• Bees and sunbirds suck the nectar of flowers; infants of humans and other mammals feed on their mother’s milk; snakes like the python swallow their prey whole; some aquatic animals filter tiny food particles from water.
• Food contains complex components — carbohydrate, protein and fat — which must be broken down into simpler forms before the body can use them.
• This breaking down happens in a long tube called the ALIMENTARY CANAL, which starts at the mouth and ends at the anus. Digestive juices secreted at different parts break the food down; the simple food is then absorbed and transported to all parts of the body.

How to explore it further

If a blue-black colour still appears in test tube B, increase the chewing time (chew longer than 60 seconds) and repeat. With more chewing, more starch is digested, so the blue-black colour becomes fainter or disappears.

Food pipe (oesophagus)

• The tongue mixes chewed food with saliva and pushes it into the oesophagus.
• The walls of the food pipe gently contract and relax in a wave-like motion, pushing the food down into the stomach.
• This movement takes place throughout the alimentary canal and keeps the food moving forward.

Stomach

The stomach walls also contract and relax to churn the food. Food leaves the stomach partially digested, as a semi-liquid mass.

• The small intestine is about 6 metres long — almost twice the height of a classroom. It is the longest part of the alimentary canal (it is called “small” because it is narrower).

Absorption

• The large intestine is about 1.5 metres long — shorter than the small intestine — but it is called “large” because it is WIDER.
• It absorbs water and some salts from the undigested food, making the waste semi-solid. This semi-solid waste is called STOOL.
• The stool is stored in the lower part of the large intestine, called the RECTUM, and is finally expelled through the anus — this process is called EGESTION.
• Eating fibre-rich food (fruits, vegetables, whole grains) makes the stool easier to pass.

Cows — rumination

Birds — the gizzard

• Birds have no teeth. They have a muscular chamber called the GIZZARD.
• Food is broken down by the contraction and relaxation of the walls of the gizzard, often with the help of grit (small stones) that birds swallow.

The real mechanism of breathing

Respiration

Food is broken down into simple substances like sugar (glucose). Oxygen helps break down glucose to release energy — this is respiration.

Note: not all the oxygen is used up — that is why exhaled air still has 16–17% oxygen. (Some other animals can use a larger fraction of the oxygen.)

Answer: (iii) To help in inhalation and exhalation

• The diaphragm is a dome-shaped muscle below the lungs. It moves down during inhalation (space in the chest increases → air enters) and up during exhalation (space decreases → air is pushed out).
• (i) is wrong — hair and mucus in the nasal passages filter the air.
• (ii) is wrong — sound is produced in the voice box, not the diaphragm.
• (iv) is wrong — oxygen is absorbed into the blood in the alveoli.

Questions Sanvi can ask

1. “If breathing and respiration are the same, then where does the energy in our body come from?” — Breathing only moves air in and out; it is respiration that releases energy from glucose.
2. “Where exactly does each process take place?” — Breathing takes place in the lungs; respiration takes place inside the body, in every part that needs energy.
3. “Is a new substance formed in each?” — In breathing, no new substance is formed (it is a physical process). In respiration, carbon dioxide and water are formed from glucose and oxygen (it is a chemical process).
4. “Then why do we need food at all — is oxygen alone enough?” — Respiration needs glucose from food as well as oxygen; breathing brings in only the oxygen.
5. “Do fish and earthworms also ‘breathe’ through lungs?” — They breathe through gills and moist skin, yet respiration goes on in their bodies just the same — so the two processes cannot be identical.

Tanu’s statement is the most correct: “We inhale air rich in oxygen.”

1. The nasal passages have tiny hair and mucus whose job is to trap dust and dirt from the air we breathe in.
2. When the air carries a lot of dust, these particles irritate the inner lining of the nose.
3. The body answers with a sneeze — a sudden, forceful rush of air out through the nose and mouth that throws the dust particles out.
4. So sneezing is a natural defence (a reflex) that stops the dust and germs from going further down the windpipe and lungs, where they could cause infection or breathing problems.

1. Anusha may have run faster, longer or harder. More muscular work → the muscles need more energy → more oxygen is needed and more carbon dioxide is produced → she must breathe faster to take in oxygen and throw out CO₂.
2. Anusha may be less used to exercise (less fit). A person who exercises regularly has more efficient lungs and heart, so the same amount of running makes them breathe less rapidly. Paridhi may be the fitter of the two.
3. Difference in body/lung capacity. A smaller lung capacity means less air per breath, so the number of breaths per minute must increase to get the same amount of oxygen.
4. Anusha may have taken less rest before counting, or may have been carrying more weight, or was not breathing properly through the nose while running.

Yadu wants to test whether SALIVA breaks down (digests) STARCH.

Why the experiment is well designed

• Test tube A is the control — everything is the same except the saliva. So any difference must be caused by the saliva.
• He waited 35–45 minutes to give the saliva enough time to act.

What she is investigating

She is trying to find out whether the air we EXHALE contains more carbon dioxide than the air we INHALE.

How she can confirm her findings

1. Repeat the experiment several times and check that the result is the same every time.
2. Take equal amounts of freshly prepared lime water in both test tubes, and pass equal volumes of air for the same length of time — so that the only difference is inhaled vs exhaled air.
3. Compare how quickly the lime water turns milky, or count the number of breaths needed to make it milky — tube B will need far fewer.
4. Ask different classmates to repeat it, so the result does not depend on one person.
5. Use a third tube as a control with lime water and no air passed through it — it should stay clear.

1. Good practices for oral hygiene

• Brush twice a day and clean the tongue; rinse the mouth with water after every meal.
• Reduce sugary and sticky food; do not sleep after eating sweets without rinsing.
• Visit a dentist for regular check-ups; change the toothbrush every few months.
• Traditional ways of our elders: chewing neem or babool twigs (datun), rinsing with warm salt water, using salt and mustard oil, and chewing fennel/clove after meals.

2. Ways to maintain a healthy digestive system

• Eat fibre-rich food — fruits, vegetables, whole grains, millets.
• Eat fermented foods — curd, buttermilk, kanji, pickles, gundruk, poita bhat — good for the helpful bacteria in the large intestine.
• Chew food well, eat at fixed timings, practise mindful eating and avoid overeating (Charaka Samhita).
• Drink enough water, exercise daily, and use digestion-friendly spices such as ginger, black pepper and cumin.

3. A 3-D clay model of the digestive system

Use coloured clay to shape the mouth, oesophagus, stomach, liver, pancreas, small intestine, large intestine, rectum and anus. Keep the small intestine coiled and longest (~6 m) and the large intestine shorter but wider (~1.5 m). Label every part with black paper strips — refer to Fig. 9.1 and Fig. 9.4.

4. Air quality and AQI

5. Box-breathing

• Breathe in for a count of 4 → hold for 4 → breathe out for 4 → hold for 4. Repeat — the four equal steps form the four sides of a “box”.
• Benefits: calms the mind, reduces stress and anxiety, improves concentration, slows the heart rate, and improves the control of the diaphragm and lungs.

6. How are birds adapted to fly where oxygen is low?

• Birds have air sacs connected to their lungs, so fresh air keeps flowing through the lungs in one direction — during both inhalation and exhalation. Human lungs, in contrast, mix fresh air with used air.
• Because of this, birds take up oxygen far more efficiently from each breath than mammals do.
• They also have light, hollow bones, strong flight muscles and blood that carries oxygen well — so they can fly high where the air is thin.