Biology · Chapter 04

🍔 Digestive System

From bite to nutrient — full journey.

💡 From Bite to Nutrient

The digestive system breaks down food into tiny molecules that cells can absorb. The journey takes 24-72 hours and travels through a ~9 metre long tube.

The 6-step journey:

1. Mouth — teeth crush, saliva (with amylase) starts breaking down starch
2. Oesophagus — muscular tube, pushes food down by peristalsis
3. Stomach — HCl + pepsin digest proteins. Food becomes thick paste (chyme).
4. Small intestine — main absorption. Bile from liver, enzymes from pancreas. Walls have villi (finger-like projections) for max surface area.
5. Large intestine — absorbs water, packs waste.
6. Rectum/anus — exit.

🧪 Three key digestive enzymes (NEET favorite)

• Amylase (in saliva, pancreas) — starch → sugar
• Pepsin (in stomach) — proteins → peptides
• Lipase (in pancreas) — fats → fatty acids

Each enzyme works at a specific pH. Pepsin needs the stomach's acidic environment (pH 1.5-2). Once food moves to small intestine (alkaline, pH 8), pepsin stops and other enzymes take over.

Human digestive system — complete pathwayWikimedia Commons / CC BY-SA 3.0

Human stomach — gastric acid and enzyme secretionWikimedia Commons / CC BY-SA 3.0

🟡 The role of bile

Bile is produced by the liver, stored in the gall bladder, and released into the small intestine. It doesn't digest fats chemically — it just emulsifies them (breaks big fat drops into tiny ones) so enzymes can attack them faster.

Without bile, fat digestion would be 90% slower.

🔍 Villi — the absorption hack

The inner wall of the small intestine has millions of villi (tiny finger-like projections). Each villus has even smaller microvilli. Total absorptive surface area = ~30 m² — about the size of a tennis court, packed into 6 metres of intestine.

🎬

Watch Food Travel Through You

Animation

A bite of food takes about 24-72 hours to fully travel. Click any organ for details!

🔬

Match Enzyme to Substrate

Interactive

Pick a food type — see which enzyme breaks it down and where.

Main nutrientStarch (carbohydrate)

Where digestion startsMouth (saliva)

Main enzymeAmylase

Broken intoGlucose (simple sugar)

Practice (NCERT): Why does bread taste sweet if you chew it for a long time?

Bread contains starch. When you chew, salivary amylase (also called ptyalin) starts breaking starch into maltose and glucose — which are simple sugars and taste sweet. The longer you chew, the more starch gets converted, so the sweeter it tastes.

Practice (NEET): Pepsin works at acidic pH but trypsin works at alkaline pH. Why?

Both pepsin and trypsin are protein-digesting enzymes, but they evolved to work in different parts of the digestive tract:
• Pepsin works in the stomach where HCl makes the pH ~1.5-2 (acidic). Pepsin's active site needs that acidity to function.
• Trypsin works in the small intestine where pancreatic juice and bile make the pH ~7.5-8 (alkaline). Trypsin's structure works only in this alkaline environment. Each enzyme is optimized for its specific location.