fish use for breathing?
(b) What does an earthworm use for gas exchange?
(c) What does an adult frog use on land?
(d) What can an adult frog use when it stays in water?
Answers: (a) Gills. (b) Moist skin. (c) Lungs. (d) Skin for gas exchange.
Section E • 5-Mark Questions
Q1. 5 Marks
Explain the journey of food through the human alimentary canal. Mention the main function of each major part.
Food journey through the human alimentary canal
Answer:
Food passes through mouth → oesophagus (food pipe) → stomach → small intestine → large intestine → rectum → anus.
• Mouth: Food is chewed and mixed with saliva; digestion of starch begins.
• Oesophagus: Wave-like contraction and relaxation pushes food towards the stomach.
• Stomach: Food is churned and mixed with digestive juice, acid and mucus; proteins begin to break down.
• Small intestine: Digestion is completed and digested nutrients are absorbed into the blood.
• Large intestine: Most remaining water and some salts are absorbed; the waste becomes semi-solid and is later expelled.
Q2. 5 Marks
Explain the mechanism of breathing in humans. Also state why breathing becomes faster after running and why we may sneeze when we inhale dust-laden air.
Answer:
During inhalation, the ribs move up and outwards and the diaphragm moves downwards. The chest space increases and air enters the lungs. During exhalation, the ribs move down and inwards and the diaphragm moves upwards. The chest space decreases and air is forced out.
After running, muscles need more energy, so the body needs more oxygen and has to remove more carbon dioxide; therefore breathing becomes faster. We may sneeze after inhaling dust-laden air as a protective response that helps deal with irritating dust particles entering the respiratory passages.
Q3. 5 Marks
Breathing and respiration are not the same. Explain the difference and describe the role of oxygen, glucose, energy and carbon dioxide in respiration.
Answer: Breathing is a physical process of inhaling and exhaling air. Respiration is a chemical process in which oxygen helps break down glucose to release usable energy.
The word equation is: Glucose + Oxygen → Carbon dioxide + Water + Energy.
Thus, breathing brings oxygen into the body and removes carbon dioxide, while respiration uses oxygen to release energy from glucose and produces carbon dioxide and water.
Q4. 5 Marks
Describe the iodine experiment used to investigate the action of saliva on starch. Include the expected observations and conclusion.
Answer:
1. Take two test tubes. Put boiled rice in test tube A and chewed boiled rice in test tube B.
2. Add a little water and a few drops of iodine solution to both test tubes.
3. The boiled rice in test tube A turns blue-black because it contains starch.
4. The chewed rice in test tube B shows no blue-black colour or only a very light blue-black colour because saliva has acted on the starch.
5. Conclusion: Saliva helps in the digestion of starch by changing it into simpler sugars.
Q5. 5 Marks
A student wants to investigate whether exhaled air contains more carbon dioxide than inhaled air. Describe the lime-water experiment, observation and conclusion.
Answer:
1. Take two test tubes, A and B, and put equal amounts of lime water in both.
2. Pass inhaled air into test tube A and exhaled air into test tube B.
3. The lime water in test tube B turns milky, while the lime water in A does not turn milky in the same way.
4. Lime water turns milky when it reacts with carbon dioxide.
5. Conclusion: Exhaled air contains more carbon dioxide than inhaled air.
Q6. 5 Marks
Answer the following in the context of the human respiratory system.
Fill in the blanks:
(A) The process of inhaling and exhaling air is called ________.
(B) The tiny balloon-like sacs in the lungs are called ________.
(C) The dome-shaped muscle below the lungs is the ________.
(D) The process by which nutrients are converted into usable energy is called ________.
(E) Fish use specialised structures called ________ for breathing.
Match the columns:
Column A
Column B
A. Nostrils
1. Exchange of gases
B. Nasal passages
2. Helps trap dust and dirt
C. Windpipe
3. Air reaches the lungs through this part
D. Alveoli
4. Fresh air from outside enters
E. Rib cage
5. Protects the lungs
Answers: Fill in the blanks: (A) breathing (B) alveoli (C) diaphragm (D) respiration (E) gills. Match: A–4, B–2, C–3, D–1, E–5.