9.1.1 Digestion in Human Beings

• Let us follow a piece of chapati as it travels through the body.
• Each stop has a specific job.

Stop 1 — The Mouth

• Food enters the mouth and the journey begins.
• Teeth crush and chew food into smaller pieces — this is called mechanical digestion .
• At the same time, saliva mixes with the food.
• Saliva contains a digestive juice that breaks starch into sugar .
• That is why chapati tastes sweet when you chew it for a long time!

• Mechanical digestion = teeth breaking food into smaller pieces.
• Saliva = digestive juice that breaks starch into sugar.
• Saliva also moistens food, making it soft and easy to swallow.

🏥 Science and Society — Oral Hygiene

A healthy mouth needs good oral hygiene.
Brush teeth and clean tongue twice a day.
Rinse mouth with water after every meal — this prevents tooth decay and bad smell.
Find out how your elders maintained oral hygiene — they may have used neem twigs (datun)!

🔬 Activity 9.1 — Let Us Investigate the Action of Saliva on Starch

Materials needed

Two test tubes labelled A and B.
Boiled rice.
Water (3–4 mL for each tube).
Iodine solution.
Dropper.

Procedure

Take one teaspoonful of boiled rice in test tube A.
Take a teaspoonful of boiled rice after chewing it for 30–60 seconds in test tube B.
Add 3–4 mL of water to both test tubes.
Note the initial colour of the rice-water mixture in Table 9.1.
Add 3–4 drops of iodine solution into each test tube using a dropper.
Mix the contents of each test tube separately and observe.
Record your observations in Table 9.1.

Table 9.1 — Action of Saliva on Starch

Test Tube	Initial Colour (Before Iodine)	Final Colour (After Iodine)	Possible Reason
A: Boiled rice (not chewed)	White/cream	Blue-black	Starch is present — iodine reacts with starch to give blue-black colour
B: Chewed boiled rice	White/cream	No change or very light blue-black	Starch broken down by saliva into sugar — little or no starch remains

Observation

Test tube A turned blue-black after iodine was added.
Test tube B showed no colour change, or only a very light blue-black.

Explanation

Iodine turns blue-black when starch is present.
Test tube A still has starch — no saliva acted on it — so it turns blue-black.
Test tube B: saliva broke the starch into sugar — no starch left — no colour change.
If colour still appears in test tube B, try chewing for a longer time and repeat the activity.

→ Saliva converts starch to sugar.

◆ Summary

• Blue-black = starch present
• No colour = starch broken down
• Saliva converts starch to sugar
• Digestion begins in the mouth

• Food is partially digested in the mouth.
• Let us follow this partially digested food further through the alimentary canal.

Stop 2 — The Food Pipe (Oesophagus)

• Saliva moistens food, making it soft and easy to swallow.
• The tongue mixes chewed food with saliva and pushes it into the food pipe (oesophagus).
• The walls of the food pipe contract and relax in a wave-like motion .
• This pushes the food down into the stomach.
• This wave-like movement happens throughout the alimentary canal and pushes food forward.

• Oesophagus = food pipe that connects mouth to stomach.
• Wave-like motion of walls pushes food forward throughout the alimentary canal.

Stop 3 — The Stomach

• The stomach walls contract and relax to churn the food.
• The churned food is mixed with a secretion from the inner lining of the stomach.
• This secretion contains three things: digestive juice, acid, and mucus .
• Food is partially digested here and becomes a semi-liquid mass.

• Digestive juice = breaks down proteins in food into simpler components.
• Acid = helps break proteins and kills many harmful bacteria.
• Mucus = protects the stomach lining from the acid.

🔬 Fascinating Fact — How Did Scientists Learn About Digestion?

In 1822, a man named Alexis St. Martin was accidentally shot in the stomach.
He was treated by Dr. William Beaumont.
His wound never fully healed — it left a small permanent hole in his stomach.
Dr. Beaumont used this opening to observe digestion as it happened.
He studied how different foods were broken down and how emotions affect digestion.

Stop 4 — The Small Intestine

• After the stomach, partially digested food moves into the small intestine.
• The small intestine is almost 6 metres long — almost twice the height of your classroom!
• It receives digestive secretions from three sources: its own inner lining, the liver , and the pancreas .
• The inner lining has thousands of finger-like projections that increase the surface area for absorption.
• Digested nutrients pass through these projections into the blood.

• Bile (from liver) = neutralises acids and breaks fats into tiny droplets.
• Pancreatic juice = breaks carbohydrates, proteins, and fats into simpler forms.
• Finger-like projections = increase surface area for efficient absorption.
• Absorption = digested nutrients pass into blood through the small intestine walls.

🏥 Science and Society — Celiac Disease

Celiac disease is a condition where the body reacts to gluten — a protein found in wheat, barley, and rye.
This reaction damages the inner lining of the small intestine.
As a result, the intestine cannot absorb nutrients properly.
The only way to manage it is to avoid foods with gluten.
Millets like jowar, bajra, and ragi are good alternatives — they are naturally gluten-free.

Stop 5 — The Large Intestine

• After most nutrients are absorbed, undigested food moves into the large intestine.
• The large intestine is about 1.5 metres long — shorter but wider than the small intestine.
• It absorbs water and some salts from the undigested food.
• The remaining waste becomes semi-solid — this is called stool .
• Stool is stored in the rectum until it is expelled through the anus — called egestion .

• Large intestine = absorbs water and salts from undigested food.
• Stool = semi-solid waste formed after water absorption.
• Rectum = lower part of large intestine where stool is stored.
• Egestion = expulsion of waste through the anus.

🔬 Fascinating Fact — Gut Bacteria

The large intestine contains many tiny living organisms, including bacteria, that help in digestion.
They break down undigested food, especially fibre, and produce essential nutrients.
Fermented foods like curd, buttermilk, shrikhand, kanji, pickles, gundruk, and poita bhat are very good for a healthy digestive system.

🏥 Science and Society — Ancient Wisdom on Digestion

The Charaka Samhita , an ancient Ayurvedic text, highlights the role of easily digestible foods.
It recommends spices like ginger, black pepper, and cumin to enhance digestion.
Modern science also emphasises eating at proper timings, mindful eating, and avoiding overeating.
Eating fibre-rich foods like fruits, vegetables, and whole grains helps the large intestine function properly.